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Frontal eddies along a western boundary current
Author’s Accepted Manuscript Frontal Eddies along a Western Boundary Current Joachim Ribbe, Liv Toaspern, Jörg-Olaf Wolff, Mochamad Furqon Azis Ismail
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S0278-4343(17)30586-1 https://doi.org/10.1016/j.csr.2018.06.007 CSR3777
To appear in: Continental Shelf Research Received date: 10 November 2017 Revised date: 6 June 2018 Accepted date: 13 June 2018 Cite this article as: Joachim Ribbe, Liv Toaspern, Jörg-Olaf Wolff and Mochamad Furqon Azis Ismail, Frontal Eddies along a Western Boundary Current, Continental Shelf Research, https://doi.org/10.1016/j.csr.2018.06.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Frontal Eddies along a Western Boundary Current
a*
b
b
Joachim Ribbe , Liv Toaspern , Jörg-Olaf Wolff , Mochamad Furqon Azis Ismail
a, c
Faculty of Health Engineering and Sciences, School of Agricultural, Computational and Environmental Sciences, University of Southern Queensland, Toowoomba 4350, Queensland, Australia. a
b
Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky University of Oldenburg, 26129 Oldenburg, Germany.
a,c
Research Center for Oceanography, Indonesian Institute of Sciences, Jakarta 14430, Indonesia.
*Corresponding author: [email protected]
1
Abstract
Cyclonic frontal eddies are distinguishable from the surrounding water due to their unique biological and physical characteristics and have been observed in all western boundary current regions. These eddies spawn from cut-off meanders and are found on the landward side of the current. Here, we report for the first time observed frontal eddies for the intensification zone (north of 28 oS) of the East Australian Current (EAC) off Southeast Queensland, Australia, by analysing remotely sensed sea surface temperature (SST) and chlorophyll-a (Chl-a) data. The frontal eddies were detected initially in the analysis of satellite tracked surface drifters. The shelfcrossing cyclonic drifter pathways indicated the presence of drifter-trapping cyclonic frontal eddies. The subsequent analysis of satellite images allows to quantify key eddy characteristics, cross-shelf volume transports associated with eddy filaments, eddy-driven shelf water renewal time scales, and export of total Chl-a and carbon per day. The observed frontal eddies have core radii of approximately 13 km and 15 km. The cold core surface SST anomaly and elevated chl-a indicates eddy entrainment of shelf water. The translational or core displacement velocity is estimated with 0.17 m.s-1 or 15 km per day and the tangential velocity quantified from tracking surface drifters is 0.28 m.s-1 to - 0.5 m.s-1. This results in a rotational period of 1.9 days to 3.9 days. We use maximum Chl-a and SST gradients to approximate the width of importing and exporting filaments associated with the
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frontal eddy to derive volume transports of 1.5 Sv and 1.9 Sv (import) and 0.3 Sv and 1.8 Sv (export), respectively. Chl-a concentrations of the exporting filaments are about 0.4 mg.m-3 to 0.6 mg.m-3 yielding a total export of 13 t to 78 t of Chl-a per day. The frontal eddy induced on-shelf transport of 130 km3 - 160 km3 per day represents between 18 % and 22 % of the shelf volume. Therefore, it would take approximately five days to renew all shelf water. We conclude that the observed frontal eddies of the northern intensification zone of the EAC potentially play an important role in determining cross-shelf exchanges, contribute to on-shelf marine conditions, enhancing coastal primary productivity and are possibly important to the export of shelf water properties such as the fish larvae of subtropical species via entrainment.
3
Graphical abstract
Keywords
Western boundary current; frontal eddy; cross-shelf transport; drifter, sea surface temperature; chlorophyll-a; East Australian Current
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1
Introduction
Cyclonic frontal eddies typically form on the land-ward side of all the western boundary currents (WBCs), are important in driving cross-shelf exchanges and are less-well understood than the larger mesoscale eddies (e.g. Koshlyakov and Monin 1978, Lee 1975, Lee et al. 1981, Yoder et al. 1981, Lutjeharms and Stockton 1987, Kasai et al. 2002, Sponaugle et al. 2005, Lorenzzetti et al. 2009, Matsuno et al. 2009, Mullaney and Suthers 2013, Schaeffer et al. 2017). The term frontal eddies appears to have been coined by Koshlyakov and Monin (1978) and Lee (1975) referred to these previously as spin-off eddies. The biological significance of frontal eddies was first described in Robinson (1983) by Angel and Fasham (1983). Their genesis is associated with the fluxes of carbon, nutrients, pollutants and the entrainment and transport of fish larvae (e.g. Lee et al. 1991, Atkinson et al. 1996, Kimura et al. 1997, Kimura et al. 2000, Nakata et al. 2000, Bakun 2006, Govoni et al. 2010, Mullaney and Suthers 2013, Matis et al. 2015, Brink 2016, Nencioli et al. 2016). Cyclonic frontal eddies are usually short-lived ( < 4 weeks) and are characterised by diameters of 10 to 100 km (e.g. Lee 1975, Sponaugle et al. 2005, Everett 2015, Roughan et al. 2017). Eddy translational (i.e. propagation or the displacement of the eddy core) and tangential velocities were observed with values ranging between 0.1 m.s-1 to 0.8 m3.s1
and 0.3 m3.s-1 to 1.3 m3.s-1, respectively (e.g. Lee and Atkinson 1983, Haus et al.
2000, Everett et al. 2011, Everett et al. 2015). Tangential velocities are generally larger than translational velocities (e.g. Everett et al. 20 15, Nagai et al. 2015, Roughan et al. 2017), and tangential velocities vary across the eddy and are larger where the off-
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shore eddy branch joins the flow of the boundary current (e.g. Schaeffer et al. 2017). A flow in the opposite direction is generated near-shore. The standard model of a cyclonic eddy identifies lower SST and elevated chlorophyll-a (Chl-a) values compared to surrounding water as characteristics (e.g. Bakun 2006).
In regards to on-shelf water renewal processes, the flushing due to a single extreme event such as a cyclonic frontal eddy (Brink 2016) and associated water renewal time scale could potentially be as or more important as the flushing and the associated timescales derived from mean property distributions and residual shelf circulations (e.g. Strutton et al. 1996; Monsen et al. 2002, Ribbe et al. 2008). Brink (2016) refers to the occurrence of mesoscale eddies as dramatic and important events that have the potential to export coastal shelf water volumes in the order of 104 m3.s-1 to 105 m3.s-1. Brink (2016) also indicated the potential for these events to play an important role off south-eastern Australia where highly energetic eddies occur (Everett et al. 2012; Ribbe and Brieva 2016). Yet, only few observational or computational studies exist to-date that quantify volume transports associated with frontal eddies and their filaments, in particular for the East Australian Current (EAC) which is the western boundary current of the South Pacific Ocean gyre. Shapiro et al. (2010) used remotely sensed Chl-a and sea surface height (SSH) anomaly to estimate cross-shelf transports induced by a large cyclonic mesoscale eddy (~ 120 km in diameter) observed in the Black Sea. The transport was calculated by multiplying the velocity within the filament with the width and the estimated vertical extent of the filament. The average velocity was estimated as 0.15 m.s-1, the width of the filament as 20 km,
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and the depth was assumed to be 100 m. This resulted in an average transport of 0.3 Sv (1 Sv = .106 m3.s-1). The eddy transported approximately 40% of the shelf water volume into the deep sea during its lifetime of 40 days. Peliz et al. (2004) applied the same approach of estimating the eddy-induced cross-shelf transport due to a large eddy filament to the shelf southwest of the Iberian Peninsula. SSH anomaly derived velocity was estimated with 0.15 m.s-1 resulting in a volume transport of 0.68 Sv and an estimate for the shelf flushing time of 5-6 days. Zhou et al. (2014) found from a computational study of cross-shelf exchanges in the Black Sea that eddies constitute the most efficient transport mechanism. Li et al. (2017) studied cross-shelf exchanges associated with cold filaments for the South China Sea. These cold filaments develop during the seasonal East Asian Monsoon transition and due to directional changes in the prevailing wind direction, thus are not frontal eddies. Nevertheless, Li et al. (2017) documented a significant offshore filament induced transport of 1.8 Sv due to the mean filament's estimated width of 100 km and a higher velocity estimate of 0.3 m.s-1.
Generally, little is known about cyclonic frontal eddies and associated cross-shelf volume exchanges in the EAC region. All previous studies of EAC frontal eddies focus on the region of the separation zone, which is located to the south of 29 oS (Suthers et al. 2011). This region is home to a complex field of mesoscale eddies (radii > 90 km, cyclonic and anticyclonic) which has been referred to as eddy avenue (Everett et al. 2012). Everett et al. (2011) and Mullaeny and Suthers (2013) observed cyclonic frontal eddies with radii of between 15 km and 40-50 km, respectively, and
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focused on biological characteristics of the core. Subsequent inspection of AVHRRSST imagery by Mullaeny and Suthers (2013) found about one submesoscale cyclonic eddy per week in this region. Everett et al. (2015) documented an entrainment of preconditioned shelf water of 0.23 Sv to 0.61 Sv due to an observed, slow moving and long-lived (4 months) large-scale cyclonic eddy (radius > 50 km). Everett et al. (2015) referred to it as a frontal eddy since it existed on the western and landward side of the boundary current. Other observational studies investigating entrainment and biological characteristics of individual ECA frontal eddies include Matis et al. (2014) and Roughan et al. (2017). Schaeffer et al. (2017) identified much smaller frontal eddies of core radii ~10 km with a frequency of approximately one per week and displacement speeds of 0.3 m.s-1 to 0.4 m.s-1 using HF radar observations over a one-year period. Modelling studies by Marchesiello and Middleton (2000), Macdonald et al. (2016) and Mantovanelli et al. (2017) propose a series of physical processes including wind forcing on surface circulation, horizontal shear, and current instabilities that drive or affect the formation of frontal eddies.
In this paper, we document for the first time the occurrence of small scale frontal eddies that appear to characterise the EAC intensification zone (i.e. north of 28 oS) and quantify their characteristics. This region is situated to the north of the previously examined EAC separation zone (Ridgway and Dunn 2003). It is referred to in this paper as the Southeast Queensland Coastal Marine Zone (SEQCMZ). This region is part of the continental shelf of eastern Australia (Figure 1). It encompasses the continental shelf between 25 °S and 27 °S and is bounded by the 150 m isobath to
8
the east. The shelf width of the SEQCMZ varies from 22 km in the south to 45 km in the north, mean water depth is 63 m and the volume occupied by shelf water is 7.4.1011 m3. The climate in this area is subtropical and the maximum rainfall occurs during the southern hemisphere spring and summer (Ribbe 2014). The SEQCMZ borders the intensification zone of the EAC located between 24 °S and 29 °S (Ridgway and Dunn 2003). The EAC is the WBC of the South Pacific Ocean gyre and most EAC studies have focused on the EAC separation zone located south of 29 oS (see e.g. review by Suthers et al. 2011).
Unique physical features of the SEQCMZ's shelf circulation include the quasistationary Southeast Fraser Island Upwelling System (Brieva et al. 2015) during spring and summer, the wind-driven shelf-encompassing cyclonic Fraser Gyre (Figure 1), which establishes itself annually during the autumn and winter months (Azis Ismail et al. 2017), and the shelf hugging EAC. The Fraser Island Upwelling System is primarily driving by bottom layer stress that is generated due to an intense and shelf encroaching EAC during summer. The north- to north-easterly winds play only a moderating role, enhancing or weakening upwelling, but do not drive directly upwelling into the surface layer (Brieva et al. 2015). The offshore branch of the Fraser Gyre joins the southward flowing EAC (Figure 1), while the western branch constitutes a northerly near shore flow of 0.15 -0.26 m.s-1. Brieva et al. (2015) estimated a mean Fraser Gyre driven shelf water renewal time scale of 3.3 days, ranging from monthly estimates of 2-8 days with the longer times during the winter period. Ribbe and Brieva (2016) included this region in a census of eddies, finding
9
that the region frequently generates large (radii > 40 km) mesoscale cyclonic eddies (frontal eddies, about 4-5 per year), however, noted that the census excluded smaller mesoscale frontal eddies (radii < 40 km) since these are not resolved in the gridded SSH anomaly maps and data used by Ribbe and Brieva (2016).
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Figure 1: (a) Geographic location of the study site along the east coast of Australia. (b) Representation of the study site referred to as the Southeast Queensland Coastal Marine Zone (SEQCMZ) as well as the 150 m (dotted), and 1000 m (solid) isobath are shown. Indicated are climatological features characterising the regional circulation and the approximate core location of observed frontal eddies referred to as Event A and Event B. Please see legend for individual geographic features shown. The larger-scale oceanographic characteristics of the study site include: (1) the southward flowing East Australian Current, (2) the Southeast Fraser Island Upwelling System characterised by elevated Chl-a concentration (dashed pattern) from September - January each year (Brieva et al. 2015) and (3) the wind-driven cyclonic on-shelf circulation of the Fraser Gyre that occupies the SEQCMZ from April to August each year (Azis Ismail et al. 2017).
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The SEQMCZ is of significant marine ecological importance and includes one of eight key ecological sites and important fishery along Australia's eastern ocean (Young et al. 2011, Dambacher et al. 2012). It is known as a spawning site for a highly diverse pelagic fish assemblage with larvae transported off-shelf and southward with the EAC away from the SEQCMZ (e.g. Zeller et al. 1996, Ward et al. 2003). Spencer et al. (2017) review physical processes that may affect the catchability of spanner crab, and argue that quantification of on-shelf transports and effects of eddies need to be assessed, including for the SEQCMZ. Everett et al. (2017) highlighted the ecological importance of this region by identifying the coastal waters of Fraser Island through Lagrangian particle tracking studies as an Eastern King Prawn larvae source region. The Fraser Island location was the most northern of all regions considered. It was characterised by the highest long-shelf dispersal speed for larvae estimated with 53 km.day-1 or 0.61 m.s-1. Impacts on the marine environment of the SEQCMZ arise from a range of human activities including tourism. This already brings several millions of visitors into the region. Fraser Island is listed as a UNESCO World Heritage site and is part of the UNESCO designed Great Sandy Biosphere Reserve which includes the surrounding coastal waters (Ribbe 2014). Furthermore, more than two thirds of Queensland's population (over 3 million people) or approximately 13% of Australia's population live nearby and potentially utilise the coastal environment of this region for a range of activities.
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We aim to document for the first time the potential role cyclonic frontal eddies play in driving regional marine environmental conditions and exchanges between the local shelf and the EAC. This includes quantifying frontal eddy characteristics such as translational and tangential velocity, volume transports for on- and off-shore flows using satellite tracked drifters and remotely sensed data, and estimating the associated export of Chl-a and carbon. Ward et al. (2003) concluded that pelagic fish larvae are transported by the EAC away from the spawning waters of the SEQCMZ, and frontal eddies potentially provide a mechanism for the cross-shelf export and injection into the EAC. Overall, little is known about the physical oceanography of this region, with most EAC studies focused on the EAC separation region (Suthers et al. 2011). Following Section 2, which outlines data and method used in this study, we present in Section 3 analysed pathways of remotely tracked drifters in comparison with remotely sensed SST and Chl-a and estimated physical characteristics of the identified frontal eddies. The final Section 4 summarises our findings in the context of the literature and discusses the limitations of the study. Overall, we conclude that frontal eddies in the SEQCMZ are likely to provide a cross-shelf transport process important to regional shelf ecology, potentially precondition the EAC and facilitate the southward advection of shelf water and associated properties into temperate regions of the EAC and the Tasman Sea.
2
Data and Method
2.1 Data 13
Sea Surface Temperature (SST) Gridded SST (oC) derived from the Advanced Very High Resolution Radiometer (AVHRR) was sourced from the Integrated Marine Observing System (IMOS, https://portal.aodn.org.au/). IMOS is a national collaborative research infrastructure, which is supported by the Australian Government. The data are available on a daily basis since August 2002. The spatial resolution of the SST data is 0.02° x 0.02°. This is detailed enough to resolve the small-scale frontal eddies of interest to this study. For the SEQMCZ, the data were previously used to identify the Fraser Gyre (Azis Ismail et al. 2017), the Southeast Fraser Upwelling System (Brieva et al. 2015) and contributed to a regional eddy census (Ribbe and Brieva 2016).
Chlorophyll-a (Chl-a) IMOS provided Chl-a concentration estimates (mg.m-3) are produced using the OC3 algorithm from the MODerate Resolution Imaging Spectroradiometer (MODIS) ocean colour measurements (https://portal.aodn.org.au/, for further details on data processing and limitations see Brieva et al. (2015) and references therein). As we aim for a case study of observed individual frontal eddies in the SEQMCZ and not a systematic eddy census, the large part of cloud covered data does not affect this study (about 63% of all images were excluded in a previous analysis, see Brieva et al. (2015) for statistics). The spatial resolution of the data is 0.01° x 0.01°, and the temporal resolution is the same as that for SST. Both Chl-a and SST data are used to identify the frontal eddies considered in this study and to quantify their
14
characteristics.
O'Reilly et al. (2000) discuss the evaluation of MODIS measurements in order to derive the Chl-a estimates used in this study. This has previously also been discussed and the data applied by Brieva et al. (2015). In shallow coastal water the usability of remotely sensed Chla is also limited due to factors such as bottom albedo, suspended sediment and coastal turbidity (Moses et al. 2012). Yet, our analysis focuses on the mid- to off-shelf region east of the 40 m isobar (within about 10 km of the coast line) with little if at all impact form bottom and suspended sediment as discussed in Brieva et al. (2015).
Drifters The data used in this study originate from drifters that are deployed as part of the Global Drifter Program (GDP) and the data are provided by the Atlantic Oceanographic and Meteorological Laboratory (AOML). Amongst other variables, the data (see http://www.aoml.noaa.gov/envids/data_available.php) includes the tracked location as well as the east- and northward velocity of each drifter four times per day. To achieve a minimal slip due to action by local wind, GDP drifters consist of two parts: A surface float with a spherical design and an attached drogue (Lumpkin and Pazos 2006). While the drogue is still attached, the wind-induced slip of drifters is less than 0.02 m.s-1 for wind velocities of less than 20 m.s-1 (Niller et al. 1987). Pazan and Niller (2001) found that drifters without a drogue have an estimated downwind slip of 8.8.10-2 m.s-1 per 10 m.s-1 of wind speed. Data from satellite-tracked drifters have previously been used for studies of the EAC and the Pacific Ocean region. Early research comprises Nilsson and Cresswell (1980) and
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Cresswell and Legeckis (1986) who investigated warm-cores eddies in the southern part of Eastern Australia. Choukroun et al. (2010) derived surface velocity maps for the Coral Sea from drifter velocities and estimated mean residence times for the Great Barrier Reef lagoon using all AOML data.
In this study, we used data from two drifters that entered the SEQMCZ in a cyclonic fashion indicative of 'trapping' by a cyclonic frontal eddy. These drifters were identified from a subset of drifters analysed in an earlier study by Azis Ismail et al. (2017). However, there are only two drifters within the SEQMCZ that also appeared to coincide with the presence of a small-scale cyclonic frontal eddy. Drifter No. 44318 was deployed on 14/07/2005 and lost its drogue on 16/01/2006 before entering the region of interest on 09/08/2006. Drifter No. 62933 was deployed on 17/05/2008, also lost its drogue on 21/09/2008 and entered the area of interest on 04/05/2009. Drifter locations are reported four times every day and are used to compute mean velocities as well as velocities on crossing the shelf-break. The latter is referred to as the tangential velocity or rotational velocity and is used to quantify both import and export by eddy filaments (Table 1).
2.2 Method
Shapiro et al. (2010) and Peliz et al. (2004) calculated the volume transport of an eddy-associated filament as F = u .d .h
16
(1)
where u (m.s-1) is the velocity within the filament, d (m) the width and h (m) the thickness of the filament. Shapiro et al. (2010) and Peliz et al. (2004) both used the geostrophic velocity derived from SSH anomaly as estimate for the velocity u within the filament. In this study, a similar approach is applied. However, the resolution of the SSH anomaly data is too coarse to compute geostrophic velocities for small-scale frontal eddies (see Ribbe and Brieva 2016). Instead, we use the velocity of the drifter, which was captured by the eddy. In order to estimate the cross-shelf transport, the velocity of the drifter perpendicular to the 150 m isobath at the time of crossing is used as u in equation (1).
The width of the filament is obtained from Chl-a data by measuring the distance between the boundaries of the filament along the 150 m isobath. We define the boundaries of the filaments as the local maxima of the horizontal gradient of the Chla concentration C, which are calculated as:
{|⃗
where ⃗
( ⁄
⁄
|}
{√(
)
(
) }
) is the horizontal gradient operator and x, y are the zonal
and meridional coordinates, respectively. The width of the filament is computed as the distance between two local maxima going along the 150 m isobath from south to north. The same procedure is carried out with the SST data for comparison.
17
Roughan et al. (2017) found a depth of approximate 1000 m for a frontal eddy which was situated at 33 °S between the 2000 m and 4000 m isobath. This shows that the potential thickness of the filament of frontal eddies can be large compared to the water depth and generally, can be considered to be similar to the depth of the shelf. The filament depth is assumed in our study to be 100 m, which is the same value used by Shapiro et al. (2010) to estimate volume transports.
3
Results
Evidence for the existence of frontal eddies was initially gained from the inspection of the pathways that were recorded for tracked surface drifters. Subsequently, examination of remotely sensed satellite SST and Chl-a data confirmed the existence of the frontal eddy events. These are referred to as Event A and Event B and are analysed further below. Key characteristics estimated and quantified for both events are summarised in Table 1, which is referred to in the following discussion.
Table 1 Frontal eddy Event A and Event B characteristics and computed quantities. Gradients are estimated for the importing (first value) and exporting filaments (second value). Event B could not be tracked in remote data due to cloud cover.
Eddy
Physical
Eddy
18
Eddy
Characteristics
Units
Event A
Event B
Core radius
(km)
15
13
Core SST
(oC)
21
24
Core Chl-a
(mg.m-3)
0.99
0.28
Core position start
-
153o48’E, 26o48’S
153°45’ E, 26°30’ S
Core position end
-
153o48’E, 27o06’S
-
Core displacement
(m.s-1)
0.17
-
Max. SST gradient
(oC.km-1)
0.15 / -
0.06 / 0.21
Max. Chl-a gradient
(mg.m-3.km-1)
0.04 / 0.03
0.09 / 0.05
Mean drifter speed
(m.s-1)
0.44
0.49
Drifter speed at shelf break
(m.s-1)
0.28
0.5
Rotational period
(days)
3.9
1.9
On-shelf transport
(Sv)
1.9
1.5
Off-shelf transport
(Sv)
0.3
1.8
Chl-a export
(t.day-1)
13
78
Assuming that the speed of the frontal eddy trapped drifter reflects a tangential velocity, the rotational period of the small frontal eddies is 1.9 days and 3.9 days, respectively. Frontal eddy characteristics found here are similar to those reported by Everett et al. (2011) who reported a displacement speed of 15-35 km per day which compares to 15 km using the 0.17 m.s-1 found for Event (A) and a period of 2.2 days for a frontal eddy with 15 km radius. 19
3.1
Detected Frontal Eddy Events
Event A A cyclonic mesoscale frontal eddy is visible in the Chl-a data with a centre located at approximately 153o 48’ E and 26o 48’ S (~60 km off-shore) and centred above the 1000 m isobaths on 09/08/2006 (Figure 2). The elevated Chl-a concentration of about 0.99 mg.m-3 within the core suggests that the eddy formed earlier and possibly entrained Chl-a enriched coastal water. However, no data for the previous days are available due to cloud obstruction. The Chl-a concentration is similar to the core values of 1-2 mg.m-3 identified by Everett et al. (2015) for an EAC cyclonic eddy which formed off the southeast of Australia entraining enriched shelf water.
The Event A core's SST of about 21oC is lower than that of any of the surrounding waters. This again could possibly be due to the entrainment of cooler shelf water. In addition, elevated Chl-a and lower SST, could be indicative of upwelling and the supply of primary productivity enhancing nutrients, which is consistent with the traditional model for an upwelling cyclonic, cold-core eddy (e.g. Bakun 2006). Increased cloud cover (i.e. the white shading seen in Figure 2) partially obscures the Chl-a for the following days, although signatures of the Chl-a enriched core are still visible the following day and potentially on 12/08/2009. In the SST data, the cold core of the eddy is visible for six days from 08/08/2006 to 13/08/2006. This allows for estimating the total displacement of the core with 15 km per day or about 0.17
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m.s-1 (Table 1). This estimated eddy core displacement speed or translational velocity (e.g. Roughan et al. 2017) appears slower than that of the mean flow of the EAC during this period of the year (see Azis Ismail et al. 2017). Rotational speeds estimated by Everett et al. (2011) were in the order of 0.45 to 0.5 m.s-1 (see their Table 2), which compares to the above estimate of 0.44 m.s-1 (see Table 1). Roughan et al. (2017) estimated a tangential and translational velocity for a small frontal eddy
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Figure 2: Chl-a (mg.m-3, top panel) and SST (oC, lower panel) for frontal eddy Event A on August 9, 2006. Indicated is drifter (ID 44318) location, speed, and direction tracked from August 8 to August 11, 2006. Isobaths are shown for 150 m and 1000 m.
(radius ~35 km diameter) with 0.7 m.s-1 (see their Table 3) and 0.18 m.s-1, respectively, with the latter being very similar to the displacement speed estimated for eddy Event A.
The eddy filament that imports water across the shelf is very wide and its width is estimated from the Chl-a concentration with 70.5 km (Figure 3). The width of the exporting filament is estimated with 14 km from Chl-a and 10 km using SST (see Figure 3), resulting in a mean of 12 km. The estimates for filament width result from measuring the distance between maximum absolute Chl-a and SST gradients and along a straight line that closely follows the shelf break (150 m isobath). The eddy
22
appears to be wedged between the shallow shelf waters and the strong southward currents of the EAC and can be regarded as a frontal eddy of the EAC.
The location of a surface drifter (ID 44318) is tracked from August 8 to August 11, 2006 and follows a cyclonic pathway indicative of being captured by the cyclonic eddy. The drifter crosses the shelf break on August 11, 2006. Mean speed of the drifter within the SEQMCZ after crossing the shelf break and along its cyclonic pathway was about 0.44 m.s-1. The drifter eventually stranded on August 15, 2006 (Azis Ismail et al. 2017).
Event B Analysis of drifter data (Drifter ID: 62933) identified a cyclonic pathway indicative of a possible frontal eddy during May 2009 (Figure 4). The subsequent inspection of remotely sensed Chl-a and SST images provided further evidence. A cold-core eddy was found to be centred at approximately 153° 45’ E and 26° 30’ S on 16/05/2009 (Figure 4). Thereafter, this eddy is referred to as Event B. Inspection of the satellite images indicate a rapid formation of the eddy since it was first detected on 15/05/2009. The eddy is wedged between the shelf and the EAC. On 16/05/2009, the width of the eddy filament extending across the shelf and that imports water is estimated with 22 km and 38 km from Chl-a and SST data, respectively, thus resulting in a mean width of approximately 30 km. The exporting filament width is estimated with 40 km and 35 km or a mean of 37.5 km (Figure 5). Although the continued development of the eddy cannot be tracked due to extensive cloud cover
23
during the following days, there is evidence that the eddy may have continued to exist. A drifter (Drifter ID: 62933) leaves the pathway of the EAC and crosses the 150 m isobath at 153° 37’ E and 26° 58’ S on 18/05/2009 and follows a cyclonic pathway indicative of a frontal eddy induced circular motion (Figure 4). The cyclonic pathway of the drifter is tracked until May 20, 2009. During is progression throughout the SEQCMZ the mean speed was estimated with about 0.42 m.s-1 and the drifter eventually stranded on May 23, 2009 (see Azis Ismail et al. 2017).
24
Figure 3: Chl-a (mg.m-3, top panel) and SST (oC, lower panel) gradients for frontal eddy Event A observed on August 9, 2006. Pink lines aligned with the shelf boundary (i.e. 150 m isobath) indicate estimated width of importing and exporting eddy filaments based on computed maximum gradients. Note the enlarged region compared to Figure 2 to assist in estimation of filament width.
25
Figure 4: Chl-a (mg.m-3, top panel) and SST (oC, lower panel) for frontal eddy Event B on May 16, 2009. Indicated is drifter (ID 62933) location, speed, and direction tracked from August 8 to August 11, 2006. Isobaths are shown for 150 m and 1000 m.
26
Figure 5: Chl-a (mg.m-3, top panel) and SST (oC, lower panel) gradients for frontal eddy Event B observed on May 16, 2009. Pink lines aligned with the shelf boundary (i.e. 150 m isobath) indicate
27
estimated width of importing and exporting eddy filaments based on computed maximum gradients. Note the enlarged region compared to Figure 4 to assist in estimation of filament width.
3.2
Estimates of Eddy On- and Off-shore Transports
Equation 1 is used to estimate the frontal eddy on- and off-shelf transports. The onshelf flow is estimated using the drifter speed perpendicular to the shelf at the time of crossing. This yields a tangential velocity of about 0.28 m.s-1 on 11/08/2006 for Event A (ID 44318) and 0.5 m.s-1 on 18/05/2009 for Event B (ID 62933). The width of the importing filaments was estimated with 70.5 km and 30 km, respectively, and a vertical extend for the filaments of 100 m was used (as per Shapiro et al. 2010). This resulted in transport estimates for the importing filaments of about 1.9 Sv (Event A) and 1.5 Sv (Event B). Using the same estimated flow speeds for the exporting filaments and estimated widths of 12 km (Event A) and 37.5 km (Event B), associated volume flows were estimated with 0.3 Sv (Event A) and 1.8 Sv (Event B). Event B export is significantly larger than in the case of Event A which is primarily due to the larger width of the exporting filament. Considering the volume of the shelf, a transport estimate of 1.5 Sv to 1.9 Sv or 130 km3 per day to 164 km3 per day equates to 18% to 22 % of the shelf volume, or in other words it would take 5 days to renew all shelf water. Entrainment exporting transports of 0.3 Sv (Event A) and 1.8 Sv (Event B) compare to estimates of 0.23 Sv to 0.61 Sv provided by Everett et al. (2015) using an eddy simulation model.
28
The exporting filaments are characterised by higher Chl-a well above the background level of less than 0.2 mg.m-3, which characterises the off-shelf water advected with the EAC. An estimate for the Chl-a concentration within the filaments varies between 0.4 mg.m-3 to 0.6 mg.m-3. Thus, volume exports by filaments estimated with 0.3 Sv and 1.8 Sv yield an export of 13 t to 78 t of Chl-a per day with a mean Chl-a concentration of 0.5 mg.m-3 . This equates to a net production and an export of 390 t to 2340 t of carbon using a carbon to Chl-a ratio of 30 (e.g. Parsons et. al. 1984, Kimura et al. 1997).
4
Summary and Conclusion
For the first time, frontal eddies (core radii < 20 km) are identified from drifter data and satellite images and their characteristics documented and quantified for the SEQCMZ. This region is situated within the intensification zone of the EAC and to the north of the EAC separation zone, which had been the focus of all previous EAC frontal eddy studies.
We find that the quantified characteristics of frontal eddy events A and B are similar to those for frontal eddies along many other western boundary current regions and for the southern and generally more dynamic region of the EAC, referred to as Eddy Avenue (Everett et al. 2012). This includes translational or core displacement speeds and tangential velocities of about 0.17 m.s-1 and 0.44 m.s-1, respectively. These compare to other translational and tangential velocity estimates ranging from 0.1
29
m.s-1 to 0.8 m3.s-1 and 0.3 m3.s-1 to 1.3 m3.s-1, respectively (e.g. Haus et al. 2000, Everett et al. 2015). At the same time, the translational velocity estimated for Event A is smaller than the drifter based estimated tangential velocity, which is again similar to findings from other studies (Everett et al. 20 15, Nagai et al. 2015, Roughan et al. 2017).
The frontal eddies A and B are characterised by a cold core and elevated Chl-a, which is indicative of coastal shelf water entrainment and possibly also upwelling. The entrainment of shelf water was previously identified as a key mechanism driving frontal eddy characteristics along the EAC (e.g. Everett et al. 2015 and studies cited within). The estimated filament associated volume transport is 0.3 Sv 1.9 Sv, which is equivalent to 18 - 22 % of the shelf water. The associated renewal time of 5 days compares to the mean renewal time of 3-4 days that is associated with the primarily wind-driven Fraser Gyre (Azis Ismail 2017). Thus, a sequence of 3-4 frontal eddy events with the characteristics identified here has the same shelf water renewal capacity than the wind-driven Fraser Gyre.
Little is known about cross-shelf transports within the SEQCMZ. The region is an important marine ecological hot spot (e.g. Brieva et al. 2015) and a spawning region for many temperate species. Ward et al. (2003) suggest that pelagic fish larvae are transport by the EAC southward away from the SEQCMZ. A potential mechanism for larvae export is that constituted by cyclonic frontal eddy driven shelf water entrainment events such as those documented here. Similar processes have been
30
found to operate in many other western boundary current regions where frontal eddies drive the entrainment of coastal water, eggs, larvae of marine species and shelf water renewal (e.g. Kasai et al. 2002; Mullaney and Suthers 2013; Gula et al. 2016; Roughan et al. 2017). Here, we propose that the same applies to the SEQCMZ.
The continental shelf of southeast Queensland appears to display similar physical characteristics to those of the southeast coast of the United States (e.g. Govoni et al. 2009). That particular region is characterised by frequent generation of small scale cyclonic frontal eddies that separate from the Gulf Stream as well as larger and longer lived cyclonic eddies. Govoni et al. (2009) refer to the region of persistent cyclonic eddy genesis as the Charleston Gyre. With regard to the SEQCMZ, Ribbe and Brieva (2016) previously found from the analysis of SSH data that the region is characterised by the generation of a larger cyclonic gyre (radius > 50 km) as well, and Azis Ismail et al. (2017) documented the existence of the quasi-permanent onshelf cyclonic Fraser Gyre.
Mullaney and Suthers (2013) visually inspected several years of AVHRR images finding that many small scale (radii < 30 km) and short-lived (a few days) eddies appear to occur within the EAC separation zone, i.e. to the south of the SEQCMZ. Similarly, inspection of satellite images for the SEQCMZ and in addition of those documenting Event A and B, identified the occurrence of many other frontal eddies of similar scale. This suggests that frontal eddies potentially play an important role in cross-shelf exchanges which in magnitude are similar to the previously assessed
31
time-averaged mean circulation driven exchange of shelf water due to the Fraser Gyre (Azis Ismail et al. 2017). Additional drifter data are currently not available to further detail and document these events. Thus, future work needs to aim at generating a complete census of frontal eddy events from additional field observations and the potential application of high-resolution operational ocean modelling. Furthermore, these studies should also aim in particular to investigate the genesis of EAC meanders and spawning of frontal eddies in the region of the SEQCMZ.
This study used a simple approach similar to that adopted by Shapiro et al. (2010), yielding results that are consistent with those documented for other WBC regions, yet several caveats to our study need to be acknowledged and considered in interpreting our results. This includes the selected tangential velocity. It is evident form our study that this velocity varies across the width of the eddy or filament, and thus would depend on the position of the drifter. Roughan et al. (2017) found that the offshore branch of a frontal eddy was characterised by a higher velocity due to that branch joining the southward flow of the EAC. Similarly, Event drifter A and B had different velocities over the period trapped within the SEQCMZ and at crossing of the shelf break (see Table 1). Furthermore, we were only able to document the trapping of two drifters since their transport into and within the SEQCZM coincided with the genesis of a frontal eddy. However, the visual inspection of Chl-a patterns, similar to the approach by Mullaney and Suthers (2013), identified about 7-13 ring like structures, aka frontal eddies events; or on average about ten per year for the
32
period 2004-2010. Thus indicates that frontal eddies are a reoccurring feature, although less frequent than further south (Mullaney and Suthers 2013) and future work is anticipated to generate an additional census for frontal eddies in the SEQCMZ. Submesoscale fronal eddies were not detected as part of the previous eddy census for the region by Ribbe and Brieva (2017). The tangential velocity estimates were also derived from undrogued drifters. Choukroun et al. (2010) compared undrogued versus drogued drifter velocities finding values of 0.18 m.s-1 and 0.13 m.s-1, respectively, and for the shelf region of the Great Barrier Reef situated just to the north of the SEQCMZ. We have made no corrections for wind slippage of the undrogued drifters. The assumed tangential velocity estimates are potentially too high. The drifter speed is a surface value that certainly reflects the maximum speed of the whole water column and represent an upper limit.
Our study is also limited since no supporting direct ship-borne observations (e.g. Everett et al. 2011, Roughan et al. 2017) and land-based high-frequency radar current measurements (e.g. Haus et al. 2000, Mantovanelli et al. 2017, Schaeffer et al. 2017) of frontal eddies are available. This has allowed for quantitative and qualitative descriptions of individual frontal eddies to the south of the SEQCMZ. However, our initial results are consistent with previous findings, and it is anticipated that this study and previous oceanographic SEQCMZ investigations (e.g. Brieva et al. 2015, Ribbe and Brieva 2016, Azis Ismail et al. 2017) are likely to inform the planning of future field missions. These will be aiming to improve our
33
understanding to the regional physical processes that drive a marine biodiverse environment and appear of importance to eastern Australian fisheries.
Yet, an important outcome from our study is that for the first time the importance of frontal eddies for cross-shelf exchanges in the northern intensification zone of the EAC was documented and quantified. It is likely that this work will inspire future computational and field work activities in the region and future work will need to focus on reducing the discussed uncertainties of this study.
Acknowledgment
The authors gratefully acknowledge agencies and colleagues that provided accesses to data and made this study possible. These include NOAA Atlantic Oceanographic and Meteorological Laboratory and the Australian Government funded Integrated Marine Observing System. Mr. Mochamad Furqon Azis Ismail acknowledges support from the Research Centre for Oceanography at the Indonesian Institute of Sciences (RCO – LIPI) and the scholarship Program for Research and Innovation in Science and Technologies (RISET-Pro) from the Ministry of Research, Technology and Higher Education of the Republic of Indonesia. Ms Liv Toaspern acknowledges financial support from the University of Oldenburg for international travel to visit the University of Southern Queensland, Australia.
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Highlights
first study to detail existence of small frontal eddies in northern EAC region
eddies likely to make significant contribution to regional cross-shelf transport
analysis appears consistent with previous western boundary current studies
44
www.elsevier.com/locate/csr
PII: DOI: Reference:
S0278-4343(17)30586-1 https://doi.org/10.1016/j.csr.2018.06.007 CSR3777
To appear in: Continental Shelf Research Received date: 10 November 2017 Revised date: 6 June 2018 Accepted date: 13 June 2018 Cite this article as: Joachim Ribbe, Liv Toaspern, Jörg-Olaf Wolff and Mochamad Furqon Azis Ismail, Frontal Eddies along a Western Boundary Current, Continental Shelf Research, https://doi.org/10.1016/j.csr.2018.06.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Frontal Eddies along a Western Boundary Current
a*
b
b
Joachim Ribbe , Liv Toaspern , Jörg-Olaf Wolff , Mochamad Furqon Azis Ismail
a, c
Faculty of Health Engineering and Sciences, School of Agricultural, Computational and Environmental Sciences, University of Southern Queensland, Toowoomba 4350, Queensland, Australia. a
b
Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky University of Oldenburg, 26129 Oldenburg, Germany.
a,c
Research Center for Oceanography, Indonesian Institute of Sciences, Jakarta 14430, Indonesia.
*Corresponding author: [email protected]
1
Abstract
Cyclonic frontal eddies are distinguishable from the surrounding water due to their unique biological and physical characteristics and have been observed in all western boundary current regions. These eddies spawn from cut-off meanders and are found on the landward side of the current. Here, we report for the first time observed frontal eddies for the intensification zone (north of 28 oS) of the East Australian Current (EAC) off Southeast Queensland, Australia, by analysing remotely sensed sea surface temperature (SST) and chlorophyll-a (Chl-a) data. The frontal eddies were detected initially in the analysis of satellite tracked surface drifters. The shelfcrossing cyclonic drifter pathways indicated the presence of drifter-trapping cyclonic frontal eddies. The subsequent analysis of satellite images allows to quantify key eddy characteristics, cross-shelf volume transports associated with eddy filaments, eddy-driven shelf water renewal time scales, and export of total Chl-a and carbon per day. The observed frontal eddies have core radii of approximately 13 km and 15 km. The cold core surface SST anomaly and elevated chl-a indicates eddy entrainment of shelf water. The translational or core displacement velocity is estimated with 0.17 m.s-1 or 15 km per day and the tangential velocity quantified from tracking surface drifters is 0.28 m.s-1 to - 0.5 m.s-1. This results in a rotational period of 1.9 days to 3.9 days. We use maximum Chl-a and SST gradients to approximate the width of importing and exporting filaments associated with the
2
frontal eddy to derive volume transports of 1.5 Sv and 1.9 Sv (import) and 0.3 Sv and 1.8 Sv (export), respectively. Chl-a concentrations of the exporting filaments are about 0.4 mg.m-3 to 0.6 mg.m-3 yielding a total export of 13 t to 78 t of Chl-a per day. The frontal eddy induced on-shelf transport of 130 km3 - 160 km3 per day represents between 18 % and 22 % of the shelf volume. Therefore, it would take approximately five days to renew all shelf water. We conclude that the observed frontal eddies of the northern intensification zone of the EAC potentially play an important role in determining cross-shelf exchanges, contribute to on-shelf marine conditions, enhancing coastal primary productivity and are possibly important to the export of shelf water properties such as the fish larvae of subtropical species via entrainment.
3
Graphical abstract
Keywords
Western boundary current; frontal eddy; cross-shelf transport; drifter, sea surface temperature; chlorophyll-a; East Australian Current
4
1
Introduction
Cyclonic frontal eddies typically form on the land-ward side of all the western boundary currents (WBCs), are important in driving cross-shelf exchanges and are less-well understood than the larger mesoscale eddies (e.g. Koshlyakov and Monin 1978, Lee 1975, Lee et al. 1981, Yoder et al. 1981, Lutjeharms and Stockton 1987, Kasai et al. 2002, Sponaugle et al. 2005, Lorenzzetti et al. 2009, Matsuno et al. 2009, Mullaney and Suthers 2013, Schaeffer et al. 2017). The term frontal eddies appears to have been coined by Koshlyakov and Monin (1978) and Lee (1975) referred to these previously as spin-off eddies. The biological significance of frontal eddies was first described in Robinson (1983) by Angel and Fasham (1983). Their genesis is associated with the fluxes of carbon, nutrients, pollutants and the entrainment and transport of fish larvae (e.g. Lee et al. 1991, Atkinson et al. 1996, Kimura et al. 1997, Kimura et al. 2000, Nakata et al. 2000, Bakun 2006, Govoni et al. 2010, Mullaney and Suthers 2013, Matis et al. 2015, Brink 2016, Nencioli et al. 2016). Cyclonic frontal eddies are usually short-lived ( < 4 weeks) and are characterised by diameters of 10 to 100 km (e.g. Lee 1975, Sponaugle et al. 2005, Everett 2015, Roughan et al. 2017). Eddy translational (i.e. propagation or the displacement of the eddy core) and tangential velocities were observed with values ranging between 0.1 m.s-1 to 0.8 m3.s1
and 0.3 m3.s-1 to 1.3 m3.s-1, respectively (e.g. Lee and Atkinson 1983, Haus et al.
2000, Everett et al. 2011, Everett et al. 2015). Tangential velocities are generally larger than translational velocities (e.g. Everett et al. 20 15, Nagai et al. 2015, Roughan et al. 2017), and tangential velocities vary across the eddy and are larger where the off-
5
shore eddy branch joins the flow of the boundary current (e.g. Schaeffer et al. 2017). A flow in the opposite direction is generated near-shore. The standard model of a cyclonic eddy identifies lower SST and elevated chlorophyll-a (Chl-a) values compared to surrounding water as characteristics (e.g. Bakun 2006).
In regards to on-shelf water renewal processes, the flushing due to a single extreme event such as a cyclonic frontal eddy (Brink 2016) and associated water renewal time scale could potentially be as or more important as the flushing and the associated timescales derived from mean property distributions and residual shelf circulations (e.g. Strutton et al. 1996; Monsen et al. 2002, Ribbe et al. 2008). Brink (2016) refers to the occurrence of mesoscale eddies as dramatic and important events that have the potential to export coastal shelf water volumes in the order of 104 m3.s-1 to 105 m3.s-1. Brink (2016) also indicated the potential for these events to play an important role off south-eastern Australia where highly energetic eddies occur (Everett et al. 2012; Ribbe and Brieva 2016). Yet, only few observational or computational studies exist to-date that quantify volume transports associated with frontal eddies and their filaments, in particular for the East Australian Current (EAC) which is the western boundary current of the South Pacific Ocean gyre. Shapiro et al. (2010) used remotely sensed Chl-a and sea surface height (SSH) anomaly to estimate cross-shelf transports induced by a large cyclonic mesoscale eddy (~ 120 km in diameter) observed in the Black Sea. The transport was calculated by multiplying the velocity within the filament with the width and the estimated vertical extent of the filament. The average velocity was estimated as 0.15 m.s-1, the width of the filament as 20 km,
6
and the depth was assumed to be 100 m. This resulted in an average transport of 0.3 Sv (1 Sv = .106 m3.s-1). The eddy transported approximately 40% of the shelf water volume into the deep sea during its lifetime of 40 days. Peliz et al. (2004) applied the same approach of estimating the eddy-induced cross-shelf transport due to a large eddy filament to the shelf southwest of the Iberian Peninsula. SSH anomaly derived velocity was estimated with 0.15 m.s-1 resulting in a volume transport of 0.68 Sv and an estimate for the shelf flushing time of 5-6 days. Zhou et al. (2014) found from a computational study of cross-shelf exchanges in the Black Sea that eddies constitute the most efficient transport mechanism. Li et al. (2017) studied cross-shelf exchanges associated with cold filaments for the South China Sea. These cold filaments develop during the seasonal East Asian Monsoon transition and due to directional changes in the prevailing wind direction, thus are not frontal eddies. Nevertheless, Li et al. (2017) documented a significant offshore filament induced transport of 1.8 Sv due to the mean filament's estimated width of 100 km and a higher velocity estimate of 0.3 m.s-1.
Generally, little is known about cyclonic frontal eddies and associated cross-shelf volume exchanges in the EAC region. All previous studies of EAC frontal eddies focus on the region of the separation zone, which is located to the south of 29 oS (Suthers et al. 2011). This region is home to a complex field of mesoscale eddies (radii > 90 km, cyclonic and anticyclonic) which has been referred to as eddy avenue (Everett et al. 2012). Everett et al. (2011) and Mullaeny and Suthers (2013) observed cyclonic frontal eddies with radii of between 15 km and 40-50 km, respectively, and
7
focused on biological characteristics of the core. Subsequent inspection of AVHRRSST imagery by Mullaeny and Suthers (2013) found about one submesoscale cyclonic eddy per week in this region. Everett et al. (2015) documented an entrainment of preconditioned shelf water of 0.23 Sv to 0.61 Sv due to an observed, slow moving and long-lived (4 months) large-scale cyclonic eddy (radius > 50 km). Everett et al. (2015) referred to it as a frontal eddy since it existed on the western and landward side of the boundary current. Other observational studies investigating entrainment and biological characteristics of individual ECA frontal eddies include Matis et al. (2014) and Roughan et al. (2017). Schaeffer et al. (2017) identified much smaller frontal eddies of core radii ~10 km with a frequency of approximately one per week and displacement speeds of 0.3 m.s-1 to 0.4 m.s-1 using HF radar observations over a one-year period. Modelling studies by Marchesiello and Middleton (2000), Macdonald et al. (2016) and Mantovanelli et al. (2017) propose a series of physical processes including wind forcing on surface circulation, horizontal shear, and current instabilities that drive or affect the formation of frontal eddies.
In this paper, we document for the first time the occurrence of small scale frontal eddies that appear to characterise the EAC intensification zone (i.e. north of 28 oS) and quantify their characteristics. This region is situated to the north of the previously examined EAC separation zone (Ridgway and Dunn 2003). It is referred to in this paper as the Southeast Queensland Coastal Marine Zone (SEQCMZ). This region is part of the continental shelf of eastern Australia (Figure 1). It encompasses the continental shelf between 25 °S and 27 °S and is bounded by the 150 m isobath to
8
the east. The shelf width of the SEQCMZ varies from 22 km in the south to 45 km in the north, mean water depth is 63 m and the volume occupied by shelf water is 7.4.1011 m3. The climate in this area is subtropical and the maximum rainfall occurs during the southern hemisphere spring and summer (Ribbe 2014). The SEQCMZ borders the intensification zone of the EAC located between 24 °S and 29 °S (Ridgway and Dunn 2003). The EAC is the WBC of the South Pacific Ocean gyre and most EAC studies have focused on the EAC separation zone located south of 29 oS (see e.g. review by Suthers et al. 2011).
Unique physical features of the SEQCMZ's shelf circulation include the quasistationary Southeast Fraser Island Upwelling System (Brieva et al. 2015) during spring and summer, the wind-driven shelf-encompassing cyclonic Fraser Gyre (Figure 1), which establishes itself annually during the autumn and winter months (Azis Ismail et al. 2017), and the shelf hugging EAC. The Fraser Island Upwelling System is primarily driving by bottom layer stress that is generated due to an intense and shelf encroaching EAC during summer. The north- to north-easterly winds play only a moderating role, enhancing or weakening upwelling, but do not drive directly upwelling into the surface layer (Brieva et al. 2015). The offshore branch of the Fraser Gyre joins the southward flowing EAC (Figure 1), while the western branch constitutes a northerly near shore flow of 0.15 -0.26 m.s-1. Brieva et al. (2015) estimated a mean Fraser Gyre driven shelf water renewal time scale of 3.3 days, ranging from monthly estimates of 2-8 days with the longer times during the winter period. Ribbe and Brieva (2016) included this region in a census of eddies, finding
9
that the region frequently generates large (radii > 40 km) mesoscale cyclonic eddies (frontal eddies, about 4-5 per year), however, noted that the census excluded smaller mesoscale frontal eddies (radii < 40 km) since these are not resolved in the gridded SSH anomaly maps and data used by Ribbe and Brieva (2016).
10
Figure 1: (a) Geographic location of the study site along the east coast of Australia. (b) Representation of the study site referred to as the Southeast Queensland Coastal Marine Zone (SEQCMZ) as well as the 150 m (dotted), and 1000 m (solid) isobath are shown. Indicated are climatological features characterising the regional circulation and the approximate core location of observed frontal eddies referred to as Event A and Event B. Please see legend for individual geographic features shown. The larger-scale oceanographic characteristics of the study site include: (1) the southward flowing East Australian Current, (2) the Southeast Fraser Island Upwelling System characterised by elevated Chl-a concentration (dashed pattern) from September - January each year (Brieva et al. 2015) and (3) the wind-driven cyclonic on-shelf circulation of the Fraser Gyre that occupies the SEQCMZ from April to August each year (Azis Ismail et al. 2017).
11
The SEQMCZ is of significant marine ecological importance and includes one of eight key ecological sites and important fishery along Australia's eastern ocean (Young et al. 2011, Dambacher et al. 2012). It is known as a spawning site for a highly diverse pelagic fish assemblage with larvae transported off-shelf and southward with the EAC away from the SEQCMZ (e.g. Zeller et al. 1996, Ward et al. 2003). Spencer et al. (2017) review physical processes that may affect the catchability of spanner crab, and argue that quantification of on-shelf transports and effects of eddies need to be assessed, including for the SEQCMZ. Everett et al. (2017) highlighted the ecological importance of this region by identifying the coastal waters of Fraser Island through Lagrangian particle tracking studies as an Eastern King Prawn larvae source region. The Fraser Island location was the most northern of all regions considered. It was characterised by the highest long-shelf dispersal speed for larvae estimated with 53 km.day-1 or 0.61 m.s-1. Impacts on the marine environment of the SEQCMZ arise from a range of human activities including tourism. This already brings several millions of visitors into the region. Fraser Island is listed as a UNESCO World Heritage site and is part of the UNESCO designed Great Sandy Biosphere Reserve which includes the surrounding coastal waters (Ribbe 2014). Furthermore, more than two thirds of Queensland's population (over 3 million people) or approximately 13% of Australia's population live nearby and potentially utilise the coastal environment of this region for a range of activities.
12
We aim to document for the first time the potential role cyclonic frontal eddies play in driving regional marine environmental conditions and exchanges between the local shelf and the EAC. This includes quantifying frontal eddy characteristics such as translational and tangential velocity, volume transports for on- and off-shore flows using satellite tracked drifters and remotely sensed data, and estimating the associated export of Chl-a and carbon. Ward et al. (2003) concluded that pelagic fish larvae are transported by the EAC away from the spawning waters of the SEQCMZ, and frontal eddies potentially provide a mechanism for the cross-shelf export and injection into the EAC. Overall, little is known about the physical oceanography of this region, with most EAC studies focused on the EAC separation region (Suthers et al. 2011). Following Section 2, which outlines data and method used in this study, we present in Section 3 analysed pathways of remotely tracked drifters in comparison with remotely sensed SST and Chl-a and estimated physical characteristics of the identified frontal eddies. The final Section 4 summarises our findings in the context of the literature and discusses the limitations of the study. Overall, we conclude that frontal eddies in the SEQCMZ are likely to provide a cross-shelf transport process important to regional shelf ecology, potentially precondition the EAC and facilitate the southward advection of shelf water and associated properties into temperate regions of the EAC and the Tasman Sea.
2
Data and Method
2.1 Data 13
Sea Surface Temperature (SST) Gridded SST (oC) derived from the Advanced Very High Resolution Radiometer (AVHRR) was sourced from the Integrated Marine Observing System (IMOS, https://portal.aodn.org.au/). IMOS is a national collaborative research infrastructure, which is supported by the Australian Government. The data are available on a daily basis since August 2002. The spatial resolution of the SST data is 0.02° x 0.02°. This is detailed enough to resolve the small-scale frontal eddies of interest to this study. For the SEQMCZ, the data were previously used to identify the Fraser Gyre (Azis Ismail et al. 2017), the Southeast Fraser Upwelling System (Brieva et al. 2015) and contributed to a regional eddy census (Ribbe and Brieva 2016).
Chlorophyll-a (Chl-a) IMOS provided Chl-a concentration estimates (mg.m-3) are produced using the OC3 algorithm from the MODerate Resolution Imaging Spectroradiometer (MODIS) ocean colour measurements (https://portal.aodn.org.au/, for further details on data processing and limitations see Brieva et al. (2015) and references therein). As we aim for a case study of observed individual frontal eddies in the SEQMCZ and not a systematic eddy census, the large part of cloud covered data does not affect this study (about 63% of all images were excluded in a previous analysis, see Brieva et al. (2015) for statistics). The spatial resolution of the data is 0.01° x 0.01°, and the temporal resolution is the same as that for SST. Both Chl-a and SST data are used to identify the frontal eddies considered in this study and to quantify their
14
characteristics.
O'Reilly et al. (2000) discuss the evaluation of MODIS measurements in order to derive the Chl-a estimates used in this study. This has previously also been discussed and the data applied by Brieva et al. (2015). In shallow coastal water the usability of remotely sensed Chla is also limited due to factors such as bottom albedo, suspended sediment and coastal turbidity (Moses et al. 2012). Yet, our analysis focuses on the mid- to off-shelf region east of the 40 m isobar (within about 10 km of the coast line) with little if at all impact form bottom and suspended sediment as discussed in Brieva et al. (2015).
Drifters The data used in this study originate from drifters that are deployed as part of the Global Drifter Program (GDP) and the data are provided by the Atlantic Oceanographic and Meteorological Laboratory (AOML). Amongst other variables, the data (see http://www.aoml.noaa.gov/envids/data_available.php) includes the tracked location as well as the east- and northward velocity of each drifter four times per day. To achieve a minimal slip due to action by local wind, GDP drifters consist of two parts: A surface float with a spherical design and an attached drogue (Lumpkin and Pazos 2006). While the drogue is still attached, the wind-induced slip of drifters is less than 0.02 m.s-1 for wind velocities of less than 20 m.s-1 (Niller et al. 1987). Pazan and Niller (2001) found that drifters without a drogue have an estimated downwind slip of 8.8.10-2 m.s-1 per 10 m.s-1 of wind speed. Data from satellite-tracked drifters have previously been used for studies of the EAC and the Pacific Ocean region. Early research comprises Nilsson and Cresswell (1980) and
15
Cresswell and Legeckis (1986) who investigated warm-cores eddies in the southern part of Eastern Australia. Choukroun et al. (2010) derived surface velocity maps for the Coral Sea from drifter velocities and estimated mean residence times for the Great Barrier Reef lagoon using all AOML data.
In this study, we used data from two drifters that entered the SEQMCZ in a cyclonic fashion indicative of 'trapping' by a cyclonic frontal eddy. These drifters were identified from a subset of drifters analysed in an earlier study by Azis Ismail et al. (2017). However, there are only two drifters within the SEQMCZ that also appeared to coincide with the presence of a small-scale cyclonic frontal eddy. Drifter No. 44318 was deployed on 14/07/2005 and lost its drogue on 16/01/2006 before entering the region of interest on 09/08/2006. Drifter No. 62933 was deployed on 17/05/2008, also lost its drogue on 21/09/2008 and entered the area of interest on 04/05/2009. Drifter locations are reported four times every day and are used to compute mean velocities as well as velocities on crossing the shelf-break. The latter is referred to as the tangential velocity or rotational velocity and is used to quantify both import and export by eddy filaments (Table 1).
2.2 Method
Shapiro et al. (2010) and Peliz et al. (2004) calculated the volume transport of an eddy-associated filament as F = u .d .h
16
(1)
where u (m.s-1) is the velocity within the filament, d (m) the width and h (m) the thickness of the filament. Shapiro et al. (2010) and Peliz et al. (2004) both used the geostrophic velocity derived from SSH anomaly as estimate for the velocity u within the filament. In this study, a similar approach is applied. However, the resolution of the SSH anomaly data is too coarse to compute geostrophic velocities for small-scale frontal eddies (see Ribbe and Brieva 2016). Instead, we use the velocity of the drifter, which was captured by the eddy. In order to estimate the cross-shelf transport, the velocity of the drifter perpendicular to the 150 m isobath at the time of crossing is used as u in equation (1).
The width of the filament is obtained from Chl-a data by measuring the distance between the boundaries of the filament along the 150 m isobath. We define the boundaries of the filaments as the local maxima of the horizontal gradient of the Chla concentration C, which are calculated as:
{|⃗
where ⃗
( ⁄
⁄
|}
{√(
)
(
) }
) is the horizontal gradient operator and x, y are the zonal
and meridional coordinates, respectively. The width of the filament is computed as the distance between two local maxima going along the 150 m isobath from south to north. The same procedure is carried out with the SST data for comparison.
17
Roughan et al. (2017) found a depth of approximate 1000 m for a frontal eddy which was situated at 33 °S between the 2000 m and 4000 m isobath. This shows that the potential thickness of the filament of frontal eddies can be large compared to the water depth and generally, can be considered to be similar to the depth of the shelf. The filament depth is assumed in our study to be 100 m, which is the same value used by Shapiro et al. (2010) to estimate volume transports.
3
Results
Evidence for the existence of frontal eddies was initially gained from the inspection of the pathways that were recorded for tracked surface drifters. Subsequently, examination of remotely sensed satellite SST and Chl-a data confirmed the existence of the frontal eddy events. These are referred to as Event A and Event B and are analysed further below. Key characteristics estimated and quantified for both events are summarised in Table 1, which is referred to in the following discussion.
Table 1 Frontal eddy Event A and Event B characteristics and computed quantities. Gradients are estimated for the importing (first value) and exporting filaments (second value). Event B could not be tracked in remote data due to cloud cover.
Eddy
Physical
Eddy
18
Eddy
Characteristics
Units
Event A
Event B
Core radius
(km)
15
13
Core SST
(oC)
21
24
Core Chl-a
(mg.m-3)
0.99
0.28
Core position start
-
153o48’E, 26o48’S
153°45’ E, 26°30’ S
Core position end
-
153o48’E, 27o06’S
-
Core displacement
(m.s-1)
0.17
-
Max. SST gradient
(oC.km-1)
0.15 / -
0.06 / 0.21
Max. Chl-a gradient
(mg.m-3.km-1)
0.04 / 0.03
0.09 / 0.05
Mean drifter speed
(m.s-1)
0.44
0.49
Drifter speed at shelf break
(m.s-1)
0.28
0.5
Rotational period
(days)
3.9
1.9
On-shelf transport
(Sv)
1.9
1.5
Off-shelf transport
(Sv)
0.3
1.8
Chl-a export
(t.day-1)
13
78
Assuming that the speed of the frontal eddy trapped drifter reflects a tangential velocity, the rotational period of the small frontal eddies is 1.9 days and 3.9 days, respectively. Frontal eddy characteristics found here are similar to those reported by Everett et al. (2011) who reported a displacement speed of 15-35 km per day which compares to 15 km using the 0.17 m.s-1 found for Event (A) and a period of 2.2 days for a frontal eddy with 15 km radius. 19
3.1
Detected Frontal Eddy Events
Event A A cyclonic mesoscale frontal eddy is visible in the Chl-a data with a centre located at approximately 153o 48’ E and 26o 48’ S (~60 km off-shore) and centred above the 1000 m isobaths on 09/08/2006 (Figure 2). The elevated Chl-a concentration of about 0.99 mg.m-3 within the core suggests that the eddy formed earlier and possibly entrained Chl-a enriched coastal water. However, no data for the previous days are available due to cloud obstruction. The Chl-a concentration is similar to the core values of 1-2 mg.m-3 identified by Everett et al. (2015) for an EAC cyclonic eddy which formed off the southeast of Australia entraining enriched shelf water.
The Event A core's SST of about 21oC is lower than that of any of the surrounding waters. This again could possibly be due to the entrainment of cooler shelf water. In addition, elevated Chl-a and lower SST, could be indicative of upwelling and the supply of primary productivity enhancing nutrients, which is consistent with the traditional model for an upwelling cyclonic, cold-core eddy (e.g. Bakun 2006). Increased cloud cover (i.e. the white shading seen in Figure 2) partially obscures the Chl-a for the following days, although signatures of the Chl-a enriched core are still visible the following day and potentially on 12/08/2009. In the SST data, the cold core of the eddy is visible for six days from 08/08/2006 to 13/08/2006. This allows for estimating the total displacement of the core with 15 km per day or about 0.17
20
m.s-1 (Table 1). This estimated eddy core displacement speed or translational velocity (e.g. Roughan et al. 2017) appears slower than that of the mean flow of the EAC during this period of the year (see Azis Ismail et al. 2017). Rotational speeds estimated by Everett et al. (2011) were in the order of 0.45 to 0.5 m.s-1 (see their Table 2), which compares to the above estimate of 0.44 m.s-1 (see Table 1). Roughan et al. (2017) estimated a tangential and translational velocity for a small frontal eddy
21
Figure 2: Chl-a (mg.m-3, top panel) and SST (oC, lower panel) for frontal eddy Event A on August 9, 2006. Indicated is drifter (ID 44318) location, speed, and direction tracked from August 8 to August 11, 2006. Isobaths are shown for 150 m and 1000 m.
(radius ~35 km diameter) with 0.7 m.s-1 (see their Table 3) and 0.18 m.s-1, respectively, with the latter being very similar to the displacement speed estimated for eddy Event A.
The eddy filament that imports water across the shelf is very wide and its width is estimated from the Chl-a concentration with 70.5 km (Figure 3). The width of the exporting filament is estimated with 14 km from Chl-a and 10 km using SST (see Figure 3), resulting in a mean of 12 km. The estimates for filament width result from measuring the distance between maximum absolute Chl-a and SST gradients and along a straight line that closely follows the shelf break (150 m isobath). The eddy
22
appears to be wedged between the shallow shelf waters and the strong southward currents of the EAC and can be regarded as a frontal eddy of the EAC.
The location of a surface drifter (ID 44318) is tracked from August 8 to August 11, 2006 and follows a cyclonic pathway indicative of being captured by the cyclonic eddy. The drifter crosses the shelf break on August 11, 2006. Mean speed of the drifter within the SEQMCZ after crossing the shelf break and along its cyclonic pathway was about 0.44 m.s-1. The drifter eventually stranded on August 15, 2006 (Azis Ismail et al. 2017).
Event B Analysis of drifter data (Drifter ID: 62933) identified a cyclonic pathway indicative of a possible frontal eddy during May 2009 (Figure 4). The subsequent inspection of remotely sensed Chl-a and SST images provided further evidence. A cold-core eddy was found to be centred at approximately 153° 45’ E and 26° 30’ S on 16/05/2009 (Figure 4). Thereafter, this eddy is referred to as Event B. Inspection of the satellite images indicate a rapid formation of the eddy since it was first detected on 15/05/2009. The eddy is wedged between the shelf and the EAC. On 16/05/2009, the width of the eddy filament extending across the shelf and that imports water is estimated with 22 km and 38 km from Chl-a and SST data, respectively, thus resulting in a mean width of approximately 30 km. The exporting filament width is estimated with 40 km and 35 km or a mean of 37.5 km (Figure 5). Although the continued development of the eddy cannot be tracked due to extensive cloud cover
23
during the following days, there is evidence that the eddy may have continued to exist. A drifter (Drifter ID: 62933) leaves the pathway of the EAC and crosses the 150 m isobath at 153° 37’ E and 26° 58’ S on 18/05/2009 and follows a cyclonic pathway indicative of a frontal eddy induced circular motion (Figure 4). The cyclonic pathway of the drifter is tracked until May 20, 2009. During is progression throughout the SEQCMZ the mean speed was estimated with about 0.42 m.s-1 and the drifter eventually stranded on May 23, 2009 (see Azis Ismail et al. 2017).
24
Figure 3: Chl-a (mg.m-3, top panel) and SST (oC, lower panel) gradients for frontal eddy Event A observed on August 9, 2006. Pink lines aligned with the shelf boundary (i.e. 150 m isobath) indicate estimated width of importing and exporting eddy filaments based on computed maximum gradients. Note the enlarged region compared to Figure 2 to assist in estimation of filament width.
25
Figure 4: Chl-a (mg.m-3, top panel) and SST (oC, lower panel) for frontal eddy Event B on May 16, 2009. Indicated is drifter (ID 62933) location, speed, and direction tracked from August 8 to August 11, 2006. Isobaths are shown for 150 m and 1000 m.
26
Figure 5: Chl-a (mg.m-3, top panel) and SST (oC, lower panel) gradients for frontal eddy Event B observed on May 16, 2009. Pink lines aligned with the shelf boundary (i.e. 150 m isobath) indicate
27
estimated width of importing and exporting eddy filaments based on computed maximum gradients. Note the enlarged region compared to Figure 4 to assist in estimation of filament width.
3.2
Estimates of Eddy On- and Off-shore Transports
Equation 1 is used to estimate the frontal eddy on- and off-shelf transports. The onshelf flow is estimated using the drifter speed perpendicular to the shelf at the time of crossing. This yields a tangential velocity of about 0.28 m.s-1 on 11/08/2006 for Event A (ID 44318) and 0.5 m.s-1 on 18/05/2009 for Event B (ID 62933). The width of the importing filaments was estimated with 70.5 km and 30 km, respectively, and a vertical extend for the filaments of 100 m was used (as per Shapiro et al. 2010). This resulted in transport estimates for the importing filaments of about 1.9 Sv (Event A) and 1.5 Sv (Event B). Using the same estimated flow speeds for the exporting filaments and estimated widths of 12 km (Event A) and 37.5 km (Event B), associated volume flows were estimated with 0.3 Sv (Event A) and 1.8 Sv (Event B). Event B export is significantly larger than in the case of Event A which is primarily due to the larger width of the exporting filament. Considering the volume of the shelf, a transport estimate of 1.5 Sv to 1.9 Sv or 130 km3 per day to 164 km3 per day equates to 18% to 22 % of the shelf volume, or in other words it would take 5 days to renew all shelf water. Entrainment exporting transports of 0.3 Sv (Event A) and 1.8 Sv (Event B) compare to estimates of 0.23 Sv to 0.61 Sv provided by Everett et al. (2015) using an eddy simulation model.
28
The exporting filaments are characterised by higher Chl-a well above the background level of less than 0.2 mg.m-3, which characterises the off-shelf water advected with the EAC. An estimate for the Chl-a concentration within the filaments varies between 0.4 mg.m-3 to 0.6 mg.m-3. Thus, volume exports by filaments estimated with 0.3 Sv and 1.8 Sv yield an export of 13 t to 78 t of Chl-a per day with a mean Chl-a concentration of 0.5 mg.m-3 . This equates to a net production and an export of 390 t to 2340 t of carbon using a carbon to Chl-a ratio of 30 (e.g. Parsons et. al. 1984, Kimura et al. 1997).
4
Summary and Conclusion
For the first time, frontal eddies (core radii < 20 km) are identified from drifter data and satellite images and their characteristics documented and quantified for the SEQCMZ. This region is situated within the intensification zone of the EAC and to the north of the EAC separation zone, which had been the focus of all previous EAC frontal eddy studies.
We find that the quantified characteristics of frontal eddy events A and B are similar to those for frontal eddies along many other western boundary current regions and for the southern and generally more dynamic region of the EAC, referred to as Eddy Avenue (Everett et al. 2012). This includes translational or core displacement speeds and tangential velocities of about 0.17 m.s-1 and 0.44 m.s-1, respectively. These compare to other translational and tangential velocity estimates ranging from 0.1
29
m.s-1 to 0.8 m3.s-1 and 0.3 m3.s-1 to 1.3 m3.s-1, respectively (e.g. Haus et al. 2000, Everett et al. 2015). At the same time, the translational velocity estimated for Event A is smaller than the drifter based estimated tangential velocity, which is again similar to findings from other studies (Everett et al. 20 15, Nagai et al. 2015, Roughan et al. 2017).
The frontal eddies A and B are characterised by a cold core and elevated Chl-a, which is indicative of coastal shelf water entrainment and possibly also upwelling. The entrainment of shelf water was previously identified as a key mechanism driving frontal eddy characteristics along the EAC (e.g. Everett et al. 2015 and studies cited within). The estimated filament associated volume transport is 0.3 Sv 1.9 Sv, which is equivalent to 18 - 22 % of the shelf water. The associated renewal time of 5 days compares to the mean renewal time of 3-4 days that is associated with the primarily wind-driven Fraser Gyre (Azis Ismail 2017). Thus, a sequence of 3-4 frontal eddy events with the characteristics identified here has the same shelf water renewal capacity than the wind-driven Fraser Gyre.
Little is known about cross-shelf transports within the SEQCMZ. The region is an important marine ecological hot spot (e.g. Brieva et al. 2015) and a spawning region for many temperate species. Ward et al. (2003) suggest that pelagic fish larvae are transport by the EAC southward away from the SEQCMZ. A potential mechanism for larvae export is that constituted by cyclonic frontal eddy driven shelf water entrainment events such as those documented here. Similar processes have been
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found to operate in many other western boundary current regions where frontal eddies drive the entrainment of coastal water, eggs, larvae of marine species and shelf water renewal (e.g. Kasai et al. 2002; Mullaney and Suthers 2013; Gula et al. 2016; Roughan et al. 2017). Here, we propose that the same applies to the SEQCMZ.
The continental shelf of southeast Queensland appears to display similar physical characteristics to those of the southeast coast of the United States (e.g. Govoni et al. 2009). That particular region is characterised by frequent generation of small scale cyclonic frontal eddies that separate from the Gulf Stream as well as larger and longer lived cyclonic eddies. Govoni et al. (2009) refer to the region of persistent cyclonic eddy genesis as the Charleston Gyre. With regard to the SEQCMZ, Ribbe and Brieva (2016) previously found from the analysis of SSH data that the region is characterised by the generation of a larger cyclonic gyre (radius > 50 km) as well, and Azis Ismail et al. (2017) documented the existence of the quasi-permanent onshelf cyclonic Fraser Gyre.
Mullaney and Suthers (2013) visually inspected several years of AVHRR images finding that many small scale (radii < 30 km) and short-lived (a few days) eddies appear to occur within the EAC separation zone, i.e. to the south of the SEQCMZ. Similarly, inspection of satellite images for the SEQCMZ and in addition of those documenting Event A and B, identified the occurrence of many other frontal eddies of similar scale. This suggests that frontal eddies potentially play an important role in cross-shelf exchanges which in magnitude are similar to the previously assessed
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time-averaged mean circulation driven exchange of shelf water due to the Fraser Gyre (Azis Ismail et al. 2017). Additional drifter data are currently not available to further detail and document these events. Thus, future work needs to aim at generating a complete census of frontal eddy events from additional field observations and the potential application of high-resolution operational ocean modelling. Furthermore, these studies should also aim in particular to investigate the genesis of EAC meanders and spawning of frontal eddies in the region of the SEQCMZ.
This study used a simple approach similar to that adopted by Shapiro et al. (2010), yielding results that are consistent with those documented for other WBC regions, yet several caveats to our study need to be acknowledged and considered in interpreting our results. This includes the selected tangential velocity. It is evident form our study that this velocity varies across the width of the eddy or filament, and thus would depend on the position of the drifter. Roughan et al. (2017) found that the offshore branch of a frontal eddy was characterised by a higher velocity due to that branch joining the southward flow of the EAC. Similarly, Event drifter A and B had different velocities over the period trapped within the SEQCMZ and at crossing of the shelf break (see Table 1). Furthermore, we were only able to document the trapping of two drifters since their transport into and within the SEQCZM coincided with the genesis of a frontal eddy. However, the visual inspection of Chl-a patterns, similar to the approach by Mullaney and Suthers (2013), identified about 7-13 ring like structures, aka frontal eddies events; or on average about ten per year for the
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period 2004-2010. Thus indicates that frontal eddies are a reoccurring feature, although less frequent than further south (Mullaney and Suthers 2013) and future work is anticipated to generate an additional census for frontal eddies in the SEQCMZ. Submesoscale fronal eddies were not detected as part of the previous eddy census for the region by Ribbe and Brieva (2017). The tangential velocity estimates were also derived from undrogued drifters. Choukroun et al. (2010) compared undrogued versus drogued drifter velocities finding values of 0.18 m.s-1 and 0.13 m.s-1, respectively, and for the shelf region of the Great Barrier Reef situated just to the north of the SEQCMZ. We have made no corrections for wind slippage of the undrogued drifters. The assumed tangential velocity estimates are potentially too high. The drifter speed is a surface value that certainly reflects the maximum speed of the whole water column and represent an upper limit.
Our study is also limited since no supporting direct ship-borne observations (e.g. Everett et al. 2011, Roughan et al. 2017) and land-based high-frequency radar current measurements (e.g. Haus et al. 2000, Mantovanelli et al. 2017, Schaeffer et al. 2017) of frontal eddies are available. This has allowed for quantitative and qualitative descriptions of individual frontal eddies to the south of the SEQCMZ. However, our initial results are consistent with previous findings, and it is anticipated that this study and previous oceanographic SEQCMZ investigations (e.g. Brieva et al. 2015, Ribbe and Brieva 2016, Azis Ismail et al. 2017) are likely to inform the planning of future field missions. These will be aiming to improve our
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understanding to the regional physical processes that drive a marine biodiverse environment and appear of importance to eastern Australian fisheries.
Yet, an important outcome from our study is that for the first time the importance of frontal eddies for cross-shelf exchanges in the northern intensification zone of the EAC was documented and quantified. It is likely that this work will inspire future computational and field work activities in the region and future work will need to focus on reducing the discussed uncertainties of this study.
Acknowledgment
The authors gratefully acknowledge agencies and colleagues that provided accesses to data and made this study possible. These include NOAA Atlantic Oceanographic and Meteorological Laboratory and the Australian Government funded Integrated Marine Observing System. Mr. Mochamad Furqon Azis Ismail acknowledges support from the Research Centre for Oceanography at the Indonesian Institute of Sciences (RCO – LIPI) and the scholarship Program for Research and Innovation in Science and Technologies (RISET-Pro) from the Ministry of Research, Technology and Higher Education of the Republic of Indonesia. Ms Liv Toaspern acknowledges financial support from the University of Oldenburg for international travel to visit the University of Southern Queensland, Australia.
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Highlights
first study to detail existence of small frontal eddies in northern EAC region
eddies likely to make significant contribution to regional cross-shelf transport
analysis appears consistent with previous western boundary current studies
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