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Chlorophyll bloom in response to tropical cyclone Hudhud in the Bay of Bengal: Bio-Argo subsurface observations
Author’s Accepted Manuscript Chlorophyll bloom in response to tropical cyclone Hudhud in the Bay of Bengal: Bio-Argo subsurface observations Neethu Chacko www.elsevier.com
PII: DOI: Reference:
S0967-0637(16)30277-1 http://dx.doi.org/10.1016/j.dsr.2017.04.010 DSRI2781
To appear in: Deep-Sea Research Part I Received date: 19 August 2016 Revised date: 7 April 2017 Accepted date: 8 April 2017 Cite this article as: Neethu Chacko, Chlorophyll bloom in response to tropical cyclone Hudhud in the Bay of Bengal: Bio-Argo subsurface observations, DeepSea Research Part I, http://dx.doi.org/10.1016/j.dsr.2017.04.010 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.
Chlorophyll bloom in response to tropical cyclone Hudhud in the Bay of Bengal: Bio-Argo subsurface observations Neethu Chacko* Regional Remote Sensing Centre-east/ National Remote Sensing Centre, ISRO, Kolkata, 700156, India * Corresponding author: Neethu Chacko. [email protected] Abstract Though previous studies have documented substantial increases in chlorophyll concentrations as a result of cyclones, most of them were based on satellite observations dealing with surface chlorophyll blooms. This study documents the subsurface biological response and the subsequent chlorophyll bloom observed in response to the tropical cyclone Hudhud as evident from a Bio-Argo float located at the central Bay of Bengal. Results show high chlorophyll concentrations of up to 4.5 mg.m-3 which is anomalous in the normally warm, stratified, and oligotrophic Bay of Bengal. The chlorophyll bloom is attributed to the combined effect of subsurface chlorophyll entrainment and nutrient injection. The presence of a preexisting cyclonic eddy and the decreased translation speed of the cyclone over this region could have played a role in inducing the biological response. This is the first ever report to document the evolution of a subsurface chlorophyll bloom in response to cyclone forcing using Bio-Argo observations.
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Key words: Bio-Argo, Subsurface Chlorophyll-a, Hudhud, Air-sea interaction, Tropical Cyclones, Eddy pumping
1. Introduction Tropical cyclones induce intense mixing of ocean waters subsequently resulting in entrainment and upwelling (Price, 1981). The upper ocean responds to the cyclone forcing physically and biologically. The implications of this physical and biological response are in the form of cooling of the sea surface temperature, chlorophyll blooms, and substantial increase in the primary productivity (Walker et al., 2005). The phytoplankton bloom following a cyclone is due to the entrainment of subsurface phytoplankton or from new production resulting from nutrient influx in to the euphotic zone (Babin et al., 2004; Liu et al., 2009; Shi and Wang, 2007; Hung and Gong, 2011; Lin et al., 2003). The drastic biological responses thus induced significantly impact marine productivity and the carbon cycle (Hung et al., 2010). A 3dimensional description of the upper ocean variability associated with the ocean-atmosphere interactions due to the tropical cyclones Nargis and Laila in the Bay of Bengal is discussed by Maneesha et al., (2012). Shi and Wang (2007) reports on the phytoplankton bloom induced by the enhanced nutrient supply brought up by the wind-induced upwelling and vertical mixing due to hurricane Katrina in the Gulf of Mexico. The bio-physical response of the ocean to the cyclone forcing depends also on the prevailing oceanic conditions (eg. mesoscale eddies) and the
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translation speed of the cyclone (Gierach and Subrahmanyam, 2008). The role of mesoscale eddies in enhancing phytoplankton bloom under cyclone forcing is well reported (e.g., Liu et al., 2009; Walker et al., 2005). Lin et al., (2003) reports that chlorophyll concentration (hereafter referred as chlorophyll-a) increased to 30 fold after the passage of the hurricane Kai-Tak in the South China Sea. Most of the reports on the biological response were based on satellite observations of chlorophyll-a depicting the surface response. Though a lot of research is published on the subsurface temperature and salinity variability based on in situ observations, subsurface biological response remains relatively unexplored. This is primarily due to the lack of subsurface chlorophyll-a observations. A study by Ye at al., (2013) documented the strong subsurface chlorophyll bloom followed by typhoon Nuri in the South China Sea which lasted for nearly 3 weeks using cruise survey data. This paper uses observations from a Bio-Argo float fortuitously located in the vicinity of cyclone Hudhud to document the biological response of the subsurface ocean to the cyclone forcing. The proximity of the Bio-Argo float to the cyclone centre provided the rare opportunity to examine the subsurface chlorophyll dynamics before and after the cyclone. Satellite-observed chlorophyll-a is also used to document the corresponding surface biological response. The aim of this study is to understand the evolution of the chlorophyll bloom in response to the cyclone forcing. The mechanisms driving the observed chlorophyll bloom are also discussed based on the prevailing conditions. 2. Data and Methods The 6-hourly track data and wind speeds of Hudhud are obtained from Regional Specialized
Meteorological
Centre
(RSMC),
3
India
Meteorological
Department
[http://www.rsmcnewdelhi.imd.gov.in/index.php]. Observations from a Bio-Argo float (WMO ID 2902114) are used in this study to document the subsurface chlorophyll-a variation associated with the cyclone. In addition to chlorophyll-a, temperature and salinity from the Bio-Argo float are also used to show the subsurface responses. Bio-Argo measurements employ fluorescence as a bio-optical proxy for chlorophyll-a concentration. The float gives observations in 5 day intervals from 5 m to 2000 m at varying depth intervals during the period January 01, 2013 to October 04, 2015. The level 3 standard chlorophyll-a products derived using OCx algorithm from Moderate Resolution Imaging Spectroradiometer (MODIS Aqua) with a spatial resolution of 9 km and a temporal resolution of 8 days are used to characterize the surface chlorophyll-a response. MODIS derived Photosynthetically available radiation (PAR) at the ocean surface is also
used
in
the
study.
MODIS
products
are
downloaded
from
http://oceancolor.gsfc.nasa.gov/cgi/l3. Daily Advanced Microwave Scanning Radiometer (AMSR-2) sea surface temperature (SST) used in this study is obtained from http://apdrc.soest.hawaii.edu/index.php. The product has a spatial resolution of 0.25 degrees. Sea surface height anomalies (SSHA) are used to observe the signatures of mesoscale eddies. Daily SSHA data from Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) data with a spatial resolution of 0.33 degrees are
used.
The
altimeter
products
were
produced
and
distributed
by
AVISO
(http://www.aviso.altimetry.fr/en/data/products/sea-surface-height-products/global/msla-h.html), as part of the Ssalto ground processing segment. Climatological vertical profiles of nitrate from World Ocean Atlas 2009 (WOA 09) are used as a proxy to show the distribution of nutrients. The translation speed of the cyclone is estimated based on its 6-hour position following Mei et al., (2012). Surface winds from Advanced Scatterometer (ASCAT) with a spatial resolution of
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0.25 degree are used for the computation of wind stress. The Ekman pumping velocity is computed from the equation below, following Ye et al., (2013), W = curl (τ/ (rho f))
(1)
where W , τ , rho and f are the Ekman pumping velocity, wind stress, density of sea water and the coriolis parameter, respectively.
3. Results and Discussion 3.1 Chlorophyll-a response using satellite data Hudhud (October 07-12, 2014) was a category 3 cyclone based on the Saffir-Simpson scale. The Saffir-Simpson wind scale is a 1 to 5 rating based on a cyclone’s sustained wind speed. Cyclones reaching category 3 and higher are considered major cyclones because of their potential for significant loss of life and damage. The track of the cyclone along with the location of the Bio-Argo float during Hudhud is shown in Fig. 1. Hudhud passed the Bio-Argo float on October 09, 2014. Fig. 2 present the 8 day chlorophyll-a in October 2014 showing the precyclone and post-cyclone variability. Though the region of interest is central Bay of Bengal where the Bio-Argo was present, satellite derived chlorophyll-a of the whole Bay of Bengal is presented to show an overall picture of the surface response induced by the Hudhud. The chlorophyll-a is very less during the pre-cyclone period (September 30, 2014 to October 07, 2014). During this period, the chlorophyll-a in the open ocean is <0.2 mg.m-3 with slightly higher values (0.2–0.4 mg.m-3) along the coastal regions. Track of the cyclone is overlaid during the period of Hudhud (Fig. 2b, f).
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Fig. 1. Track of the tropical cyclone Hudhud overlaid on sea surface height anomaly (SSHA; cm) averaged over the cyclone period (October 07-13, 2014). The 6-hourly cyclone eye locations are marked with colored circles; the colors represent the maximum sustained surface winds (MSW) in km/h. Originated at 95°E, 11.5°N Hudhud moved west-northwestwards towards the Indian subcontinent. The location of the Bio-Argo float two weeks before and after Hudhud is denoted by the star symbols. The red star indicates the position of the float during Hudhud
During this period (October 08-15, 2014), though the cloud cover is reasonably high, the chlorophyll-a maps clearly show the signatures of a strong chlorophyll bloom. The chlorophyll-a increases significantly reaching up to 2.8 mg.m-3 during this period (Fig. 2b). In the following week (October 16-23, 2014), a well defined chlorophyll bloom all along the track of Hudhud is formed. During the last week of October (October 24-31, 2014), chlorophyll-a is restored to the pre-existing values, giving a timescale for the cyclone-induced surface chlorophyll bloom of two weeks.
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Fig. 2. Tropical cyclone-induced responses of (a-d) surface chlorophyll-a (mg.m-3) and (e-h) sea surface temperature (deg C). The track of Hudhud (October 07-12, 2014) is also shown in panels b and f. Each row from top to bottom represents the 8 day averaged chlorophyll-a or sea surface temperature, the corresponding periods are September 30, 2014-October 07, 2014; October 0815, 2014; October 16-23, 2014 and October 24-31, 2014 respectively
SST averaged for the same 8-day period as that of the chlorophyll-a is presented in the Fig. 2e-f. Before the passage of Hudhud (September 30, 2014 - October 07, 2014), the SST map (Fig. 2e) showed that the whole Bay of Bengal contains warmer waters with SST greater than 30 °C. After Hudhud, significant SST cooling of up to 3 °C occurred, co-located with the chlorophyll bloom patch. Rightward bias of the cooling is noted as reported in previous studies.
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The reported reason for this rightward bias is due to the asymmetry in turning direction of the wind stress vector which in turn drives strong asymmetry in the vertical velocity in the mixed layer, and thus in the entrainment (Price, 1981; Greatbatch, 1983; Sun et al., 2014). Patchy signatures of cooler SST are observed in the following week, though the magnitude is reduced, and by the end of October the open ocean SST is restored. However, the coastal ocean still retained the cooler patch. As in the case of the surface chlorophyll bloom, the SST cooling also existed for approximately 2 weeks before restoring to the pre-existing values.
3.2 Chlorophyll-a response observed from Bio-Argo The vertical structures of chlorophyll-a, temperature and salinity in the upper 100 m of the water column as observed by the Bio-Argo float are shown in Fig. 3. The time series observations show the subsurface response during and after the cyclone activity. Both physical and biological parameters clearly indicate the changes associated with the cyclone forcing. Until Hudhud, the near surface (5 m depth) chlorophyll-a is observed to be less than 0.1 mg.m-3. Subsurface temperature (Fig. 3b) also shows response to the cyclone in a similar manner. Subsurface cooling can be observed up to 50 m from the surface. The outcropping of temperature contours reduces the surface temperature up to ~2-2.5 °C substantiating the role of entrainment. Shoaling of thermocline (depth of the 23 degree C isotherm) is also evident in the figure. The cyclone induced upwelling also results in bringing cooler subsurface water from below. The displacement of the thermocline due to the cyclone induced upwelling can be computed from the wind stress using the relation η = stress,
τ
; where η is the thermocline displacement, τ is the wind
is water density, f is the Coriolis force, and Ts is the translation speed of the cyclone
(Price, 1994). The thermocline displacement computed based on the maximum surface wind
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speed of 101.9 km/h and translation speed of 2 m.s-1 is 19 m. This is consistent with the thermocline displacement of ~17 m observed from the subsurface temperature shown in Fig. 3b. The pre-storm conditions are completely restored after October 25, 2014. Similarly, the salinity field also shows shoaling of isohalines (Fig. 3c) with the arrival of Hudhud, bringing high salinity waters to the surface.
Fig. 3. Depth-time sections of (a) chlorophyll-a (mg.m-3), (b) temperature (°C) and (c) salinity (psu) in the upper 100 m from a Bio-Argo float in the central Bay of Bengal. The black lines in the panels (a) and (b) indicates the depth of the subsurface chlorophyll maxima (SCM) and depth of 23 deg C isotherm depicting the thermocline depth, respectively Vertical profiles of chlorophyll-a in the upper 100 m are shown in Fig. 4 to precisely show the evolution of the bloom and variation in the depth of the SCM. Shown are the 9
5 day chlorophyll-a profiles from October 05-25, 2014 depicting the rapid rise in the chlorophyll-a and depth of the SCM before and after Hudhud. The SCM located at 50 m depth before Hudhud (October 05, 2014) rises very close to the surface [~10 m] after Hudhud (October 10, 2014).
Fig. 4. Vertical profiles of chlorophyll-a (mg.m-3) during the period October 05-25, 2014 observed by the Bio-Argo float At this depth the chlorophyll-a after Hudhud is enhanced to 1.6 mg.m-3. This is closer to the precyclone (October 05, 2014) subsurface chlorophyll-a. On October 15, 2014, the chlorophyll-a observed is ~3.6 mg.m-3 at 10 m depth. The concentration of chlorophyll-a increases further, reaching up to 4.6 mg.m-3 (20-30 m depth) on October 20, 2014. The pre-cyclone chlorophyll-a levels as well as depth of the SCM are restored by October 25, 2014, nearly two weeks after 10
Hudhud. The temporal evolution of the SCM and the mixed layer depth (MLD) is shown in the Fig. 5. Before Hudhud, the depth of the SCM ranges between 45-55 m depth well below the MLD. During Hudhud, the MLD deepens in response to the increased wind forcing and the subsequent entrainment mixing induced by it. It can be observed that the depth of the SCM shoals during this period and becomes shallower than the MLD. The deepened MLD gets introduced to the chlorophyll-a and the nutrients beneath it, and as a result the bloom starts to develop.
Fig. 5. Temporal evolution of mixed layer depth (m) and depth of the subsurface chlorophyll maxima (SCM; m) derived from the Bio-Argo float. The dashed vertical line indicates the date of arrival of the cyclone Hudhud
3.3 Discussion The upper ocean response to tropical cyclones is complex and depends on several atmospheric and oceanic parameters (e.g., Price, 1981; Dickey et al., 1998). The strong chlorophyll bloom is observed when the maximum surface wind speed of Hudhud was only about 101.9 km/h. While the wind speeds are relatively weaker compared to speeds during its later course, it is to be noted that the translation speed of Hudhud was ~2 m.s-1 while it was passing over this region, making it a slow moving cyclone. It can be thus considered that Hudhud
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was a near stationary cyclone which was hovering over the region. A slow moving cyclone (translation speed ≤4 m.s-1) can evoke strong upwelling in the relaxation stage (Price, 1981). An important factor which leads to phytoplankton blooms by slowly moving cyclones is the enough forcing time available for strong upwelling and entrainment (Sun et al., 2010). Zhao et al., (2008) affirms this by analyzing the case of a fast moving and slow moving cyclones in the South China Seas and observed that the slow moving cyclone induced strong phytoplankton bloom. Similarly, many reports are made on the chlorophyll bloom induced by slowly moving cyclones. A case is reported by Sun et al., (2010) for typhoon Hagibis in the South China Sea. Typhoon Hagibis was a slow moving category 1 typhoon, yet it caused a substantial increase in the chlorophyll-a. Studies of Lin (2012) and Mei et al., (2015) also showed that longer transit time of a cyclone results in stronger phytoplankton blooms. The longer lingering time of Hudhud has induced strong vertical upwelling and entrainment mixing. The wind driven vertical exchange between the surface and subsurface layers can be estimated from the Ekman pumping velocity, with positive (negative) ekman pumping velocity depicting upwelling (downwelling). The typical values of Ekman pumping velocity before the cyclone is quite low (Fig. 6). But the Ekman pumping velocity increases significantly during the cyclone suggesting strong upwelling. The strong cyclonic winds can also induce vertical mixing and thereby entrainment at the base of the mixed layer (Sun et al., 2012).
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Fig. 6 Time series of Ekman pumping velocity (m.s-1) averaged over the Bio-Argo float location
In order to examine the contribution of entrainment, the degree of chlorophyll-a entrained into the mixed layer is computed following Ravichandran et al., (2012). The chlorophyll-a entrainment is estimated using the parameter (2) In the equation 2, we represent the vertical velocity at the base of the mixed layer and h is the mixed layer depth. The terms chlah and chlah+Δh represents the chlorophyll-a averaged over the mixed layer and the layer just below the mixed layer. Fig. 7 shows the entrainment term during Hudhud. During Hudhud, the subsurface chlorophyll-a entrainment increases from near zero to 0.12 mg.m-3.day-1. The chlorophyll-a entrainment reduces after the Hudhud. This shows that the process entrainment also contributed to the cyclone induced bloom. The entrainment causes the mixed layer to deepen as shown in the Fig. 5. The combined action of strong upwelling and entrainment brings subsurface waters to the near surface and significantly increases the chlorophyll-a.
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Fig. 7 Time series of the subsurface chlorophyll-a entrainment (mg.m-3.day-1) into the mixed layer
The SSHA maps presented in Fig. 1 reveal the presence of a mesoscale feature over which the Argo float was located during the passage of Hudhud. The negative contours shown in the figure indicate the presence of cyclonic eddies. To ensure that the cyclonic eddy was preexisting before the cyclone, SSHA averaged for a week before the cyclone is also examined. It is noted that the cold core feature is a pre-existing feature (figure not shown). During the cyclone, the location of the float was near to the centre of the eddy centre. Eddy mediated biological productivity is been reported by many researchers in the Bay of Bengal (Kumar et al., 2004); and in other oceans (eg, Falkowski et al., 1991; McGillicuddy et al., 1998; Oschiles and Garcon, 1998; Sun et al., 2014; Mahadevan, 2016). It can be assumed that the cold core eddies could have strengthened the upwelling under the influence of the wind forcing of Hudhud. Phytoplankton bloom following a cyclone is due to two reasons: the upwelling of subsurface chlorophyll-a and the influx nutrients due to the cyclone induced vertical mixing (Mei et al., 2015; Babin et al., 2004). The climatological nitrate distribution obtained from WOA 09 at the float location is shown in Fig. 8. It is observed that the nitrate concentrations are negligible in the upper 30 m of the ocean. However, the nitrate concentration increases from 40 14
m depth which is fairly closer to the mixed layer depth ~35 m observed during Hudhud. Windinduced Ekman pumping and entrainment can result in the influx of nutrients to the mixed layer. The shoaling of the isotherm from below 40 m (see Fig. 3b) thus facilitates the enhancement of the nitrates to the near surface euphotic zone.
Fig. 8 Vertical profile of climatological nitrate (μmol.L-1) in October averaged over the location of the Bio-Argo float
Though observational evidence of nutrient injection is not presented here, the close visual correspondence between the climatological nitrate distribution and the observed depth of isothermal shoaling strongly suggest nutrient injection. From Fig. 3a, b and Fig. 4, it can be noted that concurrent with the temperature decrease, near-surface (at 5 m depth) chlorophyll-a increases after the Hudhud, reaching ~1.6 mg.m-3 which is nearly equivalent to the pre-cyclone 15
subsurface chlorophyll-a. This indicates the influx of subsurface chlorophyll-a in to the near surface region. Further, the chlorophyll-a increases substantially leading to a peak value of 4.75 mg.m-3 until October 20, 2014. This strong chlorophyll-a enhancement by a factor of almost 3 higher than the pre-cyclone concentration and the time lag taken for the bloom to peak strongly is due to nutrient injection. To further demonstrate the role of nutrient influx, the time series of chlorophyll-a integrated over upper 100 m depth is shown in Fig. 9. The pre-cyclone values of integrated chlorophyll-a is ~40-60 mg.m-2. Post-cyclone, the integrated chlorophyll-a increases but slowly with time. However, after October 15, 2014, it is noted that the integrated chlorophyll-a increases rapidly reaching values of ~150 mg.m-2. If the bloom is due to the injection of subsurface chlorophyll-a alone, the magnitude of the integrated chlorophyll-a would not have increased multifold. In addition to this, the duration taken for the rise in the integrated chlorophyll-a also clearly proves new production. Thus it can be concluded that observed bloom is triggered by the combined effects of subsurface chlorophyll-a entrainment and the injection of nutrients from below. Similar studies on cyclone induced chlorophyll bloom resulted by the combined effect of subsurface chlorophyll-a and nutrient influx has been reported by Gierach and Subrahmanyam (2008), Walker et al., (2005) and Subrahmanyam et al., (2002).
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Fig. 9 Time series of chlorophyll-a (mg.m-2) integrated from the surface to 100 m depth observed by the Bio-Argo float. The dashed vertical line indicates the date of arrival of the cyclone Hudhud
The availability of light also determines the bloom even if nutrients are entrained to the near surface waters. PAR is the spectral range of solar radiation from 400-700 nm that photosynthetic organisms are able to use in the process of photosynthesis, the values of which are strongly affected by water vapour/cloud. Time series of MODIS derived PAR is shown in Fig. 10. The temporal evolution of PAR shows decrease from the pre-storm values of 55 Einstein.m-2.day-1 to 28 Einstein.m-2.day-1 during Hudhud. This decrease in PAR is due to the extensive cloud cover associated with the cyclone. Post-Hudhud, the PAR increases to 48 Einstein.m-2.day-1 restoring the light availability required for photosynthesis. The better light conditions and entrained nutrients in the euphotic zone triggers new production of chlorophyll-a (Kumar et al., 2002).
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Fig. 10 Time series of MODIS derived Photosynthetically available radiation (PAR; Einstein.m.day-1)
2
4. Conclusions Observations from a Bio-Argo float provided a rare opportunity to describe the time evolution of subsurface chlorophyll bloom in response to tropical cyclone Hudhud. The lower translations speed of the cyclone as well as the presence of a mesoscale eddy facilitated enhanced vertical pumping. It can be construed that this study serves as evidence for the mesoscale eddy contribution in addition to local Ekman pumping in generating strong chlorophyll blooms under cyclone forcing. The results indicate that both entrainment and upwelling has contributed to the bloom observed. The enhanced biological response is a result of injection of the chlorophyll-a and nutrients from the subsurface to the near surface. Though existing literature report surface chlorophyll-a increase in response to cyclones in many oceanic regions, this is the first documented observation of time evolution of subsurface chlorophyll bloom in the wake of a cyclone based on in-situ observations. This study provides a preliminary insight into the subsurface biological effects that prevail under the influence of tropical cyclones. The observations presented here also emphasize 18
the utility of Bio-Argo observations of biological parameters to understand the biogeochemical processes prevailing in the oceans. It is likely that the value of chlorophyll fluorescence is affected by chromophoric dissolved organic matter (CDOM), biofouling etc. Unfortunately the Bio-Argo float presented here did not measure CDOM. CDOM being ubiquitous in the global ocean (Siegel et al., 2002; Nelson and Siegel, 2002), additional observations of biological parameters like CDOM could have been useful in understanding the biological response of the ocean more precisely. Given the importance of chlorophyll-a in influencing primary production and thereby the global carbon cycle, establishing a wide network of Bio-Argo floats will aid in understanding the subsurface chlorophyll-a dynamics and their role in the global carbon cycle.
Acknowledgements The author gr e a t l y acknowledges the support and the encouragement provided by General Manager, RRSC-east and the Director, National Remote Sensing Centre. Thanks are given to the NASA Ocean Biology Processing Group (OBPG) for providing MODIS data. The Bio-Argo data used in this work is downloaded from http://www.coriolis.eu.org/DataProducts/Data-Delivery. The Bio-Argo data is collected and are made freely available by the International Argo Program and by the national programs that contribute to it. The efforts of many international partners in planning and implementing the Argo array are gratefully acknowledged. The AMSR-2 data are produced by Remote Sensing Systems and are sponsored by the NASA AMSR-E Science Team and the NASA Earth Science MEaSUREs Program. The NOAA
Daily
(non-interpolated)
Outgoing
Longwave
Radiation
is
obtained
from
http://www.esrl.noaa.gov/psd/. Thanks also to the anonymous reviewers for their constructive comments which helped greatly in improving this paper.
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Highlights
Bio-Argo observations reveal strong chlorophyll bloom in response to cyclone Hudhud
The chlorophyll bloom is attributed to the combined effect of subsurface chlorophyll entrainment and nutrient injection.
This study highlights the utility of Bio-Argo observations in understanding ocean responses during extreme events like cyclones
24
PII: DOI: Reference:
S0967-0637(16)30277-1 http://dx.doi.org/10.1016/j.dsr.2017.04.010 DSRI2781
To appear in: Deep-Sea Research Part I Received date: 19 August 2016 Revised date: 7 April 2017 Accepted date: 8 April 2017 Cite this article as: Neethu Chacko, Chlorophyll bloom in response to tropical cyclone Hudhud in the Bay of Bengal: Bio-Argo subsurface observations, DeepSea Research Part I, http://dx.doi.org/10.1016/j.dsr.2017.04.010 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.
Chlorophyll bloom in response to tropical cyclone Hudhud in the Bay of Bengal: Bio-Argo subsurface observations Neethu Chacko* Regional Remote Sensing Centre-east/ National Remote Sensing Centre, ISRO, Kolkata, 700156, India * Corresponding author: Neethu Chacko. [email protected] Abstract Though previous studies have documented substantial increases in chlorophyll concentrations as a result of cyclones, most of them were based on satellite observations dealing with surface chlorophyll blooms. This study documents the subsurface biological response and the subsequent chlorophyll bloom observed in response to the tropical cyclone Hudhud as evident from a Bio-Argo float located at the central Bay of Bengal. Results show high chlorophyll concentrations of up to 4.5 mg.m-3 which is anomalous in the normally warm, stratified, and oligotrophic Bay of Bengal. The chlorophyll bloom is attributed to the combined effect of subsurface chlorophyll entrainment and nutrient injection. The presence of a preexisting cyclonic eddy and the decreased translation speed of the cyclone over this region could have played a role in inducing the biological response. This is the first ever report to document the evolution of a subsurface chlorophyll bloom in response to cyclone forcing using Bio-Argo observations.
1
Key words: Bio-Argo, Subsurface Chlorophyll-a, Hudhud, Air-sea interaction, Tropical Cyclones, Eddy pumping
1. Introduction Tropical cyclones induce intense mixing of ocean waters subsequently resulting in entrainment and upwelling (Price, 1981). The upper ocean responds to the cyclone forcing physically and biologically. The implications of this physical and biological response are in the form of cooling of the sea surface temperature, chlorophyll blooms, and substantial increase in the primary productivity (Walker et al., 2005). The phytoplankton bloom following a cyclone is due to the entrainment of subsurface phytoplankton or from new production resulting from nutrient influx in to the euphotic zone (Babin et al., 2004; Liu et al., 2009; Shi and Wang, 2007; Hung and Gong, 2011; Lin et al., 2003). The drastic biological responses thus induced significantly impact marine productivity and the carbon cycle (Hung et al., 2010). A 3dimensional description of the upper ocean variability associated with the ocean-atmosphere interactions due to the tropical cyclones Nargis and Laila in the Bay of Bengal is discussed by Maneesha et al., (2012). Shi and Wang (2007) reports on the phytoplankton bloom induced by the enhanced nutrient supply brought up by the wind-induced upwelling and vertical mixing due to hurricane Katrina in the Gulf of Mexico. The bio-physical response of the ocean to the cyclone forcing depends also on the prevailing oceanic conditions (eg. mesoscale eddies) and the
2
translation speed of the cyclone (Gierach and Subrahmanyam, 2008). The role of mesoscale eddies in enhancing phytoplankton bloom under cyclone forcing is well reported (e.g., Liu et al., 2009; Walker et al., 2005). Lin et al., (2003) reports that chlorophyll concentration (hereafter referred as chlorophyll-a) increased to 30 fold after the passage of the hurricane Kai-Tak in the South China Sea. Most of the reports on the biological response were based on satellite observations of chlorophyll-a depicting the surface response. Though a lot of research is published on the subsurface temperature and salinity variability based on in situ observations, subsurface biological response remains relatively unexplored. This is primarily due to the lack of subsurface chlorophyll-a observations. A study by Ye at al., (2013) documented the strong subsurface chlorophyll bloom followed by typhoon Nuri in the South China Sea which lasted for nearly 3 weeks using cruise survey data. This paper uses observations from a Bio-Argo float fortuitously located in the vicinity of cyclone Hudhud to document the biological response of the subsurface ocean to the cyclone forcing. The proximity of the Bio-Argo float to the cyclone centre provided the rare opportunity to examine the subsurface chlorophyll dynamics before and after the cyclone. Satellite-observed chlorophyll-a is also used to document the corresponding surface biological response. The aim of this study is to understand the evolution of the chlorophyll bloom in response to the cyclone forcing. The mechanisms driving the observed chlorophyll bloom are also discussed based on the prevailing conditions. 2. Data and Methods The 6-hourly track data and wind speeds of Hudhud are obtained from Regional Specialized
Meteorological
Centre
(RSMC),
3
India
Meteorological
Department
[http://www.rsmcnewdelhi.imd.gov.in/index.php]. Observations from a Bio-Argo float (WMO ID 2902114) are used in this study to document the subsurface chlorophyll-a variation associated with the cyclone. In addition to chlorophyll-a, temperature and salinity from the Bio-Argo float are also used to show the subsurface responses. Bio-Argo measurements employ fluorescence as a bio-optical proxy for chlorophyll-a concentration. The float gives observations in 5 day intervals from 5 m to 2000 m at varying depth intervals during the period January 01, 2013 to October 04, 2015. The level 3 standard chlorophyll-a products derived using OCx algorithm from Moderate Resolution Imaging Spectroradiometer (MODIS Aqua) with a spatial resolution of 9 km and a temporal resolution of 8 days are used to characterize the surface chlorophyll-a response. MODIS derived Photosynthetically available radiation (PAR) at the ocean surface is also
used
in
the
study.
MODIS
products
are
downloaded
from
http://oceancolor.gsfc.nasa.gov/cgi/l3. Daily Advanced Microwave Scanning Radiometer (AMSR-2) sea surface temperature (SST) used in this study is obtained from http://apdrc.soest.hawaii.edu/index.php. The product has a spatial resolution of 0.25 degrees. Sea surface height anomalies (SSHA) are used to observe the signatures of mesoscale eddies. Daily SSHA data from Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) data with a spatial resolution of 0.33 degrees are
used.
The
altimeter
products
were
produced
and
distributed
by
AVISO
(http://www.aviso.altimetry.fr/en/data/products/sea-surface-height-products/global/msla-h.html), as part of the Ssalto ground processing segment. Climatological vertical profiles of nitrate from World Ocean Atlas 2009 (WOA 09) are used as a proxy to show the distribution of nutrients. The translation speed of the cyclone is estimated based on its 6-hour position following Mei et al., (2012). Surface winds from Advanced Scatterometer (ASCAT) with a spatial resolution of
4
0.25 degree are used for the computation of wind stress. The Ekman pumping velocity is computed from the equation below, following Ye et al., (2013), W = curl (τ/ (rho f))
(1)
where W , τ , rho and f are the Ekman pumping velocity, wind stress, density of sea water and the coriolis parameter, respectively.
3. Results and Discussion 3.1 Chlorophyll-a response using satellite data Hudhud (October 07-12, 2014) was a category 3 cyclone based on the Saffir-Simpson scale. The Saffir-Simpson wind scale is a 1 to 5 rating based on a cyclone’s sustained wind speed. Cyclones reaching category 3 and higher are considered major cyclones because of their potential for significant loss of life and damage. The track of the cyclone along with the location of the Bio-Argo float during Hudhud is shown in Fig. 1. Hudhud passed the Bio-Argo float on October 09, 2014. Fig. 2 present the 8 day chlorophyll-a in October 2014 showing the precyclone and post-cyclone variability. Though the region of interest is central Bay of Bengal where the Bio-Argo was present, satellite derived chlorophyll-a of the whole Bay of Bengal is presented to show an overall picture of the surface response induced by the Hudhud. The chlorophyll-a is very less during the pre-cyclone period (September 30, 2014 to October 07, 2014). During this period, the chlorophyll-a in the open ocean is <0.2 mg.m-3 with slightly higher values (0.2–0.4 mg.m-3) along the coastal regions. Track of the cyclone is overlaid during the period of Hudhud (Fig. 2b, f).
5
Fig. 1. Track of the tropical cyclone Hudhud overlaid on sea surface height anomaly (SSHA; cm) averaged over the cyclone period (October 07-13, 2014). The 6-hourly cyclone eye locations are marked with colored circles; the colors represent the maximum sustained surface winds (MSW) in km/h. Originated at 95°E, 11.5°N Hudhud moved west-northwestwards towards the Indian subcontinent. The location of the Bio-Argo float two weeks before and after Hudhud is denoted by the star symbols. The red star indicates the position of the float during Hudhud
During this period (October 08-15, 2014), though the cloud cover is reasonably high, the chlorophyll-a maps clearly show the signatures of a strong chlorophyll bloom. The chlorophyll-a increases significantly reaching up to 2.8 mg.m-3 during this period (Fig. 2b). In the following week (October 16-23, 2014), a well defined chlorophyll bloom all along the track of Hudhud is formed. During the last week of October (October 24-31, 2014), chlorophyll-a is restored to the pre-existing values, giving a timescale for the cyclone-induced surface chlorophyll bloom of two weeks.
6
Fig. 2. Tropical cyclone-induced responses of (a-d) surface chlorophyll-a (mg.m-3) and (e-h) sea surface temperature (deg C). The track of Hudhud (October 07-12, 2014) is also shown in panels b and f. Each row from top to bottom represents the 8 day averaged chlorophyll-a or sea surface temperature, the corresponding periods are September 30, 2014-October 07, 2014; October 0815, 2014; October 16-23, 2014 and October 24-31, 2014 respectively
SST averaged for the same 8-day period as that of the chlorophyll-a is presented in the Fig. 2e-f. Before the passage of Hudhud (September 30, 2014 - October 07, 2014), the SST map (Fig. 2e) showed that the whole Bay of Bengal contains warmer waters with SST greater than 30 °C. After Hudhud, significant SST cooling of up to 3 °C occurred, co-located with the chlorophyll bloom patch. Rightward bias of the cooling is noted as reported in previous studies.
7
The reported reason for this rightward bias is due to the asymmetry in turning direction of the wind stress vector which in turn drives strong asymmetry in the vertical velocity in the mixed layer, and thus in the entrainment (Price, 1981; Greatbatch, 1983; Sun et al., 2014). Patchy signatures of cooler SST are observed in the following week, though the magnitude is reduced, and by the end of October the open ocean SST is restored. However, the coastal ocean still retained the cooler patch. As in the case of the surface chlorophyll bloom, the SST cooling also existed for approximately 2 weeks before restoring to the pre-existing values.
3.2 Chlorophyll-a response observed from Bio-Argo The vertical structures of chlorophyll-a, temperature and salinity in the upper 100 m of the water column as observed by the Bio-Argo float are shown in Fig. 3. The time series observations show the subsurface response during and after the cyclone activity. Both physical and biological parameters clearly indicate the changes associated with the cyclone forcing. Until Hudhud, the near surface (5 m depth) chlorophyll-a is observed to be less than 0.1 mg.m-3. Subsurface temperature (Fig. 3b) also shows response to the cyclone in a similar manner. Subsurface cooling can be observed up to 50 m from the surface. The outcropping of temperature contours reduces the surface temperature up to ~2-2.5 °C substantiating the role of entrainment. Shoaling of thermocline (depth of the 23 degree C isotherm) is also evident in the figure. The cyclone induced upwelling also results in bringing cooler subsurface water from below. The displacement of the thermocline due to the cyclone induced upwelling can be computed from the wind stress using the relation η = stress,
τ
; where η is the thermocline displacement, τ is the wind
is water density, f is the Coriolis force, and Ts is the translation speed of the cyclone
(Price, 1994). The thermocline displacement computed based on the maximum surface wind
8
speed of 101.9 km/h and translation speed of 2 m.s-1 is 19 m. This is consistent with the thermocline displacement of ~17 m observed from the subsurface temperature shown in Fig. 3b. The pre-storm conditions are completely restored after October 25, 2014. Similarly, the salinity field also shows shoaling of isohalines (Fig. 3c) with the arrival of Hudhud, bringing high salinity waters to the surface.
Fig. 3. Depth-time sections of (a) chlorophyll-a (mg.m-3), (b) temperature (°C) and (c) salinity (psu) in the upper 100 m from a Bio-Argo float in the central Bay of Bengal. The black lines in the panels (a) and (b) indicates the depth of the subsurface chlorophyll maxima (SCM) and depth of 23 deg C isotherm depicting the thermocline depth, respectively Vertical profiles of chlorophyll-a in the upper 100 m are shown in Fig. 4 to precisely show the evolution of the bloom and variation in the depth of the SCM. Shown are the 9
5 day chlorophyll-a profiles from October 05-25, 2014 depicting the rapid rise in the chlorophyll-a and depth of the SCM before and after Hudhud. The SCM located at 50 m depth before Hudhud (October 05, 2014) rises very close to the surface [~10 m] after Hudhud (October 10, 2014).
Fig. 4. Vertical profiles of chlorophyll-a (mg.m-3) during the period October 05-25, 2014 observed by the Bio-Argo float At this depth the chlorophyll-a after Hudhud is enhanced to 1.6 mg.m-3. This is closer to the precyclone (October 05, 2014) subsurface chlorophyll-a. On October 15, 2014, the chlorophyll-a observed is ~3.6 mg.m-3 at 10 m depth. The concentration of chlorophyll-a increases further, reaching up to 4.6 mg.m-3 (20-30 m depth) on October 20, 2014. The pre-cyclone chlorophyll-a levels as well as depth of the SCM are restored by October 25, 2014, nearly two weeks after 10
Hudhud. The temporal evolution of the SCM and the mixed layer depth (MLD) is shown in the Fig. 5. Before Hudhud, the depth of the SCM ranges between 45-55 m depth well below the MLD. During Hudhud, the MLD deepens in response to the increased wind forcing and the subsequent entrainment mixing induced by it. It can be observed that the depth of the SCM shoals during this period and becomes shallower than the MLD. The deepened MLD gets introduced to the chlorophyll-a and the nutrients beneath it, and as a result the bloom starts to develop.
Fig. 5. Temporal evolution of mixed layer depth (m) and depth of the subsurface chlorophyll maxima (SCM; m) derived from the Bio-Argo float. The dashed vertical line indicates the date of arrival of the cyclone Hudhud
3.3 Discussion The upper ocean response to tropical cyclones is complex and depends on several atmospheric and oceanic parameters (e.g., Price, 1981; Dickey et al., 1998). The strong chlorophyll bloom is observed when the maximum surface wind speed of Hudhud was only about 101.9 km/h. While the wind speeds are relatively weaker compared to speeds during its later course, it is to be noted that the translation speed of Hudhud was ~2 m.s-1 while it was passing over this region, making it a slow moving cyclone. It can be thus considered that Hudhud
11
was a near stationary cyclone which was hovering over the region. A slow moving cyclone (translation speed ≤4 m.s-1) can evoke strong upwelling in the relaxation stage (Price, 1981). An important factor which leads to phytoplankton blooms by slowly moving cyclones is the enough forcing time available for strong upwelling and entrainment (Sun et al., 2010). Zhao et al., (2008) affirms this by analyzing the case of a fast moving and slow moving cyclones in the South China Seas and observed that the slow moving cyclone induced strong phytoplankton bloom. Similarly, many reports are made on the chlorophyll bloom induced by slowly moving cyclones. A case is reported by Sun et al., (2010) for typhoon Hagibis in the South China Sea. Typhoon Hagibis was a slow moving category 1 typhoon, yet it caused a substantial increase in the chlorophyll-a. Studies of Lin (2012) and Mei et al., (2015) also showed that longer transit time of a cyclone results in stronger phytoplankton blooms. The longer lingering time of Hudhud has induced strong vertical upwelling and entrainment mixing. The wind driven vertical exchange between the surface and subsurface layers can be estimated from the Ekman pumping velocity, with positive (negative) ekman pumping velocity depicting upwelling (downwelling). The typical values of Ekman pumping velocity before the cyclone is quite low (Fig. 6). But the Ekman pumping velocity increases significantly during the cyclone suggesting strong upwelling. The strong cyclonic winds can also induce vertical mixing and thereby entrainment at the base of the mixed layer (Sun et al., 2012).
12
Fig. 6 Time series of Ekman pumping velocity (m.s-1) averaged over the Bio-Argo float location
In order to examine the contribution of entrainment, the degree of chlorophyll-a entrained into the mixed layer is computed following Ravichandran et al., (2012). The chlorophyll-a entrainment is estimated using the parameter (2) In the equation 2, we represent the vertical velocity at the base of the mixed layer and h is the mixed layer depth. The terms chlah and chlah+Δh represents the chlorophyll-a averaged over the mixed layer and the layer just below the mixed layer. Fig. 7 shows the entrainment term during Hudhud. During Hudhud, the subsurface chlorophyll-a entrainment increases from near zero to 0.12 mg.m-3.day-1. The chlorophyll-a entrainment reduces after the Hudhud. This shows that the process entrainment also contributed to the cyclone induced bloom. The entrainment causes the mixed layer to deepen as shown in the Fig. 5. The combined action of strong upwelling and entrainment brings subsurface waters to the near surface and significantly increases the chlorophyll-a.
13
Fig. 7 Time series of the subsurface chlorophyll-a entrainment (mg.m-3.day-1) into the mixed layer
The SSHA maps presented in Fig. 1 reveal the presence of a mesoscale feature over which the Argo float was located during the passage of Hudhud. The negative contours shown in the figure indicate the presence of cyclonic eddies. To ensure that the cyclonic eddy was preexisting before the cyclone, SSHA averaged for a week before the cyclone is also examined. It is noted that the cold core feature is a pre-existing feature (figure not shown). During the cyclone, the location of the float was near to the centre of the eddy centre. Eddy mediated biological productivity is been reported by many researchers in the Bay of Bengal (Kumar et al., 2004); and in other oceans (eg, Falkowski et al., 1991; McGillicuddy et al., 1998; Oschiles and Garcon, 1998; Sun et al., 2014; Mahadevan, 2016). It can be assumed that the cold core eddies could have strengthened the upwelling under the influence of the wind forcing of Hudhud. Phytoplankton bloom following a cyclone is due to two reasons: the upwelling of subsurface chlorophyll-a and the influx nutrients due to the cyclone induced vertical mixing (Mei et al., 2015; Babin et al., 2004). The climatological nitrate distribution obtained from WOA 09 at the float location is shown in Fig. 8. It is observed that the nitrate concentrations are negligible in the upper 30 m of the ocean. However, the nitrate concentration increases from 40 14
m depth which is fairly closer to the mixed layer depth ~35 m observed during Hudhud. Windinduced Ekman pumping and entrainment can result in the influx of nutrients to the mixed layer. The shoaling of the isotherm from below 40 m (see Fig. 3b) thus facilitates the enhancement of the nitrates to the near surface euphotic zone.
Fig. 8 Vertical profile of climatological nitrate (μmol.L-1) in October averaged over the location of the Bio-Argo float
Though observational evidence of nutrient injection is not presented here, the close visual correspondence between the climatological nitrate distribution and the observed depth of isothermal shoaling strongly suggest nutrient injection. From Fig. 3a, b and Fig. 4, it can be noted that concurrent with the temperature decrease, near-surface (at 5 m depth) chlorophyll-a increases after the Hudhud, reaching ~1.6 mg.m-3 which is nearly equivalent to the pre-cyclone 15
subsurface chlorophyll-a. This indicates the influx of subsurface chlorophyll-a in to the near surface region. Further, the chlorophyll-a increases substantially leading to a peak value of 4.75 mg.m-3 until October 20, 2014. This strong chlorophyll-a enhancement by a factor of almost 3 higher than the pre-cyclone concentration and the time lag taken for the bloom to peak strongly is due to nutrient injection. To further demonstrate the role of nutrient influx, the time series of chlorophyll-a integrated over upper 100 m depth is shown in Fig. 9. The pre-cyclone values of integrated chlorophyll-a is ~40-60 mg.m-2. Post-cyclone, the integrated chlorophyll-a increases but slowly with time. However, after October 15, 2014, it is noted that the integrated chlorophyll-a increases rapidly reaching values of ~150 mg.m-2. If the bloom is due to the injection of subsurface chlorophyll-a alone, the magnitude of the integrated chlorophyll-a would not have increased multifold. In addition to this, the duration taken for the rise in the integrated chlorophyll-a also clearly proves new production. Thus it can be concluded that observed bloom is triggered by the combined effects of subsurface chlorophyll-a entrainment and the injection of nutrients from below. Similar studies on cyclone induced chlorophyll bloom resulted by the combined effect of subsurface chlorophyll-a and nutrient influx has been reported by Gierach and Subrahmanyam (2008), Walker et al., (2005) and Subrahmanyam et al., (2002).
16
Fig. 9 Time series of chlorophyll-a (mg.m-2) integrated from the surface to 100 m depth observed by the Bio-Argo float. The dashed vertical line indicates the date of arrival of the cyclone Hudhud
The availability of light also determines the bloom even if nutrients are entrained to the near surface waters. PAR is the spectral range of solar radiation from 400-700 nm that photosynthetic organisms are able to use in the process of photosynthesis, the values of which are strongly affected by water vapour/cloud. Time series of MODIS derived PAR is shown in Fig. 10. The temporal evolution of PAR shows decrease from the pre-storm values of 55 Einstein.m-2.day-1 to 28 Einstein.m-2.day-1 during Hudhud. This decrease in PAR is due to the extensive cloud cover associated with the cyclone. Post-Hudhud, the PAR increases to 48 Einstein.m-2.day-1 restoring the light availability required for photosynthesis. The better light conditions and entrained nutrients in the euphotic zone triggers new production of chlorophyll-a (Kumar et al., 2002).
17
Fig. 10 Time series of MODIS derived Photosynthetically available radiation (PAR; Einstein.m.day-1)
2
4. Conclusions Observations from a Bio-Argo float provided a rare opportunity to describe the time evolution of subsurface chlorophyll bloom in response to tropical cyclone Hudhud. The lower translations speed of the cyclone as well as the presence of a mesoscale eddy facilitated enhanced vertical pumping. It can be construed that this study serves as evidence for the mesoscale eddy contribution in addition to local Ekman pumping in generating strong chlorophyll blooms under cyclone forcing. The results indicate that both entrainment and upwelling has contributed to the bloom observed. The enhanced biological response is a result of injection of the chlorophyll-a and nutrients from the subsurface to the near surface. Though existing literature report surface chlorophyll-a increase in response to cyclones in many oceanic regions, this is the first documented observation of time evolution of subsurface chlorophyll bloom in the wake of a cyclone based on in-situ observations. This study provides a preliminary insight into the subsurface biological effects that prevail under the influence of tropical cyclones. The observations presented here also emphasize 18
the utility of Bio-Argo observations of biological parameters to understand the biogeochemical processes prevailing in the oceans. It is likely that the value of chlorophyll fluorescence is affected by chromophoric dissolved organic matter (CDOM), biofouling etc. Unfortunately the Bio-Argo float presented here did not measure CDOM. CDOM being ubiquitous in the global ocean (Siegel et al., 2002; Nelson and Siegel, 2002), additional observations of biological parameters like CDOM could have been useful in understanding the biological response of the ocean more precisely. Given the importance of chlorophyll-a in influencing primary production and thereby the global carbon cycle, establishing a wide network of Bio-Argo floats will aid in understanding the subsurface chlorophyll-a dynamics and their role in the global carbon cycle.
Acknowledgements The author gr e a t l y acknowledges the support and the encouragement provided by General Manager, RRSC-east and the Director, National Remote Sensing Centre. Thanks are given to the NASA Ocean Biology Processing Group (OBPG) for providing MODIS data. The Bio-Argo data used in this work is downloaded from http://www.coriolis.eu.org/DataProducts/Data-Delivery. The Bio-Argo data is collected and are made freely available by the International Argo Program and by the national programs that contribute to it. The efforts of many international partners in planning and implementing the Argo array are gratefully acknowledged. The AMSR-2 data are produced by Remote Sensing Systems and are sponsored by the NASA AMSR-E Science Team and the NASA Earth Science MEaSUREs Program. The NOAA
Daily
(non-interpolated)
Outgoing
Longwave
Radiation
is
obtained
from
http://www.esrl.noaa.gov/psd/. Thanks also to the anonymous reviewers for their constructive comments which helped greatly in improving this paper.
19
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Highlights
Bio-Argo observations reveal strong chlorophyll bloom in response to cyclone Hudhud
The chlorophyll bloom is attributed to the combined effect of subsurface chlorophyll entrainment and nutrient injection.
This study highlights the utility of Bio-Argo observations in understanding ocean responses during extreme events like cyclones
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