Shelf-break upwelling on a very narrow continental shelf adjacent to a western boundary current formation zone

Journal of Marine Systems 194 (2019) 52–65

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Shelf-break upwelling on a very narrow continental shelf adjacent to a western boundary current formation zone M.R. Thévenina,

⁎,1

T

, J. Pereirab,1, G.C. Lessac,1

a Programa de Pós-graduação em Geofísica, Instituto de Geociências, Universidade Federal da Bahia, Campus de Ondina, Travessa Barão de Jeremoabo, s/n, Salvador, BA, 40170-280, Brazil b Departamento de Física da Terra e do Meio Ambiente, Instituto de Física, Universidade Federal da Bahia, Campus de Ondina, Travessa Barão de Jeremoabo, s/n, Salvador, BA 40170-280, Brazil c Departamento de Oceanografia, Instituto de Geociências, Universidade Federal da Bahia, Campus de Ondina, Travessa Barão de Jeremoabo, s/n, Salvador, BA 40170280, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords: Coastal upwelling Ekman transport Ekman pumping Cyclonic eddy Current driven upwelling Brazil current

Two years of hydrodynamic monitoring with an ADCP deployed at 32 m of depth at 12.5°S, in association with remotely sensed and numerical modeling data were used to investigate the uplift forcing mechanisms at the narrowest continental shelf close the formation zone of the South Atlantic western boundary currents (Brazil Current and North Brazil Undercurrent). Fifteen uplift events were successfully identified by shelf-bottom temperature anomalies. Their duration varied between 3 and 21 days, presented negative anomalies as large as 2.8 °C, temperature amplitudes of 4.2 °C, maximum vertical temperature stratification of 3.5 °C. Wind driven processes were the main drivers for most of the mapped uplift events, especially the Ekman transport, second by cyclonic eddies that frequently acted as pre-conditioners for the elevation of the isotherms, associated with 75% of events. Although the lowest absolute temperature associated with the events was relatively high (23.3 °C) due to a depressed regional thermocline, their duration, intensity and vertical extent were consistent with upwelling/ uplift events from well-known upwelling regions.

1. Introduction A negative thermal anomaly of the sea surface temperature (SST) on a coastal ocean is a well-known proxy for the advection of deeper water masses up the continental slope and onto the continental shelf, a process called upwelling. The process may fail to transport colder water all the way to the surface, and in such cases it is called shelf-break upwelling or uplift (Campos et al., 2000; Roughan and Middleton, 2002; Schaeffer et al., 2014). Upwelling and uplift can be caused by several processes that may work in concert, such as surface wind stress, current shear on the slope, current divergence and eddies. The shear generated by an upwelling favorable wind on the water surface engenders vertical (upward Ekman pumping, w) and horizontal (offshore Ekman transport, TEK) water movements that are amply acknowledged as the main upwelling driver according to a revision of Kämpf and Chapman (2016). Several authors have shown that the upwelling strength is enhanced, or upwelling is even established, when the isotherms are deflected upwards by the

other subordinate drivers (Gibbs et al., 1998; Roughan and Middleton, 2002; Calado et al., 2010; Palóczy et al., 2014). This second order drivers are: i) interaction of the boundary current with the slope topography, as it drifts closer to the slope (Lee and Pietrafesa, 1987; Oke and Middleton, 2000; Rodrigues and Lorenzzetti, 2001; Palma et al., 2008; Palma and Matano, 2009), ii) the detaching of the boundary current from the shelf break (Janowitz and Pietrafesa, 1982; Roughan and Middleton, 2002; Xie et al., 2007), or iii) the presence of stationary and non-stationary cyclonic eddies (Lutjeharms et al., 1989; Campos et al., 2000; Calado et al., 2010; Palóczy et al., 2014). These oceanographic processes are mainly observed in the western ocean coasts at mid latitudes, where western boundary currents (WBC) are faster and closer to the continental shelf (Loder et al., 1998). Upwelling processes are not well studied or documented in low latitudes of the western ocean margins, where i) the boundary currents are embryonic and ii) the upwelled water is both not as cold and nutrient rich as its counterparts form well known upwelling regions. However, given the highly oligotrophic conditions of the water masses

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Corresponding author. E-mail address: [email protected] (M.R. Thévenin). 1 Tropical Oceanography Group – http://www.goat.fis.ufba.br/index.php/pagina/ver/1. https://doi.org/10.1016/j.jmarsys.2019.02.008 Received 22 October 2018; Received in revised form 31 January 2019; Accepted 21 February 2019 Available online 25 February 2019 0924-7963/ © 2019 Elsevier B.V. All rights reserved.

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predominant flow direction at the shelf break between 13°S and 14°S alternates between southwards, associated with the start of BC formation, from October to February (upwelling favorable conditions) and northwards, associated with the NBC/NBUC, from March to September (Amorim et al., 2012; Amorim et al., 2013). The SEC bifurcation moves southwards with increasing depth, reaching ~22°S at the South Atlantic Central Water (SACW) depth (Pereira et al., 2014). Therefore, the BC in the study region is a very shallow current transporting essentially Tropical Water (TW) close to the shelf break (Silveira et al., 2000). The warm (T > 20 °C) and salty (S > 36.4) TW (Emílsson, 1961), is succeeded by the SACW at depths varying from 150 m to 220 m (Amorim et al., 2012). The SACW is formed in the Brazil-Malvinas Confluence zone (38°S) (Stramma, 1999; Silveira et al., 2000), being colder (6–20 °C), less salty (34.6–36.4) and richer in nutrients than TW (de Miranda, 1985). The SACW is the water mass associated with upwelling processes along the SE Brazilian coast. During the months when the SEC bifurcation is at its northernmost position, the shear between the opposing upper southward flow and the northward NBUC is apparently the reason behind the large number of eddies in this region (Soutelino et al., 2011). The SEC bifurcation and BC intensity varies seasonally as a result of the latitudinal position and intensity of the South Atlantic subtropical high (SASH) pressure system (Rodrigues et al., 2007; Amorim et al., 2013). The SASH is positioned further south and east during the austral summer (Sun et al., 2017), generating ENE trade winds which pushes the bSEC northwards and strengthens the BC (Castro and de Miranda, 1998; Castelao and Barth, 2006; Amorim et al., 2013). In the winter a more intense SASH drifts north and west, and sets in SSE trades which, along with transient cold fronts, decelerate the BC (Bittencourt et al., 2010; Amorim et al., 2013). Shelf currents are mainly wind-driven, present a clear seasonality, and are increasingly more influenced by the boundary current close to the shelf break, especially into southern latitudes (Amorim et al., 2013). Depth integrated circulation in the coastal zone is towards SW with ENE winds in the summer, and to NE with SSE winds in the winter (Cirano and Lessa, 2007). Bottom topography strongly influences shelf circulation at around 13°S where Salvador Canyon locally intensifies wind driven uplift of the isotherms and creates a cyclonic circulation that can advect deeper water over the shelf to the north (Amorim et al., 2012; Aguiar et al., 2018). Tides are semi-diurnal with a mean spring range on the shelf of 1.86 m. Tidal, as well as riverine flows are overall of secondary importance on the shelf, although tide currents are strongly accelerated close to Todos os Santos Bay inlet (BTS) (Cirano and Lessa, 2007; Santana et al., 2018). The BTS is a well-mixed estuary with stronger ebb-tidal flows and an ebb-tidal delta (Banco de Santo Antônio), and although mean density gradient across the bay are small, gravitational circulation is a common feature (Cirano and Lessa, 2007; Santana et al., 2018).

in these areas, the event can be of local, or even regional, importance. For instance, coastal upwelling events off the coast of Colombia cause changes in the depth of the upwelled Subtropical Subwater (Rueda-Roa and Muller-Karger, 2013), which with a thermal signature of 22 °C is important for the maintenance of the small pelagic fish fauna (RuedaRoa and Muller-Karger, 2013), benthic and macro-algae communities (Eidens et al., 2014). Around the formation zone of the Eastern Australian Current (EAC) (14–20°S - Brinkman et al., 2002), the shelf break upwelling with temperatures around 22 °C aid in the provision of nutrients to the maintenance of the Great Barrier Reef between 16°S and 19°S (Andrews and Gentien, 1982; Andrews, 1983; Andrews and Furnas, 1986; Berkelmans et al., 2010). At the oligotrophic Brazilian northeast coast (Castro and de Miranda, 1998), around the Brazil Current (BC) formation zone, Santos et al. (2014) documented, by means of remotely sensed SST with very low temporal resolution, a coastal upwelling process at 13°S with a thermal signature of 24 °C. Although no concurrent chlorophyll anomaly existed, a higher diversity of fish and more efficient fishing activities during the summer (upwelling) season in this region (Olavo et al., 2005, 2011; Braga and Costa, 2014) suggest a positive impact of the upwelling on the trophic system. Here, mesoscale wind circulation was well correlated with upwelling events at 13°S, but some of them were found to coexist with little-favorable or unfavorable wind conditions, indicating that other unknown oceanographic processes may also have a role in the upwelling process (Santos et al., 2014). This upwelling region has the narrowest (< 15 km wide) continental shelf along the Eastern South America (Castro and de Miranda, 1998) and is located within the bifurcation zone of the South Equatorial Current (SEC), which gives origin to the southward BC (Stramma, 1991) and the northward North Brazil Current and Undercurrent (NBC/NBUC) (Silveira et al., 1994; Stramma et al., 1995). The SEC bifurcation position above 200 m of depth varies approximately between 13°S in November and 17°S in July, thus causing seasonal reversions of the current direction on the shelf-break (Rodrigues et al., 2007; Pereira et al., 2014). Narrow continental shelves allow for a more extensive influence of the WBCs on the shelf circulation (Loder et al., 1998; Jiang et al., 2011). Accordingly, numerical modeling of the shelf circulation in this region (Amorim et al., 2013) showed that the WBC may influence the outer shelf region. Besides, the presence of Salvador Canyon at 13°S (Fig. 1) is reported to locally intensify the upward deflection of the isotherms in the summer (Amorim et al., 2012; Aguiar et al., 2018). Aguiar et al. (2018) showed that the canyon driven upwelling can create a northward coastal undercurrent when the BC is not encroached onto the shelf. This undercurrent, in turn, advects uplifted water from the canyon to the shelf area a few tens of kilometers to the north, where Santos et al. (2014) identified an upwelling hotspot. Hence, the peculiarity of the regional morphology and hydrodynamics allows for the coexistence of various mechanisms forcing upwelling and uplift events. To investigate the uplift's forcing mechanisms in the area, we deployed two mooring arrays at 32 m of depth between 12.6°S and 13°S. This is the first long term hydrodynamic monitoring in this section of the coast, and possibly the longest analyzed shelf time series in the country. Our observations were confronted with CTD profiles, remotely sensed SST and altimetry, local wind records, and numerical modeling of the wind, ocean circulation, sea surface height (SSH) and temperature.

3. Methods We deployed two 600 KHz acoustic Doppler current profiler (ADCP) from Teledyne RD Instruments on November 2014. One instrument was deployed at 32 m of depth and 8 km away from the shelf break (#PF, −12.60°/−37.97°, Fig. 1), whereas the other was deployed at 31 m of depth in front of BTS (#BTS, −13.08°/−38.51°). The northern (#PF) instrument is still active, but data analysis will be restricted to the first two years of the record (up to November 2016). The southern (#BTS) instrument worked for 1 full year, until October 2015. The instruments were configured to record 1-min averages every hour, resolving 1 m vertical cells between 6 m and 30 m below the surface. Temperature records at the sea bed were averaged accordingly. Given the proximity to the Todos os Santos Bay inlet, the sub-inertial circulation at #BTS was highly contaminated by the gravitational circulation imposed by the bay. Therefore, only temperature results from this location were analyzed. Temperature anomalies at the shelf bed were used as a proxy

2. Regional settings The study region is located at the northern coast of the Eastern Brazilian shelf (8–15°S, Castro and de Miranda, 1998), around the narrowest shelf width (~8 km) along the Brazilian continental margin (Knoppers et al., 1999). The continental shelf in this area is shallow with the shelf break situated at a water depth of approximately 70–80 m (Campos and Dominguez, 2010; Dominguez et al., 2013). Due to the latitudinal excursion of the SEC bifurcation, the 53

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Fig. 1. Bathymetric contour of the study area. Red crosses indicate the mooring sites, and the green triangle the meteorological station at Salvador International Airport (AERO). Blue and orange dots indicate location of the CTD profiles in October 2015 and March 2016, respectively. The dotted orange square refers to the region of CFS wind data, whereas the dotted black square indicates the region from where modeled (HYN) hydrodynamic results were obtained. Green square indicates the location from where GHRSST temporal series were obtained. BTS stands for Todos os Santos Bay, BSA for Santo Antônio sandbank and CS for Salvador Canyon. Thick arrows indicate the cross (u) and alongshore (v) orientation of the marine currents and wind. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. Correlation between (a) alongshore (VW, gray circles) and cross-shore (UW, red dots) wind components from CFS and AERO. (b) Between the surface temperature from CTD and HYN (HYN-TS) (black triangles), between CTD and GHRSST (red circles), and between GHRSST and HYN-TS (blue dots) for the entire time series, and (c) between shelf-bottom temperature (Tf) at #PF and HYN's temperature at 30 m (HYN-T30). (d) Time series of the sub-inertial signal of HYN-T30, Tf at #PF and #BTS, and bottom (30 m) temperatures measured by the CTD casts at #PF. All correlations are statistically significant at a 95% confidence interval. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

processing Teledyne package WinADCP. Spurious data were identified as data points falling outside an envelope produced by the mean ± 2 standard deviations of a 3 h window (Thomson and Emery, 2014). Data gaps were filled in by cubic interpolation, vertical gaps first. Only horizontal gaps smaller than 6 h were interpolated. A low-pass Lanczos spectral filter (Lee and Pietrafesa, 1987; Thomson and Emery, 2014) with a cut-off period of 53 h, relative to the local inertial period, was

for the uplift events, as explained in Section 3.1. Current vectors were corrected for magnetic declination (−23.43°), aligned with the coastal orientation (−40°) and then decomposed into alongshore and cross-shore current components. Data quality was assessed, and bad data removed, via a measure of error (data with error mean > 2 standard deviations) and relative goodness of the acoustic signal (data with percentage of good < 99%) provided by the post54

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coastwatch.pfeg.noaa.gov/erddap/index.html), for the entire period of study. The average temperature of 7 HYN grid points around #PF (−12.63°/−37.90°, Fig. 1) was used to check the model ability to reproduce realistic surface temperatures. Additionally, maps of GHRSST data, for the days of highest negative thermal anomalies measured by the ADCPs, were used to verify the vertical temperature gradient and assess whether negative thermal anomalies reached the surface. In such occasions, GHRSST anomalies were calculated by subtracting the mean SST at the coast (mean of 1 km2) and that at 500 km further offshore (mean of 10 km2) at the same latitude, according to Nykjær and Van Camp (1994). HYN-TS and GHRSST data were well correlated with the surface temperature obtained by the CTD casts (Fig. 2b). The CTD temperature explained 90% of the HYN-TS and 81% of the GHRSST variability. Likewise, the GHRSST explained 92% of the HYN-TS variability, for the entire time series. All correlations are statistically significant at a 95% confidence interval. The bottom temperature (Tf) at #PF explained 76% of the variability of HYN temperature at 30 m deep (HYN-T30) (Fig. 2c). Because of HYN's coarse bathymetric representation close to the boundary, the #PF station is at 400 m depth in HYN's domain. Even though modeled temperatures were generally consistent with observed ones, HYN-T30 did not capture the uplift events that occur close to the bottom (Fig. 2d). Therefore, although the model was skillful in reproducing local temperature variations, it was not adequate for an investigation of uplift events in this area. In order to check the ability of the model to produce realistic circulation patterns, daily gridded absolute dynamic topography (ADT) and corresponding absolute geostrophic velocity at 1/4° × 1/4° was used. The altimeter products in the delayed time version were produced by Ssalto/Duacs and distributed by AVISO (Archiving, Validation, and Interpretation of Satellite Oceanographic), with support from French Centre National d'Études Spatiales (Cnes) (http://www.aviso.altimetry. fr/duacs/). It is important to note that the AVISO altimetric product errors show higher values in the coastal areas (< 200 km) (Pujol et al., 2016), which basically encompasses our entire region of interest. The AVISO altimetry fields were interpolated for the HYN grid. The AVISO ADT spatial average was significantly higher than the corresponding HYN-SSH over the entire region. Thus, an offset was added such that the ADT had a spatial average similar to that of the SSH (Yan et al., 2015).

used to isolate the sub-inertial signal at every depth cell. Gaps smaller than 24 h in the filtered time series were cubic interpolated. Larger gaps were filled in with white noise to allow further signal processing. Sea-Bird CTD casts (SBE 19 plus) with a sampling rate of 2 Hz were performed in ten occasions between February 2015 and March 2016 at the #PF mooring site and depths of 200 m and 500 m (Fig. 1). Profile data were processed to remove spurious data and averaged at 1 m interval according to Thomson and Emery (2014). Wind data from CFSv2 (NCEP Climate Forecast System Version 2), with temporal and spatial resolution of 6 h and 38 km (Saha et al., 2011), was obtained for the grid element closest to #PF, without reaching the continent (−12.98°/−38.04°, Fig. 1). CFS data was ground-truthed with wind time-series obtained from a METAR station (SBSV) at the Salvador International Airport (AERO) (−12.9°/−38.3°), located 50 km to the south of #PF and 3 km inland (Fig. 1). Spurious data were identified as data points falling outside an envelope mean ± 2 standard deviations of a 3 h window, and data gaps smaller than 24 h were cubic interpolated. The wind vectors were rotated 180° (oceanographic reference frame), aligned with the coast and decomposed into alongshore (VW) and cross-shore (UW) wind components. The effects of the sea and land breezes were removed by applying a Lanczos loss-pass spectral filter with a cut-off period of 53 h (local inertial period). The sub-inertial AERO and CFS wind time series were well correlated. The offshore (CFS) winds explained 78% and 90% of the crossshore and alongshore variability, respectively, at the shoreline (AERO) (Fig. 2a). Despite the high level of correlation, there was considerable difference between the wind intensities, with CFS data showing winds on average 30% stronger, which is typical of offshore wind data when compared with mainland data (Hsu, 1981; Amorim et al., 2012). Given its larger spatial representativeness, the CFS data was used to calculate both Ekman pumping and transport (w and TEK). The wind stress was calculated according to τ ⃑ = ρa Cd | v |⃑ v ⃑ , where ρa is the air density (1.22 kg m−3), Cd is the drag coefficient (103Cd = 1.1, for υ < 6 m/s or 103Cd = 0.61 + 0.063| v ⃑ |, for υ ≥ 6 m/ s) and υ is the wind velocity (Smith, 1981). The offshore TEK was obtained through Talley et al. (2011) formulation: TEK = τy ⃑/ ρf , where τy ⃑ is the wind stress parallel to the coastline, ρ is the sea water density (1024 kg m−3) and f is the Coriolis parameter. Ekman pumping was ̂ × τ ⃑/ ρf , where k ̂ is a local vertical unit vector calculated as: w= k ∙∇ (Talley et al., 2011). To better compare the transport volumes associated with w and TEK, the w velocity was integrated along a cross-shore distance of 80 km (approximate extension of the negative wind stress curl) to obtain the vertical transport due to Ekman pumping (TPUMP) (Castelao and Barth, 2006). Negative TEK and TPUMP were considered upwelling favorable. Velocity, SSH and temperature data were extracted from the Hybrid Coordinate Ocean Model with the Navy Coupled Ocean Data Assimilation - HYCOM/NCODA 2.2. (henceforth HYN), with daily global solutions at 1/12° horizontal resolution (~7 km). The surface circulation and SSH were mapped to verify mesoscale features (WBC's meanders and eddies). Time series of the surface temperature (HYN-TS), temperature at 30 m deep (HYN-T30) and the alongshore current component were extracted from a grid point close to shelf break (−12.64°/ −37.91°, Fig. 1). The alongshore velocity on the slope (VHYN) was aligned with the coast, averaged over the first 100 m of depth, low-pass filtered at cut-off period of 53 h, and then used as a proxy for the behavior of the WBC. The HYN-TS and HYN-T30 were filtered accordingly. Additionally, shore-normal transects of velocity and temperature close to #PF were extracted from HYN. Again, the alongshore velocity component was obtained after rotating the direction of the current vector in -40°. Daily-mean SST data from the Group for High Resolution Sea Surface Temperature Pilot Project (GHRSST-PP), with spatial resolution of 1 km, were obtained through the ERDDAP platform (https://

3.1. Thermal index Different indexes have been used in the literature to identify and measure the intensity of upwelling events. In the realm of physical oceanography these indexes (henceforth UI) are mainly based on the offshore Ekman transport (UIET) (Bakun, 1973; Alvarez et al., 2011) and on water temperature variability (Demarcq and Faure, 2000; Chen et al., 2012). While the former only addresses the wind as a forcing mechanism, the latter identifies the phenomena without addressing its cause. In tropical regions, where vertical temperature gradients tend to be steeper, upwelled waters are easily detectable through a measure of negative temperature anomaly (McClean-Padman and Padman, 1991; Benazzouz et al., 2014) or, in places where the vertical thermal structure is well mapped, when a specific isotherm outcrops at the surface (Campos et al., 2000; Andrade and Barton, 2005; Palóczy et al., 2014). Temperature anomalies can be calculated by spatial temperature differences or by site-specific temperature anomalies. Nykjær and Van Camp (1994) and Alvarez et al. (2011) used the temperature difference between coastal and oceanic points at the same latitude to identify surface upwelling through satellite imagery covering several years. Tapia et al. (2009) and Aguiar et al. (2014) used running means of 30 and 90 days, respectively, from temperature time series to obtain a reference value to calculate discrete anomalies associated with small scale (101 km) upwelling events. In Aguiar et al. (2014) a −1 °C anomaly was considered as the threshold for an upwelling event. 55

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Temperature anomaly (°C)

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Fig. 3. Time series of the shelf-bottom temperature anomalies at #PF calculated with a 30 days and 90 days running means.

speed of 0.5 (−0.6) m/s occurred in June 2015 (November 2015), respectively. Positive, up coast directed, monthly-mean VSC occurred between May and June, with a maximum mean value of 0.1 ± 0.1 m/s in June (Fig. 5c). Negative, downcoast directed, monthly-mean water flows occurred between August and April, with a maximum speed (−0.4 ± 0.1 m/s) occurring in November. The speed of the bottom flow (VBC) was much smaller, with maximum positive (negative) values of approximately 0.2 (−0.2) m/s in June 2016 (November 2016) (Fig. 4c). The seasonality was less evident (maximum monthly-mean value of 0.1 ± 0.1 m/s in June) than that on the surface, and an opposing mean direction to the surface flow indicate that vertical shear was frequent in February and August (Fig. 5c). The cross-shore shelf circulation (UC, Fig. 4d) was one order of magnitude weaker than the alongshore flows, with maximum surface (bottom) positive speed 0.1 m/s in March 2015 (November 2015) and maximum negative speed of about −0.1 m/s in November 2015 (September 2015). The data distribution indicates that vertical shear was common throughout the year, particularly between September and January when monthly-mean flows has opposing signs, i.e., negative (landward directed) on the bottom and positive (seaward directed) at the surface (Fig. 5d). Vertical shear was much less frequent between May and August, when monthly-mean flows were positive. Larger standard deviations in both along- and cross-shore current components indicate that reversions in the direction of the current field occur more frequently, and for a longer part of year, than in the wind field (Fig. 5). The time series of the alongshore flow on the slope (VHYN) showed several peaks of positive speeds (up coast) (Fig. 4c), with velocity maximum of 1.0 m/s in June 2016. Maximum negative speed was −0.7 m/s in February 2016. There was also a well-marked seasonality of the VHYN, although with more extensive positive, northbound currents than on the shelf. Monthly-mean flows associated with the incipient NBC/NBUC prevailed between March and October, reaching a maximum in June (0.3 ± 0.03 m/s) (Fig. 5e). The embryonic southbound BC was predominant between November and February, with a maximum monthly-mean value in January (−0.2 ± 0.1 m/s). Relatively large standard deviations indicate that current reversions were frequent between November and March. Shelf-bottom temperatures (Tf) showed a well-defined intra-annual pattern in both #PF and #BTS, with a large (small) temperature oscillation between September and April (May and August) (Fig. 4e). The highest temperature recorded at #PF and #BTS were 29.4 °C (March 2016) and 29.0 °C (April 2015), respectively. The two lowest temperatures were 23.3 °C and 23.5 °C at both #PF and #BTS, respectively, in September 2015. The seasonality of the monthly-mean Tf at #PF and #BTS was well defined (Fig. 5e), with an amplitude of 2.9 °C in both stations. Maximum monthly-mean Tf of 28.0 ± 0.8 °C occurred in April at both stations. Minimum monthly-mean Tf took place in September (25.3 ± 0.6 °C at #PF, 25.1 ± 0.7 °C at #BTS) and November (25.1 ± 0.5 °C at #PF, 25.3 ± 0.3 °C at #BTS). Largest (smallest) standard deviations were observed between February and May (June and August) (Fig. 5e). Larger differences between #PF and #BTS monthly-mean values occurred in January and March. While the former can be explained by a data gap in #BTS, the latter is ascribed to high

We calculated temperature anomalies at the shelf bed using running-mean windows of 30 and 90 days (Fig. 3) and found that the less extensive smoothing associated with the 30 days running mean identified more uplift events (with anomalies equal or larger than −1 °C). For instance, the events on February 2015, April 2015 and October 2015 were not identified with the use of a 90-day running mean (Fig. 3). Hence, our analysis will be based on temperature anomalies using a running-mean window of 30 days. The temperature amplitude (ΔTf) and duration (De) of the identified events were determined according to Tapia et al. (2009). The event commences in the beginning of the anomaly downward trend and it is over when the anomaly returns to 0. The ΔTf value was calculated by the difference between the temperature anomaly in the beginning of the event and its maximum value, thus indicating the temperature drop. 4. Results 4.1. Mean conditions and seasonality Time series of wind speeds, Ekman pumping and transport, current, water temperature, and shelf-bottom temperature anomalies are shown in Fig. 4. Monthly-mean values of these same variables were calculated for the full 2 years of data (Fig. 5). The wind field showed a clear seasonality of the alongshore velocity component (VW) (Fig. 4a). Upwelling-favorable winds (negative VW, northeast oriented) occurred throughout the year but were dominant between September and March. Maximum positive (negative) velocity was 9.9 m/s (−9.3 m/s) in June (November) 2016. The cross-shore speed (UW) was predominantly negative (winds blowing from the East), with maximum speeds of −11.2 m/s in July 2016 and 4.3 m/s in January 2016, respectively. While the monthly-mean UW (Fig. 5a) varied little (between −2.5 ± 1.1 m/s and − 5.1 ± 2.4 m/s), and maintained its direction, the monthly-mean VW described a sinusoidal cycle with a positive (up coast) extreme in May and June (~1.0 ± 2.7 m/s) and a negative maximum in November (−4.5 ± 1.7 m/s). Direction reversals of the alongshore wind flow became frequent between April and August, as indicated by higher standard deviations (Fig. 5a). The TEK followed the oscillations of the alongshore wind (Fig. 4b), more upwelling-favorable between September and March, while the TPUMP was mainly negative (upwelling favorable) throughout the year (Fig. 4b). Maximum positive and negative TEK (TPUMP) were 4.3 (4.4) m2/s in June 2016 and − 4.0 (−4.6) m2/s in November 2016, respectively. Monthly-means values of TEK and TPUMP were negative (upwelling favorable) throughout the years (Fig. 5b), but were most (least) intense in the summer (winter), with a maximum (minimum) strength in November (May–June), with monthly-means values of −1.6 ± 0.8 m2/s (0 ± 0.5 m2/s) for TEK and − 1.4 ± 0.7 m2/s (−0.5 ± 1.0 m2/s) for TPUMP. Downwelling favorable (positive TEK and TPUMP) conditions occurred in May and June (Fig. 5b). The alongshore shelf circulation close to the surface (~5 m of depth, VSC) followed the wind pattern (Fig. 4c), with the alongshore wind explaining 55% of the VSC variability. Maximum positive (negative) 56

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Fig. 4. Time series of the sub-inertial signal of the (a) crossshore (UW) and alongshore (VW) CFS wind velocity. (b) Ekman transport (TEK) and pumping (TPUMP). (c) Surface (VSC) and bottom (VBC) alongshore velocity on the shelf, from ADCP, and mean alongshore velocity on the slope, depthaveraged for the first 100 m of the water column (VHYC), from HYN. (d) Surface (USC) and bottom (UBC) cross-shore velocity on the shelf (different scale), from ADCP. (e) Sea surface temperature from GHRSST (GHRSST), HYN's surface temperature (HYN-TS), shelf-bottom temperature (Tf) at #PF and #BTS, and the surface (0 m) and bottom (30 m) temperatures measured by the CTD casts at #PF. (f) Shelf-bottom temperature anomalies at #PF and #BTS. Shaded areas indicate the duration of the event.

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temperature (0.3 ± 0.2 °C). The CTD profiles executed in October 2015 and March 2016 on the slope showed the Tropical Water (TW) and South Atlantic Central Water (SACW) interface located at 161 m and 158 m, respectively (Fig. 6). Although temperature gradients were higher in October, one minor thermocline was observed in both profiles at around 80 m of depth where a fast temperature drop exceeded 2 °C, from 24.4 to 22.2 °C (24.9 to 23.6 °C) in October (March). This temperature difference coincided with sharp salinity decline of 0.5 psu. Seven uplift events were identified in the first monitored year, and other eight events were detected in the second year (Table S1 – Supplementary material). Although only three out of the first seven events were concurrent at both stations (February, early March and April 2015) (there was a data gap at #BTS in the first event), it is observed that the associated temperature drops were omnipresent where data from the two stations exists (Fig. 4f). The detected events took place between September and April, with no clearly defined climax since they were not concentrated, nor more intense, in any month. The duration of the events (De) varied from a minimum of 4 days to a maximum of 21 days. Concurrent events in both stations lasted 14.9 ± 6.6 days in average. For the full two years of record at #PF the

temperatures recorded in March 2016 at #PF (Fig. 4e). Surface temperature time series from GHRSST and HYN-TS were quite similar, even though higher frequency oscillations exist at GHRSST. Distinct from Tf, GHRSST and HYN-TS variations were small throughout the time (Fig. 4e). The highest (lowest) temperature was 29.4 (25.2) °C in April 2015 (October 2016) for HYN-TS and was 29.4 (24.4) °C in March 2015 (November 2014) for GHRSST. Both time series showed a clear seasonal cycle, with amplitude of 3 °C (Fig. 4e). Monthly-mean values of surface temperature were higher than Tf between September and April (> 1 °C in the summer). HYN-TS and GHRSST maximum monthly-mean values were also reached in April (28.7 ± 0.3 °C, for both), but the minimum monthly-mean value occurred earlier (August) than in the observed data, with 25.7 ± 0.3 °C and 25.8 ± 0.3 °C, respectively (Fig. 5e). The CTD profiles at the mooring site showed bottom (surface) temperatures similar to those recorded by the ADCP (recorded by GHRSST and modeled by HYN) (Fig. 4e). Differences between measured bottom temperatures (< 0.07 °C) were smaller than differences between modeled and observed surface mean temperature (0.3 ± 0.3 °C), and between satellite (GHRSST) and observed surface mean 57

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become surface upwelling events (Fig. S1 l, m and n – Supplementary material), and other three events showed very local negative anomalies away from #PF (Fig. S1c, g and k – Supplementary material). Similar patterns of SST anomalies from GHRSST can also be identified in HYN's SST anomalies shown in Fig. S2.

average De was 11.6 ± 5.6 days. The average amplitude of temperature drops (ΔTf) for concurrent events were 2.0 ± 0.7 °C and 2.6 ± 0.3 °C for #PF and #BTS, respectively, and 2.3 ± 0.9 °C for the two years at #PF. Maximum ΔTf was 4.2 °C at #PF on April/May 2016. The GHRSST anomalies indicate that all but three uplift events failed to 58

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HYN's outputs of daily mean surface circulation and SSH (Fig. S3 – Supplementary material) show that the model was able to reproduce the main features observed with AVISO altimetry (Fig. S4 – Supplementary material), somehow a significant ability for a global circulation model with 1/12° resolution. The spatial pattern of the surface circulation and the SSH/ADT from HYN and AVISO indicate the presence of cyclonic eddies at the beginning of eleven (out of fifteen) observed upwelling events (Fig. S3 and S4). Amongst these eleven events, two were associated with the detachment of the CB off the shelf break (Fig. S3 h and i). These features were also visible on AVISO altimetry fields (Fig. S4 h and i), showing the model ability to reproduce realistic circulation patterns, as will be shown ahead in the cases studied.

than those observed in upwelling events on the Brazilian SE coast, where SST is smaller than 20 °C. This can be ascribed to the deeper (shallower) contact between TW and SACW in lower (higher) latitudes, where BC is poorly (well) established. As Roughan and Middleton (2002) and Hood et al. (2017) noted, sluggish WBC at low latitudes depress the elevation of the thermocline. According to Amorim et al. (2012), the above-mentioned interface lies between 150 m (summer) and 220 m (winter) at 13°S, which is in agreement with our CTD profile data (Fig. 6), and between 100 m and 150 m at 23°S in accordance to Pereira et al. (2014) and Castro (2014). At this higher latitude Tf (at 50 m of depth) during an upwelling event can be as low as 14 °C (Cerda and Castro, 2014). An additional, worth-mentioning fact, is that because of the deeper (120 m to 200 m) break of the SE Brazilian shelf (Castro and Lee, 1995), the shallower TW-SACW interface causes more frequent and intense uplift events (Campos et al., 2000; Castelao et al., 2004) or even a permanent placement of the SACW onto the outer shelf (Palma et al., 2008). Uplift events associated with warm waters (T > 21.5 °C) located at water depths between 80 and 100 m were also observed in the formation zone of the EAC between 14 and 20°S (Andrews, 1983; Andrews and Furnas, 1986; Berkelmans et al., 2010), in the southern Caribbean Sea (Rueda-Roa and Muller-Karger, 2013; Eidens et al., 2014), and in the northeastern South China Sea (Gan and Guo, 2009; Gan et al., 2015). At all these sites the uplifted water was reported to have an important role in the regional productivity. Thermal stratification of the water column at #PF was observed between September and May, as indicated by ADCP temperature records (Tf) and both by modeled (HYN-TS, Fig. 4e) and observed (CTD and GHRSST, Fig. 4e) surface temperature data. The stratification was enhanced during uplift events not succeeded by surface upwelling, a situation that accounted for 40% of the uplift events. For instance, during the strongest uplift event in May 2016, with ΔTf equal to 4.2 °C (Fig. 4f), the colder bottom water did not reach the surface, as indicated by a high surface-to-bottom temperature difference of 3.8 °C (HYNTs = 28.5 °C, Tf = 24.7 °C) and GHRSST horizontal differences smaller than 1 °C. Mean Tf at #PF and #BTS did not follow the increasing solar radiation through the austral spring and summer because of the uplift events, as similarly reported by Franchito et al. (2008) for SST in the upwelling region of Cabo Frio and by Wood et al. (2013) for Tf off the Sydney (Australia) coast. Although the minimum HYN-TS took place in August, the smallest Tf occurred well into the austral spring season, and the mean surface-to-bottom temperature difference stayed high (> 1 °C) until the month of March. Surface heat loss to the atmosphere apparently occurred between May and August, when Tf was higher than HYN-TS. Uplift events may encompass hundreds of kilometers in extension along the shelf-break in the Australian northeast, positively impacting a large extension of the Great Barrier Reef (Furnas and Mitchell, 1996; Brinkman et al., 2002). Here, well correlated temperature time series between #BTS and #PF as well as concurrent uplift events with similar magnitudes, suggest that the events extend for at least 80 km along the shelf. Highest correlation (+0.76) between the entire time series was obtained with a time lag of 17 h, indicating that the process is established initially in the south and then propagates to the north. This agrees with the findings of Santos et al. (2014) who, after analyzing 10 years of SST imagery, pointed out that more frequent and larger temperature anomalies occurred in front of Todos os Santos Bay. Such local intensification can be ascribed to flow disturbances caused by the presence of Salvador Canyon close by. The canyon disturbs along-shelf upwelling-favorable currents, forcing a cyclonic circulation at the canyon head and enhancing cross-shore exchanges and upwelling intensities, when compared to the surrounding shelf-break regions (Amorim et al., 2012; Aguiar et al., 2018).

5. Discussion 5.1. Uplift events Fifteen uplift events were identified between the months of September and April, which correspond to the “upwelling season” in the Brazilian coast between 19°S and 24°S (Castro and de Miranda, 1998; Rodrigues and Lorenzzetti, 2001; Aguiar et al., 2014). The observed De and ΔTf, with averages of 11.6 ± 5.6 days and 2.3 ± 0.9 °C, were similar to those reported by Tapia et al. (2009) regarding upwelling events on the US west coast and Chilean coast, two of the world's most important upwelling systems. In those locations, 10 years of monitoring of surface temperatures in several locations resulted in averages of De in the US West (Chilean) coast varying between 9.5 and 21.3 days (8.8 and 15.7 days) and mean values of ΔT (related to surface temperatures) oscillating from 1.63 °C to 3.85 °C (1.67 °C to 2.80 °C). In addition, Roughan and Middleton (2004) showed that ΔTf (graphically inferred) measured at 50 m of depth on the Australian shelf around 32°S can reach almost 4 °C. This value was also similar to the highest ΔTf we have recorded (4.2 °C) and shows that low latitude events have similar thermal amplitudes to its counterparts from well-known upwelling systems, even though lowest temperatures are still high for traditional thermal upwelling standards. We must point, however, that comparison of these metrics deserves some caution because i. there is great diversity amongst the methods used to identify these events, with resulting indexes not always comparable (absolute temperature value) (Campos et al., 2000; Andrade and Barton, 2005; Palma and Matano, 2009; Palóczy et al., 2014; Cerda and Castro, 2014; Alonso et al., 2015), difference between in- and offshore temperature (Nykjær and Van Camp, 1994; Alvarez et al., 2011; Chen et al., 2012; Benazzouz et al., 2014), difference between observed and running mean (Tapia et al., 2009; Aguiar et al., 2014; Santos et al., 2014) and still others temperature-related (McCleanPadman and Padman, 1991; Demarcq and Faure, 2000), ii. observations derived from different sources, including direct observation (Ikeda et al., 1974; Andrews, 1983; Andrews and Furnas, 1986; Tapia et al., 2009; Berkelmans et al., 2010; Schaeffer et al., 2014; Cerda and Castro, 2014; Schaeffer and Roughan, 2015), remote sensing of skin SST (Demarcq and Faure, 2000; Zavala-Hidalgo et al., 2006; Rueda-Roa and Muller-Karger, 2013; Mazzini and Barth, 2013) and numerical modeling (Palma and Matano, 2009; Calado et al., 2010; Jiang et al., 2011; Palóczy et al., 2014; Aguiar et al., 2014). As for the latter, while Santos et al. (2014) observed SST anomalies of about −2,7 °C between 12°S and 13°S using climatological and 90-days running means, Aguiar et al. (2014) found further south, between 17°S and 23°S (where strongest upwelling occur on the Brazilian coast), smaller SST anomalies (> − 2 °C) using the same method. The minimum temperature associated with uplift events was 23.3 °C, which in accordance with the CTD profiles shown in Fig. 6a, was related to water advected from a depth between 80 m and 100 m, consistent with reports from important upwelling regions (Messié et al., 2009; Lathuilière et al., 2010). This temperature is at least 3 °C higher 59

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negative, upwelling favorable, TEK and TPUMP (Fig. 4b). Aguiar et al. (2018), based on 5 years of numerical simulation of the coastal circulation (model ROMS with 500 m spatial resolution over the shelf at 13°S), identified the predominance of surface upwelling in the study area in the spring and summer, with uplift events better distributed along the year especially close to Salvador canyon (Fig. 1). Their analysis of the forcing mechanisms revealed the wind as the main driver for these processes, with eddy driven upwelling being identified in only 1 occasion over the 5 modeled years. Uplift of the isotherms can be caused by propagating or stationary cyclonic meanders and eddies. The quasi-stationary ones – that may grow nearly without propagation – cause divergence in the water column by drawing shelf waters offshore (Lutjeharms et al., 1989; Calado et al., 2010). Moving eddies, on the other hand, while interacting with the shelf break, can advect water upwards on its leading edge, which is then transported on to the shelf in a clockwise sense (Campos et al., 1995, 2000). Both eddies have been shown to cause or intensify the intrusion of SACW onto the shelf in southeast Brazil (Campos et al., 2000; Calado et al., 2010; Palóczy et al., 2014). Furthermore, the lateral drift of the WBC axis away from the shelf break generates divergent surface flows, negative SSH anomalies and potential conditions for the formation of cyclonic eddies and uplift. This can occur in a wide range of length scales, as previously reported by Roughan and Middleton (2002) and Wu et al. (2008), describing the dynamics of the Eastern Australia and Kuroshio currents as they move offshore in southeast Australia and northeast Taiwan, respectively. Our results show the interaction between coastal and ocean circulation, and that the presence of cyclonic eddies can enhance a primarily winddriven coastal upwelling or cause uplift events, as it will be shown with the cases studied ahead.

5.2. Forcing mechanism Our results showed a clear seasonality of VW (Fig. 4a), whose intensity is governed by the meridional position of the SASH (Castro and de Miranda, 1998; Bittencourt et al., 2010; Amorim et al., 2012). The uplift events occurred mainly between September–April, the period when SASH is farther away from the equator and entails northeast winds (with negative VW) that cause coast-divergent, negative TEK, in the study region (Fig. 4b). This upwelling-favorable condition brought about an often-well-marked vertical shear of the UC, with negative speeds close to the bottom and positive speeds near the surface (Fig. 4d). Likewise, given the persistence of negative VW speeds, the VC (Fig. 4c) was seldom negative during the spring and summer. Conversely, prevailing positive VW speeds between May and August forced positive VC speeds and a downwelling process, with negative (positive) UC values near the surface (bottom), such as in May and June 2016. Therefore, shelf circulation was mainly wind driven, ensuing Ekman dynamics associated with both up- and downwelling (Csanady, 1982) in consonance with the alongshore wind component. These distinct shelf circulation scenarios are characteristic of the northeastern Brazilian coast (Castro and de Miranda, 1998; Cirano and Lessa, 2007; Amorim et al., 2011; Teixeira et al., 2013). Upwelling events along the southeast (19°S-24°S) Brazilian coast are mainly attributed to coast-divergent transport driven by TEK between September and April (Franchito et al., 2008; Mazzini and Barth, 2013; Aguiar et al., 2014). Castelao and Barth (2006) and Aguiar et al. (2014) pointed out that TPUMP was also an important component where changes in the orientation of the continental margin occurs, making a significant contribution for the onset of upwelling events around Cabo Frio (Castelao and Barth, 2006). It was observed in our study area that uplift events were almost always associated with upwelling favorable TEK and TPUMP. However, several upwelling conducive TPUMP conditions between May and August (especially in 2015) did not induce negative temperature anomalies. This could be ascribed to the fact that, in this period, the SEC bifurcation sits in its most austral position and a better established northbound NBUC at the shelf break is observed, leading to the deepening of the thermocline. The correlation between TEK and Tf, for the complete time series was higher (+0.42 with 1-day lag) than that between TPUMP and Tf (+0.35 with 2-days lag), suggesting a greater importance of TEK for uplift events in this area. The intensity of upwelling favorable TEK and TPUMP between December and April was notably higher in 2015–16 then in the year before, with averages rising from approximately -0.7 ± 0.4 m2/s to −1.0 ± 0.6 m2/s. This caused more frequent and stronger uplift events, with the number of events increasing from 4 to 6, and ΔTf jumping from 1.3 ± 0.7 °C to 2.4 ± 1.1 °C. Our results suggest that the wind is indeed the primary forcing upwelling mechanism. However, we have identified a few uplift events where the wind forcing was only mildly upwelling conducive. Between November and February, the SEC bifurcation is in its boreal most position, just north of the study area (~13°S). As a result, southbound flows prevail close to the shelf break, where geophysical instabilities, caused by opposing BC-NBUC flows and/or topographical constraints, cause intense mesoscale activity (Soutelino et al., 2011). This explains the frequent reversion episodes of the current direction (Fig. 5c). The inspection of the spatial distribution of the surface current field and SSH (Fig. S3 – Supplementary material) showed that eleven uplift events (73%) were associated with the presence of cyclonic eddies. Few moving eddies propagated northwards, contrary to the travelling direction of the eddies identified on SE Brazilian coast (Campos et al., 2000). Most of the eddies were quasi-stationeries and lasted on average 5 days. A special situation occurred between November and December 2015, when a cyclonic eddy approaching from the south pushed the embryonic BC offshore. The BC stayed detached from the coast for > 30 days which was coincident with a general cooling process (Fig. 4e). Within these 30 days, two closely spaced uplift events coincided with

5.2.1. Case 1: wind and eddy-driven uplift The combined actions of a quasi-stationary cyclonic eddy and an upwelling conducive wind caused the largest negative thermal anomaly of the monitored period. This event was recorded in the early spring, between September and October 2015, coincident with persistent ENE winds. Fig. 7a shows that an almost constant GHRSST and HYN-TS (25.8 °C) coincided with a Tf drop at #PF and #BTS between September 19th and October 1st, with ΔTf reaching 2.5 °C and 2.3 °C, respectively (Fig. 7b). Water temperature measured by CTD casts in two occasions corroborated the vertical temperature gradient (Fig. 7a) although the measured surface temperature at the event climax was a little lower than the remotely sensed or simulated temperatures. During the event, the wind caused shore-divergent Ekman transport (Fig. 7c), vertical shear of the cross-shore current component (Fig. 7d), and negative alongshore speeds on the slope and shelf alike (Fig. 7e). The current field averaged over 1 day showed a cyclonic eddy close to #PF on September 25th (Fig. 7f) centered at 37°W. As it moved closer to shore on September 28th TEK and TPUMP slackened (Fig. 7c), but negative temperature anomalies grew higher (Fig. 7b). On October 1st the eddy was well established over the shelf (Fig. 7g), and the uplift event eventually gave rise to a relatively ample surface upwelling, as indicated by SST anomalies derived from GHRSST data (Fig. 7h). The eddy dynamic signature was also detected by AVISO's altimetry on October 1st (Fig. 7i). It is worth noting that shelf waters were being advected offshore under the eddy influence, similarly as reported by Calado et al. (2010) at Cabo de São Tomé (SE Brazilian coast). Even though HYN's resolution produce coarse bathymetric representations that end up causing differences in simulated dynamic fields, it was still possible to observe the interaction between the eddy and the shelf dynamic in the vertical profiles of temperature and alongshore current component. Fig. 7j shows the initial stages of the eddy formation, spanning > 500 m vertically, although the thermal signature was almost unnoticeable across the mixing layer. As the eddy intensified and touched the shelf break the isotherms were uplifted and colder water climbed onto the shelf as observed in Fig. 7k (e.g., the 60

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Fig. 7. Case study of a wind- and eddy-driven uplift on September 2015 illustrated by (a) HYN's surface temperature (HYN-TS), GHRSST, bottom temperature (Tf) at #PF and #BTS and the surface (blue dots) and bottom (black dots) temperatures measured by the CTD casts at #PF. (b) bottom temperature anomalies at #PF and #BTS. (c) Ekman transport (TEK) and pumping (TPUMP). (d) Surface (USC) and bottom (UBC) cross-shore velocity on the shelf (different scale), from ADCP. (e) Mean alongshore velocity on the shelf (VC), from ADCP, and on the slope, depth-averaged for the first 100 m of the water column (VHYC), from HYN. Mean surface circulation and SSH from HYN coincident with the formation of the cyclonic eddy (f) and at the apex of the uplift event (g). (h) GHRSST anomalies at the apex of the uplift event. (i) AVISO altimetry at the apex of the uplift event. Vertical sections of temperature and alongshore current speed from HYN, for initial stages of eddy development (j), and at the apex of the uplift event (k). Shaded areas indicate the duration of the event, in boxes a-e. Dark lines in f-g represent the positions of vertical sections. Red crosses indicate the location of the #PF (north) and #BTS (south) stations in boxes f-i, and the location of #PF in boxes j-k. Dashed lines in j-k represent the bathymetric contour obtained from nautical charts and gray line indicates the location of VHYN. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

intense mesoscale activity, with a stronger influence of the northward flowing NBUC on the shelf break. In both events, VC was negative, but VHYN oscillated between positive and negative values, indicating the presence of four eddies over that period (Fig. 8e). The location of theses eddies is shown in Fig. 8f-h and Fig. 8g-i for the moments associated with the beginning and the intensification of the water temperature declines, respectively. It is interesting to note that the intensification (relaxation) of the upwelling events was accompanied by the formation (dissipation) of cyclonic eddies. For instance, the eddy centered at 13°S on April 26 (Fig. 8i), also showed at AVISO's altimetry (Fig. 8l), intensified almost without any propagation until April 30, causing uplift of the isotherms and the most intense temperature drop of the entire time series. These uplift events did not evolve into surface upwelling as indicated by the mapped SST anomalies (Fig. 8j and Fig. 8k), which is a characteristic phenomenon when oceanographic forcing is the main uplift agents (Roughan and Middleton, 2002). Cross sections of temperature and alongshore current speeds from HYN (Fig. 8m) show the

25 °C isotherm moved from 70 m to 50 m). More important changes in wind direction after September 30th caused decreasing values of TEK and TPUMP and the termination of the event. 5.2.2. Case 2: eddy-induced uplift The two strongest events, observed in April 2016, which are portrayed in Fig. 8, took place with little-favorable wind conditions, and were driven by four quasi-stationary cyclonic eddies. During these events, the Tf underwent a drop along several days (Fig. 8a), with maximum ΔTf of 3.0 °C and 4.2 °C in the first and second events, respectively (Fig. 8b). Although TPUMP was slightly upwelling favorable in both events, positive TEK values were unfavorable in the first event (Fig. 8c), when no vertical shear of the cross-shore velocity component was observed (Fig. 8d). Although a mild vertical shear was present during the second event, its magnitude was one order of magnitude smaller than that observed in Fig. 7d, when the wind was the predominant uplift driver. These events coincided with the initial stages of migration of the SEC bifurcation to the south, which generates an 61

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Fig. 8. Case study of uplift events associated with cyclonic eddies, as illustrated by: (a-e) as in Fig. 7, but for April–May 2016. Mean surface circulation and SSH from HYN for the beginning and intensification of (f-g) the 1st uplift event and, (h-i) the 2nd uplift event. GHRSST anomalies at the apex of (j) the 1st uplift event and, (k) the2nd uplift event. (l) AVISO altimetry coincident with intensification of the 2nd uplift event. Vertical sections of temperature and alongshore current speed from HYN for (m) initial stages of eddy development and, (n) at the apex of the uplift event. Shaded areas indicate the duration of the events in boxes a-e. Dark line in box i represent the positions of vertical sections. Red crosses indicate the location of the #PF (north) and #BTS (south) stations, in boxes f-l and, the location of #PF in boxes m-n. Dashed lines in m-n represent the bathymetric contour obtained from nautical charts and gray line indicates the location of VHYN. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

initial development stages of a cyclonic eddy on April 26. Even though the eddy influence was felt to depths of at least 500 m, the thermal signature in the mixing layer was too weak. However, as the eddy

become stronger and moved closer to the shelf break the isotherms were uplifted and colder water climbed onto the shelf, as observed on April 30 (Fig. 8n). 62

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6. Conclusions

Appendix A. Supplementary data

Two years of hydrodynamic monitoring in association with remotely sensed and numerical modeling data were used to investigate the uplift forcing mechanisms close the formation zone of the South Atlantic western boundary currents (BC and NBC/NBUC). Fifteen uplift events were successfully identified by shelf-bottom temperature anomalies higher than 1 °C based on a 30-days running-mean window. The uplift duration varied between 3 and 21 days, presented negative anomalies as large as 2.8 °C, temperature amplitudes (ΔTf) of 4.2 °C, lowest absolute temperature of 23.3 °C and maximum vertical temperature stratification of 3.5 °C. The uplift episodes identified here at around 12.5°S extend for a length of at least 80 km, have a clear seasonality (Sep-Apr), and present duration and intensities consistent with upwelling/uplift events from well-known upwelling regions. Although vertical advection of water occurred down to 80–100 m of depth, again similar to the depth range of upwelling systems further south, the thermal signature of the upwelled waters was higher than its counterparts from well-known upwelling systems. This is ascribed to the depressed thermocline at lower latitudes in the western ocean margins, which prevents the ascension of < 20 °C, nutrient rich water masses to the continental shelf. From all the recorded uplift events, three did develop into an upwelling, other three possibly caused a surface temperature signal, and nine succeeded to become a full upwelling event. Our results may provide insight into the little-known cross-shelf dynamics of other WBC formation zone where similar mesoscale activity exist. We identified three mechanisms responsible for the uplift process, two associated with the wind field (Ekman pumping - TPUMP, and Ekman transport - TEK), and one related to the dynamics of ocean currents (cyclonic eddies). The wind driven processes were the main drivers for most of the mapped uplift events, especially TEK, being associated with fourteen events. Eleven uplift events (73% of the total events) were associated with cyclonic eddies, including the two longest (> 30 days) events that were associated with the combined action of wind and eddies that started with the lateral drift of the BC away from the coast. The coldest upwelling event was forced by the wind and intensified by a quasi-stationary cyclonic eddy, demonstrating the interaction between the shelf and ocean circulation. Finally, the two strongest recorded uplift events, not followed by surface upwelling, took place with little-favorable wind conditions and were driven by four subsequent quasi-stationary cyclonic eddies. Hence, we conclude that cyclonic eddies are also important drivers for shelf-break upwelling, frequently elevating the isotherms prior to the onset of wind processes around the BC formation zone. Although we have shown that the uplifting processes are hydrodynamically significant, there is yet no concrete observation of its importance in the primary production of this largely oligotrophic shelf.

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Acknowledgments Data from CFSv2, HYN, AVISO and GHRSST were fundamental for this work, and their availability is greatly appreciated. The current meter was acquired with funds made available by the Post-Graduate Program in Geophysics – UFBA. Field support provided by the sea turtle TAMAR Project was essential to the conclusion of this research. Special thanks go to Paulo Lara. Financial support for the field work was provided by CNPq UNIVERSAL/2014, 443695/2014-8 and REDE ONDAS. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) – Finance Code 001. We also thank Julia Porto, Lucas Fonseca and André Brandão, undergraduate Oceanography students at UFBA, for their help with the equipment recovery/deployment.

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