cool, elevated chlorophyll-a waters off northern mozambique

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Cool, elevated chlorophyll-a waters off northern Mozambique B.S. Malauene a,b,n , F.A. Shillington c , M.J. Roberts d , C.L. Moloney a a Department of Biological Sciences and Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa b Instituto Nacional de Investigação Pesqueira, Av. Mao Tse Tung 309, Maputo, Mozambique c Department of Oceanography & Nansen-Tutu Centre for Marine Research, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa d Oceans & Coasts Research, Department of Environmental Affairs, PO Box 52126, Victoria & Alfred Waterfront, Cape Town 8000, South Africa article info Available online 7 November 2013 Keywords: Mozambique Channel Angoche Shelf edge upwelling Wind driven Eddy driven abstract Direct in-situ observations from a shallow underwater temperature recorder on the continental shelf and from a shipboard oceanographic survey, were combined with MODIS satellite data (sea surface temperature and chlorophyll-a) to assess the temporal and spatial variability of temperature and chlorophyll-a in the Mozambique Channel near the coastal town of Angoche, 161S. Intermittent, relatively cool surface water and elevated chlorophyll-a signatures were found, indicating upwelling near Angoche over an area between 151S and 181S. A 5-year (20022007) analysis of temperature (from both in-situ and satellite) revealed two distinct periods: (1) the AugustMarch period with highly variable intermittent cool waterevents and (2) the AprilJuly period with little temperature variability. Generally, periods of cooling occurred at about 2 months intervals, but shorter period occurrences (830 days) of cool coastal events were also observed. Two possible forcing mechanisms are discussed: (1) wind derived coastal upwelling (using satellite blended sea surface wind derived from NOAA/NCDC) and (2) the effect of passing transient southward moving eddies (using sea level anomalies from AVISO altimetry). It is suggested that the cool surface, elevated chlorophyll-a waters are primed and formed by favourable wind-driven Ekman-type coastal upwelling, responding to alongshore northeasterly monsoon winds prevailing between August and March. These waters are then enhanced in chlorophyll-a and advected further offshore by anti-cyclonic/cyclonic eddy pairs interacting with the shelf. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction High chlorophyll-a (Chl-a) waters off northern Mozambique, near Angoche ( 161S), were rst noticed by Nehring et al. (1987) using ship acquired data. They suggested that its formation was driven by a continuous southward Mozambique Current, together with the orientation of the coast. Lutjeharms (2006) and Tew-Kai and Marsac (2009) have subsequently suggested that the enhanced Chl-a near Angoche forms during the southward passage of anticyclonic eddies. The ow in the Mozambique Channel is dominated by both cyclonic and anticyclonic southward propagating eddies (Fig. 1), the majority being anticyclonic in nature (Biastoch and Krauss, 1999; de Ruijter et al., 2002; Lutjeharms, 2006; Quartly and Srokosz, 2004; Sætre and da Silva, 1982, 1984). Such anticyclonic eddies have spatial scales of approximately 300350 km and an average of 56 have been observed per year (Backeberg et al., 2008). The eddies propagate southwards at approximately 36 km d 1 with current speeds reaching up to 2 m s 1 at their edges (Schouten et al., 2002, 2003). The average southward water volume transported by the passage of such eddies at 171S has been estimated from current meter observations to be 14 Sv (Ridderinkhof and de Ruijter, 2003). During the passage of an anticyclonic eddy off northern Mozambique, a strong poleward current forms along the coast north of Angoche. As the continental shelf begins to widen and the coast has a more southwestly orientation at the termination of the northsouth orientation of the coast near Angoche (Fig. 1), the poleward propagating ow detaches from the coast generating a semi-permanent cyclonic lee eddy to the south off Angoche (Nehring et al., 1987; Ridderinkhof and de Ruijter, 2003; Lutjeharms, 2006; Tew-Kai and Marsac, 2009). This occurs in an analogous manner to the theory proposed by Gill and Schumann (1979). It has been shown that such a cyclonic lee eddy can have a diameter of 100 km and that deep water is upwelled in its central core (Nehring et al., 1987; Schemainda and Hagen, 1983). This feature is associated with enhanced Chl-a concentrations (Nehring et al., 1987). High Chl-a waters off Angoche ( 161S) could raise the productivity in this region and hence be important Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr2.2013.10.017 n Corresponding author at: Department of Biological Sciences and Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa. E-mail address: [email protected] (B.S. Malauene). Deep-Sea Research II 100 (2014) 6878

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Cool, elevated chlorophyll-a waters off northern Mozambique

B.S. Malauene a,b,n, F.A. Shillington c, M.J. Roberts d, C.L. Moloney a

a Department of Biological Sciences and Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africab Instituto Nacional de Investigação Pesqueira, Av. Mao Tse Tung 309, Maputo, Mozambiquec Department of Oceanography & Nansen-Tutu Centre for Marine Research, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africad Oceans & Coasts Research, Department of Environmental Affairs, PO Box 52126, Victoria & Alfred Waterfront, Cape Town 8000, South Africa

a r t i c l e i n f o

Available online 7 November 2013

Keywords:Mozambique ChannelAngocheShelf edge upwellingWind drivenEddy driven

a b s t r a c t

Direct in-situ observations from a shallow underwater temperature recorder on the continental shelf andfrom a shipboard oceanographic survey, were combined with MODIS satellite data (sea surfacetemperature and chlorophyll-a) to assess the temporal and spatial variability of temperature andchlorophyll-a in the Mozambique Channel near the coastal town of Angoche, 161S. Intermittent, relativelycool surface water and elevated chlorophyll-a signatures were found, indicating upwelling near Angocheover an area between 151S and 181S. A 5-year (2002–2007) analysis of temperature (from both in-situand satellite) revealed two distinct periods: (1) the August–March period with highly variableintermittent “cool water” events and (2) the April–July period with little temperature variability.Generally, periods of cooling occurred at about 2 months intervals, but shorter period occurrences (8–30 days) of cool coastal events were also observed. Two possible forcing mechanisms are discussed:(1) wind derived coastal upwelling (using satellite blended sea surface wind derived from NOAA/NCDC)and (2) the effect of passing transient southward moving eddies (using sea level anomalies from AVISOaltimetry). It is suggested that the cool surface, elevated chlorophyll-a waters are primed and formed byfavourable wind-driven Ekman-type coastal upwelling, responding to alongshore northeasterly monsoonwinds prevailing between August and March. These waters are then enhanced in chlorophyll-a andadvected further offshore by anti-cyclonic/cyclonic eddy pairs interacting with the shelf.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

High chlorophyll-a (Chl-a) waters off northern Mozambique,near Angoche (�161S), were first noticed by Nehring et al. (1987)using ship acquired data. They suggested that its formation wasdriven by a continuous southward “Mozambique Current”,together with the orientation of the coast. Lutjeharms (2006)and Tew-Kai and Marsac (2009) have subsequently suggested thatthe enhanced Chl-a near Angoche forms during the southwardpassage of anticyclonic eddies.

The flow in the Mozambique Channel is dominated by bothcyclonic and anticyclonic southward propagating eddies (Fig. 1),the majority being anticyclonic in nature (Biastoch and Krauss,1999; de Ruijter et al., 2002; Lutjeharms, 2006; Quartly andSrokosz, 2004; Sætre and da Silva, 1982, 1984). Such anticycloniceddies have spatial scales of approximately 300–350 km and anaverage of 5–6 have been observed per year (Backeberg et al.,

2008). The eddies propagate southwards at approximately3–6 km d�1 with current speeds reaching up to 2 m s�1 at theiredges (Schouten et al., 2002, 2003). The average southward watervolume transported by the passage of such eddies at 171S has beenestimated from current meter observations to be �14 Sv(Ridderinkhof and de Ruijter, 2003). During the passage of ananticyclonic eddy off northern Mozambique, a strong polewardcurrent forms along the coast north of Angoche. As the continentalshelf begins to widen and the coast has a more southwestlyorientation at the termination of the north–south orientation ofthe coast near Angoche (Fig. 1), the poleward propagating flowdetaches from the coast generating a semi-permanent cyclonic leeeddy to the south off Angoche (Nehring et al., 1987; Ridderinkhofand de Ruijter, 2003; Lutjeharms, 2006; Tew-Kai and Marsac,2009). This occurs in an analogous manner to the theory proposedby Gill and Schumann (1979).

It has been shown that such a cyclonic lee eddy can have adiameter of �100 km and that deep water is upwelled in itscentral core (Nehring et al., 1987; Schemainda and Hagen, 1983).This feature is associated with enhanced Chl-a concentrations(Nehring et al., 1987). High Chl-a waters off Angoche (�161S)could raise the productivity in this region and hence be important

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/dsr2

Deep-Sea Research II

0967-0645/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.dsr2.2013.10.017

n Corresponding author at: Department of Biological Sciences and MarineResearch Institute, University of Cape Town, Private Bag X3, Rondebosch 7701,South Africa.

E-mail address: [email protected] (B.S. Malauene).

Deep-Sea Research II 100 (2014) 68–78

for the shrimp fishery. Angoche is an important area because it islocated at the northernmost limit of the Sofala Bank (16–211S),which is an important fishing ground for shallow water shrimps(family PENAEIDAE). The Sofala Bank is the principal source ofshrimps for the Mozambique fisheries export market. The nearestfishing ground to Angoche contributes �50% to the total catcheach year, approximately 4000 t with a monetary value of $40million in 2007 (de Sousa et al., 2008). This area seems to be themost productive on the bank, possibly related to the enhancedprimary production associated with the enhanced Chl-a surfacewaters near Angoche.

As far as we can ascertain, these high Chl-a waters have notbeen studied previously in any detail. It is unknown whether theAngoche cyclonic lee eddy and its associated upwelled waters arepersistent, or what the forcing mechanisms are. Lutjeharms (2006)and Tew-Kai and Marsac (2009) suggested that passing eddies areimportant in the formation of this feature. Not mentioned thus faris that local winds can also act as a forcing mechanism for thisphenomenon. The northern region of the Mozambique Channel isinfluenced by the southern extension of the East African monsoonwinds that blow from the southwest (SW) during the australwinter (April–August) and the northeast (NE) during the australsummer between September and March (Sætre and da Silva,1982). The NE Monsoon wind that blows poleward, alongshoreand parallel to the coast off Angoche is favourable for offshoresurface Ekman transport and consequently can induce coastalupwelling of cooler water. This study investigates the temporaland spatial variability of these cool, elevated Chl-a events offnorthern Mozambique and attempts to identify which of thesetwo potential forcing mechanisms (eddy and wind) are active.

2. Data and methods

In this study, a cool water event is defined as a decrease intemperature (a trough in the time series) of more than 2 1C. Thisvalue corresponds to half of the largest event scale temperaturedifference (�4 1C) observed in the time series. An elevated Chl-aevent is considered as Chl-a above 0.35 mg m�3, which corre-sponds to values of Chl-a in the highest 25% (above the 75thpercentile) of the data set.

2.1. Study area

The study area focused on the region near Angoche at �161S,situated on the western side of the Mozambique Channel on thenorthern Mozambique coast (Fig. 1). This is the area where cool,elevated Chl-a waters have been observed. In-situ observationswere made in the region near �161S and remotely sensed satellitedata was collected for an area limited to 12–201S and 35–451E(Fig. 1).

2.2. In-situ data

In-situ observations are very scarce in the western part of theMozambique Channel. For this reason, an underwater temperaturerecorder (UTR, Seamon mini temperature recorder, accuracy70.01 1C) was deployed at a depth of 18 m near Angoche(Fig. 1). The UTR recorded hourly measurements of temperaturefrom October 2002 to September 2007. These hourly data wereaveraged to obtain daily and weekly time series, which were thenanalysed.

Fig. 1. Map of the study area showing the UTR mooring (star) and the CTD stations (triangles), with the numbers 1–3 representing the three selected offshore transects forthe research cruise, 14–16 August 2009. A schematic of eddy circulation (rings, not scaled) is superimposed on the composite area of the spatial extent of cool, elevatedchlorophyll-a waters near Angoche.

B.S. Malauene et al. / Deep-Sea Research II 100 (2014) 68–78 69

Although not part of the UTR time interval deployment, cruisedata was also available from an oceanographic survey conductedalong the northern Mozambique coast by the RV Dr. Fridtjof Nansenas part of the ASCLME and MESOBIO programmes. Three offshoretransects of seven CTD stations each (transect 1 in the north;transect 2 in the centre; transect 3 in the south; 14–16 August2009 respectively) were chosen to elucidate the vertical hydro-graphic structure (Fig. 1). Vertical profiles of temperature, salinityand Chl-a fluorescence were measured with a CTD-Seabird 911þ .Data were sampled at 1 m intervals during the downcast to amaximum depth of 10 m above the sea floor. Due to limited seatime, the maximum depth of the CTD casts extended to 1500 monly. In this study, only data down to 200 m are presented, as themain objective was to investigate the near-surface signature. (Notethat UTR data were not available for the period of the cruisein 2009.)

2.3. Satellite data

Daily level-2 sea surface temperature (SST, 1C) and chlorophyll-a(Chl-a, mg m�3) data from 2003 to 2007 were derived from theMODIS-Aqua satellite sensors (http://modis.gsfc.nasa.gov/) assupplied by the Marine Remote Sensing Unit at the University ofCape Town. The data have a spatial resolution of 1 km, which isadequate for resolving features in coastal areas. The data wereatmospherically corrected and geolocated, and then resampledinto standard gridded map coordinates. Spatial and temporalresampling of the data was performed in a consistent manner ata 1 km spatial scale and a daily temporal scale for both SST andChl-a.

Daily blended sea surface wind speed (W, m s�1) and windvelocity vector components (u, zonal; v, meridional) at a referenceheight of 10 m and at 0.251 grid resolution were used in this study.Five years (2003–2007) of level-3 data products were derived fromcontinuous observations of multiple satellite sensors, namelyAdvanced Microwave Scanning Radiometer – Earth ObservationSystem (AMSR-E), Spatial Sensor Microwave Image (SSMI)-F13,SSMI-F14, SSMI-F15 , Quick Scatterometer (QuikSCAT) and TropicalMicrowave Image (TMI). Blending the various wind productsincreased the temporal and spatial resolution of the individualsatellite products by filling in gaps and reducing the sub-samplingand random errors. The data are distributed by NOAA/NCDC(http://www.ncdc.noaa.gov/oa/rsad/air-sea/seawinds.html). Thewind vectors followed the oceanographic current convention: i.e.positive east is a westerly wind, and positive northwards is asoutherly wind.

To assess the influence of meandering currents and eddies oncool, elevated-Chl-a events, weekly composite sea level anomalies(SLA, cm) and geostrophic velocity vectors (u and v, cm s�1) from2003 to 2007 were used. These data are provided on a 1/31gridded Mercator projection, and the Level-2 data product ismerged from multi-satellite altimeter missions, including TOPEX/Poseidon, Jason-1, GFO, ERS-1, ERS-2 and ENVISAT. The DelayedTime (DT) reference data product was used, which is the histori-cally homogeneous data product. The data are readily availablefrom SSALTO/Duacs and distributed by AVISO with support fromCentre National d'Etudes Spatiales (CNES, http://www.aviso.oceanobs.com).

2.4. Analysis

Time series from 2003 to 2007 for daily and weekly MODIS SSTand Chl-a, daily blended NOAA/NCDC wind velocity vectors, andweekly AVISO SLA and geostrophic velocities were plotted. For adirect comparison with the in-situ UTR data set, these time series

were constructed at the nearest pixel to the UTR mooring site(Fig. 1).

To better understand the UTR variability from short to longtime scales, wavelet analysis was applied to the daily UTRtemperature data. This technique decomposes the time series intotime-period, amplitude and phase space, so as to detect thedominant modes of variability and their changes with time(Emery and Thomson, 2001; Thompson and Demirov, 2006). Priorto performing the wavelet analysis, the UTR time series wasnormalized by subtracting the 5-year daily means from thecorresponding daily values of the original 5-year data set, andthen divided by the averaged standard deviation. This removed theseasonal cycle from the original UTR time series. The waveletanalysis revealed distinct short and long time variations infrequency of occurrence and amplitude of cool water events fromthe UTR time series for the period 2003–2007. (The Morlet waveletwas chosen as the “mother” wavelet. Further aspects of themethod are described in detail by Torrence and Compo (1998),at URL: http://paos.colorado.edu/research/wavelets/.) The statisti-cal significance test for wavelet power spectra was set at a 95%confidence level.

In order to assess the occurrence of Chl-a events off northernMozambique, a 5-year time series of offshore Ekman transport (My)was computed at the UTR location near Angoche according to Myersand Drinkwater (1988/1989), Bakun (1996) and Halpern (2002):

My ¼ �τalongρ� f

ð1Þ

where τalong is the alongshore surface wind stress component, ρ is thedensity of the seawater (1024 kg m�3) and f the Coriolis parameternear Angoche. τalong and f were calculated using the followingformulas, respectively:

τalong ¼ ρaCdjUjU ð2Þ

f ¼ 2Ω sin θ ð3Þwhere the constant ρa is density of air (1.3 kg m�3) and Cd is thedimensionless drag coefficient (�0.0011), U is the alongshore windvelocity component from daily blended NOAA/NCDC wind at 10 mheight at the UTR mooring site, jUj is the wind speed, Ω isthe angularvelocity of the Earth (2π/24�3600E7.272�10�5 rad s�1), and θ isthe latitude of Angoche (161S).

3. Results

3.1. Time series of in-situ UTR, and MODIS SST and Chl-a

The satellite derived time series of daily MODIS SST and Chl-adata from 2003 to 2007 in the study area had many missing valuesdue to cloud cover, particularly during summer which is the localrainy season (not shown). To overcome this problem, weeklyaveraged time series were produced for both MODIS SST and Chl-a. The MODIS weekly SST time series and the in-situ UTR data(averaged over weekly intervals) exhibited similar variability(Fig. 2A). Strong seasonal oscillations were observed in both timeseries, with an amplitude range of �5 1C between hot (�30 1C inFebruary–April) and cool (�25 1C in July–September) periods. Shorttime scale intermittent cool water events were evident in both timeseries from the end of August to March every year (Fig. 2A). Theperiod of the cool events generally coincided with the warm seasonwith a few exceptions (Fig. 2A) and the events occurred consistentlyin both the MODIS SST and the UTR datasets (Fig. 2A). However, theamplitudes of cool events in the MODIS SST (�2 1C) were smallerthan the UTR time series (�4 1C). For example, in January 2005 theMODIS SST changed by 2.5 1C, compared with a 4 1C change insubsurface temperature (UTR); in January–February 2006 there was

B.S. Malauene et al. / Deep-Sea Research II 100 (2014) 68–7870

a change of �1.5 1C (MODIS SST) compared with 4 1C (UTR). Attimes, the amplitude of cool water events at 18 m (UTR) exceededthe amplitude of the observed seasonal cycle. For example, inOctober 2003 and February 2006, large anomalies were apparentin the normalized daily UTR time series (Fig. 3A). The comparisonalso showed that MODIS SSTs were warmer than those measured bythe UTR, particularly during summer peak temperatures, as onemight expect, as the MODIS instrument only measures the tem-perature in a very thin surface layer.

The time series of MODIS Chl-a concentrations showed that theregion is generally characterized by low Chl-a levels (Fig. 2B) whencompared to the major global eastern boundary current systems(Patti et al., 2008). There was no seasonal variation in Chl-a, butoccasional intra-seasonal variability was observed, with relativelyelevated (for the region) concentrations. Such elevated Chl-aevents (40.35 mg m�3), with concentrations greater than themean, were particularly observed during the hot spring–summermonths from late August to April. In contrast, low Chl-a was

Fig. 3. (A) Time series of normalized daily UTR temperature with the seasonal cycle removed for the period October 2002–September 2007. (B) Morlet wavelet powerspectrum. White solid line encloses the region with greater than 95% confidence level for red noise. White dashed line indicates the region where edge effects becomeimportant. (C) The global wavelet power spectrum. Red dashed line indicates the 95% significance level.

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31

Tem

pera

ture

( C

)o

MODIS SSTUTR

Jan/04 Apr Jul/04 Oct Jan/05 Apr Jul/05 Oct Jan/06 Apr Jul/06 Oct Jan/07 Apr Jul/07 Oct0

0.5

1

1.5

2

Time (weekly)

MO

DIS

(Chl

−a) (

mg

m

)−3 MODIS Chl−a

Fig. 2. Time series of weekly averaged temperature derived fromMODIS (surface) and the UTR (18 m) for the period January 2003 to December 2007 at the UTR location, andMODIS Chl-a for the same period and location as the MODIS SST.

B.S. Malauene et al. / Deep-Sea Research II 100 (2014) 68–78 71

observed in winter from April to August (Fig. 2B). Some of thiselevated Chl-a coincided with cool water events (Fig. 2A). Exam-ples are in August–September 2004, in December 2004, in Octoberand December 2005, and in February and October 2006.

3.2. Wavelet power spectrum analysis of the in-situ UTR time series

Time series of daily UTR anomalies, with the seasonal cycleremoved, showed that the intra-seasonal variability was domi-nated by temperature variation with a range up to 4 1C inamplitude between August–March every year (Fig. 3A). Thewavelet power spectrum showed high amplitude (dark red) at aperiod of about 16 days between October and March (australspring–summer), and low amplitude at the same period betweenMarch and July (winter months) every year (Fig. 3B). The highpower indicated periods of high variability for a range of periodsbetween 16 and 64 days, but only the �60-day period showed as atime-integrated peak in the spectrum (Fig. 3C). Significant (at 95%confidence level) cool water events of large amplitude (dark redspectrum enclosed by white line) were observed in spring–summer of 2003, 2004 and 2006, with an unusual low amplitudeevent in the summer of 2007. The dominant period of �60 daysfor significant cool water events was observed each year, except in2005 when it had a slightly shorter period (Fig. 3B).

3.3. Horizontal distribution of SST and Chl-a

The 5-year (2003–2007) long term mean of the daily MODIS SSTand Chl-a showed quasi-homogeneous (weak gradients), warm(�28 1C) and low (o0.2 mg m�3) Chl-a waters in the region offAngoche (not shown). The available satellite time series was care-fully examined for evidence of surface signatures of cool, elevatedChl-a waters. It should be noted that the region was subjected topersistent cloud cover, particularly from October to February, which

is the rainy season, and this results in considerable satellite data loss.Despite the presence of clouds, enough data were available toobserve a number of such signatures. Cool, elevated Chl-a eventswere observed from August to March (not shown), and one of thebest case studies was chosen for detailed description, and to showthe possible spatial extent of such an event (Fig. 4). Four sequentialimages over 3 weeks of daily surface Chl-a are illustrated in Fig. 4showing the evolution of an elevated Chl-a event. The spatial extentincreased with time and propagated southward off Angoche. On 22August 2004 there was little Chl-a at the surface off Angoche. A slightincrease in Chl-a was evident on the shelf to the north of Angoche at�151S, representing the early phase of the elevated Chl-a event(Fig. 4A). On 26 August (Fig. 4B) there were elevated levels of Chl-a(40.35 mgm�3) in the shape of a triangle off Angoche, suggestingthat the elevated Chl-a event was well developed. Five days later, on31 August, the elevated Chl-a had moved southward along the coast,increasing in size and spreading over a larger area (Fig. 4C). In the lastimage on 9 September 2004, the elevated Chl-a had moved to thesouth and appeared as a sub-mesoscale filament which spreadsoutheastwards into the centre of the Channel (Fig. 4D). MODIS SSTobservations for the same period highlighted a similar sequence oftemperature changes as described for Chl-a (not shown).

Another 10 events with similar spatial extent of elevated Chl-awaters to that observed on 31 August 2004 (Fig. 4C) were observedwhen there was cloud free data. A composite image (total area inFig. 1) comprising all the cool, elevated Chl-a events observed in thetime series was produced using a Geographical Information System(GIS). The spatial extent of cool, elevated Chl-a waters off northernMozambique near Angoche was estimated to be 60,000 km2.

3.4. Altimetry data – eddies

Weekly composite images of geostrophic velocity vectors(cm s�1) for the period 2003–2007, superimposed on sea level

Fig. 4. Snapshots of daily 1 km MODIS Chl-a concentrations (mg m�3) showing the evolution of an elevated Chl-a event between 22 August and 9 September 2004:(A) 22 August, (B) 26 August, (C) 31 August and (D) 9 September 2004.

B.S. Malauene et al. / Deep-Sea Research II 100 (2014) 68–7872

anomalies (SLA, cm), showed strong mesoscale eddy variability (inboth space and time) within the study domain. Such variability wascharacterized by several anti-cyclonic and cyclonic eddies propagat-ing southward, which is consistent with previous studies (Backebergand Reason, 2010; Backeberg et al., 2009; de Ruijter et al., 2002;Lutjeharms, 2006; Ridderinkhof and de Ruijter, 2003; Tew-Kai andMarsac, 2009). To further understand the influence of eddy activityon the elevated Chl-a events, a case study of SLA between 18 Augustand 15 September 2004 was selected (Fig. 5). These data corre-sponded to the elevated Chl-a event depicted in Fig. 4.

On 18 August 2004 (Fig. 5A) weak negative SLA features(��10 cm, C1 and C2), unlikely to be cyclonic eddies, were foundalong the Angoche coast at 15–171S. Two cyclonic eddies appearedto be centred at 171S, 421E (C3), and at 191S, 401E (C4) respectively.Anticyclonic features were observed adjacent to these cyclonicfeatures. In particular, a relatively strong anticyclonic eddy (AC1)was observed entering the study domain from the north. In thesecond image on 25 August 2004, the anticyclonic eddy (AC1) hadmoved south, merging with an anticyclonic feature (AC2) on theeastern side of the Channel off Madagascar at 171S (Fig. 5B). Thispassing anticyclonic eddy had a strong rotating velocity on itsedge, particularly on the Mozambican side. The coastal, negative

SLA features (C1 and C2) had developed and merged with thecyclonic eddies to the south (C3 and C4 respectively), forming twolarge cyclonic features centred at 161S (C1*¼C1þC3, the Angochecyclonic lee eddy) and 191S (C2*¼C2þC4) (Fig. 5B).

By 8 September 2004 (Fig. 5C), the anticyclonic eddy (AC1) andthe anticyclonic feature (AC2) had merged into one large antic-yclonic eddy (AC*¼AC1þAC2), located across the entire narrowsection of the Mozambique Channel (16–181S). Two cyclonicfeatures were also observed adjacent to this anticyclone (AC*),one to the north (C5, centred at �141S, 42.51E) and the other tothe south (C1*, centred at �17.51S, 40.51E). The latter (C1*,Angoche cyclonic lee eddy) was stronger with an SLA of ��25 cmand appeared to be an eddy that had extended to the continentalshelf. Another cyclonic eddy (C2*) was observed to be exiting thestudy area to the southwest.

The large anticyclonic eddy (AC*) was observed to be propagat-ing southward on 15 September and decreasing in size (Fig. 5D),indicating that the eddy had started to dissipate. The cyclonic eddy(C1*, Angoche cyclonic lee eddy) to the south of the anticycloniceddy was still present but with relatively smaller amplitude(SLA�20 cm). The interaction between the pair of eddies withopposite polarity (one large anticyclonic (AC*) to the north and the

Fig. 5. Snapshots of weekly sea level anomaly (cm) and geostrophic velocity vectors (cm s�1) derived from 1/31 gridded AVISO altimetry data for (A) 18 August 2004, (B) 25August 2004, (C) 08 September 2004 and (D) 15 September 2004 during the elevated Chl-a event depicted in Fig. 4. Letters AC1, AC2, AC* (AC1þAC2) refer to different anti-cyclonic features and C1, C2, C3, C4, C5, C1* (C1þC3) and C2* (C2þC4) to cyclonic features.

B.S. Malauene et al. / Deep-Sea Research II 100 (2014) 68–78 73

Angoche cyclonic lee eddy (C1*) to the south) generated a dipolepair with a current moving offshore from the coast in between theeddies (Fig. 5B–D).

3.5. Sea surface wind

The daily satellite sea surface wind images for 2003–2007showed two distinct regimes. One was the southwesterly (SW)monsoon during austral autumn–winter from the end of April to

August, and the other was the northeasterly (NE) monsoon duringaustral spring–summer from August to April. The daily snapshotsof wind fields on 8 May 2004 and 4 September 2004 highlightedthese 2 regimes (Fig. 6). Fig. 6B corresponded with the elevatedChl-a event shown in Fig. 4. Importantly, the NE wind that blowsparallel to the coast at Angoche (�161S) is the alongshore windcomponent (Fig. 6B) that is favourable for offshore surface Ekmantransport, leading to wind induced coastal upwelling. The 5-yeartime series of daily alongshore surface wind component at the UTR

Fig. 6. Snapshots of daily 0.251 gridded blended sea surface wind speed (m s�1) and velocity vectors (m s�1) derived from multi-satellite data products showing the twowind regimes (a) SW winds on 08 May 2004, and (b) NE winds on 04 September 2004, which is the upwelling favourable wind off Angoche and corresponds with theelevated Chl-a event depicted in Fig. 4.

Fig. 7. (A) Time series of daily temperature anomaly (1C) derived from the UTR and daily alongshore blended sea surface wind velocity component (m s�1) derived from0.251 grid multi-satellite observation (NOAA/NCDC) at the UTR site for the period January 2003–September 2007. Positive y-axis wind values refer to SW wind and negativewind value to NE wind. (B) Time series of Ekman transport (m2 s�1) calculated from the NOAA/NCDC wind at the same location and period as (a). Positive y-axis valuesindicate offshore surface Ekman transport and negative values indicate onshore surface Ekman transport.

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site showed that winds were rarely steady, oscillating withseasonal and intra-seasonal periods (Fig. 7A).

The wavelet analysis of the normalized daily wind speed (notshown) showed significant (95% confidence level) high amplitude at2–16-day periods, representing short periods of wind accelerationand deceleration. The two distinct wind regimes were also note-worthy in the time series: (1) persistent wind blowing from the SW,which prevailed during austral autumn–winter, and (2) wind blow-ing from the NE, that dominated during austral spring–summer(Fig. 7A). The SW wind was stronger and had higher speeds(�15 m s�1) than the NE wind (�10 m s�1) (Fig. 7A). Time seriesof daily UTR anomalies was compared with the time series of dailyblended sea surface wind velocity vectors (Fig. 7A). There werecoherent variations between the cool water events and the NEwinds (August–April) throughout the time series. There were nocool water events during the SW winds (April–mid-August). How-ever, the correlation between the amplitude of cool water eventsand the magnitude of the NE wind vectors was not very strong(Fig. 7A). For example, on 19 October 2003 a cool event of 4 1Camplitude range was observed in response to a NE wind blowing at7 m s�1, whereas on 08 November 2003, a cool event of 2 1Camplitude was observed also in response to a 7 m s�1 NE wind.Note the second cool water event had smaller amplitude but wasinduced by similar NE wind strength.

Fig. 7B shows the time series of surface Ekman transportcalculated by Eq. (1) near Angoche at the UTR location. Positivesurface Ekman transport indicated offshore, upwelling favourableflow, while negative values indicated onshore transport. Fig. 7Brevealed that surface Ekman transport near Angoche exhibited aclear seasonal cycle, with strong onshore transport (�4 m2 s�1)during the winter and weak offshore transport (2 m2 s�1) duringthe summer. Such onshore Ekman transport was related topersistent SW wind while the offshore Ekman transport wasrelated to a weaker and highly variable NE wind (Fig. 7A and B).Furthermore, the offshore surface Ekman transport coincided withcool water events suggesting that there was wind induced coastalupwelling at this time (Fig. 7A and B). Although in January 2007,strong offshore Ekman transport was observed, resulting in mini-mal surface cooling (Fig. 7A and B).

3.6. Case study to demonstrate the vertical structure of an event

An oceanographic survey carried out in August 2009 (later thanthe time series period 2003–2007) provided evidence of cool,elevated Chl-a surface waters close to 161S. For transect 1 (Fig. 1),the vertical distributions of temperature showed nearshore iso-therms rising on the continental shelf in the region off Angoche(Fig. 8, T1). The 17 1C isothermwas raised from a depth of �200 mat 50 km offshore to a depth of 125 m at the shelf. Slightly elevatedChl-a was observed in the coastal area from the surface to �50 m,with a maximum of 40.2 mg m�3 at �25 m (Fig. 8B, C1). Off-shore, a deep chlorophyll maximum was noted, with low concen-trations (�0.1 mg m�3) between 50 and 75 m (Fig. 8B, C1).

For transect 2, an upward doming of isotherms, consistent withelevated Chl-a, was apparent in the middle of the transect (Fig. 8,T2, C2). Isotherms were found to decline at the coast and at theoffshore sides of transect 2, and were shallow, reaching the surfaceat the centre of the transect between 25 and 45 km from the coast(Fig. 8, T2). The 16 1C isothermwas located quite deep at both endsof the transect, but shallower in the middle. In addition, the 25 1Cisotherm was raised from a depth of�20 m at the coast andoffshore, reaching the surface at a distance of 25–45 km offshore(Fig. 8, T2). Corresponding elevated Chl-a values (40.35 mg m�3)were also apparent at the surface to 50 m in the middle of thetransect 2 (Fig. 8, C2). Such doming suggested the occurrenceof a cyclonic eddy, also noted at a similar location by Nehringet al. (1987). A deep chlorophyll maximum was observed intransect 2 centred at 50 m, but restricted to the coastal side(Fig. 8, T2, C2).

On the southern transect 3 (Fig. 1), there was a warm (25 1C)and oligotrophic surface mixed layer. Below the mixed layer,isotherms were horizontal and parallel from the coast to offshore,with a weak vertical temperature gradient (Fig. 8, T3). A deepchlorophyll maximum was apparent along the entire transect,with a maximum of about 0.2 mg m�3 in the middle of thetransect (22–50 km offshore) at a depth of 75 m (Fig. 8, C3). Asnapshot of the SLA for 13 August 2009 during the cruise showed alarge anticyclonic eddy moving southwards and a cyclonic eddywas apparent to the southeast of this anti-cyclonic eddy (not

Fig. 8. Vertical distribution of temperature (1C) and chlorophyll-a (mg m�3) along transect 1 (14 August 2009), transect 2 (15 August 2009), and transect 3 (16 August 2009)(top to bottom panels). Vertical lines represent station positions. Distance offshore (x-axis) is from the westernmost station on each transect.

B.S. Malauene et al. / Deep-Sea Research II 100 (2014) 68–78 75

shown). Such anticyclonic/cyclonic eddy pairs presumably gener-ated a dipole with offshore flow in the form of a filament betweenthe eddies as described before.

4. Discussion and conclusion

4.1. Occurrence of cool, elevated-Chl-a waters

The northern region of Mozambique is generally oligotrophicand characterized by warm surface water (up to 30 1C) and lowChl-a (o0.2 mg m�3) compared with eastern boundary currentsystems (Patti et al., 2008). Such low levels of Chl-a have also beenreported by Omta et al. (2009) and Tew-Kai and Marsac (2009).In the study area, a seasonal cycle was observed in temperature,mainly due to solar insolation. No seasonal variability was foundfor Chl-a, in agreement with Tew-Kai and Marsac (2009) who alsodid not find seasonal variability in Chl-a in their central study areabetween 161S and 241S, an area similar to our study. In contrast,Omta et al. (2009) reported a seasonal cycle in Chl-a, because theyanalysed a much larger area of the entire Mozambique Channel.Tew-Kai and Marsac (2009) also found a seasonal cycle in Chl-a tothe south of our study zone. In this study, intra-seasonal variabilitywas observed in both temperature and Chl-a, apparent as cool,elevated Chl-a waters. Cool water events were apparent in thein-situ UTR time series (Fig. 2A), MODIS SST (Fig. 2A) and waveletanalysis of UTR data (Fig. 3). Additionally, elevated Chl-a waterswere also apparent in the MODIS Chl-a (Figs. 2B and 4). These cool,elevated Chl-a signatures suggested the existence of upwellingnear Angoche (Nehring et al., 1987). Although the concentrationsof elevated Chl-a were relatively low, such events might still beimportant for these low productivity waters which support animportant commercial fishery, particularly on the Sofala Bank.

The cruise survey carried out in August 2009 provided evidencefrom vertical profiles of cool, elevated-Chl-awaters (Fig. 8), support-ing the finding that weak upwelling does occur off northernMozambique near Angoche. Deep chlorophyll maxima and lowsurface Chl-a suggested oligotrophic waters in the study area(Fig. 8). Such phenomena are a typical feature in tropical and sub-tropical oligotrophic oceans (Cullen, 1982; Huisman et al., 2006;Venrick et al., 1973). A deep chlorophyll maximum implies that thesurface layer was depleted in nutrients and, as a result, surface Chl-awas low. Surface elevated Chl-a was apparent nearshore in transect1 and in the middle of transect 2, between 30 and 50 km offshore(Fig. 8C1 and C2), suggesting that upwelling increased nutrientsupply from deeper, nutrient-rich waters. This process leads to anabsence of a deep chlorophyll maximum and stimulated increasedsurface Chl-a (Cullen, 1982; Venrick et al., 1973).

Comparisons between temperature at the surface and at adepth revealed that the amplitude of cool events at the surface(�2 1C) was smaller than at 18 m (�4 1C). Some small cool waterevents were not visible at the surface (Fig. 2), whereas large coolwater events (44 1C), exceeding the amplitude of the observedtemperature seasonal cycle, were apparent at 18 m (Figs. 2 and 3).Cruise data also showed that cool, elevated Chl-a water signaturesbecame more prominent with depth (Fig. 8). These results suggestthat cool, elevated Chl-a waters off northern Mozambique aremainly located at depth and not well manifested at the surface,making it difficult to distinguish events using satellite surfaceobservations.

4.2. Temporal variability of cool, elevated Chl-a waters

At the UTR site, the weekly derived MODIS SSTs were similar tothe weekly temperatures at 18 m. However, the UTR time seriescaptured the cool water signatures better at daily time scales

(Figs. 2 and 3), and therefore the UTR time series was consideredto be the best proxy to describe the timing of cool waters nearAngoche. Two distinct time periods, corresponding to two distinctregimes were clearly apparent (Figs. 2 and 3): (1) the period April–July (winter), which was characterized by an absence of coolingevents every year and (2) the period August–March (summer),which was characterized by intermittent cooling events at intra-seasonal periods. The observations from the cruise survey inAugust 2009 coincided with the second period. The upwellingnoted by Nehring et al. (1987) near Angoche also occurred withinthe same period. A research cruise carried out in late April 2010(Ternon et al., 2013) also surveyed the region off Angoche that wasstudied in summer 2009, and did not find any cool, elevated Chl-awater. This is consistent with findings in this paper that April–July(winter) is a period with few cool events.

The periodicity of cool water events off Angoche duringAugust–March is particularly evident in the daily UTR waveletanalysis (Fig. 3). Although shorter periods of 1–4 weeks betweencool events were observed, the dominant interval for the coolevents was about 2 months (�60 days, Fig. 3). The statisticallysignificant dominant periodicity of �60 days observed in theglobal wavelet spectrum coincides with the period of the Mad-den–Julian oscillation (MJO, Zhang, 2005), suggesting a possibleconnection between the cool, elevated Chl-a water events and theMJO as a potential driver. The MJO is the dominant component ofthe atmospheric variability at the intra-seasonal periods prevailingin the tropical Indian and Pacific oceans (Zhang, 2005). It isassociated with wind anomalies particularly during the australsummer (Zhang, 2005; Pohl et al., 2007). The MJO has beenreported to modulate monsoon systems in West Africa (Zhang,2005), Asia (Yasunari, 1979, 1981; Lawrence and Webster, 2002)and Australia (Hendon and Liebmann, 1990). Furthermore, the SSTdecreases in response to the MJO (Zhang, 2005). The fact that theMJO and the cool, elevated Chl-a waters occur at the same time ofthe year (summer) with the same dominant intervals (�60 days)and the effect of the MJO in decreasing the SST, supports thefinding that the cool, elevated Chl-a waters near Angoche areinduced by atmospheric forcing, in this case the NE monsoonwinds. Five-year maps of quasi-daily MODIS SST and Chl-aexhibited several sustained periods of cool, elevated Chl-a events.However, observations of these events were contaminated byperiods of persistent cloud cover. Caution needs to be exercisedto not interpret these as one long, single event.

4.3. Spatial variability of the cool, elevated Chl-a waters

Maps of MODIS SST and Chl-a distribution showed that most ofthe cool, elevated Chl-a water started in the northern region of theshelf, propagating southward, and then spreading to offshorewaters as illustrated in Fig. 4. Sometimes this water did not reachas far offshore, suggesting that the elevated Chl-a events were morefrequent in the shelf region than further offshore. This supports therole of wind induced upwelling. The triangle seen in Fig. 4B and Cwas a common shape for the cool, elevated Chl-a water and thetotal area estimated to be roughly 60,000 km2 within the region of15–181S and from the coast to �411E (shaded area in Fig. 1). Cautionneeds to be exercised when interpreting these values because thisarea was estimated using a GIS-based composite image comprisingthe best daily MODIS SST and Chl-a observations of the studyperiod. It should be noted that satellite observations, in particularocean colour used here, are not ideal for investigating this regionbecause of persistent cloud cover (Tew-Kai and Marsac, 2009). Thisis particularly evident during the rainy season, which coincides withthe period of the cool, elevated Chl-a events. This factor limits theuse of available satellite data products, which could not be includedin the calculations presented above. Even though some parts of the

B.S. Malauene et al. / Deep-Sea Research II 100 (2014) 68–7876

area might have shown cool, elevated Chl-a signatures only once inthe study period, they are included in the calculation of thestandard area. The shape and the extent of this area appears rathertoo large to be driven by local, weak NE upwelling favourable windsalone, implying that other meso- to large scale forcings are mostlikely involved.

4.4. Cool, elevated Chl-a events and wind

The blended sea surface wind data from 2003 to 2007 exhib-ited two distinct wind regimes (Figs. 6 and 7): (1) a persistentand strong (�14 m s�1) SW wind regime from April to July and(2) a low velocity (10 m s�1) NE wind regime from August toMarch (Figs. 6 and 7). These two wind regimes correspond to thesouthern extension of the East African monsoon, which was alsonoted by Biastoch and Krauss (1999) and Sætre and da Silva(1982).

Cool water events appeared to be mostly connected to thealongshore, low velocity NE monsoon winds between Augustand March and associated offshore Ekman transport (Fig. 7). Incontrast, the periods without cool water events appeared to beassociated with high velocity SW monsoon winds between Apriland July and onshore Ekman transport (Fig. 7). This was consistentthroughout the study period implying that cool, elevated Chl-aevents near Angoche are partly due to wind driven Ekman typecoastal upwelling, in response to the weak alongshore NE mon-soon winds. Weak NE wind, which blows poleward, parallel,alongshore to the coast of Angoche can cause offshore Ekmantransport in magnitude of�2 m2 s�1 m�1 of coastline. Such off-shore Ekman transport is weak compared with the vigorouseastern boundary upwelling systems. For instance, offshore Ekmantransport of �5 m2 s�1 was reported in the central Chile upwel-ling region (Landaeta et al., 2008). Nontheless, the resultantcoastal upwelling off Angoche appears to be an important sourceof shelf productivity and hence for fisheries in the region.

Wind induced upwelling, particularly monsoon driven, hasbeen observed in other parts of the world ocean in the north-western Indian Ocean, the Somali upwelling system (Bakun et al.,1998), in the western Philippines Sea (Udarbe-Walker and Villa-noy, 2001), in the South China Sea (Liu et al., 2002), in the ArabianSea (Habeebrehman et al., 2008) and in the western Taiwan Strait(Hong et al., 2009). Interestingly, the upwelling along the westernboundary of the South China Sea (Liu et al., 2002) and TaiwanStrait (Hong et al., 2009) are driven by the prevailing SW monsoonwinds whereas in the western Mozambique Channel the elevatedChl-a waters are related to the NE monsoon winds. Both windregimes blow toward the poles of the corresponding hemisphere(north and south, respectively) leading to offshore Ekman transport.

The intermittent nature of the cooling events is partly due tofluctuations in the wind speed and direction during the NE windseason. Although clear evidence of NE wind driven cooling eventsis apparent, sometimes cooling the water by more than 4 1C, thestrength of the wind and the amplitude of cooling events do notagree well with the wind speed variations (Fig. 7). This impliesthat another remote forcing could be involved in driving thecoastal elevated Chl-a events off northern Mozambique.

4.5. Cool, elevated Chl-a events and eddies

Analysis of SLAs showed that the events could be related to thesouthward passage of an anticyclonic eddy (Fig. 5). This forcingmechanism had been proposed before (Lutjeharms, 2006; Omtaet al., 2009; Tew-Kai and Marsac, 2009). In addition, an associatedcyclonic eddy centred at �171S (C1*, the Angoche cyclonic lee eddy)was apparent during the passing of the anticyclonic eddy, when itsflow detached from the coastline near Angoche (Fig. 5). The upward

doming in the middle of transect 2 (Fig. 8, T2 and C2) confirmed thepresence of the Angoche cyclonic lee eddy in August 2009 duringthe cruise, also apparent in the corresponding SLA image (notshown). Such upward doming of isotherms and the correspondingelevated Chl-a support the finding that deep, cool, enhanced Chl-awater is upwelled in the core of the Angoche cyclonic lee eddy(Nehring et al., 1987). Everett et al. (2012) also found elevated Chl-aconnected with a cyclonic eddy in the Tasman Sea.

A third mechanism associated with passing eddies observed hereis the formation of a dipole and a subsequent offshore filament,with strong currents and diverging surface waters in the coastalregion, resulting in deeper cool, elevated Chl-a water beingupwelled into the upper mixed layer. An offshore current withinthe dipole can then advect the elevated Chl-a water away from thecoast. It is not clear whether eddy activity was generating elevatedChl-a water or whether it was simply responsible for transportingthis water southward and then offshore. Siegel et al. (2011)demonstrated through a sequence of statistical analyses that asingle mechanism is unlikely to be the driving force, but ratherthe interaction of several mechanisms (i.e. eddy pumping, eddyEkman pumping and eddy advection) are responsible for the Chl-apatterns generated by mesoscale eddies. Nevertheless, eddyinduced horizontal advection is believed to be the dominantmechanism (Chelton et al., 2011; Siegel et al., 2011). During theperiod April–July, there were no cooling events but passing eddiesand/or dipoles were observed. This raises the question as to whythere were no cooling events when passing eddies created appar-ently favourable conditions for elevated Chl-a waters off Angoche.This is an open question at present as to whether there could beother remote sub-mesoscale forcings involved.

Neither coastal upwelling favourable winds, nor passingmesoscale eddies, can by themselves explain the elevated Chl-aevents observed off northern Mozambique. This suggests that theoccurrence of these events are due to a combined process of initialforcing by the local winds and then enhanced both vertically andlaterally by passing eddies. At present it is not clear whichmechanism might dominate. Deep eddies could raise the thermo-cline at the shelf so that cool water at depth is upwelled to thesurface under the influence of the alongshore NE winds. These, orother, eddies might then transport the elevated Chl-a waterssouthward and offshore. Chung-Ru et al. (2004) also found thatenhanced Chl-a in the upwelling region off southeastern Mada-gascar was induced by a combination of local winds and currentswhen the Eastern Madagascar Current became separated from thecoastline at the southernmost end of the island.

Further investigation is necessary to clarify the roles of possibleforcing mechanisms responsible for the elevated Chl-a events.New numerical model simulations of sub-mesoscale processes,and applied to improved in-situ observations in this area, willfurther elucidate the nature of the vertical uplift of nutrientsleading to enhanced Chl-a filaments at the surface. This paper didnot attempt to investigate the role of topography and islands (Ilhade Mozambique, Ilhas primeiras e segundas) to the north of thestudy area on the formation of cool, elevated Chl-a waters offnorthern Mozambique near Angoche.

Acknowledgements

Financial support was provided by the South African ResearchChairs Initiative of the Department of Science and Technology andthe National Research Foundation (NRF), the Applied Centre forClimate and Earth Systems Science (ACCESS), and the AfricanCoelacanth Ecosystem Project (ACEP). We thank Dr. Bjorn Backebergfor his excellent contributions and assistance in MATLAB processing,and Edward Hill for helping with GIS. Thanks go to Christo Whittle

B.S. Malauene et al. / Deep-Sea Research II 100 (2014) 68–78 77

for supplying the remotely sensed data, and the ASCLME project andall the participants in the cruise in the Mozambique Channel in2009 for the shipboard data. The authors are grateful to the guesteditor and anonymous referees for detailed comments that led to animproved manuscript. Thanks go to W. de Ruijter for the personaldiscussion and for the advice towards publishing this work.

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