Offshore Ekman Transport Research Articles

Upwelling in the Alaskan Beaufort Sea: Atmospheric forcing and local versus non-local response

The spin up and relaxation of an autumn upwelling event on the Beaufort slope is investigated using a combination of oceanic and atmospheric data and numerical models. The event occurred in November 2002 and was driven by an Aleutian low storm. The wind field was strongly influenced by the pack-ice distribution, resulting in enhanced winds over the open water of the Chukchi Sea. Flow distortion due to the Brooks mountain range was also evident. Mooring observations east of Barrow Canyon show that the Beaufort shelfbreak jet reversed to the west under strong easterly winds, followed by upwelling of Atlantic Water onto the shelf. After the winds subsided a deep eastward jet of Atlantic Water developed, centered at 250 m depth. An idealized numerical model reproduces these results and suggests that the oceanic response to the local winds is modulated by a propagating signal from the western edge of the storm. The disparity in wave speeds between the sea surface height signal—traveling at the fast barotropic shelf wave speed—versus the interior density signal—traveling at the slow baroclinic wave speed—leads to the deep eastward jet. The broad-scale response to the storm over the Chukchi Sea is investigated using a regional numerical model. The strong gradient in windspeed at the ice edge results in convergence of the offshore Ekman transport, leading to the establishment of an anti-cyclonic gyre in the northern Chukchi Sea. Accordingly, the Chukchi shelfbreak jet accelerates to the east into the wind during the storm, and no upwelling occurs west of Barrow Canyon. Hence the storm response is fundamentally different on the Beaufort slope (upwelling) versus the Chukchi slope (no upwelling). The regional numerical model results are supported by additional mooring data in the Chukchi Sea.

Surface wind variability and its implications for the Yucatan basin‐Caribbean Sea dynamics

Sea surface wind data from the QuikSCAT satellite and two meteorological stations collected over 1 decade (1999–2008) were used to study the dominant time scales of the wind regime in the Yucatan basin. Wind data were used to quantify its implications on regional hydrodynamics by evaluating Ekman pumping and Ekman transport. The annual cycle of surface wind revealed different features than the Caribbean wind regime with a relative wind stress maximum observed in the Yucatan basin in June caused by the Caribbean low‐level jet. The strongest wind stress occurred during November presumably due to the collapse of the Atlantic warm pool. This feature provides new evidence of the importance of local atmosphere‐ocean coupling. Synoptic scale variability was detected during fall and winter, leaving spring and summer almost free from these activities. Empirical orthogonal functions revealed three dominant time scales of variability: an annual scale in mode 1 (46.3% of the total variance), a synoptic scale in mode 2 (27.7%), and a semiannual scale in mode 3 (6.4%). The third mode featured extreme pulses (six tropical storms and nine hurricanes). Ekman transport was found to contribute the most (93%) to total transport, whereas Ekman pumping contributed only 7%. However, upwelling induced by Ekman pumping persisted during some synoptic pulses, underscoring the importance of tropical storms and hurricanes. Offshore Ekman transport over the continental shelf north of the Yucatan Peninsula favored upwelling all year long. Onshore transport toward the coasts of southern Cuba and the eastern Yucatan Peninsula favored downwelling.

Coastal upwelling along the north coast of Papua New Guinea and El Niño events during 1981–2005

We investigate the relationship between sea surface temperature (SST) cooling and upwelling along Papua New Guinea’s (PNG) north coast before the onset of El Nino events using a hindcast experiment with a high-resolution ocean general circulation model. Coastal upwelling and related SST cooling appear along PNG north coast during the boreal winter before the onsets of six El Nino events occurring during 1981–2005. Relatively cool SSTs appear along PNG north coast during that time, when anomalous northwesterly surface wind stress, which can cause coastal upwelling by offshore Ekman transport appearing over the region. In addition, anomalous cooling tendencies of SST are observed, accompanying anomalous upward velocities at the base of the mixed layer and shallow anomalies of 27°C isotherm depth. It is also shown that entrainment cooling plays an important role in the cooling of the mixed layer temperature in this region.

Control of lipophilic shellfish poisoning outbreaks by seasonal upwelling and continental runoff

Closures of mussel rafts in the Ría de Ares–Betanzos (NW Spain) because of the occurrence of lipophilic shellfish poisoning outbreaks over the period 1999–2007 have been studied in relation to the coastal wind and precipitation regimes of the area. More than 85% of the episodes concentrate on the summer and autumn that coincide with the upwelling-favourable/dry season and the transition to the downwelling-favourable/wet season, respectively. We obtain that the percentage of days closed in summer can be predicted by the average continental runoff ( Qr) during the preceding spring (86% of the variance explained by the average Qr of May) and the percentage of days closed in autumn by the average offshore Ekman transport (Ek L) during the preceding summer (91% of the variance explained by the average Ek L of August). A rainy spring will produce extensive closures in summer and a windy summer extensive closures in autumn. We speculate that this seasonal lag between atmospheric forcing and lipophilic shellfish poisoning outbreaks, which provides a chance to forecast mussel toxicity, is compatible with the obligate mixotrophic nutritional mode of the Dynophysis acuminata complex, the dominant lipophilic shellfish poisoning species in the Ría de Ares–Betanzos.

Mixed layer depth variability in the tropical boundary of the California Current, 1997–2007

The variability of the mixed layer depth (MLD) is examined over a decade (1997–2007) for the tropical boundary of the California Current (24–32°N), using conductivity‐temperature‐depth observations collected by quarterly survey cruises. Results indicate that salinity gradients control MLD rather than temperature gradients. The mean state of the upper ocean indicates that contours of constant MLD are parallel to the coast, with mixed layer thickness decreasing toward the coastal zone. The deepest (∼70 m) thickness is reached in January and the shallowest (∼15 m) occurs in July. The warmer conditions (summer) are reproduced for a simple thermal energy equation. The rest of the seasons are reproduced for a one‐dimensional momentum balance for the upper ocean, which includes Ekman dynamics and stratification. This comparison indicates that the variability of MLD is mainly due to wind‐driven phenomena except during the heating period. In particular, seasonal and interannual variability of the MLD are correlated with offshore Ekman transport. An abrupt MLD change occurs between January 1998 and January 2000 associated with the strong El Niño‐La Niña cycle shift that occurred in this period.

Interannual variability of shellfish toxicity in the Gulf of Maine: Time and space patterns and links to environmental variability

Six metrics characterize annual aspects of the magnitude and timing of shellfish toxicity resulting from dinoflagellates of the genus Alexandrium at >100 monitoring stations over 21 years (1985–2005) along the coast of Maine, northeastern USA. Climatologies at each station show distinct geographic patterns, generally consistent with previous reports based fewer stations/years. Earliest initiation and dates of maxima are at far western coastal stations, latest in far eastern stations. Toxicity magnitudes are highest at exposed western coastal stations and at far eastern stations inside a bay with minima in the center of the study region. Multivariate statistics group years according to similarity in station toxicity magnitude and timing. Results (a) confirm that a year of extreme toxicity (2005) differs from recent years but show it to be similar to strongly toxic years of the late 1980s, (b) show 3 year-groups, 1980s–early 1990s of high toxicity, mid-1990s–early 2000s of lower toxicity, and 2004–2005 similar to the early years, and (c) show temporal autocorrelation suggesting that processes carrying from year to year are important in controlling toxicity. Multivariate statistics then group stations according to interannual co-variability. Resulting station-groups show strong regionality consistent with known hydrographic and circulation patterns and identify stations that differ from their neighbors. Annual means calculated within station-groups provide 21-year time series that are cross-correlated with concurrent monthly environmental metrics of Gulf of Maine oceanographic conditions calculated from field measurements, satellite data and numerical circulation model hindcasts. These analyses provide three main results. First, toxicity time series show an overall pattern of elevated magnitudes and increased duration in the 1980s, minimum values in the mid-late 1990s and then increasing again in the 2000s. Second, only station-groups geographically located in the western portion of the study area have correlations with any tested environmental metric. Third, toxicity in many western station-groups was positively correlated to interannual variability in early season (April–May) wind stress driving onshore Ekman transport, negatively correlated with summer (June–July) wind stress driving offshore Ekman transport and negatively correlated with summer (June–July) cross-shelf surface temperature gradients indicative of relatively warm coastal surface temperature patterns, consistent with patterns expected from the wind transport correlations. The data do not show significant correlation between station-group toxicity and along-shelf temperature structure (an indicator of fronts and alongshore flow connection), river discharge, surface temperature anomalies, modeled surface salinities or alongshore current velocities. These data provide a quantitative summary of Maine coastal toxicity over 21 years, a spatial toxicity geography and isolate dominant environmental forcing responsible for interannual variability.

Interannual variability of upwelling indices in the Southeastern Arabian Sea: A satellite based study

Increase in sea surface temperature with global warming has an impact on coastal upwelling. Past two decades (1988 to 2007) of satellite observed sea surface temperatures and space borne scatterometer measured winds have provided an insight into the dynamics of coastal upwelling in the southeastern Arabian Sea, in the global warming scenario. These high resolution data products have shown inconsistent variability with a rapid rise in sea surface temperature between 1992 and 1998 and again from 2004 to 2007. The upwelling indices derived from both sea surface temperature and wind have shown that there is an increase in the intensity of upwelling during the period 1998 to 2004 than the previous decade. These indices have been modulated by the extreme climatic events like El-Nino and Indian Ocean Dipole that happened during 1991–92 and 1997–98. A considerable drop in the intensity of upwelling was observed concurrent with these events. Apart from the impact of global warming on the upwelling, the present study also provides an insight into spatial variability of upwelling along the coast. Noticeable fact is that the intensity of offshore Ekman transport off 8°N during the winter monsoon is as high as that during the usual upwelling season in summer monsoon. A drop in the meridional wind speed during the years 2005, 2006 and 2007 has resulted in extreme decrease in upwelling though the zonal wind and the total wind magnitude are a notch higher than the previous years. This decrease in upwelling strength has resulted in reduced productivity too.

Numerical study on the summer upwelling system in the northern continental shelf of the South China Sea

A three-dimensional baroclinic nonlinear numerical model is employed to investigate the summer upwelling in the northern continental shelf of the South China Sea (NCSCS) and the mechanisms of the local winds inducing the coastal upwelling, associated with the QuikSCAT wind data. First, the persistent signals of the summer upwelling are illustrated by the climatological the Advanced Very High-Resolution Radiometer (AVHRR) Sea Surface Temperature (SST) image over 1985–2006 and field observations in 2006 summer. Then, after the successful simulation of the summer upwelling in the NCSCS, four numerical experiments are conducted to explore the different effects of local winds, including the wind stress and wind stress curl, on the coastal upwelling in two typical strong summer upwelling regions of the NCSCS. The modeled results indicate that the summer upwelling is a seasonal common phenomenon during June–September in the NCSCS with the spatial extent of a basin-scale. Typical continental shelf upwelling characteristics are clearly shown in the coastal surface and subsurface water, such as low temperature, high salinity and high potential density in the east of the Hainan Island, the east of the Leizhou Peninsula and the southeast of the Zhanjiang Bay (noted as the Qiongdong-QD), and the inshore areas from the Shantou Coast to the Nanri Islands of the Fujian Coast (noted as the Yuedong-YD). The analysis of the QuikSCAT wind data and modeled upwelling index suggests that the local winds play significant roles in causing the coastal upwelling, but the alongshore wind stress and wind stress curl have different contributions to the upwelling in the Qiongdong (QDU) and the coastal upwelling in the Yuedong (YDU), respectively. Furthermore, model results from the numerical experiments show that in the YD the stable alongshore wind stress is a very important dynamic factor to induce the coastal upwelling but the wind stress curl has little contribution and even unfavorable to the YDU. However, in the QD the coastal upwelling is strongly linked to the local wind stress curl. It is also found that not only the offshore Ekman transport driven by the alongshore wind stress, the wind stress curl-induced Ekman pumping also plays a crucial effect on the QDU. Generally, the wind stress curl even has more contributions to the QDU than the alongshore wind stress.

Seasonal variation in horizontal and vertical structure of larval fish assemblages off south-western Australia, with implications for larval transport

The coastal ocean off south-western Australia is characterised by the southward-flowing Leeuwin Current, which suppresses the upwelling typically associated with other eastern boundary currents in the southern hemisphere. This results in a unique environment for the transport and survival of planktonic fish larvae. The horizontal and vertical structure of larval fish assemblages off south-western Australia was investigated during winter (August 2003) and summer (January 2004), and related to these unusual regional oceanographic and biological processes. Larval fish were sampled along a four station transect running from the inner continental shelf to offshore waters, using depth-integrated bongo net tows and depth-stratified EZ net tows. The distribution of taxa across the shelf and offshore was strongly influenced by the current regime at the time of sampling. Larval fish assemblage structure reflected the distinctive oceanographic conditions found during each season, and vertical depth distributions of larvae affected their horizontal location. Continental shelf samples were dominated by larvae of pelagic fishes, such as clupeiform species (e.g. Sardinops sagax), whereas offshore assemblages were characterised by larvae of oceanic families, such as Myctophidae and Phosichthyidae. The winter cruise (August 2003) was completed during a time of strong, southerly Leeuwin Current flow, whereas the northward-flowing Capes Current, in combination with surface offshore Ekman transport, predominated during summer. The vertical depth preferences of larvae were particularly influential in affecting their horizontal position; especially so for surface-dwelling larval fishes found during summer. This study represents the first documentation of the vertical structure of ichthyoplankton assemblages in the oligotrophic waters off south-western Australia.

Spatial and temporal variability of microplankton and detritus, and their export to the shelf sediments in the upwelling area off Concepción, Chile (∼36°S), during 2002–2005

Seasonal variations in diversity and biomass of diatoms, tintinnids, and dinoflagellates and the contribution of microplankton and faecal material to the vertical flux of particulates were investigated at one time series station T (station 18) between 2002 and 2005 and at a grid of stations during November 2004 in the coastal and oceanic area off Concepción (36°S), Chile. The variations were analysed in relation to water column temperature, dissolved oxygen, nutrient concentration, offshore Ekman transport, and chlorophyll- a concentration. Abundance was estimated as cell numbers per litre and biomass in terms of biovolume and carbon units. A sharp decrease with depth was observed in the abundance of both phytoplankton and microzooplankton during the whole annual cycle; over 70% of their abundance was concentrated in the upper 10 m of the water column. Also, a clear seasonality in microplankton distribution was observed at station T, with maxima for diatoms, tintinnids, and dinoflagellates every summer (centred on January) from 2002 to 2005. On the grid of stations, the maximum integrated (0–50 m) micro-phytoplankton abundances (>1 × 10 9 cells m −2) occurred at the coastal stations, an area directly influenced by upwelling. A similar spatial distribution was observed for the integrated (0–200 m) faecal carbon (with values up to 632 mg C m −2). Tintinnids were distributed in all the first 300 miles from the coast and dinoflagellates were more abundant in oceanic waters. At station T, the average POC export production (below 50 m depth) was 16.6% (SD = 17%; range 2–67%; n = 16). The biological-mediated fluxes of carbon between the upper productive layer and the sediments of the continental shelf off Concepción depend upon key groups of phytoplankton ( Thalassiosira spp., Chaetoceros spp.) and zooplankton (euphausiids) through the export of either cells or faecal material, respectively.

Factors responsible for the differences in satellite-based chlorophyll a concentration between the major global upwelling areas

The aim of this study was to identify the factors responsible for the differences in chlorophyll a concentration (Chl- a) observed between the California, Canary, Humboldt and Benguela upwelling areas. Monthly climatologic values of Chl- a obtained from satellite images, covering the years 1998–2004, revealed that this pigment was higher in the Benguela system than in the other areas. Upwelling intensity, as derived from offshore Ekman transport computations, was higher in the Benguela and Humboldt regions and, for the same upwelling intensity, Chl- a was higher in Benguela than in the other regions. Upwelling intensity appears to be able to drive Chl- a densities through nutrient supply, as nutrients are correlated to offshore Ekman transport. A linear regression model including the fraction of sea surface over the shelf in each 1° × 1° box, nitrate, silicate, turbulence and variability of offshore Ekman transport explained the 84.8% of the variance in Chl- a among the areas. Differences in offshore Ekman transport explained the lower Chl- a observed in Canary and California and the higher Chl- a observed in Benguela and Peru-Humboldt. A narrow continental shelf and low water column stability also contribute to reducing phytoplankton pigment biomass in the Canary and California areas. The higher Chl- a values observed in Benguela compared to Humboldt-Peru are due to a wider extension of the continental shelf in the Benguela region.

Local diurnal upwelling driven by sea breezes in northern Monterey Bay

Sea breezes often have significant impacts on nearshore physical and biological processes. We document the effects of a diurnal sea breeze on the nearshore thermal structure and circulation of northern Monterey Bay, California, using an array of moorings during the summer upwelling season in 2006. Moorings were equipped with thermistors and Acoustic Doppler Current Profilers (ADCPs) to measure temperature and currents along the inner shelf in the bay. Temperature and current data were characteristic of traditional regional scale upwelling conditions along the central California coast during the study period. However, large diurnal fluctuations in temperature (up to 5 °C) were observed at all moorings inshore of the 60-m isobath. Examination of tidal, current, temperature, and wind records revealed that the observed temperature fluctuations were the result of local diurnal upwelling, and not a result of nearshore mixing events. Westerly diurnal sea breezes led to offshore Ekman transport of surface waters. Resulting currents in the upper mixed layer were up to 0.10 m s −1 directed offshore during the afternoon upwelling period. Surface water temperatures rapidly decreased in response to offshore advection of surface waters and upwelling of cold, subsurface water, despite occurring in the mid-afternoon during the period of highest solar heat flux. Surface waters then warmed again during the night and early morning as winds relaxed and the upwelling shadow moved back to shore due to an unbalanced onshore pressure gradient. Examination of season-long, moored time series showed that local diurnal upwelling is a common, persistent feature in this location. Local diurnal upwelling may supply nutrients to nearshore kelp beds, and transport larvae to nearshore habitats.

A Seasonal Heat Budget across the Extent of the California Current

Abstract A seasonal heat budget is based on observations that span the broad California Current (CC) region. Budget terms are estimated from satellite data (oceanic heat advection), repeat ship transects (heat storage rate), and the Comprehensive Ocean–Atmosphere Data Set (COADS) (surface heat flux). The balance between terms differs with distance from shore. Offshore, a local balance between the heat storage rate and net heat flux (Q0) holds; the latter is dominated by its shortwave component QSW. Shoreward of ∼500 km, oceanic heat advection shifts the phase of the heat storage rate to earlier in the year and partially offsets an increase in Q0 due to cloud clearing. During the summer maximum of Q0, the ∼500-km-wide CC region loses heat to alongshore geostrophic transport, offshore Ekman transport, and, to a lesser degree, cross-shore geostrophic transport and eddy transport. The advective heat loss is neither uniform in space nor temporal phase; instead, the region of geostrophic and eddy heat loss expands cross shore with the annual widening of the California Current to ∼500 km. This expansion begins in spring with the onset of equatorward winds. A region of relatively positive wind stress curl widens at the same gradual rate as the CC, suggesting a coupling mechanism between the two.

Effect of 1998 El Niño on the distribution of phytoplankton in the South China Sea

The present study analyzed interannual variation of phytoplankton/Chlorophyll‐a (Chl‐a) distribution in the South China Sea (SCS) for the period from 1997 to 2005 using SeaWiFS‐derived Chl‐a data and other oceanographic data. The results show high spatial variation of Chl‐a concentrations in the SCS and revealed an anomalous event in 1998. High Chl‐a concentrations in the southwestern SCS in the summer season (June to August) may be related with strong Ekman Pumping and strong wind stress, whereas a jet‐shape high Chl‐a region offshore in western SCS was associated with coastal upwelling driven by offshore Ekman transport and Vietnamese offshore current. Chl‐a concentrations in 1998 summer in the SCS were the lowest among the 7 years and were particularly low in the western SCS. At the same season, the jet‐shape Chl‐a region offshore of southeast Vietnam almost disappeared, and southwesterly monsoon winds and offshore current were relatively weaker in 1998. This anomalous event of low phytoplankton biomass in the SCS coincided with an El Niño year in 1998.

Seasonal phytoplankton blooms associated with monsoonal influences and coastal environments in the sea areas either side of the Indochina Peninsula

The Gulf of Thailand (GoT) is a semienclosed sea on the west and southwest side of the Indochina Peninsula and connects with the near‐coastal waters of the South China Sea (SCS) on the east and northeast side of the Malay Peninsula. The objective of the present study is to understand dynamic features of the phytoplankton biology in the GoT and the nearby SCS, on both sides of the Indochina Peninsula, using remote‐sensing measurements of chlorophyll‐a (Chl a), sea surface temperature (SST), and surface vector winds obtained during the period from September 1997 to March 2003. Results show that seasonal variations of the phytoplankton blooms are primarily controlled by the monsoonal winds and related coastal environments. The GoT and the near‐coastal SCS have a peak in the averaged monthly Chl a in December and January, which is associated with the winter northeaster monsoon. The near‐coastal SCS have another big peak in the averaged monthly Chl a in summer (July to September), which is associated with the summer southwest monsoon. The offshore bloom in the GoT occurs in its southern part and enhances the December–January peak of averaged monthly Chl a. By contrast, the offshore bloom in the nearby SCS is observed northeast of the Peninsula, and represents the primary source of the July–September peak Chl a. Here the coastal upwelling associated with the offshore Ekman transport caused by the coastal surface winds parallel to the Vietnam east coast gives physical conditions favorable to the development of offshore phytoplankton blooms. The Mekong River discharge waters flow in different directions, depending on the monsoon winds, and contributes to seasonal blooms on both sides of the Peninsula.

Coastal upwelling process on a continental shelf limited by submarine canyons, Concepción, central Chile

The subinertial flow on the continental shelf off Concepción (central Chile) and its relation with the local wind‐forcing and the topography are studied. Emphasis is placed on the three‐dimensional aspects and the effect of the topography on the classical two‐dimensional approach to the coastal upwelling. Time series coming from 18 current meters installed on six moorings during March–April 1994 were analyzed. Hydrographic data were collected during two hydrographic cruises (6–7 March and 30–31 March). Complementary information includes wind data and sea level. Empirical orthogonal functions using all series showed two principal subinertial flow patterns. The first mode corresponds to an alongshore flow responding to the intensification of southwesterly wind on a 3‐ to 10‐day scale. The second mode is dominated by the cross‐shore fluctuations at the shelf break. The importance of the local wind‐forcing appears clearly when comparing the theoretical and observed Ekman transport in the north and central shelf (R = 0.8). Near the bottom a mean southward flow was found with a clear southward intensification induced by the bathymetry and probably by a meridional pressure gradient. This bottom flow modifies the typical coastal upwelling pattern through an offshore Ekman transport in the surface and bottom layers. This implies that the compensation flow for the coastal divergence comes from intermediate waters that arrive onto the shelf through ageostrophic process related with the submarine canyons located toward the north and south of the shelf.

Wind‐forced setup of upwelling, geographical origins, and numerical models: The role of bottom drag

An analysis is made to determine the properties and scales for the setup of upwelling driven by an impulsively started, steady, uniform wind of semi‐infinite fetch. The backward edge of the wind field acts as a geographical origin for the generation of coastal‐trapped waves (CTWs). A perturbation analysis is used to show that prior to the arrival of the nth mode CTW, the alongshore velocity is shown to asymptote to a constant value, (set by bottom drag) and over a timescale Tn. As this viscous limit is achieved, the upwelling becomes increasingly confined to the bottom boundary layer and bottom stress balances wind stress: Interior upwelling is shut down. The viscous limit may not be achieved, since after the arrival of the nth CTW mode, a gradient in the alongshore velocity is set up and its divergence acts to feed the offshore Ekman transport and shut the upwelling down. Numerical results are used to show that the frictional length and timescales predicted by the perturbation analysis are unrealistically small due to an underestimation of thermal wind shear. Moreover, the theory underpredicts the importance of the first CTW mode, which is shown to largely determine the setup and shutdown of upwelling. A simple first‐mode model for upwelling is developed and used to evaluate the validity of a numerical, open boundary condition in determining the correct degree of upwelling. A case study for the Chilean shelf is presented.

Convectively Driven Mixing in the Bottom Boundary Layer

Closely spaced vertical profiles through the bottom boundary layer over a sloping continental shelf during relaxation from coastal upwelling reveal structure that is consistent with convectively driven mixing. Parcels of fluid were observed adjacent to the bottom that were warm (by several millikelvin) relative to fluid immediately above. On average, the vertical gradient of potential temperature in the superadiabatic (statically unstable) bottom layer was found to be 21.7 3 1024 Km 21, or 6.0 3 1025 kg m24 in potential density. Turbulent dissipation rates («) increased toward the bottom but were relatively constant over the dimensionless depth range 0.4‐1.0z/D (where D is the mixed layer height). The Rayleigh number Ra associated with buoyancy anomalies in the bottom mixed layer is estimated to be approximately 1011, much larger than the value of approximately 10 3 required to initiate convection in simple laboratory or numerical experiments. An evaluation of the data in which the bottom boundary layer was unstably stratified indicates that the greater the buoyancy anomaly is, the greater the turbulent dissipation rate in the neutral layer away from the bottom will be. The vertical structures of averaged profiles of potential density, potential temperature, and turbulent dissipation rate versus nondimensional depth are similar to their distinctive structure in the upper ocean during convection. Nearby moored observations indicate that periods of static instability near the bottom follow events of northward flow and local fluid warming by lateral advection. The rate of local fluid warming is consistent with several estimates of offshore buoyancy transport near the bottom. It is suggested that the concentration of offshore Ekman transport near the bottom of the Ekman layer when the flow atop the layer is northward can provide the differential transport of buoyant bottom fluid when the density in the bottom boundary layer decreases up the slope.

Comparisons of chlorophyll variability between the four major global eastern boundary currents

The first two years of SeaWiFS (Sea viewing Wide Field of view Sensor) data (1997–1999) are used to document the variability of large-scale surface chlorophyll patterns within the coastal region along the full latitudinal extent of each of the four major global eastern boundary currents; the California, Humboldt, Benguela and Canary Currents. Seasonal chlorophyll patterns are compared to coincident seasonal cycles of Ekman transport calculated from satellite scatterometer data. In all four regions, maximum chlorophyll concentrations are generally temporally and latitudinally coincident with the seasonal maximum in upwelling (offshore Ekman transport) over most of their latitudinal range, but exceptions are documented. Interannual differences are evident in each region, most notably in the two Pacific regions where the 1997–1998 chlorophyll seasonality was affected by El Niño conditions. Significant differences between previously published chlorophyll seasonality deduced from the relatively sparse coverage of the Coastal Zone Color Scanner (CZCS) and the more complete coverage of SeaWiFS in both Southern Hemisphere regions are evident.

Seasonal circulation on the western shelf of the Gulf of Mexico using a high‐resolution numerical model

The seasonal circulation on the western shelf of the Gulf of Mexico is studied using a high‐resolution numerical simulation, historical hydrographic data, sea level data, and satellite images. Three regions are distinguished, the Tamaulipas‐Veracruz (TAVE) shelf, the Louisiana‐Texas (LATEX) shelf, and the western Campeche Bank. On the TAVE shelf there is a swift reversal of the along‐shelf current, downcoast from September to March and upcoast from May to August when there is upwelling due to offshore Ekman transport. Circulation on the western Campeche Bank is upcoast throughout the year. The LATEX shelf has a cyclonic circulation, except during summer months when the flow is eastward. During spring‐summer the upcoast current on the TAVE shelf reaches the southern Texas shelf where it encounters a downcoast coastal current favoring offshore transports. In the fall‐winter, the downcoast current reaches the southern Bay of Campeche where it meets an opposing along‐shelf current, generating seasonal offshore transports. During fall and winter, cool low‐salinity water from the Mississippi and Atchafalaya Rivers is advected westward along the LATEX shelf onto the TAVE shelf, developing along‐shelf fronts and temperature inversions commonly observed over the outer shelf and shelf break. The main forcing over the western shelf of the gulf is the along‐coast wind stress component. The existence of the cross‐shelf transports in the confluence regions is supported by chlorophyll a data. Up to 80% of the seasonal sea level variability is explained by the along shelf currents and the low‐frequency variability of the atmospheric sea level pressure.