Variability of Ekman transport and resulting upwelling Comparison of Southeastern Brazil Coast to Hainan Island This project will discuss and compare the occurrence of upwelling in two distinct coastal regions and how different physical processes contribute to this phenomenon.

1. Introduction and background definitions Alongshore upwelling fronts are a commonly studied subject in oceanography and can be observed in a large number of the worlds coastal oceans. The wind-induced upwelling is the prevailing process but there are several other mechanisms that can generate and influence upwelling near coastal regions such as bottom bathymetry, instabilities in boundary currents, Coriolis effect, Ekman transport, eddies, seasonal variations, among others. However, this project will present and discuss only the influence of cyclonic meanders and topography (or bottom bathymetry) in upwelling systems and, in this introduction, a simple definition of both processes will be given. A meander trough consists of a clockwise rotating dome of cold upwelled water, which lies between a given boundary current and the continental slope. They tend to be formed after spin off stronger currents, lasting for weeks to months. One rotation typically takes place at every 10-30 days in a horizontal scale that ranges from 10 to 100 km, often called meso-scale. According to Gille et al. (2003), seafloor topography influences ocean circulation and is relevant because it steers ocean flows, but also because it inhibits or enhances the mixing and transport of waters from different regions. Most major currents respond to sea floor topography and as an example of this we can mention the following currents: Antarctic Circumpolar Current (ACC); the Gulf Stream and the Kuroshio Extension all steer around ridges and seamounts. a. Shelf break upwelling driven by Brazil Current cyclonic meanders Knowing a little more about the definition of those two ocean variables we can now introduce two case studies that seek to relate current meandering and topographic effects with upwelling. Campos et al., [2000] developed a numerical ocean model for a specific region of the Brazilian coast that shows the occurrence of upwelling associated with cyclonic meanders of the Brazil Current, in the region known as the Southeast Brazil Bight (SBB) or Santos Bight. It also shows that, in the summer, the South Atlantic Central Water (SACW) was detected in the SBB as close as 50 Km to the coast and that, in the wintertime, the SACW retreats to near the shelf break, being detected only in the middle and outer shelves (depths greater than 100m). The wind regime can be designated as the controlling forcing for this seasonal behavior but it cannot explain why the SACW is found in the outer regions of the shelf year round, since the

SACWs core is usually found at depths greater than 200m. Campos et al., [2000] suggests that during the summer the mechanism responsible for pumping that water onto the shelf could be a combination of shelf-break upwelling, induced by the Brazil Current (BC) cyclonic meanders, and coastal wind-driven upwelling.

By analyzing the horizontal distributions of temperature at a depth of 100m, the presence of the leading part of cyclonic meanders can be determined. Vertical sections of temperature and salinity indicate that the SACW was found climbing the shelf. The fact that the SACW reached the shallower regions only in the summer can be justified by additional action of the wind driven Ekman pumping, schematically represented in Figure 2.

Figure 1. Horizontal distribution of temperature at 100m depths. Notice the signature of a cyclonic meander in the southeastern quadrant of the surveyed area.

Figure 2. Schematic of the combined effect of coastal wind-driven and meander-induced upwelling. During the summer, coastal upwelling occurs in response to the offshore Ekman transport near the surface (CW: Coastal Water; SACW: South Atlantic Central Water; TW: Tropical Water).

b. Wind and topography influence on an upwelling system at the eastern Hainan coast The Hainan Island is located along the northern South China Sea (SCS) coast where upwelling can be verified by measurements and also by satellite data. In summer, under the southwesterly monsoon forcing, a northeastward current flows along the Vietnam coast, the south and east coast of Hainan Island, and the northern SCS coast, denoted as a western boundary current. Chu et al. [1999] calculated the transport volume of this current to be 5.5 Sv. There are some studies concerning the dynamics and inter-annual variability of upwelling near Hainan Island but they dont assess or dont explain what is the mechanism controlling the structure of patchiness and what is the role of topographic effects. As an effort to further understand this process, Su, J., and T. Pohlmann (2009) applied a baroclinic free surface model to an area of the island with dipole structures (i.e., an upwelling center behind a cape while a downwelling center exists in front of a cape looking in the direction of the flow). Whereas in the control run the vertical velocity in shelf areas indicates the combination of upwelling and downwelling centers around the capes, only upwelling centers were produced in the experiment. This indicates that the capes distort the potential vorticity balance, which leads to localized up- and downwelling patches. When upwelling favorable winds are strong enough, a sea surface temperature (SST) anomaly is formed at the downstream of the cape, causing an upward lift of isopycnals toward the coast. The additional density induced pressure gradient will result in further movement of the upwelling center. The existence of several capes along such a short coastline is relatively exceptional, and thus the resulting number of patches of up- and downwelling is the dominant feature in these areas. So the main dynamics around these capes are topographically steered.

Su, J., and T. Pohlmann (2009) affirm that this finding could be extended to other coastal areas that have similar complex coastline. This can be confirmed by the work of Tony et al. (2001), which relates three observed recurrent upwelling centers off the New Jersey coast with consistent solutions of their oceanic model and three topographic heights at the locations of Barnegat Inlet, the Mullica River estuary, and Townsend Hereford Inlet, respectively (Figure 3). Their research also shows that the existence of a topographic perturbation component in the upwelling solution of their theoretical model indicates that the alongshore topography plays an important role in controlling the formation of upwelling centers as it enhances upwelling at some locations and induces downwelling at others.

Figure 3. Tony et al.s (2001) satellite image of upwelling events along the southern New Jersey coast. The image indicates the upwelled surface water has converged into a series three cold patches. CTD transects through the Mullica upwelling center are show in the figure.

2. Discussion It is very important to consider the implications behind a comparison between different ocean processes happening in distinct regions, under various circumstances and timelines. To a certain extent currents and winds formed in one side of the planet can generate upwelling in the other side and have a significant impact on the circulation at different latitudes and longitudes. However, when we focus on smaller scales or specific systems, each measured property and confirmed process is a result of the combination of each forcing - that is also valid for global circulation - consequently, topographic and meandering effects will be better understood if analyzed as part of the equation that governs ocean and atmosphere circulation, not as a separate process. As a result of various oceanographic studies and researches it is widely known that the major forcing responsible for coastal upwelling is the Ekman transport and that other processes play a secondary role on generating upwelling phenomena, nonetheless, by carefully considering the effects of seafloor topography and current meandering, among other, we can have a more thoroughly answer for the unanswered questions that Ekman transport alone cannot answer. Finally it can be concluded that the Brazil Current cyclonic meandering inside the SBB causes the presence of the SACW on the outer regions of the continental shelf. Thus, the mechanism responsible for pumping the SACW onto the shelf was mainly due to meander induced shelf break upwelling, during the winter, and a combination of shelf break and wind-driven upwelling in the summertime. Highlighting topographic and cyclonic meander influence is not contradictory to the classical understanding of Ekman upwelling. The classic Ekman theory considers only large-scale processes, whereas in a narrow upwelling system the topography and current meandering can also play an important role. These smaller-scale processes might have a profound impact on the biology, chemistry and geology at similar scales.

3. Summary Table The following table is a summary of the comparison between the Hainan Island and the Southeast Brazil Bight, containing the processes that better describe the physical environment in a concise way.

4. References Anne F. Sell (2002), An Introduction: Impact on Marine Food Chains, University of Hamburg Chai, F., H. Xue, and M. Shi (2001), Upwelling east of Hainan Island, Oceanogr. China, 13, 129137. Edmo J. D. Campos, Denise Velhote and Ilson C.A. Silveira Geophysical research letters, vol. 27, no. 6, pages 751-754, march 15, 2000 Gille et al. (2003) Seafloor Topography and Ocean Circulation, Oceanography, Volume 17, Number 1. Guo, F., M. C. Shi, and Z. W. Xia (1998), Two-dimensional diagnose model to calculate upwelling on offshore of the east coast of hainan island, Acta Oceanol. Sinica, 20(6), 109116. Hu, J. Y., H. Kawamura, and D. L. Tang (2003), Tidal front around the Hainan Island, northwest of the South China Sea, J. Geophys. Res., 108(C12), 3342, doi:10.1029/2003JC001883. Sergio R. Signorini (1977), On the circulation and the volume transport of the Brazil Current between the Cape of So Tom and Guanabara Bay Su, J., and T. Pohlmann (2009), Wind and topography influence on an upwelling system at the eastern Hainan coast, J. Geophys. Res., 114, C06017, doi:10.1029/2008JC005018. Chu, P. C., N. L. Edmons, and C. Fan (1999), Dynamical mechanisms for the South China Sea seasonal circulation and thermohaline variabilities, J. Phys. Oceanogr., 29(11), 29712989.