Ekman transport

As a result of these factors (wind, Ekman transport, Coriolis force) the surface ocean circulation in the mid-latitudes is characterized by clockwise gyres in the northern hemisphere and the counterclockwise gyres in the southern hemisphere. The main surface currents around these gyres for the world s oceans are shown in Fig. 10-6. The regions where Ekman transport tends to push water together are called convergences. Divergences result when surface waters are pushed apart. 

Ekman transport can also cause coastal upwelling and downwelling as shown in Figures 4.5c and d for the southern hemisphere. The patterns woifld be reversed for the northern hemisphere southerly winds induce downwelling and northerly winds lead to upwelling. 

In upwelling areas, the resupply of DSi via Ekman transport leads to a silica trapping effect as illustrated in Figure 16.4. Under these conditions, growth rates in the surface waters are controlled by the supply rate of DSi via upwelling. An example of nutrient... 

Regions of offshore upwelling driven by Ekman transport. 

Coastal upwelling The upward advection of water from the base of the mixed layer toward the sea surface caused by Ekman Transport. This water motion brings nutrient-rich water to the sea surface. 

Ekman transport The advection of water in the mixed layer caused by the winds and the Coriolis... 

Early N-cycle models were incorporated into dynamic, spatiaUy-explicit frameworks in the mid-1970s (e.g., Walsh, 1975 Wroblewski, 1977). For example, Wroblewski (1977) incorporated an N-cycle model into a two-dimensional up-weUing model. This coupled system allowed simulation of primary production and ecosystem response in an idealized, wind-forced coastal upweUing circulation with NO3 infusion from depth and offshore surface Ekman transport (Fig. 33.1). 

Schematic diagrams illustrating Ekman transport in response to wind forcing at the air—water interface (A) at the Equator and (B, C) near the coastline. (Redrafted from Thurman (1994).)...

Although the details of the vertical structure of the wind-driven current in the surface mixed layer depend on the vertical distribution of the Reynolds stress in the surface layer, the vertical integrated wind-driven current, the Ekman transport, depends only on the wind stress at the sea surface. 

For the Ekman transport, we obtain after vertical integration of (2.23) from a depth of vanishing Reynolds stress to the sea surface... 

Vertical currents, either upwelling or downwelling, can be generated if the divergence of the Ekman transport in the surface mixed layer is different from zero. The divergence of the Ekman transport can be derived from Equation 2.24 in the stationary state... 

In contrast to the ocean, no significant permanent vertical component of the wind stress curl exists at the surface of the Baltic Sea since it is located entirely in one climate belt, namely the west wind belt. This implies that no permanent divergence of the Ekman transport in the open Baltic Sea and subsequently no up- or downwelling and hence no permanent geostrophic currents can be excited by the wind in the open Baltic Sea. 

Price, J. E, Weller, R. A., Schudlich, R. R., 1987. Wind-driven ocean currents and Ekman transport. Science, 238, 1534—1538. 

The surface circulation of the western basin apparently remains anticyclonic, and the deep circulation cyclonic, under the predominant winds. As far as the relatively fast surface currents are concerned, the transversal spatial scale of today s sea is too small for the Coriollis force to be significant, so that the direct wind drag matters rather than the Ekman transport. On the other hand, the bottom layer circulation seems to immediately follow the sea surface slopes in the classic barotropic manner, so the Coriollis force is still effective for the slower, near-bottom currents. 

Sharpies (1997) made observations on the intrusion of subtropical water into the coastal zone of northeast New Zealand from late winter to midsummer 1994-1995, 1 year after the Karenia spp. bloom. He showed that the intrusion of subtropical water into the gulf was associated with high salinity nutrient depleted surface layers and near bottom waters with high nitrate N levels (3-5 mmol/m ). Southeasterly winds that occurred in late-November and mid-December 1992, prior to the bloom, are consistent with the hypothesis that Ekman transport effects on the East Auckland current induced the movement of a subtropical water mass, possibly carrying an established dinoflagellate community, into the Hauraki Gulf in late-December 1992. 

The thermal structure reveals the existence of a barrier layer in the SCS. The barrier layer usually weakens the cooling effect entrained at the bottom of the mixed layer. There are barrier layers in both the NSCS and SSCS, but they are thinner than that in the western equatorial Pacific. A barrier layer in the SSCS has a seasonal variation, and its depth has a positive correlation with temperature in the mixed layer. In addition, the barrier layer often exists in summer and autumn. The structure of the barrier layer in the SSCS is significantly modulated by the wind field, as well as by development of the mixed layer. In summer, relatively fresh water in the upper layer in the SSCS piles up in the southeast SCS because of the combined action of southeastward Ekman transport and downwelling in the eastern SCS. The high temperature water at the bottom of the mixed layer remains in a thermally uniform layer after separating from the mixed layer. The deepest barrier layer lies in the southeastern SCS, at about 30 m depth. The location of the thickest barrier layer almost overlaps the SCS Warm Water, which suggests that the heat barrier effect may stimulate the development of the SCS Warm Water. 

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