While his ship was frozen in sea ice in the 1890s, Norwegian polar explorer, author, and oceanographer Fridtjof Nansen observed that icebergs, ice, and his ship were moving 45° to the right of the wind rather than in the same direction as the wind. He asked a student, Vagn Walfrid Ekman to explain this. Ekmans theory considers the steady state balance of friction and the Coriolis acceleration caused by Earths rotation. In this balance, the wind driven surface current is 45° to the right of the wind in the Northern Hemisphere (to the left in the Southern Hemisphere), the wind driven current decreases exponentially with depth, and it veers further to the right in the Northern Hemisphere (left in the Southern Hemisphere) with depth, forming the Ekman Spiral (see Figure 1.07). The integrated net transport of the wind driven layer (the Ekman Spiral) is 90° to the right of the wind in the Northern Hemisphere (to the left in the Southern Hemisphere). This net mass transport of water (called the Ekman Transport) creates vertical motions of water (upwelling or downwelling) in order to conserve mass.
4.1 Coastal, Wind Driven Upwelling
When the winds along the west coasts of continents (eastern side of ocean basins) blow equatorward (from the north in northern hemisphere, from the south in the southern hemisphere), or have a component in this direction, there is a net offshore movement of water in the wind driven layer due to the Ekman Transport. This creates a divergence along the coast, as depicted in Figure 1.08 (top panel). Deeper waters must move onshore and towards the surface, to replace the water which is moving offshore. This phenomenon is called upwelling. Conversely, when the winds blow poleward or have a poleward component, there is a net onshore movement of water in the wind driven layer due to the Ekman Transport (convergence near the coast), and the surface waters must move downward (downwelling) near the coast.
Because the deeper waters are colder and higher in nutrient concentrations, the conditions that lead to upwelling cause colder surface temperatures and increased biological activity near the west coasts of the continents in both hemispheres. Figure 1.09 shows the monthly average SST distribution along the west coast of the United States in August 1988. The cold SSTs (approximately 10°C) along the coast are clear indicators that wind driven coastal upwelling was occurring in August 1988.
4.2 Open Ocean Wind Driven Upwelling Near the Equator
As shown in Figure 1.10, the global surface wind patterns are such that the winds are from the northeast north of the equator and from the southeast south of the equator (dark arrows). The net movement of water in the wind-driven layer as a result of the Ekman Transport will be away from the equator both north (90° to the right of the winds) and south (90° to the left of the winds) of the equator (gray arrows). This divergence of surface waters near the equator leads to upwelling of colder (and higher in nutrient concentrations) subsurface waters. The cooler SSTs associated with this upwelling along the equator can be seen in Figures 1.02 and 1.04.
In addition to the movement of water by winds and currents affecting SST, warming or cooling on the sea surface can occur on daily, seasonal, or interannual time scales. For example, each hemisphere warms up during its summer because it receives more energy from the Sun than it loses; conversely, each hemisphere cools during its winter because it loses more heat than it receives from the Sun.