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Coastal Up/Down-Welling Ocean Surface Topography

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Coastal Up/Down-Welling Ocean Surface Topography phytoplankton divergence of water masses Open ocean upwelling pigment results in upwelling and high productivity in subpolar waters (oceanic divergence at the equator) convergence of water masses causes near-surface waters Map view Profile view to pile-up in the subtropics Northeast Trade Winds Ekman SE Trades NE Trades divergence of water masses transport winds moving results in upwelling and high into the page productivity at the equator CZ photic zone IT equator convergence of water masses causes near-surface waters to pile-up in the subtropics Southeast Trade Winds Change in the direction of the Surface waters are replaced by divergence of water masses Coriolis effect at the equator nutrient-rich deeper waters results in upwelling and high causes divergence of the resulting in high biological productivity in subpolar waters surface waters productivity Coastal up/down-welling Ocean surface topography • Along coastal areas Ekman • Ekman transport and the Coriolis effect cause surface waters to converge transport can induce downwelling (“pile-up”) in the subtropics, and diverge (move apart) at the equator and in or upwelling by driving water subpolar waters. towards or away from the coast, respectively. • Convergence and divergence of surface water masses create subtle relief (“domes” and “depressions”) on the ocean surface (~1 m). • Gravity acts on the water to pull it downslope (down the pressure gradient), while the Coriolis effect is working in the opposite direction. • Geostrophic flow represents the balance between pressure gradient and Coriolis. NOTE! TOPEX/ Poseidon downwelling • Notice the red (high phytoplankton pigment) area off southwest Africa – this is coastal upwelling in action! • A similar (& important!) process happens upwelling off the west coast of South America 1 Geostrophic flow Global geostrophic flow • Coriolis effect deflects water into the center of the gyres, forming a low mound of water. • Water flowing down pressure gradients on the ocean’s irregular surface is deflected by the Coriolis effect. The amount of deflection is a function of latitude and current speed. • As height of the mound increases, the pressure gradient steepens, pushing the water outward in an attempt to level the mound. • When the pressure gradient equals Coriolis deflection, the current flows parallel to the wind around the mound. – flows as a geostrophic current – called geostrophic flow • Notice the warm currents flowing poleward on the western sides of the ocean basins and the cool currents flowing equatorward on the eastern sides. • Also note the Antarctic Circumpolar Current that flows around the isolated continent of Antarctica; this current serves to connect ocean circulation among the three major ocean basins (Atlantic, Indian, Pacific). • Most surface currents are geostrophic. • Gyres rotate clockwise in the NH and • Essentially all these currents are directed by a balance between pressure gradient counterclockwise in the SH. and Coriolis; that is, they are geostrophic. Sargasso Sea Western Boundary Currents • The Sargasso Sea is a large, deep lens of warm water encircled by • Earth’s rotation from west to east, compounded by the typically strong Trade the North Atlantic gyre and separated from cold waters below and Winds and weak Coriolis deflection near the equator, cause tropical & subtropical laterally by a strong thermocline. surface waters to “pile-up” on the western sides of the ocean basins. • It is named for the Sargassum seaweed found in this area. • This off-center “hill” causes water on the western side of the hill to flow faster than the eastern side; this is called “western intensification”. • It lies at the center of the North Atlantic gyre and is very low in nutrients due to lack of upwelling. • Strong western boundary currents mark the western sides of subtropical gyres. – Its eastern boundary is the Canary Current. • The flow pattern in gyres is asymmetrical – Its western boundary is the Gulf Stream. – narrow, deep, and swift currents western boundary currents along the basin’s western edge – broad, shallow, and slower currents along the basin’s eastern edge geostrophic flow around the North Atlantic Ocean 2 Gulf Stream – classic w.b. current Gulf Stream movie • First recognized (and mapped) by Gulf Stream movie Ben Franklin as part of shipping route between USA and Europe • characteristics – up to 5 miles per hour – avg. 70 km wide, 500 m deep – moves ~300x more water volume than Amazon R. • The Gulf Stream, like other western boundary currents, forms a meandering front separating coastal waters from warmer waters in the gyre’s center. • Meanders can be cut off to form warm-core and cold-core rings – both travel southwest – eventually re-join the main flow 3.
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