Eddy-Mediated Transport Along Eastern Boundaries
P. Ted Strub(1) and Corinne James(1)
Oregon State University,
104 COAS Admin Bldg,
Corvallis, Oregon, 97331-5503,
One of the advances in our understanding of the ocean that altimeters have demonstrated (in combination with other satellite and in situ data) is the ubiquitous presence of mesoscale eddies along the eastern boundaries of the Pacific and Atlantic Oceans, at all latitudes. Several different processes create both cyclonic and anticyclonic eddies over or next to the continental shelves, which then propagate to the west. These eddies affect the offshore transport of water with coastal characteristics (including higher nutrients and plankton populations) in at least two ways. The first is the alternating onshore-offshore currents associated with the eddies. Combinations of altimeter data with satellite-derived pigment concentrations (from ocean color) and SST (from IR) demonstrate this at mid- and high-latitudes. The second mechanism involves the transport of water in the core of the eddies, as they propagate to the west. An unanticipated result is that both anticyclonic and cyclonic eddies can trap richer, coastal water and transport that water offshore within their centers, maintaining higher phytoplankton populations than the surrounding water. The normal expectation is that open ocean anticyclonic eddies depress the nutricline and suppress primary production.
At higher latitudes in the NE Pacific, anticyclonic eddies in the Gulf of Alaska have been examined in detail by a number of studies. These form along the eastern boundary and propagate westward in the open ocean and along the shelf-break of the northern boundary. They both pull shelf water offshore around them and maintain a core of richer water that continues to support phytoplankton populations for many months. At similar latitudes off southern Chile, anticyclonic eddies also form along the boundary and carry richer coastal water offshore, as shown by combinations of altimeter and ocean color data. An example is shown of a particularly strong and persistent eddy that formed during the 1997-98 El Niño and continued to have both SSH and color signals for over a year. Eddies off southern Chile have not yet been studied in detail.
At (higher) mid-latitudes, similar formations of anticyclonic eddies occur in winter, under downwelling conditions. Examples are presented in both hemispheres, using altimeter and ocean color fields. During persistent upwelling conditions, alternating cyclonic and anticyclonic eddies pull water off the shelf, often keeping the richer water inshore of a meandering jet. Field data off Chile show that cyclonic eddies that break off from the jet can also carry coastal water (with undercurrent characteristics) in their core. Subsurface (undercurrent) water is also seen in eddies at low latitudes in the SE Pacific. Eddies appear less long lived at mid-latitudes (weeks to several months) than at higher latitudes. At lower latitudes in the NE Pacific, a unique process forms eddies of both signs off Central America, due to wind jets through the mountain gaps. Anticyclonic eddies persist longer than cyclonic eddie, also carrying water with characteristics of the eastern margin.