The Gulf Stream and Kuroshio Extensions are the regions where these two currents have separated from their respective western boundaries, to flow as free inertial jets in deep water. In approximate thermal wind balance with this baroclinic jet, a vigorously meandering front separates the warm subtropical and cold northern waters. The current systems also include important barotropic flow, particularly in tight recirculation gyres on their flanks.
Over the last couple of decades, results from a number of substantial western boundary current programs have fundamentally changed the scientific community′s understanding of their interconnected system of currents, recirculations, eddies, cross-frontal exchange mechanisms, and processes governing the upper ocean heat budget, and in turn affecting midlatitude storm tracks and climate. A brief synopsis for the Gulf Stream follows, which has guided the design for a Kuroshio Extension System Study (KESS) to occur in 2004-2006.
After separating from the coast, the narrow WBC is unstable, baroclinically and barotropically, and vigorous meanders develop. Meanders of the baroclinic front exert vortex stretching to spin up deep barotropic eddies, which are shifted slightly downstream, consistent with baroclinic instability, in a sense that enhances cross-frontal exchange of both heat and PV. The PV flux serves to reduce the anomaly carried from lower latitudes by the current, and forces anticyclonic and cyclonic recirculation gyres to the south and north. In turn, these gyres entrap fluid, to enhance deep wintertime convection, form mode water, and serve as heat reservoirs.
In the Gulf Stream, essentially all energy and momentum exchanges occurred in a few steep meander events, rather than an accumulation of many contributions spread more evenly over time; hence case-studies mapping the 3-D structure of large events are particularly informative. A 2-D array of inverted echo sounders (IESs) can map out the 3-D structure of horizontal baroclinic velocity and density field. Additional deep pressure and current measurements provide referencing to make the mapped velocity profiles absolute. (These three measurements have now been combined into one instrument, a C-PIES.)
The Atlantic and Pacific Oceans differ qualitatively concerning topography, stratification, wind curl, and thermohaline circulation, so KESS observations are designed to identify and quantify the dynamics and variability in the Kuroshio Extension and recirculation gyre: baroclinic-barotropic coupling; cross-frontal exchange; and processes governing the strength and structure of the recirculation gyre. Mesoscale-resolving time series measurements of the density and velocity field are required to understand these processes. The KESS experimental design employs an array of C-PIES, moored profilers with an upward-looking ADCP and RCMs at deeper levels, combined with floats, altimeter SSH, and hydrographic surveys.
(KESS PIs are Donohue, Hacker, Hogg, Jayne, Mitsudera, Qiu, and Watts, from URI, WHOI, and UH. See http://www.po.gso.uri.edu/kess/) |
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