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Surface currents in the North Western Pacific from WOCE / TOGA drifters

 

Nikolai A. Maximenko*1, Gleb G. Panteleev*2, Pearn P. Niiler*3 and Toshio Yamagata*4

*1 P. Shirshov Institute of Oceanology

*2 Dalhousie University

*3 Scripps Institution of Oceanography

*4 University of Tokyo

 

To obtain mean velocities in the upper mixed layer of the North Western Pacific (20-60N, 120-180E) we analyzed data of 296 WOCE / TOGA Surface Velocity Programme Lagrangian drifters (106.8 drifter years in total), deployed in 1987-96, with a drogue centered at 15m depth. Along with fast Kuroshio, Warm Tsushima, and Kamchatka Currents, Subtropical Countercurrent, we discovered a double-jet structure in the Kuroshio Extension (KE) / Subarctic Front (SF) zone, with two fast eastward flows, associated with KE and SF and weak, but seen in mean, westward countercurrent between them. When KE and SF jets have similar mean velocity structure, SF is more stable, so that velocity variability demonstrates the only maximum on meridional sections, coinciding with the middle part of KE.

Analysis of individual drifter trajectories revealed the lack of horizontal water exchange between the Subtropical and Subpolar Gyres. Two groups of buoy tracks (selected by location of their release point either north or south of 35N) overlap in the transitional zone, width of which is about 3N and does not increase remarkably eastward through 180E.

Representative mean velocity field is calculated with variational interpolation scheme. Its main advantage is the inclusion of advective nonlinear term into the cost function weak constraints. This idea is based on weakness of ageostrophic currents and results, mathematically, in a dominance of along-stream velocity averaging, when cross-stream flow structure is mostly remained. Optimized mean velocities not only keep main parameters of mean jets unchanged, but even allow discussion of KE and SF interaction with the Shatsky Rise and Emperor Seamounts.

 

 

 

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