Submesoscale anomalies in the North Pacific Subarctic Front

Data analysis of the experiment “Megapolygon‐87”, targeted at large‐scale and mesoscale currents, revealed a submesoscale coherent eddy with a core of anomalously cold water south of the North Pacific Subarctic Front (SF). TS characteristics of the core water are similar to the ones of the Subarctic water mass north of SF. The core occupies a depth range from 950 through at least 1500 m and has an outer diameter of 40 km. The maximum value of thermal anomaly of −0.5°C is located at 1000 m depth. Anticyclonic orbital velocity reaches 30 cm/s at 10‐km radius at the edge of the internal core, rotating nearly as a solid body. The 7 cm/s translating eddy is governed by the larger‐scale currents. Significant seawater density anomalies above the lens are explained by the interaction of the lens with a mean horizontal flow with strong vertical shear. The lens was probably formed either by upper layer cooling during the previous winter‐spring season, able to generate deep convection processes and move surface water to the middle and lower part of the main thermocline, or by instability of the countercurrent existing below SF. No direct observations of those processes in the northwestern Pacific are available at present. Other similar submesoscale anomalies have been found in the area south of SF.