Keywords:mesoscale eddy, western subarctic North Pacific
Mesoscale eddies in the subarctic North Pacific have a significant impact on the heat, freshwater, macro- and micro-nutrient and biota exchanges between shelf and offshore regions and thus play an important role in the marine ecosystem of the offshore region. In the offshore western subarctic North Pacific, we conducted a cross-section observation of two mesoscale eddies (Eddies A and B) on board T/S Oshoro-maru in June–July 2016. Satellite data analysis revealed that Eddy A formed along the Alaskan Peninsula, and Eddy B formed in the observation area. Unexpectedly, it was found that iron and chlorophyll concentrations were low both inside and outside of the eddies. Satellite chlorophyll concentration was high inside of Eddy A along the Aleutian Islands and for a while after it detached from the Aleutian Islands, suggesting that iron originated from coastal region was lost during 2-year journey in the offshore region to the Oshoro-maru observation area. Oshoro-maru observation also revealed that the vertical diffusivity was low both inside and outside of Eddies A and B. This weak vertical mixing could be another reason for low iron and thus low chlorophyll concentration inside Eddies A and B. Analysis of the output data of high-resolution model suggested that the weak vertical mixing could be attributed to the seasonality of the vertical velocity inside mesoscale eddies and the weakness of Eddies A and B.