Low-frequency Variability of Kuroshio and Oyashio Extensions and Associated Ocean-Atmosphere Coupling

Interannual to decadal variability in the latitude and strength of the oceanic fronts along the Kuroshio Extension (KE) and Oyashio Extension (OE) is investigated based on observational datasets as well as an eddy-resolving hindcast simulation using the ocean general circulation model for the Earth Simulator (OFES). A particular emphasis is placed on the comparison between the two periods, 1958-1977 and 1988-2012, which exhibit changes in the relationships between the frontal variability of the KE and OE and also in the leading modes of atmospheric variability in the North Pacific. The index time series for the OE latitude and strength are defined based on the meridional gradient of sea surface temperature, while the KE indices are based on the sea surface height. In the early period, the KE latitude and strength changes are significantly anti-correlated to each other, but they become significantly positively correlated in the later period. Similarly, the OE latitude and strength become significantly positively correlated only in the later period. These changes are accompanied by the increased influence of the North Pacific Oscillation (NPO)-related atmospheric variability on the KE and OE variability in the later period. Consistent with these changes, the leading mode of the wind stress curl variability over the North Pacific is associated with the Pacific-North American (PNA) teleconnection in the early period, but with the NPO in the later period. The role of atmospheric variability in the KE and OE variability as well as the associated ocean-to-atmosphere feedback in the two periods will be discussed.