Recent thermohaline transition in the Pacific western subarctic boundary currents and their fresh core eddies: the response of sound-scattering layers

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The Oyashio and Kamchatka Currents are contiguous western-boundary currents of the western subarctic Pacific that transport cold surface waters southward. At the same time, large eddies within the Oyashio propagate against the current and carry warm waters of subtropical origin northwards. Off the Kuril Islands, the south setting flow bifurcates around these eddies to affect the Oyashio Coastal and Oyashio Offshore branches. During the 1990s, a major thermohaline transition occurred within this current system, linked to the relative fluxes of two major water masses entering the Sea of Okhotsk: warm salty waters through Soya Strait and cold fresh waters through Kuril Island straits. This thermohaline transition was manifested by (1) changes of the temperature and salinity structure of the boundary current regime, (2) alteration of the Oyashio's path and (3) strengthening and restratification of the Coastal Oyashio branch. In concert with physical changes, the structure of sound-scattering layers (SSL), reflecting mesopelagic fish communities, likewise changed. These large-scale adjustments in the western subarctic Pacific show that the regional ocean climate and its associated biota strongly depend on the pathway and strength of the Oyashio. We describe here the physical changes that occurred in the 1990s and relate these to observations of the SSL. In particular, we show that large eddies significantly affect the structure of the SSL and that there is clear evidence of substantial SSL response to the recent thermohaline transition.