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Temporal changes in the properties of the North Pacific Subtropical Mode Water (NPSTMW) and the North Pacific Central Mode Water (NPCMW) that occurred around the mid 1970s is investigated using temperature data composited for the two decades bounded by the mid 1970s: 1966-1975 and 1976-1985.
Properties of these mode waters changed greatly after the mid 1970s. The colder NPCMW was formed and widely distributed during 1976-1985. In the NPSTMW formation area, warmer water occupied the southwestern part, and colder water occupied the northeastern part during 1976-1985. The cause of this change is discussed with regards to heat flux and wind stress data. The cooling can be explained as a result of changes in surface heat flux and heat divergence in the Ekman layer (i.e., a larger amount of heat released from the ocean surface and an increased southward Ekman transport of cold water due to intensification of the westerlies). In particular, the latter played a dominant role in the observed cooling. On the other hand, the warming in the southwestern part of the NPSTMW area cannot be explained by the above mechanism alone. Time series of Sverdrup transport, Kuroshio transport and the thermal structure of the upper ocean reveal that the subtropical gyre intensified after the mid 1970s, suggesting that increased advection of warm water by the Kuroshio from lower latitudes to the south of Japan contributed to the observed warming.
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