Changes in salinity of the North Pacific subtropical mode water (STMW) were investigated using profiles for 2003–2011. In 2009 and 2010, the STMW freshened markedly (S < 34.7 psu), with salinity 0.1 psu lower than other years. Freshening in 2009 was caused by excess rainfall in the 2008 warm season, which was related to an increase in the number of low pressures passing over the STMW formation region associated with a southeastward shrinkage of the summertime North Pacific subtropical high. The freshening signal persisted under the seasonal pycnocline during the 2009 warm season. This resulted in the freshening of STMW in 2010.
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