Interannual Variability of the Water Masses Observed in the Tropical Northwestern Pacific

Abstract

The interannual variability of the water masses was analyzed from the CTD data measured in the tropical northwestern Pacific from 2006 to 2014. There are two typical water masses NPTW and NPIW that reveal the interannual variability in the survey area, in addition to two other water masses; the surface water mass TSW with a large seasonal variability and the deep water mass AACDW with a constant temperature-salinity characteristic at the depths deeper than 2,000 meters. In 2012 and 2014 NPTW was the most widely extended horizontally and thicker than 100 meters vertically, which was found over the entire survey area. However, NPTW was reduced and became much narrower in 2009 than in the other years. NPIW seemed to expand southwards from the north of 21˚N to 15oN in 2008 and in 2012, which showed the salinity minimum in 2013 (＜ 34.15 psu). The sea surface height estimated by Absolute Dynamic Topography (ADT) approximately along 135˚E section showed the high peaks (＞ 1.45 dyn•m) between 16˚N and 18˚N during the periods between 2007 and 2009 and between 2012 and 2013; the former peak lasted wider and longer in latitude and time (about three times) than the latter. The vertical section of the geostrophic currents in the upper 1,000 meters shows that there was a mesoscale pattern of repeated eastward and westward flows a few times in some years (2010 and 2014), which seemed to disappear in some other years (2008 and 2012); the former was closely related to the mesoscale eddies and the latter implied the pattern with the permanent currents. The persistent eastward flow between 17oN and 19oN seems to be related to the Subtropical Countercurrent (STCC).