Variability of the subtropical mode water in the Southwest Pacific

The variability of Subtropical Mode Water(STMW) in the Southwest Pacific is investigated using a 28-year-long time series (1986 to 2014) of high-resolution expendable bathythermographdata north of New Zealand (PX06) and a shorter time series, the Roemmich-Gilson monthly Argooptimal interpolation for the 2004-2014 period. The variability in STMW inventories is compared to the variability in air-sea heat fluxes, mixed layerdepths and transport of the East Auckland Current (EAUC) to assess both the atmospheric and oceanic roles influencing the formation and decay of STMW. The STMW north of New Zealand has a short lifespan with little persistence of the water massfrom one year to the next one. Deeper mixed layersand negative anomalies in surface heat fluxes are correlated with increased formation of STMW. The heat content of the STMW layer is anticorrelated with inventories, particularly during the El Nino years. This suggests that large volumes of STMW are coincident with cooler conditions in the prior winter and less oceanic heat storage. There is significant seasonal and interannual variability in STMW inventories, however there are no trends in STMW properties, including its core layer temperature over the last decade. The variability of the winter EAUC transport is highly correlated with the STMW inventories and thermoclinedepth in the following spring, suggesting ocean dynamicsdeepen the thermoclineand precondition for deeper mixed layers.