South Pacific Eastern Subtropical Mode Water

Abstract

The structure, formation, and destruction of South Pacific Eastern Subtropical Mode Water (SPESTMW) are analyzed. Geographic extent and water properties are discussed by using high-quality CTD sections collected between 1991 and 1996. Defined as having a planetary potential vorticity magnitude of less than 3 ? 10?10 m?1 s?1, SPESMTW has a volume of about 1.1 ? 1015 m3, estimated from CTD data. The ventilation of this mode water is described by using data from a high-resolution XBT section in concert with 30-month time series from profiling CTD floats, some of the first Argo deployments. Published subduction rates allow a mode-water formation rate estimate of 8.7 ? 106 m3 s?1. Combining this estimate with the volume yields a residence time of about 4 years. The density-compensating covarying patterns of late winter surface temperature and salinity in the ventilation region of SPESTMW are shown to contribute to the strength of the mode water. However, while the destabilizing salinity gradient in SPESTMW contributes to its formation, it may also hasten its destruction by leaving it susceptible to double-diffusive convective mixing. SPESTMW spreads northwestward from its ventilation region within the subtropical gyre, eventually joining the South Equatorial Current. It is speculated that the proximity of the SPESTMW ventilation region to the Tropics, where winds and sea surface temperatures vary significantly, coupled with a direct interior circulation pathway to the equator, may allow SPESTMW to effect modulation of ENSO dynamics.