Southwest Pacific subtropical mode water: A climatology

The large-scale distribution and changes in Southwest Pacific subtropical mode water (STMW) are investigated and discussed. The paper presents for the first time geographic maps showing the spatial distribution of STMW thicknesses, with a vertical temperature gradient <2.0 degrees C/100 m occupying the 14-20 degrees C range below the mixed layer depth, across the entire Southwest Pacific region. STMW changes in areal thickness extent, vertical cross-sectional area along selected transects, and total volume, are examined on seasonal and interannual time scales between 1973 and 1988. We find that STMW extends across the entire width of the Tasman Sea in a very broad swath between the Tropical Convergence in the north Oust to the south of New Caledonia), the southeast Australian coast in the west to as far south as 39 degrees S (likely due to the southward extension of the EAC) and eastwards along the Southern STMW boundary in a meandering pathway that broadly follows the Tasman Front. The total STMW volume across the region (i.e., west of 180 degrees) varies seasonally by a factor of more than three between the estimated maximum of 6.6 ( +/- 0.5) x 10(14) m(3) in October and minimum of 1.9 (+/- 0.4) x 10(14) m(3) in May. Interannual variations O (+/- 0.5 x 10(14) m(3)) are also observed in the spatial extent of the thick mode water and its total volume. El Nino composite maps show an anomalous thickening of the STMW during the El Nino year with October positive thickness anomalies in excess of +20 m (total volume anomaly of +0.6 x 10(14) m(3)) manifested throughout the subtropical gyre interior as far north as New Caledonia. Total volume anomalies tend to be positive from January of the El Nino year through to the July following (18 months). The maximum correlation coefficient r = -0.3 between 3-monthly STMW volume anomalies and the Southern Oscillation index is statistically significant at the 95% confidence level. We conclude that during the anomalous cooling of the upper Southwest Pacific Ocean in the El Nino year, winter-time convection and STMW formation is enhanced across the region resulting in an El Nifio - Southern Oscillation climate signal that is identifiable below the mixed layer by the increased STMW volume which persists through to the following winter. Finally, some evidence for the possible decadal modulation of the STMW variability is also discussed. (C) 2008 Elsevier Ltd. All rights reserved.