Late-Winter Generation of Spiciness on Subducted Isopycnals

Abstract

The origins of density-compensating anomalies of temperature and salinity (spice) are investigated using a model forced with the most realistic surface products available over the 40 years 1958?97. In this hindcast, the largest interannual spiciness anomalies are found in the Pacific Ocean near the isopycnal σ0 = 25.5, where deviations as great as 1.2°C and 0.6 psu are generated equatorward of winter outcropping in the eastern subtropics in both hemispheres. These source regions are characterized by very unstable salinity gradients and low mean density stratification in winter. Two related signatures of winter mixing in the southeast Pacific (SEP) are density that is well mixed deeper than either temperature or salinity and subsurface density ratios that approach 1. All ocean basins in the model are shown to have regions with these characteristics and signatures; however, the resultant spiciness signals are focused on different isopycnals ranging from σ0 = 25.0 in the northeast Pacific to σ0 = 26.5 in the south Indian Ocean. A detailed examination of the SEP finds that large positive anomalies are generated by diapycnal mixing across subducted isopycnals (e.g., σ0 = 25.5), whereas negative anomalies are the result of a steady isopycnal advection, moderated by vertical advection and heave. There is considerable interannual variability in the strength of anomalies and in the density on which they occur. Historical observations are consistent with the model results but are insufficient to verify all aspects of the hindcast, including the processes of anomaly generation in the SEP. It was not possible to relate isopycnal anomaly genesis to local surface forcing of any kind. A complex scenario involving basinwide circulation of both the ocean and atmosphere, especially of surface water through the subtropical evaporation zones, is put forward to explain the decadal time scale evident in SEP salinity anomalies on σ0 = 25.5. Pacific anomalies generated on σ0 = 25.5 can be traced along mean geostrophic streamlines to the western boundary, where decadal salinity variations at ≈7°S are about 2 times as large (order ±0.1 psu) as at ≈12°N, although there may be more variance on shallower isopycnals in the north. At least portions of the σ0 = 25.5 signals appear to continue along the boundary to a convergence at the equator, suggesting that the most robust sources of Pacific spiciness variance coincide with equatorial exchange pathways.