q 2004 American Meteorological Society Late-Winter Generation of Spiciness on Subducted Isopycnals

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 s0 5 25.5, where deviations as great as 1.28C 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 s0 5 25.0 in the northeast Pacific to s0 5 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., s0 5 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 th