SIMULATION OF HEAT STORAGES AND ASSOCIATED HEAT BUDGETS IN THE PACIFIC-OCEAN - 2 - INTERDECADAL TIMESCALE

We use a primitive equation isopycnal model of the Pacific Ocean to si mulate and diagnose the anomalous heat balance on interdecadal timesca les associated with heat storage changes observed from 1970-1988 in th e expendable bathythermograph (XBT) data set. Given the smallness of t he interdecadal signals compared to the El Ni (n) over tilde o-Souther n Oscillation (ENSO) signal, the agreement between model and observati ons is remarkably good. The total anomalous heat balance is made up of two parts, the diabatic part (from the model temperature equation) an d the adiabatic part (from the model mass conservation equation) due t o thermocline heave. We therefore describe our analysis of both the to tal and diabatic anomalous heat balances in four areas of the tropical and subtropical North Pacific Ocean in the upper 400 m. The interdeca dal total (diabatic plus adiabatic) heat balance in the North Pacific Ocean is characterized by a complicated interplay of different physica l processes, especially revealed in basin-scale averages of the heat b udget components that have comparable amounts of variance. In smaller subregions, simpler balances hold. For example, in the western equator ial Pacific (area 1) the total heat content tendency term is nearly ze ro, so that a simple balance exists between surface heat flux, vertica l heat transport, and horizontal mixing. In the western subtropical Pa cific the total heat content tendency balances the three-dimensional d ivergence of the heat flux. We speculate that this complexity is indic ative of multiple physical mechanisms involved in the generation of No rth Pacific interdecadal variability. The diabatic heat balance north of 24 degrees N, a region of special interest to The World Ocean Circu lation Experiment (WOCE), can be simplified to a balance between the t endency term, surface heat flux, and meridional advection, the last te rm dominated by anomalous advection of mean temperature gradients. For the western equatorial region the diabatic heat content tendency is n early zero and the steady balance involves simply horizontal advection and the surface heat flux, which at these latitudes has a damping rol e in the model, hn important finding of this study is the identificati on of two interdecadal timescales, roughly 10 and 20 years, both simil ar to those reported by other investigators in recent years. [Tourre e t al., 1998; Latif and Barnett, 1994; Robertson, 1995; White et al., 1 997; Gu and Philander, 1997; Jacobs et al., 1994]. The 20-year timesca le is only present in diabatic heat budget components, while the 10-ye ar timescale is present in both diabatic and adiabatic components. The 10-year timescale can also be seen in the surface heat flux time seri es, but it occurs in the ocean adiabatic components which demonstrates the importance of oceanic adjustment through Rossby wave dynamics on decadal timescales.