We use a primitive equation isopycnal model of the Pacific Ocean to simulate and diagnose the anomalous heat balance on El Ni¿o-Southern Oscillation, ENSO, timescales associated with heat storage changes observed in the expendable bathytermograph (XBT) data set. We focus on the analysis of the total (diabatic plus adiabatic) and diabatic anomalous heat balances in six areas of the tropical and subtropical North Pacific Ocean in the upper 400 m. The diabatic (i.e., from the model conservation temperature equation) and adiabatic (i.e., from the model mass conservation equation) anomalous heat balances add up to the total anomalous heat balance. We computed the adiabatic/diabatic ratios to infer the relative importance of both contributions in different areas and found that they are smaller than 2.0 in only two regions (western equatorial and central North Pacific). The larger ratios (>2) were found along the corridor where adiabatic anomalies propagate westward in the form of Rossby waves and at the eastern equatorial Pacific. For those areas where the adiabatic/diabatic ratio is higher than about 2 the total anomalous heat balance is dominantly between the temporal change of heat and the three-dimensional divergence of the heat flux. At the central North Pacific area the total anomalous heat balance is between the temporal changes in anomalous heat, the surface heat flux and the vertical advection of heat. Different ENSO events are not always controlled by the same physical processes in the different areas. In many cases these differences are associated with the relative importance of adiabatic to diabatic processes. For instance, the western equatorial Pacific is controlled in general by diabatic processes, while the eastern equatorial Pacific is dominated by adiabatic physics most of the time. ¿ 1998 American Geophysical Union