Response of the Indo‐Pacific warm pool to interannual variations in net atmospheric freshwater

Abstract

We used the ocean general circulation model (OGCM) developed in the Massachusetts Institute of Technology to study the response of the Indo‐Pacific warm pool (IPWP) to net atmospheric freshwater flux (NAFW, equal to evaporation minus precipitation) at interannual timescales. The OGCM is forced by observed, monthly NAFW from 1988 to 2000. Our simulations show that the magnitude of interannual anomalies of salinity and temperature reaches about 0.7 practical salinity unit and 0.4°C, respectively. The typical timescale of these interannual variabilities is about 3–5 years. Averaged over 1988–2000, a decrease (increase) of temperature is accompanied with a decrease (increase) of salinity in the western Pacific and South Indian Ocean. A decrease of temperature, however, is accompanied with an increase of salinity in the surface layer (0–50 m) of the North Indian Ocean. The diagnosed budgets of salinity and temperature (heat) are analyzed to estimate the role of advection and vertical mixing in response to the surface NAFW forcing. The analyses indicate that the salinity anomaly in the IPWP is largely due to vertical mixing, especially in the surface layer. The vertical mixing of salinity, in turn, is associated with the surface NAFW anomaly. In contrast, the temperature anomaly above 300 m is primarily due to changes in advection forced by the NAFW, which is associated with basin‐wide changes in major ocean currents. Because of the strong effect of advection on the interannual variability of temperature, the temperature anomaly in the surface layer lags the salinity anomaly about 14–15 months. The results of our simulations are consistent with previous studies about the nearly immediate response of the tropical upper ocean to NAFW forcing due to vertical mixing. The slower response due to changes in basin‐scale heat advection suggests the possibility that ocean variability at interannual and longer timescales can be generated by large‐scale NAFW forcing at seasonal and longer timescales.