Oceanic Response to Idealized Net Atmospheric Freshwater in the Pacific at the Decadal Timescale

In the study of decadal variations of the Pacific Ocean circulations and temperature, the role of anomalous net atmospheric freshwater (NAFW; evaporation minus precipitation) has received scant attention even though ocean salinity anomalies are long lived and can be expected to have more variance at low frequencies than at high frequencies. To explore the magnitude of salinity and temperature anomalies and their generation processes, we studied the response of the Pacific Ocean to idealized NAFW anomalies in the tropics and subtropics, using an ocean general circulation model developed in the Massachusetts Institute of Technology. Simulations showed that salinity anomalies generated by the anomalous NAFW were spread throughout the Pacific basin by mean flow advection. This redistribution of salinity anomalies caused adjustments of basin-scale ocean currents, which further resulted in basin-scale temperature anomalies due to changes in heat advection caused by anomalous currents. In our study, the response of the Pacific Ocean to magnitudes and locations of anomalous NAFW was linear. When forced with a positive NAFW anomaly (anomalous evaporation) in the subtropical North (South) Pacific, a cooling appeared in the western North (South) Pacific, which extended to the tropical and South (North) Pacific; and a warming emerged in the eastern North (South) Pacific. When forced with a negative NAFW anomaly (anomalous precipitation) in the tropical Pacific, a warming occurred in the tropical Pacific and western North and South Pacific; and a cooling occurred in the eastern North Pacific near 30\deg N and the South Pacific near 30\deg S. The warming (cooling) in the tropical Pacific was associated with the weakening (strengthening) of the South Equatorial Current. The warming (cooling) in the east and cooling (warming) in the west in the subtropical North and South Pacific were associated with a spin-down (spin-up) of the subtropical gyres. The temperature anomalies propagated from the tropical Pacific to the subtropical North and South Pacific via equatorial divergent Ekman flows and poleward western boundary currents; and they propagated from the subtropical North and South Pacific to the western tropical Pacific via equatorward coastal Kelvin waves and to the eastern tropical Pacific via eastward equatorial Kelvin waves. The timescale of temperature response was typically much longer than that of salinity response due to slow adjustment times of ocean circulations. These results imply that the slow response of ocean temperature due to anomalous NAFW in the tropics and subtropics may play an important role in the Pacific decadal variability.