Seasonal variability of zonal heat advection in the mixed layer of the tropical Pacific

Zonal heat advection (ZHA) plays an important role in the variability of the thermal structure in the tropical Pacific Ocean, especially in the western Pacific warm pool (WPWP). Using the Simple Ocean Data Assimilation (SODA) Version 2.02/4 for the period 1958–2007, this paper presents a detailed analysis of the climatological and seasonal ZHA in the tropical Pacific Ocean. Climatologically, ZHA shows a zonal-band spatial pattern associated with equatorial currents and contributes to forming the irregular eastern boundary of the WPWP (EBWP). Seasonal variation of ZHA with a positive peak from February to July is most prominent in the Niño3.4 region, where the EBWP is located. The physical mechanism of the seasonal cycle in this region is examined. The mean advection of anomalous temperature, anomalous advection of mean temperature and eddy advection account for 31%, 51%, and 18% of the total seasonal variations, respectively. This suggests that seasonal changes of the South Equatorial Current induced by variability of the trade winds are the dominant contributor to the anomalous advection of mean temperature and hence, the seasonality of ZHA. Heat budget analysis shows that ZHA and surface heat flux make comparable contributions to the seasonal heat variation in the Niño3.4 region, and that ZHA cools the upper ocean throughout the calendar year except in late boreal spring. The connection between ZHA and EBWP is further explored and a statistical relationship between EBWP, ZHA and surface heat flux is established based on least squares fitting.