A see-saw pattern of the Intertropical Convergence Zone over the eastern Pacific

Over the eastern Pacific Ocean, the Intertropical Convergence Zone (ITCZ) is a prominent feature in the atmospheric general circulation. The ITCZ over the eastern Pacific prefers the Northern Hemisphere, resulting from the ocean-atmosphere interactions and coastal geometries. The northern ITCZ (hereafter NITCZ) reaches the northernmost location (20°N) in June, July and August, and then returns to the lower latitude (5°N) in February, March and April. In addition to this major NITCZ, a band of convergence zone located south of it, the southeast Pacific intertropical convergence zone (hereafter SITCZ), appears in March and April in the Southern Hemisphere. According to the average of rainfall and low-level divergence, NITCZ and SITCZ co-exist in March and April. For the rest of the year, only NITCZ exists and is well-studied in previous works. While a number of studies have documented the existence of the SITCZ, no study considers the interannual variation of the SITCZ.

This study analyzes the interannual variation of the SITCZ based on several independent observational datasets, including satellite-derived cloud, precipitation and sea surface temperature (SST) data. In addition, this study uses Q1 (apparent heat source) and Q2 (apparent moisture sink) extensively. They are obtained as the residuals of heat and moisture budgets as described in Yanai et al. (1973). Results show that the annual cycle of the local SST over the SITCZ region merely contributes to the seasonal evolution of the convection, but does not contribute to the interannual variation. The strength of the SITCZ does not correlate with its local SST. Nonetheless, the intensity of the NITCZ over the eastern Pacific has a negative correlation with the interannual variation of the SITCZ. In other words, when the SST is weak over the entire eastern Pacific, it is expect to observe a weak NITCZ. Such a weak NITCZ produces a weak meridional circulation which results in a better large-scale condition for convections to grow to its south such that a stronger SITCZ is evident. The present study, with different time-scale datasets, finds this passive characteristic of the SITCZ on the interannual variations. The intensity of the SITCZ strongly depends on the condition of the NITCZ which is related to the SST over the eastern Pacific. Our findings and conclusions suggest a see-saw pattern for the interannual variations of the NITCZ and SITCZ intensities over the eastern Pacific.