Seasonal to Interannual variations of salinity fronts under Pacific ITCZ from spaceborne observations and their associations with ENSO

Abstract:

One decade of sea surface salinity (SSS) data are available from Aquarius, SMOS and SMAP satellite mission. One of the most noticeable salinity structures in the global map is a band of freshwater under Pacific intertropical convergence zone (ITCZ) due to the heavy rainfall. Our results show that under Pacific ITCZ, SSS is fresher in SMAP data than in Aquarius data. To further compare with SMOS data, it is suggested that SSS differences between SMAP and Aquarius under Pacific ITCZ are actually showing interannual variations instead of observation errors.

The satellites observe sharp salinity fronts on both the northern and southern boundaries of the freshwater band. The salinity front in this area plays an important role modulating the barrier layer thickness and dominating the density variations. Every year, the salinity fronts at the southern bound of the freshwater band propagate from equator to around 8N from January to July, which is related to the shift of ITCZ. In this study, we further investigate the dynamics that drives the continuous northward propagation of the salinity fronts at the northern bound of the freshwater band from August to December.

Four El Niño (2014/15, 2015/16, 2018/19, 2019/20) and four La Niña (2010/11, 2011/12, 2016/17, 2017/18) events occur within the 10-year study period. By simply observing the SSS, there’s no evident correlation between SSS values and El Niño Southern Oscillation (ENSO) activities. However, we noticed that during La Niña years, the salinity fronts at the southern boundary of the freshwater band stay at 8N from July to December. On the other hands, the salinity fronts propagate southward in the latter half of the year during El Niño years. The southward propagation of the salinity fronts may be the results of higher sea surface temperature in El Niño years, leading ITCZ going southward.

The results in this study conclude that the salinity fronts show distinct structures correlated with ENSO activities and the investigation of salinity fronts could be useful to study the water cycle in El Niño and La Niña years.