Using 2005-06 Argo daily real-time and delayed mode temperature and salinity profiles, world ocean atlas 2005 (WOA2005) monthly climatological objectively analyzed sea temperature and salinity fields on a one-degree grid, and NOAA monthly extended reconstructed sea surface temperature, version 2 (ERSST.v2), multi-temporal scale variability of the tropical Pacific barrier layer thickness (BLT) and the relationship between BLT and the upper ocean thermodynamic balance are studied, and then the variation of BLT during ENSO is shown. BLT extends over four latitude belts, that is, the 20°S-3°S latitude belt, the 3°S-3°N near the equator and the 3°N-12°N latitude belt, respectively, and the southwest-northeast directed extension in the 12°N-25°N latitude belt. BLT has an annual cycle variability, usually with strongest thickness over the winter hemisphere and weakest thickness over the summer hemisphere, however, BLT near the equator has a semi-annual cycle fluctuation; moreover, BLT has both interannual and higher frequency variability, with extending polarward from the equator, the temporal scale of BLT has a trend of high-frequency shift.

BLT has a significant effect on the upper ocean dynamic height (DH) mainly on interannual variability, and BLT might be thick enough to have a direct positive feedback on DH, which contributes to the increase in the upper ocean heat content, and according to the geostrophic balance, the variation of DH will produce the variation of the upper ocean geostrophic current, so, BLT has an indirect influence on the upper ocean circulation. There exist large values of BLT in the western Pacific warm pool (WPWP), and the spatial extent is far more widespread than other regions of large BLT outside WPWP; when SST in the equatorial eastern Pacific is getting more and more warmer than normal state, WPWP moves eastward and the region of large BLT in WPWP also moves eastward, on the contrary, they propagate westward. So, by a direct significant effect on WPWP thermodynamic balance, BLT has an indirect influence on ENSO evolution and then global climate change.