Barrier layer variability in the western Pacific warm pool from 2000 to 2007

Major features of the equatorial portion of the western Pacific warm pool (WP) were brought to light through the analysis of an unprecedented collection of temperature and salinity profiles derived from Argo floats from 2000 to 2007. A region of thick (>15-25 m) and quasi-permanent barrier layers (BLs) was found to occur in a band from 10 degrees to 20 degrees longitude to the west of the maximum zonal sea surface salinity gradient (partial derivative S/partial derivative x), which occurs at the eastern edge of the WP. In this region, thick BLs and associated maxima (partial derivative S/partial derivative x) were displaced eastward (westward) during El Nino ( La Nina) over a distance of more than 6000 km. The thickness of the BL in this region is, to the first order, proportional to partial derivative S/partial derivative x and quasi-permanently associated with the occurrence of sea surface temperatures warmer than 28-29 degrees C, which are a good proxy for maximum atmospheric convection for the current Pacific climate. Statistics indicated that a thick BL forms preferentially under low wind conditions, heavy precipitation, eastward advection of low sea surface salinity, zonal current vertical shear, and/or in conjunction with equatorial downwelling Kelvin and Rossby waves (favoring the vertical stretching of the upper water column). None of these processes seemed to dominate the others, indicating that the formation of a thick BL results from a combination of different and complex mechanisms. The fact that a thick BL represents a quasi-permanent feature in the WP signifies that its specific stratification and likely impact on the sea surface temperature balance should be accounted for in coupled models.