Seasonality and Formation of Barrier Layers and Associated Temperature Inversions in the Eastern Tropical North Pacific

Abstract

Seasonality and formation of barrier layers (BLs) and associated temperature inversions (TIs) in the eastern tropical North Pacific Ocean were investigated using raw and gridded Argo profiling float data, satellite data, and various sea surface flux data. BLs were observed frequently in boreal summer and autumn along the sea surface salinity (SSS) front south of the eastern Pacific fresh pool. TIs were found within the gap between the western and eastern Pacific warm pools in autumn when BLs were thickest. A mixed layer salinity budget was constructed to determine the formation mechanism responsible for BLs with TIs. This budget revealed that Ekman advection works to both freshen and cool the eastern tropical North Pacific in autumn and contributes to the formation of the thickest BLs with the warmest TIs through the tilting of the SSS front. Precipitation is a secondary contributor to BL formation in autumn. The BLs are also prevalent during summer but are thinner, are without associated TIs, and are primarily formed through precipitation. The largest rainfall associated with the intertropical convergence zone mostly occurred north of the band of thickest BLs in both summer and autumn. The geostrophic advection of salinity did not coherently contribute to the formation of BLs or TIs. The idea that Ekman advection contributes most to the formation of the thickest BLs with warm TIs was further corroborated because the horizontal salinity gradient was the dominant contributor to the density gradient and so is favorable for BL and TI formation.

What Role Does the Barrier Layer Play During Extreme El Niño Events?

“…The maximum BLT in the mean is found in the equatorial western Pacific Ocean (about 30 m) and decreases eastward (about 10 m near 140°W). Note that this is the mean picture, which may reflect that the BL is only seasonally or sporadically present, such as in the eastern Pacific Ocean (e.g., Katsura & Sprintall, 2020), or alternatively that the ECCO vertical resolution is not sufficient to resolve the thin BLTs and shallow MLDs in this region.…”

Growth and Decay of Northwestern Tropical Atlantic Barrier Layers

“…In fact, some signals seen in EN4‐derived BLT have no correspondence in the profiles that serve as basis for the objective analysis. Studies suggest that interpolation of in situ profile data in gridded data sets creates BLT artifacts (de Boyer Montégut et al., 2007; Katsura & Sprintall, 2020; Mignot et al., 2007; Sato et al., 2006). We take therefore the EN4 results as being only illustrative rather than giving true BLT amplitudes.…”

“…In an analysis of Argo observations from January 2000 to June 2005, Sato et al (2006) noticed patchy and thick barrier layer structures, which were explained by large-scale and small-scale (100 km) subduction processes in the subtropical gyres of the World Ocean. As those synoptic thick barrier layers appear in areas where the quasi-permanent climatological barrier layer is also thick, the climatological barrier layer is claimed to be a spatially and temporally smoothed picture of the synoptic barrier layers ( de Boyer Montégut et al, 2007;Katsura & Sprintall, 2020;Mignot et al, 2007;Sato et al, 2006). According to the authors, since the Argo floats sample mesoscale features but do not resolve them, the role of mesoscale eddies to describe the barrier layer formation process needs to be assessed.…”

Upper Ocean Stratification in the Eastern Pacific During the SPURS‐2 Field Campaign

“…A seasonal climatological study of BLs and TIs based on Argo profiling float data by Katsura and Sprintall (2020; henceforth KS2020) revealed that BLs are distributed along the SSS front south of the EPFP with a seasonal peak in boreal summer and autumn. Bingham et al (2020) show an example of this from a high-resolution model.…”