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

Abstract

The region between a shallow mixed layer and a deep isothermal layer resulting from salinity stratification is called a barrier layer (BL). Since BLs hinder the surface heat and momentum exchange with the ocean subsurface, they play an important role in air‐sea interaction. Synoptic features and formation of BLs and associated temperature inversions (TIs) in the Eastern Pacific Fresh Pool (EPFP) were investigated using shipboard observations. BLs and TIs were found in the high‐precipitation EPFP, a frontal transition zone (FTZ) and in the sea surface salinity (SSS) front during a 2016 boreal summer cruise. During a 2017 boreal autumn cruise, BLs and TIs were found from the southern part of the EPFP to the SSS front. The BLs in the SSS front and the FTZ were meridionally displaced from the precipitation maximum during both the 2016 and 2017 cruises. Freshening and cooling by tilting of both the isohalines and isotherms of the SSS front occurred via Ekman and geostrophic flow during both cruises, and also within the EPFP during 2016. The SSS front was a dominant contributor to the density gradient in the horizontal plane. A high dissolved oxygen content observed within BLs confirmed that the surface flow plays an important role in the BL and TI formation through the tilting process. Patchy distribution of freshening and cooling by tilting implies an intermittent BL and TI formation with short time scales.

The Influence of Preexisting Stratification and Tropical Rain Modes on the Mixed Layer Salinity Response to Rainfall

“…In any case, the model results suggest that stratification is dominant compared to rain forcing in modulating the salinity response to rain: when preexisting stratification is present, freshwater from rain is confined near the surface and slowly mixes downward. Preexisting stratification in the eastern tropical Pacific results from longer‐term freshwater input from the ITCZ and ocean dynamics (Bingham et al., 2020; Katsura & Sprintall, 2020; Katsura et al., 2021). Stratification is also produced locally by individual rain events (Iyer & Drushka, 2021).…”

“…Ship‐based radar and in situ observations from SPURS‐2, which took place during 2016–2017 in the eastern tropical Pacific Ocean, show significant variations in the patterns of convective and stratiform rain (Rutledge et al., 2019; Thompson et al., 2019). As a result of rainfall, stratified layers were commonly observed near the surface during the SPURS‐2 field campaign (Bingham et al., 2020; Katsura & Sprintall, 2020; Katsura et al., 2021). Hence, a wide‐range of preexisting stratification conditions are present in this region (Figure 2e).…”

Revisit the Upper Portion of the Japan Sea Proper Water: A Recent Structural Change and Freshening in the Formation Area

“…Therefore, the Akima spline has been used not only in oceanography (ex. Katsura et al, 2021;Oka et al, 2011;Sato et al, 2006), but also in wide scientific and engineering fields (Boor, 1978;Evenden, 1989). The potential temperature (θ) and potential density (σ θ ) were calculated from the interpolated values.…”

The Impact of the Eastern Pacific Fresh and Warm Pools on the Bimodal Seasonality of Barrier Layers

“…They attributed these thick BLs and associated TIs to southward Ekman advection, which transports cold and fresh water driven by northeastward winds. Katsura et al (2021) used the in-situ observations from a 2017 boreal autumn cruise and suggested that the sharp SSS front and surface flow are primary contributors to the formation 3 of 21 of the BLs and TIs in the NETP. A high-resolution model study focused on two specific thick BL events along 125°W and suggested a sharp salinity front tilt associated with vertical circulation as an additional mechanism for the NETP BLs (Bingham et al, 2020).…”