Near-Surface Temperature Warming Modulated by Rain Layers at the Edge of a Warm Eddy

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Abstract This study uses observations from two semi-Lagrangian EM-APEX floats to investigate how rain layers (RLs) influenced near-surface ocean stratification and sea surface temperature (SST) warming in June 2023. An RL, likely formed by precipitation prior to the field experiment at the edge of a warm eddy, induced strong salinity stratification around 40-m depth on yearday 169. From the floats’ initial passage over the intersected points (yearday 169) to their return (yearday 175), the SST increased by over 1°C, accompanied by a salinity increase of 0.2 psu in the upper 20 m. Using the heat budget approach, we identify surface radiative flux as the primary driver of upper-ocean warming. In the ocean interior, the RL suppressed the penetration depth of nighttime convection, reducing the entrainment of cooler water from below. This preexisting salinity stratification allowed the surface heat to accumulate, favorable for the restratification of the temperature structure in the upper 40 m over subsequent days. Although the RL was weakened by the strong horizontal salinity advection after yearday 173, the increased temperature stratification continued to restrict nighttime convective mixing in the upper 15 m. These results highlight the important role of transient rain-induced stratification in amplifying SST warming under calm, clear-sky conditions and offer valuable insights for improving SST prediction in complex air–sea interaction processes. Significance Statement In this study, we explore how a preexisting freshwater layer influences sea surface temperature (SST) warming and the evolution of upper-ocean stratification over the course of 1 week. Rainfall prior to the field experiment generated a surface freshwater layer overlying saltier subsurface water, establishing a strong barrier layer to vertical mixing. This “rain layer” suppressed the upward transport of cooler water from below, thus allowing solar radiation to more efficiently warm the ocean surface. Although the rain layer gradually weakened due to lateral advection, surface warming continued to intensify, resulting in a highly stratified temperature structure. Nighttime mixing was less effective at cooling the upper ocean compared to less stratified conditions, allowing atmospheric heat to accumulate near the surface and increase the SST by over 1°C. These findings underscore the complexity of predicting heat and moisture exchanges in the marine boundary layer, particularly between successive convective weather events.

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