Cold Pools Drive Short-Term Thermohaline Variability in the Ocean Skin Layer

Atmospheric cold pools are transient sub-mesoscale to mesoscale weather phenomena generated by convective precipitation. They are characterized by cool, dense air spreading laterally, which strongly alters near-surface atmospheric conditions and the thermohaline properties of the ocean skin layer (< 1 mm) and near-surface layer (NSL: < 1 m). The spatial extent and thermodynamic intensity of atmospheric cold pools are moderated by mixing within the moist marine boundary layer (MBL), the lowest ~0.5–1 km of air above the ocean. Despite this moderation, accompanying wind surges and intense precipitation drive short-lived intensifications of atmosphere–ocean fluxes that can abruptly restructure upper-ocean thermohaline structure, an aspect that remains poorly constrained by in situ measurements and is underrepresented in coupled atmosphere–ocean models.Here we present observations of atmospheric cold pool passages over the tropical Atlantic Ocean, combining ship-based meteorological measurements, aerial drones, weather balloon profiles, and high-resolution observations from the autonomous surface vehicle HALOBATES. Equipped with meteorological and oceanographic sensors sampling at high-resolution (1 Hz), HALOBATES resolves the ocean skin layer and the NSL dynamics, allowing us to identify and characterize cold pools as they passed over the study area during the R/V METEOR M211 field campaign (14 June–27 July 2025).Between 13 and 18 July 2025, cold pool passages were identified by rapid air-temperature drops of 0.6–1.8 °C and, during rainy events, rainfall rates of up to 37 mm h⁻¹. All cold pools induced transient ocean surface cooling, leading to sea surface temperature anomalies (Tskin-TNSL) of 0.02 – 0.35 °C, and amplified cooling in the skin layer relative to the NSL. To quantify the cooling and recovery of the ocean skin layer following the cold pool passage, we consider two different skin-layer depths: the infrared (IR) skin layer observed by thermal cameras and the skin layer measured in situ by HALOBATES. This comparison shows that skin-temperature responses evolve on timescales of only a few minutes and therefore require high temporal resolution in situ measurements to be adequately resolved.Salinity responses depended critically on precipitation: cold pools that passed the study area without measurable rainfall produced negligible changes, whereas intense-rain events freshened the skin by up to 1.3 g kg-1 and the NSL by 0.4 g kg-1, forming shallow freshwater lenses that re-stratified the upper meter within approximately 15 minutes. Pronounced cold pool passages were accompanied by enhanced latent and sensible heat fluxes, with maximum increases of −140 W m⁻² and −30 W m⁻², respectively, driven primarily by increased wind speeds and indicating intensified ocean-to-atmosphere heat exchange. These observations demonstrate that cold pools strongly affect short-term variability in upper-ocean thermohaline structure through short-lived intense peaks in atmosphere–ocean fluxes, emphasizing the need to include these transient events in future coupled atmosphere–ocean models.