Effects of a Cold Ocean Eddy on Local Atmospheric Boundary Layer near the Kuroshio Extension: In Situ Observations and Model Experiments

Ship-board observations captured clear modulations of the marine atmospheric boundary layer (MABL) over a cold ocean eddy in the Kuroshio Extension in April 2014. Regional atmospheric model experiments are conducted to aid the interpretations of the in situ observations. Over the warm water west of the eddy, the surface turbulent heat fluxes enhanced, promoting a well-mixed MABL up to 1900 m deep and a layer of stratocumulus capped by a strong temperature inversion. Over the cold sea surface of the eddy, by contrast, the turbulent heat fluxes were suppressed, resulting in a clear-sky MABL with height of 1200 m. Striking variation was observed in the transition zone from the warm water in the west to the cold eddy. Though the air-sea interface was thermally stable, the MABL deepened and convective cumulus developed under a temperature inversion at 2800 m (Figure 1). Model results indicate that the cold eddy is responsible for the abrupt variations in the MABL. The low sea surface temperatures of the cold eddy change the surface winds through two physical processes: (1) the reduced surface heat fluxes suppress the downward transfer of momentum to decelerate the westerly winds; (2) a high-pressure anomaly induced by the cold eddy drives easterly winds. The two processes moderate the surface northwesterly to northeasterly winds in the transition zone, causing wind convergence (Figure 2) to deepen the MABL and convective cumulus (Figure 3). The model further shows that the atmospheric asymmetric responses in surface divergence are due to the combined effects of the vertical mixing and pressure adjustment mechanisms. This study reveals the physical processes in atmospheric responses to cold ocean eddies, which can improve the understanding of ocean eddy-atmosphere interactions.