Effects Of The East China Sea Kuroshio And Gulf Stream Fronts On Sea Surface Winds And Low-level Clouds In Winter And Spring

Sea surface temperature (SST) fronts form between the warm water of the EastChina Sea Kuroshio and Gulf Stream and cold continental shelf water. Strong SSTfronts have substantial impacts on the marine boundary layer. The present studyanalyzes and synthesizes high-resolution satellite observations and reanalysis data toexamine how SST fronts influence sea surface winds and low-level clouds and theirphysical mechanisms. Two creative results are achieved.In winter and spring, the climatological vector wind is strongest right on the SSTfront of the East China Sea Kuroshio while the scalar wind speed reaches a maximumon the warm flank of the front and is collocated with the maximum differencebetween SST and surface air temperature (SST-SAT). The distinction is due to thechange in relative importance of two physical processes of SST-wind interaction atdifferent time scales. The SST front-induced sea surface level pressure (SLP)adjustment (SF-SLP) contributes to strong vector wind above the front at the longtime scale, consistent with the collocation of baroclinicity in marine boundary layerand corroborated by the similarity between the thermal wind and observed wind shearbetween1000hPa and850hPa. By contrast, the SST modulation of synoptic winds ismore evident on the warm flank of the SST front. Large thermal instability (SST-SAT)of the near-surface layer strengthens temporal synoptic wind perturbations byintensifying vertical mixing, resulting in a scalar wind maximum.Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)captures a sharp low-level cloud transition across the Gulf Stream front, a structurefrequently observed under the northerly condition. Low-level cloud top (<4km)increases by about500m from the cold to the warm flank of the front. The sea surface temperature front induces a secondary low-level circulation through sea level pressureadjustment with ascending motion over the warm water and descending motion overcold water. The secondary circulation further contributes to the cross-frontal transitionof low-level clouds. Composite analysis shows that surface meridional advection overthe front plays an important role in the development of the marine atmosphericboundary layer and low-level clouds. Under cold northerly advection over the GulfStream front, strong near-surface instability leads to a well-mixed boundary layer overthe Gulf Stream, causing southward deepening of low-level clouds across the seasurface temperature front. Moreover, the front affects the freezing level bytransferring heat to the atmosphere and therefore influences the cross-frontal variationof the cloud phase.The results unify mechnasims of the SF-SLP adjustment and vertical mixing inaccording to their time scales, which helps to deepen the understanding of small scaleair-sea interaction. The present study applies the direct cloud observations fromspaceborne lidar in the field of small scale air-sea interaction and serves as a templateto study cloud transition near other strong SST fronts.