A Modeling Study Of The Atmospheric Responses To Mesoscale SST Perturbations In The Kuroshio-Oyashio Extension Region

The ubiquitous mesoscale air–sea interaction in the global oceans has been recognized and extensively investigated.Currently,however,how mesoscale sea surface temperature?SST?perturbations(SST_meso)affect the atmosphere in mid-latitude regions as well as the related climatic effects have not been fully understood.In this study,high-resolution satellite observations and the WRF model?Weather Research and Forecasting Model?are used to investigate the atmospheric responses to SST_meso in the Kuroshio-Oyashio Extension?KOE?region.The main approach is to perform a pair of WRF simulations,in which the control run?CTRL?is forced by low-resolution SST fields?almost no mesoscale SST signals?and the perturbation run?PER?by superimposing additional SST_meso fields extracted from high-resolution satellite observations.To extract SST_meso,the LOESS?locally weighted smoothing?method is adopted,which can effectively isolate mesoscale signals.The atmospheric responses to SST_mesoeso are isolated by calculating the differences between the output from CTRL and PER.It is the first time this experimental setup is used to study the atmosphere at the mid-latitude.Our analysis of the monthly mean atmospheric responses and satellite observations shows that the WRF model reproduces the boundary layer atmospheric responses to SST_meso well,and the responses can affect the middle and lower troposphere.The spatial patterns of mesoscale atmospheric responses agree quite well with the satellite observations and other previous model results.Results of the linear regression between the 10-m wind perturbations and SST_meso indicate that in the WRF model,the high positive correlation between the two fields is well captured but the coupling strength is relatively weak,suggesting the boundary layer parameterization needs to be improved.An examination on the underlying mechanism suggests that in the KOE region the pressure adjustment mechanism and the vertical mixing mechanism are both at work.The dominant mechanism at work should be assessed based on specific environmental conditions.The climatic effects of mesoscale SST fields are further investigated using model output from 2005 to 2013.The time-mean SST_meso in the KOE region is characterized by three SST fronts,which have a significant impact on local climatological precipitation.Climatological precipitation and its variability increase?decrease?on the warmer?colder?flank of the fronts and the net effect is a rectification on local precipitation.This effect improves the spatial distribution of the climatological precipitation simulated by PER and makes it agree better with observations,which indicates the important role of high-resolution SST boundary conditions in atmospheric modeling.The mechanism of the climatological precipitation response is further analyzed and the result shows that the pressure adjustment mechanism previously proposed to explain the rain band along the Gulf Stream is also valid in the KOE region and successfully explains several climatological features near the fronts.It is also noted that the pressure adjustment alone is not sufficient to explain the variability in the climatological precipitation response,and thus other factors such as the environmental conditions and the thermodynamic effect of SST_mesoeso on extratropical cyclones need to be taken into consideration.