Interannual,Seasonal,Sub-seasonal Variation And Impact Factors Of Low Cloud Fractions In Different North Pacific Ocean Regions

Based on the long-term satellite data provided by Moderate-resolution Imaging Spectroradiometer(MODIS)and reanalysis data provided by European Center,the seasonal,interannual and subseasonal variability of low-cloud fraction(LCF)as well as the associated controlling factors over the eastern and western North Pacific on different time scales are investigated.The main conclusions are follows.(1)The dominating factors associated with the seasonal cycle of LCF are different over the eastern and western North Pacific regions.Over the eastern North Pacific,the estimated inversion strength(EIS)dominates the enhancement of LCF in summer,together with the700-hPa relative humidity(RH).In summer,the equatorward winds induce coastal upwelling,upper-ocean mixing,and surface evaporation,acting to maintain relatively low sea surface temperature(SST).Meanwhile,the mid-troposphere subsidence associated with the subtropical high acts to warm the mid-troposphere.The combination of cool SST and warm mid-troposphere maintains a strong EIS at the top of the boundary layer,inhibiting cloud-top entrainment of dry air,further increasing LCF.Besides,the enhanced RH indicates an increase in vapor concentration in summer,which provides positive condition to the formation of low-level clouds.Over the western North Pacific,the enhancement of LCF in winter is mostly due to enhanced SHF and cold advection.In this area,the cold advection in winter over the western region destabilizes the surface layer,increasing the difference between sea surface temperature(SST)and surface air temperature(SAT),further resulting in large upward sensible heat flux(SHF).The wintertime enhancement of upward SHF facilitates shallow convection in the boundary layer,and further increases LCF.(2)On the interannual time scale,the increased LCF over the eastern North Pacific in summer is associated with increased EIS and decreased sea surface temperatures,in which the El Ni?o plays an important role.The El Ni?o-related SSTs have negative feedback on the LCF over the eastern North Pacific,in which the EIS plays an important role.Over the western North Pacific,the enhanced LCF in spring and winter has a positive correlation with enhanced SHF and tropospheric low-level cold advection,which can be partly explained by the subpolar frontal zone(SPFZ)intensity.The SPFZ-related cold advection has a positive effect on the enhanced LCF by interacting with the SHF over the western North Pacific.(3)There are obvious sub-seasonal signals of LCF over the eastern and western North Pacific.Over the eastern north Pacific,the sub-seasonal variation of LCF is positively correlated with sea surface wind speed and EIS,negatively correlated with SST andtemperature advection.The peak of sea surface wind speed and temperature advection is 1-2days ahead of LCF while the peak of SST lags LCF by 1 day.Besides,there is no lag relationship between EIS and LCF,which indicates that the enhanced sea surface wind field and cold advection in this region can promote the formation of low-level clouds by increasing EIS.As a result,the increase of LCF will reduce the short-wave solar radiation reaching the sea surface,thereby reducing the SST.Over the western North Pacific,the subseasonal variation of LCF is mainly affected by sea surface wind speed,cold advection and SHF,and is positively correlated with these factors.The peak of sea surface wind speed,SST and temperature advection are 1 day ahead of the peak of LCF,and there is no lag relationship between SHF and LCF.This indicates that the enhanced sea surface wind speed and cold advection over the western North Pacific can promote the formation of low-level clouds by increasing the SHF at the subseasonal time scales.