Interannual Variability Of Sea Fog Frequency In The Northwestern Pacific In July-with A Projection Under Global Warming

The Interannual variabilities in the sea fog frequency (SFF) in July in themidlatitude Northwestern Pacific (40oN-50oN,140oE-170oW) from1979to2009isinvestigated with observations and reanalysis datasets. The SFF-proxy from thereanalysis data is suggested as compensation for the lack of observations in the broadNorthwestern Pacific. Composite analysis shows that in high-SSF years (theSFF-proxy is much higher than normal), the center of the Northwestern Pacificsubtropical high (SH) shifts eastward and a strengthened ridge exists in themidlatitude Northwestern Pacific. Under such conditions, large amount of moisturesfrom the subtropics are transported northward by the southerlies over the west flankof the SH and the ridge is helpful for stable stratification, conductive to fog formationin the midlatitude Northwestern Pacific. In contrast, in low-SFF years (the SFF-proxyis much lower than normal), the center of the SH expands westward and drifts furthersouth; thus moistures can hardly reach to the midlatitudes. Meanwhile, an anomaloustrough in the midlatitudes and the associated anomalous northerlies both weaken thesoutherlies and reduce the stability, which are unfavorable for fog occurrence. Thecase studies confirm that the air parcels, moving from the subtropical zone to themidlatitudes controlled by the SH, keep higher temperature and humidity when theyflow across the Kuroshio Extension, and then cool over the cold oceanic surface infog case. The air parcels in non-fog case do not have these features. The SFF in theNorthwestern Pacific would decline under the conditions of global warming due to thevariations of the SH in CMIP5(Coupled Model Intercomparison Project Phase5)RCP (Representative Concentration Pathway)4.5and8.5data.