Decadal Changes Of Hydrological Cycle Factors Associated With PDO And NPGO

This study investigates the features of large-scale circulation,storm track,multi-scale water vapor transport and atmospheric river(AR)associated with the Pacific Decadal Oscillation(PDO)and North Pacific Gyre Oscillation(NPGO).The datasets used in this study include NCEP/NCAR reanalysis dataset,HadISST dataset,and Global Precipitation Climatology Centre monthly precipitation.Based on the moisture budget equation,the divergence and evaporation anomalies and their effects on anomalous precipitation are also analyzedDuring the positive phase of PDO,the geopotential height anomaly at 500-hPa exhibits a Pacific-North America(PNA)-like pattern.The subtropical westerly jet is strengthened and extends downstream.During the positive phase of NPGO,the geopotential height anomaly at 500-hPa is characterized by a meridional dipole over the North Pacific region.The subtropical jet moves poleward.The anomalous patterns of baroclinicity and storm track are consistent with those of zonal wind anomalies at 250-hPa for both PDO and NPGO modesDuring the positive phase of PDO,the zonally ocean-to-land moisture transport is enhanced along 30°N and reduced over the northeast Pacific for the time-mean integrated vapor transport(IVT).The synoptic poleward transport is suppressed/enhanced north/south of 40°N.During the positive phase of NPGO,the zonal moisture transport is intensified south of 20°N and along 45°-50°N for the time-mean IVT,and weakened over the west coast of North America for the low-frequency IVT.The synoptic poleward transport is suppressed south of 30°N.The low-frequency poleward transport is reduced during the PDO/NPGO positive phase The eastern part of the North Pacific AR band moves slightly southward during the PDO positive phase,while the whole AR band shifts northward during the NPGO positive phase.During the positive phase of PDO,anomalously wet areas are seen in the east part of Western Siberia,the coast Gulf of Alaska and southwest part of North America,while anomalously dry areas are located in the east part of China and the west coast of North America within 40° to 50°N.During the positive phase of NPGO,anomalously wet areas are seen in the islands around the South China Sea,the North China Plain and the west coast of North America north of 45°N,while anomalously dry areas are located in Northeast Asia,southern rim of Tibetan Plateau to South China,and the west coast of North America south of 45°N.Based on the moisture budget equation,the precipitation anomalies are mainly influenced by the anomalous divergence of IVT and evaporation from the surface.During the positive phase of PDO,both the convergence of IVT and the increase of evaporation contribute to the increased precipitation over the east part of Western Siberia and southwest part of North America.While the divergence of IVT and the decrease of evaporation lead to the decrease of precipitation over the east part of China.While over the west coast of North America within 40°to 50°N,the divergence of IVT is more important During the positive phase of NPGO,the anomalously divergent IVT and decreased evaporation are both contribute to the decrease of precipitation over the Northeast Asia and the west coast of North America south of 45°N.While over the southern rim of Tibetan Plateau to South China,the divergence of IVT is more important.In the Alaskan region,the precipitation anomaly depends on either divergence anomaly or evaporation anomaly for both the PDO and NPGO mode.