IPO-BT Mode And Its Effect On Circulation Changes Affecting Tropical Cyclone Activity

In the past 40 years,the global annual mean surface temperature has experienced a nonuniform warming,differing from the spatially uniform warming simulated by the forced responses of large multi-model ensembles to anthropogenic forcing.Rather,it exhibits significant asymmetry between the Arctic and Antarctic,intermittent and spatially varying warming trends along the Northern Hemisphere(NH)mid-latitudes and a slight cooling in the tropical eastern Pacific.Using six reanalyses,we find that this teleconnection can be consistently obtained as a leading circulation mode in the past century.The non-uniform features of surface temperature changes are likely tied together by tropical eastern Pacific sea surface temperature(SST),via this global atmospheric teleconnection.The tropically-driven teleconnection is associated with a Pacific SST pattern resembling the Interdecadal Pacific Oscillation(IPO),and hereafter referred to as the IPO-related Bipolar Teleconnection(IPOBT).Further,proxy records and two paleo-reanalysis reconstruction datasets show that the IPOBT is a robust recurrent mode over the past 2000 years.The IPO-BT mode may thus serve as an important internal mode that regulates high-latitude climate variability on multidecadal time scales,favoring a warming(cooling)episode in the Arctic accompanied by cooling(warming)over Eurasia and the Southern Ocean(SO).Thus,the spatial non-uniformity of recent surface temperature trends may be partially explained by the enhanced appearance of the IPO-BT mode by a transition of the IPO toward a cooling phase in the eastern Pacific in the past decades.The 200-h Pa geopotential height(Z200)anomalous dipole over the Asian mid-latitudes is associated with the IPO-BT mode in boreal summer.Using four reanalysis datasets in the past century,the surface and tropospheric warming in the Asian mid-latitudes is related to a strong upper-level geopotential height rise north of the Tibetan Plateau(TP).The Asian warming decreases meridional gradient of geopotential height,resulting in the weakening of westerly winds over the TP and East Asian summer monsoon.While the South Asia High(SAH)intensity is insensitive to the dipole,the westward retreat of the northwest-to-southeast monsoon trough(MT)associated with the internal variability and the accompanied westward extension of the tropical upper tropospheric trough(TUTT)reduce the tropical cyclone(TC)formation in the eastern part of the western North Pacific(WNP)basin when the tropical eastern Pacific cooling SST extending westward.It is indicated that the observed poleward shift of TC formation was partially attributed to natural variability in the recent decades.Further research is required to quantify contributions of natural variability and anthropogenic influence.Importantly,while the newest set of climate models(CMIP6)appear to successfully simulate the winter global teleconnection mode and its temporal characteristics,the vast majority of models have limitations to reproduce the summer IPO-BT mode likely stemming from models' biases in simulating low-frequency SST variability and vertical convection over the tropical eastern Pacific and climatological mean state over the same region during boreal summer.Moreover,CMIP6 models have limitations in simulating large-scale circulation trend over the WNP in summer.This CMIP6 model's biases should be considered in the prediction of climate change and TC activity.