Spatiotemporal Variation In Diurnal Temperature Range On The Eastern Tibetan Plateau And Its Linkage To ENSO

The Tibetan Plateau(TP),as one of the most sensitive regions to global change,can affect the atmospheric circulation and climate in East Asia,and changes in atmosphere and hydrological cycle here will trigger a series of environmental changes.The reduction in diurnal temperature range(DTR)due to a greater increase in the minimum temperature(T_min)than that in the maximum temperature(T_max)has been widely observed since the 1950s in many regions of the world,including the TP.The variation in DTR includes the information both in temperature and hydrological variables,its reduction under the recent global warming has also witnessed the warming and moistening on the TP.However,the length of instrumental records limits the understanding of DTR variation on a longer timescale.To date,it is still unknown whether the decreasing trend in DTR is a phenomenon unique to the recent global warming,and the internal forcing mechanisms of its regional difference remain largely uncertain.In this study,we focus on the eastern TP and aim to identify the spatiotemporal variation in DTR using instrumental observations,and compare it with the changes during the past two and a half centuries by extending DTR records from tree rings.To understand the internal forcing mechanisms of DTR variation,we also investigate the linkage between DTR and ENSO using their observed and reconstructed series.According to the seasonal variation in precipitation on the eastern TP,we first focus on the spatiotemporal characteristics in DTR during the rainy season(May-September).Rotated empirical orthogonal function(REOF)reveals the distinct regional modes on the northeastern and southeastern TP(NETP and SETP).Correlation coefficients between DTR and other climatic variables indicate that DTR is mainly affected by T_min on the NETP,but T_max on the SETP.This regional discrepancy is mainly caused by precipitation,which also has distinct regional modes.Comparison between the composited anomalies and trends in T_max,T_min,DTR during wet days and dry days indicates that precipitation can moderate the trend of warming,with a stronger effect on the drier NETP.The regional difference in precipitation between the NETP and SETP also alters its effect on DTR.On the NETP,where the total precipitation amount is less and evapotranspiration is limited,precipitation mainly affects T_min at nighttime through radiative forcing;while on the wetter SETP,precipitation mainly affects DTR by increasing soil moisture and the associated evaporative cooling on T_max at daytime.Tree-ring chronologies were significantly and negatively correlated with May-June DTR during the early rainy season.Therefore,we present two regional records of May-June DTR since 1753 reconstructed from tree rings on the NETP and SETP,respectively.Spatially,both the instrumental and reconstructed DTR show different modes on the NETP and SETP in the results of regime shift detection and moving linear trends.The instrumental DTR changes on these two regions are not in step with each other.The DTR reconstructions reveal fluctuations during the past two and a half centuries but with different high and low periods,and their regime shifts did not happen at the same time.The decreasing trends and mean levels of the observed DTR also occurred in the time earlier than the 1950s in reconstructed series,indicating that the reduction in DTR under the recent global warming is a notable phenomenon because of the asymmetric increase in T_min and T_max,but not unique compared with its past change.El Ni(?)o-Southern Oscillation(ENSO)is one of the leading climate variability modes,results here reveal the significant linkage between the DTR on the SETP and ENSO.Composite analysis of sea surface temperature anomalies in the tropical Pacific for extreme DTR years on the SETP show typical ENSO patterns,and May-June DTR on the SETP is significantly and positively correlated with Ni(?)o3.4index from the previous summer to the current spring.Results of Superposed Epoch Analysis(SEA)indicate that,on the SETP,higher and lower DTR values in May-June usually follow the El Ni(?)o and La Ni(?)a events in the previous year,respectively.Composite analysis of large-scale atmospheric circulation patterns suggests that the responses of the DTR on the SETP to ENSO events are sustained by the teleconnection between ENSO and Indian summer monsoon(ISM).ENSO can generate the anomalous intensity of early ISM during May-June,with the changes in atmospheric circulation related to rainy weather on the SETP,ENSO will affect the DTR on this region.By comparison with the reconstructed winter Ni(?)o 3.4 index,the positive correlation between the DTR on the SETP and Ni(?)o 3.4 index over the past two and a half centuries is accompanied by a stable response of the decreased DTR to La Ni(?)a events,suggesting that ENSO is a crucial driver for the DTR variation on the SETP through oceanic-atmospheric modulation.