Uncertainty in projections of the South Asian summer monsoon under global warming by CMIP6 models:Role of tropospheric meridional thermal contrast

Driven by the increase in CO_(2)concentration,climate models reach a consensus that the large-scale circulation of the South Asian summer monsoon(SASM) becomes weakened but with different magnitudes.This study investigates the major uncertainty sources of the SASM response to an abrupt quadrupling of CO_(2)(abrupt-4×CO_(2))in 18 models of phase 6 of the Coupled Model Intercomparison Project.The projected weakening of the SASM indicated by both zonal and meridional monsoon circulation indices is closely linked to decreases in the meridional gradient of upper-tropospheric temperature between Eurasia and the Indian Ocean(EUTT-IUTT).A climate feedback-response analysis method is applied to linearly decompose the uncertainty of changes in EUTT-IUTT into the partial changes due to external forcing and internal processes of the earth-atmosphere column.Results show that the uncertainty of changes in EUTT-IUTT is contributed positively by the dominant atmospheric dynamic process,followed by the cloud shortwave radiative effect,and negatively by the surface latent heat flux and cloud longwave radiative effect.Contributions from CO_(2)forcing and other internal processes including albedo and water vapor feedbacks,oceanic heat storage,and sensible heat flux are found to be minor.

Impact of ENSO events on the interannual variability of Hadley circulation extents in boreal winter

The interannual variability of the boreal winter Hadley circulation extents during the period of 1979e2014 and its links to El Ni^no-Southern Oscillation(ENSO) were investigated by using reanalysis datasets. Results showed that the El Ni^no(La Ni^na) events can induce the shrinking(expansion) of Hadley circulation extent in the Southern Hemisphere. For the Northern Hemisphere, El Ni^no(La Ni^na) mainly leads to shrinking(expansion) of the Hadley circulation extent in the middle and lower troposphere and expansion(shrinking) of the Hadley circulation extent in the upper troposphere. The ENSO associated meridional temperature gradients have close relationship with the Hadley circulation extents in both Hemispheres. But in the Northern Hemisphere, the ENSO associated eddy momentum flux divergence plays more important role in affecting the Hadley circulation extent than the meridional temperature gradient because of the small local Rossby number. In the Southern Hemisphere, as the ENSO induced eddy momentum flux divergence is small, the meridional temperature gradient dominates the change of the Hadley circulation extent.

STUDY ON THE VARIATION IN THE CONFIGURATION OF SUBTROPICAL ANTICYCLONE AND ITS MECHANISM DURING SEASONAL TRANSITION-PART Ⅱ:SEASONAL TRANSITION INDICES OVER THE ASIAN MONSOON REGION

Based on the studies in Part Ⅰ (see Mao et al.2003),this paper further examines the relationship between the Asian summer monsoon onset and variation in meridional position of the warm temperature ridge with a zonal orientation in mid-upper troposphere.During the Asian monsoon bursting consequentially over the Bay of Bengal,South China Sea,and South Asia,in addition to the reversal of winds in the lower and upper troposphere and deep convection before and after the onset,the atmospheric meridional temperature gradient (MTG) in the vicinity of the ridge-surface of subtropical high (WEB defined in Part Ⅰ) exhibits a significant reversal.Since the establishment of temperature structure with higher over north than over south of the WEB in the mid-upper troposphere (200-500 hPa) characterizes the collective essential that the Asian summer monsoon bursts over different areas,the MTG in mid-upper troposphere,based on the thermodynamics associated with the seasonal transition,should be a reasonable index to measure the Asian monsoon onset.The definition for onset date is proposed,and the time series of onset date for different sections are determined.As compared with the onset dates determined by other indices such as 850-hPa zonal wind and OLR.correlation analyses indicate that the 850-hPa zonal wind is only regional index,but the MTG index is applicable universally to the Asian monsoon regime.

STUDY ON THE VARIATION IN THE CONFIGURATION OF SUBTROPICAL ANTICYCLONE AND ITS MECHANISM DURING SEASONAL TRANSITION-PART Ⅲ:THERMODYNAMIC DIAGNOSES

The mechanisms for the variation in the configuration of subtropical anticyclone during seasonal transition are explored from energy budget using the NCEP/NCAR reanalysis data.Based on the seasonal variations of temperature and heating fields,it is found that the significant diabatic heating associated with spring precipitation over southern China has impacts on subsequent Asian seasonal transition.The reversal of meridional temperature gradient in the vicinity of the WEB (westerly-easterly boundary) in the middle and upper troposphere also depends on the latitudinal position where temperature ridge locates.The northward shift of the warm temperature ridge results from the fact that the local temperature increase to the north of the WEB is more than that in its vicinity.The diagnostic results through thermodynamic equation show that physical mechanism responsible for seasonal transition is different from area to area over the Asian monsoon region.The dominant factors responsible for northward shift of the Bay of Bengal warm ridge are the meridional temperature in initial stages of the onset and the descending motion after the onset. The factors for causing the northward jump of the South China Sea warm ridge involve the zonal temperature advection,meridional temperature advection,and diabatic heating associated with the southern China spring rainfall.The subsidence is the factor leading to the northward migration of the South Asia warm ridge.

Climatological Features of the Western Pacific Subtropical High Southward Retreat Process in Late Spring and Early Summer

Based on the climatological daily mean NCEP/NCAR reanalysis data, NOAA outgoing longwave radiation （OLR） data, and pentad NOAA CMAP precipitation from 1979 to 2006, the variation of the western Pacific subtropical high （WPSH） ridge during late spring and early summer （LSES） and its relationship with the onset of the Asian summer monsoon is discussed from a climatological perspective. It is found that a remarkable southward retreat process （SRP） of the WPSH during LSES appears at both lower and higher levels of the troposphere, with a lifespan of approximate two weeks. Afterwards, the first northward jump of the WPSH occurs. The end date of the WPSH SRP in the upper troposphere is about 10 days earlier than the beginning of the WPSH SRP in the lower troposphere, showing a meaningful leading signal for predicting the WPSH SRP in the lower troposphere and the subsequent northward jump of the WPSH. The WPSH SRP at lower levels happens simultaneously with a notable eastward shift of the WPSH. After the WPSH SRP at lower levels comes to the southernmost position around the end of May, the WPSH ridge axis inclines northward rather than southward with altitude due to the change of the meridional gradient of air temperature. The Asian summer monsoon onset and associated variations in strong convection and rainfall in Asia are closely related to the variations of W＇PSH SRP during LSES. In the mid-late period of the higher-level WPSH SRP, around the end of April, the summer monsoon onset takes place in the Andaman Sea and the Bay of Bengal. Following the start of the lower-level WPSH SRP, the South China Sea （SCS） summer monsoon breaks out （May 14-15）. By the end of the lower-level WPSH SRP, in the beginning of June, the Indian summer monsoon kicks off. Upon the end of the lower-level WPSH return stage, the East Asian summer monsoon begins. The commencement of each component of the Asian summer monsoon system corresponds nicely to a particular stage of the WPSH SRP in the lower or higher troposphere. This offers valuable information for monsoon onset prediction in different sectors of Asia. In addition, it is found that there is a typical wet-dry-wet sandwich precipitation pattern, with two rainfall belts in the regions south and north to the WPSH main body, and a dry belt under it. The variation of this rainfall pattern is related to the shift of the WPSH ridge.