The inverse relationship between the warm phase of the El Nino Southern Oscillation(ENSO)and the Indian Summer Monsoon Rainfall(ISMR)is well established.Yet,some El Nino events that occur in the early months of the ye...The inverse relationship between the warm phase of the El Nino Southern Oscillation(ENSO)and the Indian Summer Monsoon Rainfall(ISMR)is well established.Yet,some El Nino events that occur in the early months of the year(boreal spring)transform into a neutral phase before the start of summer,whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season.This study investigates the distinct influences of an exhausted spring El Nino(springtime)and emerging summer El Nino(summertime)on the regional variability of ISMR.The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Nino-3.4 region in the Pacific.The ISMR’s dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations,vertically integrated moisture flux convergence(VIMFC),wind shear,and upper atmospheric circulation.The monsoon circulation features are remarkably different in response to the exhausted spring El Nino and emerging summer El Nino phases,which distinctly dictate regional rainfall variability.The dynamic and thermodynamic responses reveal that exhausted spring El Nino events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India.In contrast,emerging summer El Nino events negatively impact the seasonal rainfall over the country,except for a few regions along the west coast and northeast India.展开更多
In this paper we examine the epochal changes in the frequency of cyclones over the North Indian Ocean during the pre-onset and onset phases of the monsoon.We consider three epochs;namely,the early(1955-74),middle(1975...In this paper we examine the epochal changes in the frequency of cyclones over the North Indian Ocean during the pre-onset and onset phases of the monsoon.We consider three epochs;namely,the early(1955-74),middle(1975-94)and recent(1995-2014)epochs.It is found that the number of cyclones in the Bay of Bengal(BOB)decreases throughout the three epochs.Over the Arabian Sea(ARB),however,there is a decrease in the early epoch,before then reaching a minimum in the middle epoch followed by an increase in the recent epoch,thus exhibiting epochal variability.Dynamic and thermodynamic parameters along with Genesis Potential Index(GPI)are examined to understand the frequency variation in cyclogenesis over the ARB and BOB.Over the ARB,thermodynamic factors such as mid-level moisture,surface latent heat flux and sensible heat flux,and dynamic parameters such as lower-level convergence and upper-level divergence,are favorable during the early and recent epochs but unfavorable during the middle epoch,and these results are found to be consistent with the observed epochal variability in the frequency of cyclogenesis.However,all these influential parameters are found to have decreased over the BOB during the entire 60-year period.展开更多
基金funding support from the National Monsoon Mission program of the Ministry of Earth Sciences(MoES),New Delhi。
文摘The inverse relationship between the warm phase of the El Nino Southern Oscillation(ENSO)and the Indian Summer Monsoon Rainfall(ISMR)is well established.Yet,some El Nino events that occur in the early months of the year(boreal spring)transform into a neutral phase before the start of summer,whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season.This study investigates the distinct influences of an exhausted spring El Nino(springtime)and emerging summer El Nino(summertime)on the regional variability of ISMR.The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Nino-3.4 region in the Pacific.The ISMR’s dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations,vertically integrated moisture flux convergence(VIMFC),wind shear,and upper atmospheric circulation.The monsoon circulation features are remarkably different in response to the exhausted spring El Nino and emerging summer El Nino phases,which distinctly dictate regional rainfall variability.The dynamic and thermodynamic responses reveal that exhausted spring El Nino events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India.In contrast,emerging summer El Nino events negatively impact the seasonal rainfall over the country,except for a few regions along the west coast and northeast India.
文摘In this paper we examine the epochal changes in the frequency of cyclones over the North Indian Ocean during the pre-onset and onset phases of the monsoon.We consider three epochs;namely,the early(1955-74),middle(1975-94)and recent(1995-2014)epochs.It is found that the number of cyclones in the Bay of Bengal(BOB)decreases throughout the three epochs.Over the Arabian Sea(ARB),however,there is a decrease in the early epoch,before then reaching a minimum in the middle epoch followed by an increase in the recent epoch,thus exhibiting epochal variability.Dynamic and thermodynamic parameters along with Genesis Potential Index(GPI)are examined to understand the frequency variation in cyclogenesis over the ARB and BOB.Over the ARB,thermodynamic factors such as mid-level moisture,surface latent heat flux and sensible heat flux,and dynamic parameters such as lower-level convergence and upper-level divergence,are favorable during the early and recent epochs but unfavorable during the middle epoch,and these results are found to be consistent with the observed epochal variability in the frequency of cyclogenesis.However,all these influential parameters are found to have decreased over the BOB during the entire 60-year period.
