Paradox in South Asian summer monsoon circulation change: Lower tropospheric strengthening and upper tropospheric weakening

Jian Ma,Jin‐Yi Yu

doi:10.1002/2014gl059891

Abstract

AbstractIn the literature, there exist contradictory conclusions on the South Asian summer monsoon (SASM) precipitation and circulation changes: whether the circulation change contributes positively by strengthening or negatively by weakening to the rainfall enhancement, on a background of moisture content increase. Based on Representative Concentration Pathway 4.5 simulations by 18 Coupled Model Intercomparison Project phase 5 models, this study explains these puzzles by illustrating that the SASM circulation changes oppositely between the lower and upper troposphere, with tipping point at 450 hPa. However, this indicates a new paradox, created by competing mechanisms. By analyzing the intermodel variability, we determine that the mean advection of stratification change mechanism weakens the upper tropospheric circulation, while the enhanced surface land‐sea thermal contrast strengthens the lower level and surface winds. Our moisture budget analysis shows that the SASM precipitation enhancement (8% K−1) attributes to moisture increase (5% K−1) and lower tropospheric circulation strengthening (3% K−1).

Highlights

Constituting the most spectacular manifestation of the global climate system resulting from land-sea thermal contrast and orographic features, the South Asian summer monsoon (SASM) provides a major percentage of annual precipitation over the world’s most densely populated regions, e.g., India, and has tremendous impacts on agriculture, health, water resources, economies, and ecosystems throughout South Asia [Webster et al, 1998]
Our moisture budget analysis shows that the SASM precipitation enhancement (8% KÀ1) attributes to moisture increase (5% KÀ1) and lower tropospheric circulation strengthening (3% KÀ1)
While consistent evidences are found for the increase of the equilibrium SASM precipitation [Douville et al, 2000; Ashrit et al, 2005], contradictions stand among former studies on the monsoon circulation change in relation to the moisture and rainfall increase

Summary

Introduction

Constituting the most spectacular manifestation of the global climate system resulting from land-sea thermal contrast and orographic features, the South Asian summer monsoon (SASM) provides a major percentage (up to 80%) of annual precipitation over the world’s most densely populated regions, e.g., India, and has tremendous impacts on agriculture, health, water resources, economies, and ecosystems throughout South Asia [Webster et al, 1998]. Despite a larger continental-scale land-sea thermal contrast in summer, the intensity of the monsoon circulation is predicted to reduce by 14% per century [Tanaka et al, 2005]. This weakening can be explained by the thermodynamic constraint discussed in Held and Soden [2006] or by the mean advection of stratification change (MASC) mechanism raised in Ma et al [2012]. MASC is presumably an invariant independent of model or scenario but highly sensitive to the extent of the sea surface temperature SST warming. The vertical profile of SASM circulation change is illustrated, and a rainfall change budget is calculated, attributing to moisture content increase and lower/upper level circulation change according to the profile

Data and Method

Results

Conclusions