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.2017 May 30;Volume 53(Iss 2):181-194.
doi: 10.1007/s13143-017-0033-4.

Competing influences of greenhouse warming and aerosols on Asian Summer Monsoon circulation and rainfall

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Competing influences of greenhouse warming and aerosols on Asian Summer Monsoon circulation and rainfall

William K M Lau et al. Asia Pac J Atmos Sci..

Abstract

In this paper, we have compared and contrasted competing and amplifying influences on the global and regional drivers, circulation and rainfall responses of the Asian monsoon under global greenhouse warming (GHG) and aerosol forcing, based on CMIP5 historical simulations. Under GHG-only forcing, the land warms much faster than the ocean, magnifying the pre-industrial climatological land-ocean thermal contrast and hemispheric asymmetry,i.e., warmer northern than southern hemisphere. A steady increasing warm-ocean-warmer-land (WOWL) trend has been in effect since the 1950's substantially increasing moisture transport from adjacent oceans, and enhancing rainfall over the Asian monsoon regions. However, under GHG warming, increased atmospheric stability due to strong reduction in mid-tropospheric and near surface relative humidity coupled to an expanding subsidence areas, associated with the Deep Tropical Squeeze (DTS, Lau and Kim, 2015b) strongly suppress monsoon convection and rainfall over subtropical and extratropical land, leading to a weakening of the Asian monsoon meridional circulation. The inclusion of aerosol emissions strongly masks WOWL, by over 60% over the northern hemisphere, negating to a large extent the rainfall increase due to GHG warming, and leading to a further weakening of the monsoon circulation, through increasing atmospheric stability, most likely associated with aerosol solar dimming and semi-direct effects. Overall, we find that GHG exerts stronger positive rainfall sensitivity, but less negative circulation sensitivity in SASM compared to EASM. In contrast, aerosols exert stronger negative impacts on rainfall, but less negative impacts on circulation in EASM compared to SASM.

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Figures

Figure 1
Figure 1
Spatial distribution of June-July-August MMM surface temperature anomalies (°C) for GHG (a), and ALL (b), and time series of warm-ocean-warmer-land (WOWL) index (see text for definition) for ALL, GHG, Nat, and inferred anthropogenic aerosol effects, IAA = ALL-(GHG+Nat), and pre-industrial (PI) control respectively (c). Grid points where more than 75% (15 out of 19), and 65% (13 out of 19), of the percentage of ensemble members having the same sign as the MMM anomalies are indicted by black, and open circles, respectively.
Figure 2
Figure 2
June-July-August latitude-height profiles of zonally averaged climatological mean (contour) and anomalous (color) relative humidity (RH) in percentage for a) GHG, and b) ALL. Grid points where more than 75% (65%) of ensemble members have the same sign as the MMM anomalies are indicated by black (open) circles.
Figure 3
Figure 3
Left panels showing spatial distributions of RH anomalies at a) 500 hPa, b) 850 hPa, c) SST anomaly (°C), and d) negative anomalous vertical p-velocity (10−2hPa s−1) for GHG, with contour showing climatology. Right panels, e), f), g) and h) are the same as corresponding left panels, except for ALL. Green dots in a), b), e) and f) denote regions of anomalous descent.
Figure 4
Figure 4
Spatial distribution of anomalous moisture transport (ms−1 gKg−1) and 25-year (1981–2005) mean distribution of total precipitable water (g Kg−1) for a) GHG and b) ALL. Key cross-sections for transport of moisture to SASM and EASM are shown in a).
Figure 5
Figure 5
MMM Moist Static Energy (MSE) anomalies for the SASM for a) GHG and b) IAA, and same for c) and d), except for EASM. Units in kJ/Kg. Grid points with black (open) circles indicate where more than 75% (65%) of the ensemble members having the same sign as the MMM anomaly.
Figure 6
Figure 6
Left panels show spatial distribution and magnitude of June-July-August vertical velocity (hPa s−1) associated with the monsoon meridional circulation of the SASM, for a) PI climatology, b) GHG induced anomalies, c) IAA induced anomalies, and d) and rainfall anomalies due to GHG (red) and due to IAA (blue), with error bars indicating model spread. Right panels e), f), g) and h) are the same as corresponding left panels, except for the EASM.
Figure 7
Figure 7
Spatial distribution of June-July-August MMM rainfall anomalies (mm) for a) GHG, b) ALL, c) IAA, and d) trends (mm decade−1) during 1961–2007 from APHRODITE rainfall observations. In a) and b), grid points in which more than 75% of ensemble members having the same sign as the MMM anomalies are indicated by green solid (open) circles.
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