a global perspective of southern asian monsoon...

Cooperative Institute for Climate and SatellitesUniversity of Maryland5825 University Research Court, (Suite 4001)College Park, MD 20740-3823Tel: (301) 405-2147 Fax: (301) 405-8648http://www.essic.umd.edu/cics

Institute Director: Dr. Phillip [email protected] Director: Andrew [email protected]

Monterey, Oct 26-30, 2009 NOAA 34th Climate Diagnostic and Prediction Workshop

A GLOBAL PERSPECTIVE OF SOUTHERN ASIAN MONSOON ANNUAL PRECIPITATION VARIATIONS IN GPCP (1979-2006)

(The co-variability of annual monsoon precipitation and global surface temperature with a focus on

the southern Asian monsoon region)

Investigators: A. Sudradjat, A. Gruber, J. Janowiak, and R. R. Ferraro



• Background• Data & methods• Results• Summary

Outline

This study is a co-variability study (not a trend-focused study)



Why the study?• IPCC AR4 (2007): Global surface

temperature decreasing 1940-1980 and increasing 1980-recent

• The Asian monsoon system is a large scale, coupled land-ocean-atmosphere system driven by the surface temperature gradientbetween the Indian and Pacific Oceans and Eurasian continent

SST

Land-surface air

Warming: Land >> SST



Why the study?• Q: How does recent (1979-recent) global land and ocean warming affect

monsoon P spatio-temporally? • Comprehensive observational global datasets available: short (1979-recent)

for interdecadal analyses [Smith et al. 2006], but useful for covariabilityanalyses

Recent observational monsoon & global P interdecadal studies:• Wang and Ding (2006): global oceanic monsoon P in GPCP increasing

1979-2003. Global land monsoon P decreasing 1948-1980 and leveling off 1980-2003 (in line with Chase et al. 2003).

• Smith et al. (2006): global P in GPCP (1979-2004) vary spatially over land and ocean with a near-zero global average. Increase convective P over tropical oceans due to SST >29°C.

• Adler et al. (2008): linear change occurs predominantly in tropical P• Zhou et al. (2008): global land P variations arise from ocean forcing (SST)• The first three studies warrant the sensitivity of trend analysis to the record

length



Data & MethodsData: 1979-2006• GPCP Version 2.1 [Huffman et al.

2009]• Global merged surface

temperature [Smith et al. 2008]• ERSST v. 3 [Smith et al. 2008]• Global land daily max & min

temperature from GTS station [Xieet al. 1996]

• NCEP/NCAR reanalysis winds [class “A”; Kalnay et al. 1996]

Methods:EOF, correlation, low frequency variability

EOF-1 (long-term mean removed) (EV=32.37%)



• PC-1 v. SOI [U. East Anglia/CRU]: R=0.90

• Red region: lower P in El Niño • Most variations are in Asian-

Australian & SPCZ• Anti-correlation: the south Asian

monsoon v. El Niño

ENSO signals

EOF-1 annual P (EV=26.79%)



• PC-2 v. TNI [Trenberth and Stepaniak 2001]: R=0.75

• Mostly response to strong El Niño events of 82/83 & 97/98

• Combining with EOF-1: in collocated red region, even dryer P during the strong El Niño

• Most variations are in Asian-Australian & SPCZ

ENSO signals


Niño-4

Niño-1+2



• Positive change dominates and are in Asian-Australian & SPCZ

• Smith et al. [2006] & Zhou et al. [2008]: monsoon P variations arise from ocean forcing (i.e. SST)

• High R between monsoon P v. SST only? High R are both in land and ocean

• SPCZ: increase convective P due to SST >29°C [Smith et al. 2006]

• Similar results with ERSST and GTS (independent datasets)

• Southern Asian monsoon region is sandwiched by correlation regions of the 30°N Asian land and equatorial Indian Ocean.

P v. SST-Tsfc land




P v. SST-Tsfc land

R: P v. Gradient 0.60 & P v. PC-3 0.63

Increasing concurrently

getting higher

Increasing: (Spearman-Conley test) • U850 (α=10%)• V850 (α=5%)• The Webster-Yang monsoon index [u*850-u*200] [Webster and Yang, 1992] (α=10%)

• P in the Bay of Bengal (α=5%)



• Monsoon P in GPCP co-varies with land and sea Tsfc• Most P variations can be associated with ENSO• Higher P is expected with increasing SST in tropical convective oceanic

monsoon regions• Where moisture import is the main source of monsoon P, such as the

southern Asian monsoon region, variations in the land-sea Tsfc gradient can affect the monsoon circulation by changing the low level wind fields and hence moisture transport and ultimately P in the region.

Modeling efforts focusing on dynamical physical connections between the variations of surface temperature gradient and monsoon precipitation should be done especially under the global warming scenario. These should incorporate the dynamic changes of the gradient from observations as land warms faster than ocean.

Summary



Wang [2006]:• Related to the large scale vertical shear in the zonal winds [u*850-u*200] of

the Asian monsoon system• Measures the broad-scale monsoon circulation • Depicts the thermally driven nature of the monsoon and thus recognizes the

importance of horizontal temperature gradient• Higher Webster-Yang index corresponds to increase P in the Northeastern

India, Bay of Bengal, Southeast Asia, South China Sea, and western Pacific

The W-Y index



The warm pool

SST

P

P limit?

140°E-180° & 6°S-6°N



• Seasonal cycle temperature amplitude: land > oceans• Specific heat capacity: ocean >>> land.• Mixing in the ocean: wind and buoyancy-generated turbulence.• Land surface conducts heat slowly.• Air over land warms faster and reaches a higher temperature than air over

the ocean → creating low pressure → low level wind bring moisture from ocean.

• Rainfall: moist ocean air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface.

• In winter, the land cools off quickly, but the ocean retains heat longer

Monsoon process

a global perspective of southern asian monsoon...

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