Zeng-Zhen Hu (1)Renguang Wu (1)

James Kinter III (1)Song Yang (2)

(1) Center for Ocean-Land-Atmosphere Studies4041 Powder Mill Road, Suite 302

Calverton, MD 20705E-amil: hu@cola.iges.org

(2) Climate Prediction CenterNCEP/NWS/NOAA

Camp Springs, MD 20746

Connection of summer rainfall variations in South and East Asia: Dependences on ENSO and time

scales

Some of previous works:Tao and Chen 1987Huang and Tang 1987Ding 1994Wang et al. 2001.Guo and Wang 1988Kripalani and Singh 199Hu and Nitta 1996Kripalani and Kulkarni 1997, 2001Krishnan and Sugi 2001

It is evident for the connection between the summer rainfall variations

in East and South Asian

The connection is accomplished through a wave-type anomalous circulation pattern over mid-latitude Asia

between 30 and 50oN

Some of previous works:

Krishnan and Sugi 2001

Wu 2002

Wu and Wang 2002

Questions and Objectives

Q1: What is the influence of ENSO on this connection ? Both East and South Asian rainfall variations are associated with ENSO (Webster and Yang 1992; Kripalani and Kulkarni 1997; Weng et al. 1999; Wang et al. 2000; Lau and Wu 2001; Kinter et al. 2002; Wu et al. 2003; Chou et al. 2003).

Q2: What are the differences of the connection pattern on interannual and interdecadal time scales ? Hu and Nitta (1996) showed that the significant positive correlations between summer rainfall over north China and India are concentrated in two time-scale bands: shorter than 7 years and longer than14 years.

Data

• Precipitation in East Asia: 1951-2000, JJAS, 206 stations (160 stations in mainland China, 33 stations in Japan, 9 stations in South Korea, and 4 stations in Taiwan of China)

• All-Indian summer monsoon rainfall index (ISMR):

An areal average of the rainfall in 29 subdivisions in India (Parthasarathy et al. 1995). The subdivisional rainfall data were derived from areally averaged district rainfall that was computed by averaging all stations in the district.

• NCEP/NCAR reanalysis data (2.5X2.5): v 200 hPa, h200, h500

• Reconstructed SST (2X2): (Reynolds and Smith 1994)

• NINO3.4 index: from NCEP

• ALL DATA are JJAS mean, span 1951-2000

Methodologies

• ENSO influence: By using a linear regression of the total

ISMR with respect to the summer NINO3.4, IMSR is separated into two parts: ENSO-related and ENSO-unrelated.

• Time scale separation: After removing the 50-year trend by

linear regression, a high-pass filter is applied, ISMR is separated into two parts: interannual (<10 years) and interdecadal (> 10 years, including the trend) variations. The high-pass filter is based on a fast Fourier transform (Press et al. 1992).

All Indian summer monsoon rainfall index (ISMR)

Most (73%) of the ISMR variance is not explained by ENSO;Only 27% of the ISMR variance linearly relates to ENSO;

9.6%, 1.3%, and 8.3% of ISMR variance is accounted for by the interdecadal components of the total, ENSO-related, and ENSO-unrelated parts of ISMR

Correlations of SST and ISMR (<10 years)ENSO-related part is closely related with SST in the Indian & Pacific Oceans;

ENSO-unrelated is almost without significance correlation with global SST

Correlations of SST and ISMR (>10 years)ENSO-related part is closely related with SST in the Indian/Pacific Oceans;

which is similar to the interdecadal mode in the Pacific Ocean

Corr of ISMR & East Asian rainfall (<10 years)Positive (negative) correlations in northern China (southern Japan) result from ENSO;Negative correlations in southwestern China are associated with ENSO-unrelated part;The similarity of the correlation patterns in Figs. b and c indicates that the

occurrence of ENSO generally reinforces the connection

Corr of ISMR & East Asian rainfall (>10 years)Positive correlations in northern China mainly result from ENSO-unrelated part;The location of the significant positive correlation in northern China is

shifted eastward from interannual time scales to interdecadal time scales

Regression (contour), correlation (shading) and wave activity fluxes (arrows) of v200 (<10 years)

The ENSO-related part affects East Asian summer rainfall variations through two meridional propagation teleconnection patterns: one over the East Asian continent and the other over the western Pacific Ocean;The ENSO-unrelated part connects with East Asian rainfall variations

by a zonal propagation pattern over mid-latitudes in the Asian continent

How to calculate the wave activity flux ?

• Divergence (convergence) of wave activity fluxes indicates a source (sink) of stationary wave activity. Therefore, you can trace the origins and regions of influence of stationary waves.

• Details in Plumb (1985), Karoly et al. (1989), and Yang and Gutowski (1994), Wu et al. (2003).

What is the mean of the wave activity flux ?

The equation:

• FS =(FFλ,Fφ)

=σcosφ{v*2- ə (v*Φ*)/əλ/(2Ωasin(2φ),

- {v*u*- ə(u*Φ*)/əλ/(2Ωasin(2φ)}

• Where σ=pressure/1000 hPa, (λ,φ) are longitude and latitude, (u, v) are the time mean of horizontal geostrophic velocities from the geopotential height Φ, and Ω and a are the earth’s rotation rate and radius, respectively. The asterisk indicates the departure from the zonal mean.

JJAS V200 composite based on ISMR, NINO3.4, and ISMR without ENSO

Regression (contour), correlation (shading) and wave activity fluxes (arrows) of v200 (>10 years)

The wave patterns are not as clear as those on interannual time scales;The anomalies in high latitudes seems involved

Main Results

• Most (73%) of the Indian summer monsoon variance is not explained by ENSO; Only 27% of the monsoon variance linearly relates to ENSO.

• The ENSO-related part is significantly positively (negatively) correlated with the rainfall variations in northern China (southern Japan). The connection is completed through two meridionally propagating teleconnection patterns: one over the East Asian continent and the other over the western Pacific Ocean.

• The ENSO-unrelated part has significantly negative correlations with the rainfall variations in southwestern China. The connect is associated with a zonal propagation pattern over mid-latitudes in the Asian continent, which is forced by the ENSO-unrelated part of the South Asian summer monsoon.

• The occurrence of ENSO generally reinforces the connection between the South Asian summer monsoon and East Asian summer rainfall variations.

• In general, the connection is less evident on interdecadal time scales than on interannual time scales. The location of the significant positive correlation in northern China is shifted eastward from interannual time scales to interdecadal time scales.

Discussion• It should be noted that the linear regression of ISMR with

respect to the NINO3.4 index does not necessarily capture the total influence of ENSO on the South Asian summer monsoon for two reasons. First, the influence of ENSO on the South Asian summer monsoon may be nonlinear. Second, the spatial and subseasonal variations of the South Asian summer monsoon are not reflected in ISMR.

• Robustness and significance of the connection on interdecadal time scales needs to be further investigated, since the data record is too short and the reanalysis data are questionable for interdecadal analysis (Yang et al. 2002; Wu et al. 2004). For example, it is not clear what is the impact of human activity (Qian and Giorgi 1999; Hu et al. 2000; Menon et al. 2002; Hu et al. 2003) or what is the origin of the interdecadal shift in the middle-1970s (Yatagai and Yasunari 1994; Hu 1997; Chang et al. 2000; Wu 2002) in the summer rainfall in East Asia on the connection between the South Asian summer monsoon and East Asian summer rainfall variations.

• The influence of the changing relationship between the South Asian monsoon and ENSO (Kumar et al. 1999; Krishnamurty and Goswami, 2000; Chang et al. 2001; Kinter et al. 2002) on the connection is also an interesting future research topic.

Further Information and Acknowledgements

Further Information Published as an internal report:

(www.iges.org/pub/ctr/ctr_163.pdf )Submitted: International J. Climatol.E-mail: hu@cola.iges.org

Acknowledgements :We are grateful for the helpful advice of D. Straus. The authors thank Jinhong Zhu, Dao-Yi Gong, and Mong-Ming Lu for providing some of the station data. Some rainfall data were obtained from http://www.ncdc.noaa.gov/.

This work was supported by grants from the National Science Foundation (ATM-0122859), and the U. S. Department of Energy (De-FG02-01ER63256).

Correlations of SST and ISMR (all time scales)ENSO-related part is closely related with SST in Indian & Pacific Oceans;ENSO-unrelated is almost without significance correlation with global SST

Corr of ISMR & East Asian rainfall (all scales)Positive (negative) correlations in northern China (southern Japan) result from ENSO;Negative correlations in southwestern China are associated with ENSO-unrelated part;

The similarity of the correlation patterns in Figs. b & c indicates that the occurrence of ENSO generally reinforces the connection

Regression (contour), correlation (shading) and wave activity fluxes (arrows) of v200 (all time scales)

The ENSO-related part affects East Asian summer rainfall variations through two meridional propagation teleconnection patterns: one over the East Asian continent and the other over the western Pacific Ocean;The ENSO-unrelated part connects with East Asian rainfall variations

by a zonal propagation pattern over mid-latitudes in the Asian continent