JSPS 5th University Allied Workshop on Climate and Environmental Studies for Global Sustainability Decadal change in the relationship between East Asian-western North Pacific summer monsoons and ENSO in the mid-1990s So-Young Yim and Jong-Ghap Jhun July 1, 2008 School of Earth and Environmental Sciences, Seoul National University

Backgrounds (1) Asian-Pacific summer monsoon Asian-Pacific summer monsoon system consists of three different sub-monsoons: Indian, western North Pacific, and East Asian summer monsoons (ISM, WNPSM, and EASM) (Wang and LinHo 2002)

Backgrounds (2) Previous studies Negative relationship between the EASM and the WNPSM The EASM correlates with ENSO during its decaying phase (e.g. Shen and Lau 1995, Wang et al. 2001). Anomalous anticyclone over the WNP -> Impacts on weather and climate over East Asia (Nitta 1987, Wu and Wang 2000, Lu 2004, Lee et al. 2005)

i.e., an ENSO-related mode and a WNPSM-related mode Backgrounds (3) Previous studies A climate shift in summertime circulation over East Asia : at the mid-1990s (Kwon et al. 2007) The first two leading modes of summer-mean precipitation over the East Asia and WNP region (Kwon et al. 2005); i.e., an ENSO-related mode and a WNPSM-related mode The first leading mode of summer-mean precipitation over the East Asia and WNP region has changed from an ENSO-related mode in 1979-1993 (previous period) to a WNPSM-related mode in 1994-2004 (recent period).

Motivation However, Kwon et al. (2005) left an important question why first two leading modes were changed in their order. Significant change in the SST distribution of the tropical Pacific in the recent decade Ashok et al. (2007) and Weng et al. (2007) - The anomalous warming events different from conventional El Nino events occur in the equatorial central Pacific during recent decade - The influence of such change in El Nino events on the EASM and WNPSM has been less understood.

Objectives To identify the tropical Pacific SST pattern associated with the two leading modes before and after the mid-1990s To examine the relationship between the first two leading modes of summertime precipitation over the East Asia and WNP region and the recent change in the SST distribution in the tropical Pacific

Datasets NCEP/NCAR reanalysis NOAA CMAP –Global precipitation NOAA OLR NCDC SST Analyzed period : 1979-2006 (28 years)

Used indices ENSO indices NINO3 SST index [5S-5N, 150W-90W] NINO4 SST index [5S-5N, 160E-150W] 160E 150W 90W NINO4 NINO3 5S 5N

1st mode in 1979-1993 vs 1994-2006 ENSO-related mode WNPSM-related mode Kwon et al. (2005)

SVD (Precipitation & SST) (1) 1979-1993 1994-2006 Change in the ENSO with the location of largest SST in the tropical Pacific -> decadal change in rainfall variability over the East Asia and the WNP region

Regressed fields (1979-1993 )-SST (2) SVD1 PC (1979-1993) NINO3 NINO4 DJF(-1) 0.54** 0.60* MAM(0) 0.71** 0.82** JJA(0) 0.78** 0.81** SON(0) 0.65** 0.56* DJF(0) 0.49* 0.46 * 95% confidence level ** 99% confidence level 1979-1993 (P1) : Developing/lasting phase of ENSO in the NINO3 region

Regressed fields (1994-2006 )-SST (3) SVD1 PC (1994-2006) NINO3 NINO4 DJF(-1) -0.46 -0.05 MAM(0) -0.24 0.19 JJA(0) 0.51 0.74** SON(0) 0.59* 0.87** DJF(0) 0.62* 0.80** * 95% confidence level ** 99% confidence level 1994-2006 (P2) : Developing phase of ENSO in the NINO4 region These results imply that the spatial distribution of SST in the equatorial Pacific has significantly different impacts on the precipitation variability over the East Asia and WNP region.

Regressed fields-OLR &VVEL (4) Averaged (120E-130E) ; VVEL

Regressed fields-H500 & UV850 (5)

Summary and conclusions The EASM-WNPSM system in the 1979-1993 period is mainly linked to the NINO3 SST warming during the developing ENSO phase, while that in the 1994-2006 period is associated with the NINO4 SST warming during the developing ENSO phase. Impacts of NINO4 SST anomalies on the EASM-WNPSM system are opposite compared to those of NINO3 SST anomalies. When NINO3 (NINO4) SST anomalies are strong in the equatorial Pacific, the PJ-like (wave-like) pattern is dominant. The dynamics for the relationship between EASM-WNPSM and NINO4 SST anomalies remains unclear. Further studies are need for more complete understandings of dynamics involved in this relationship.

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