The impact of convective activity in the western tropical Pacific on rainfall over East Asia is investigated to develop a statistical and empirical method using the inter-relationship between western Pacific convection and rainfall variance in East Asia. We examine the prediction skill of the reconstructed summer rainfall over the East Asian region by applying the developed method to the individual models and multi-model ensemble (MME). The model data used in this diagnostic study consists of seasonal retrospective forecasts for the 23-year period (1983 – 2005) from the seven operational coupled forecast models of the Asia-Pacific Economic Cooperation (APEC) Climate Center (APCC). National Centers for Environmental Prediction (NCEP)-Department of Energy (DOE) Reanalysis 2, Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and NOAA Optimum Interpolation (OI) Sea Surface Temperature (SST) V2 are used for the same period as observational data. The target season is summer (June-August).

The East Asian summer monsoon region is subject to complex variability and spatial and temporal structures, and is not only influenced by climate variations originating in the tropics, but also by those from mid-latitudes. In this study, for the boreal summer, lower-tropospheric circulation anomalies over the western tropical Pacific affect moisture transport into East Asia and thus have an important role in influencing precipitation in East Asia. Given this relationship, we try to overcome the limitations of dynamical model prediction for summer precipitation in East Asia.

We examine the characteristics of convection in the western Pacific affecting the East Asian monsoon rainfall. We found two types of distinct structure patterns in the western tropical Pacific. It is noteworthy that the first and second patterns are related to El Niño – Southern Oscillation (ENSO) developing and decaying mode, respectively. Normalized Maritime continent-western tropical Pacific Indices (MPIs), as the major influences on the East Asian summer rainfall, are developed using two distinct western Pacific convection related patterns. We investigate the inter-relationship, in terms of teleconnection pattern for precipitation, atmospheric low-level wind, 500 hPa geopotential height, and sea level pressure to assess the possible use of the developed MPIs. As a result, we find teleconnection patterns with a meridional tripole structure for each variable, confirming the suitability of the MPIs in representing the East Asian summer monsoon variability. These patterns remind us of the well-known Pacific-Japan (PJ) and East Asia/Pacific (EAP) teleconnection patterns. Based on these results, it turns out that MPIs are a significantly useful predictor for precipitation prediction in the East Asia region. We also find that the regressed precipitation on MPIs and the interannual variability of MPIs have a statistically significant and strong correlation with the pattern and variability of observed precipitation over East Asia.

In this study, we select the principal component time series of the first EOF mode for precipitation over East Asia as a predictand and two newly developed MP indices as predictors and develop the East Asia Rainfall Index (EARI), which indicates the interannual variability of the East Asian monsoon rainfall using a multiple linear regression method. It can be seen that the regressed rainfall pattern from EARI has better performance than those from the East Asia summer monsoon index (EASMI) or each MP index. This result demonstrates the necessity of two new MP indices representing western Pacific convection to improve the prediction skill of East Asian precipitation. From a practical perspective as far as application to real-time forecasts, we use the leave-one-out cross-validation for each target year for the study period from 1981-2003 and obtain multiple linear regression coefficients for 23 years by calculating the observed and cross-validated EARI (EARIcv). The normalized predicted MP indices obtained from the individual climate model and MME prediction are applied to multiple regression to produce the predicted and cross-validated EARI (EARIcvm). The developed EARIcvm of the individual climate model and MME is applied statistically to the observed and regressed precipitation pattern to reconstruct a novel empirical predicted precipitation over the East Asia region. We evaluate the reconstructed East Asia summer precipitation of the individual models and MME, and find that the observed characteristics over East Asia are captured well. In general, the predictions of the individual models and MME using the statistical and empirical method show better performance in comparison to those of the dynamical models and MME. In addition, it can be discerned that the prediction skills of the MME are generally superior to those of individual models.

In the present study, the relatively small size of the available hindcast datasets poses a sampling limitation. In addition, we only considered climate drivers from the western tropical Pacific convection for the summer rainfall variability in East Asia, which is influenced by various climate drivers, originating not only in the tropics but also from the mid- and high-latitudes. Nonetheless, it is noteworthy that the prediction skill of the reconstructed precipitation, through the empirical method producing the predictors in a statistically significant region, has been enhanced. Based on this, the developed approach method can be applied to the current operational forecast system for further improvement of forecast information.

1. 서론

2. 연구 자료 및 방법

2.1 사용 자료

2.2 통계적 방법

3. 관측에서의 서태평양 대류 활동과 동아시아 몬순 강우와의 관련성

3.1 서태평양 대류 활동의 특성 분석

3.2 EASM 지역에 대한 서태평양 대류 활동의 영향

3.3 서태평양 대류 활동에 대한 관측과 모델의 비교 검증

4. 동아시아 몬순 강우 지수 개발과 재생산 강수의 평가

4.1 MP 지수를 이용한 새로운 동아시아 강우 지수

4.2 동아시아 강우 지수를 이용한 재생산 강수의 생산 및 평가

5. 요약 및 결론