Consecutive below-normal precipitation years (multi-year droughts) is a critical issue in California, as made evident by the recent 2012-2015 drought that caused tremendous socio-economic damages. However, studies on the causes of the multi-year droughts are limited. In this study, we investigated the atmospheric and oceanic characteristics of the three multi-year drought events (1999-2002, 2007-2009, and 2012-2015) in California during the last two decades for the winter (December-February, DJF) in order to understand large-scale circulations that are responsible for the initiation, maintenance, and termination of droughts. It was found that abnormally developed upper-tropospheric ridges over the North Pacific are primarily responsible for precipitation deficits and the ensuing droughts. These ridges develop when negative sea surface temperature (SST) anomalies including La Niña events are pervasive in the tropical Pacific. After 3 or 4 years, it was found that the droughts ended under the opposite conditions; upper-tropospheric troughs in the North Pacific with El Niño events in the tropics. Results of Empirical Orthogonal Function (EOF) analysis for the 41-year (1974/75-2014/15) 500 hPa geopotential height in DJF revealed that during the drought periods, the positive phases of the first and second EOF mode (EOF1+ and EOF2+, respectively) were active one by one, positioning upper-tropospheric ridges over the North Pacific. While EOF1+ is associated with cold tropical central Pacific and negative Pacific Decadal Oscillation (PDO), EOF2+ is associated with the tropical east-west SST dipole pattern (i.e., warm western tropical Pacific and cool eastern tropical Pacific

near the southern Peru). Based on these results, we developed a regression model for winter precipitation. While dominant SST factors differ by decades, for the recent two decades (1994/1995-2014/2015), 56% variability of DJF precipitation is explained by the tropical east-west SST dipole pattern and PDO (NINO3.4 signal removed) together. In addition, this model performs well in detecting slowly-varying precipitation variability and multi-year drought patterns. These results suggest that SST variability, not only in the western/eastern tropical Pacific but also in the North Pacific, independently contribute to precipitation variability and long-term droughts in California.

1. INTRODUCTION

2. DATA AND METHODOLOGY

3. RESEARCH RESULTS

3.1 가뭄사례 분석

3.2. 상층지위고도 EOF 분석을 통한 가뭄관련 대규모 순환의 이해

3.2.1. EOF 모드

3.2.2. Correlation Analysis

3.2.3. Composite analysis

3.3 중태평양 동태평양 ENSO와의 연관성

3.4 강수량 회귀분석

4. CONCLUDING REMARKS

REFERENCE