The origin of low frequency modulation of ENSO amplitude in a CGCM

The dominant El Nino and Southern Oscillation period increased from 2-3 years during 1960-1975 to 4-6 years after the late 190s. The amplitude of ENSO also tends to increase. However, the causes of ENSO amplitude modulation on decadal timescales are currently unknown. Recently, Moon et al (2004) suggested that changes in ENSO statistics is due to redistribution of baroclinic energy by change of vertical stratification in the equatorial Pacific, but they did not provide why the vertical stratification changes on decadal timescales. In this study we use the results from simulation to investigate the change in the equatorial subsurface structure and its impact on the ENSO decadal modulation.2

GAIA 과제: 통합 기후예측 시스템을 위한 기후예측 시뮬레이터 개발 및 대양 관측

본 연구는 교육과학기술부의 지원을 받아 3단계 10년간 수행되는 사업으로 장주기 기후변화를 예측·파악 할 수 있는 전 지구 고해상도 기후 예측 시뮬레이터 개발과 함께 장기 기후자료를 생산하고 IPCC 6차 보고서에 독자적인 기후변화 예측 결과의 제출을 목표로 하고 있다. 이를 위하여 기후모델 요소모형들의 결합 및 실험, 모델 활용을 위한 해양 및 대기 경계층의 물리과정(적운대류, 해양상층 수직혼합, 수온약층 수직혼합) 모수화 방안 제시,열대 서태평양 해양 관측 및 활용, 해양-대기 상호작용 및 기후변화 관련 해양과정 분석, 중ㆍ장기 (10-50년) 기후 예측 수행을 위한 해양자료동화체계 개발, 기후 변화 메커니즘 규명 및 기후 변동성 원인 분석 등과 관련된 연구를 수행하고 있다. 이번 발표회에서는 1단계의 연구 내용 중 일부를 취합 발표하고자 한다.2

Vertical structure variability in the tropical Pacific and its relation to ENSO modulation in a CGCM

We study the relationship between changes in the equatorial stratification and low frequency ENSO modulation in a coupled general circulation model (CGCM). The stratification is intensified at upper levels in the western and central equatorial Pacific ocean during periods of high ENSO amplitude. Furthermore, changes in the stratification are connected with a westward-equatorward propagation of subsurface temperature anomalies originating in the central-south tropical Pacific. The analysis of the wind stress projection coefficient onto the oceanic baroclinic modes suggests that the contribution of the higher modes increases in the western and central equatorial Pacific during periods of high ENSO amplitude. This result supports the hypothesis of oceanic tunnels from the South tropical Pacific to explain the low frequency ENSO modulation.1

State-dependent atmospheric noise associated with ENSO

Concurrently with the continuous rise of atmospheric greenhouse gas concentrations, tropical Pacific sea-surface temperatures (SST) have increased significantly during the second half of the 20th century. Our observational analysis suggests that in this period the equatorial-Pacific intraseasonal surface-wind variability is significantly increased. In addition, its nonlinear interaction with the interannual El Niño-Southern Oscillation (ENSO) have intensified substantially. These changes led to an increase in the El Niño activity and the emergence of a new warm-pool El Niño regime. State-of-the-art coupled general circulation models (CGCMs) simulate a future enhancement of the intraseasonal surface-wind variability in the equatorial Pacific in response to an atmospheric greenhouse gas increase, with potential repercussions for future El Niño activity.2