이학박사학위논문

Global warming, low-frequency variability, and biennial oscillation contribution to the Pacific climate variability
and its decadal changes around the late-1990s
태평양 기후의 온난화, 장 주기, 2년 주기 변동 성분과 1990년대 후반에 나타난 10년 변화의 특징




2013년 8월
서울대학교 대학원
지구환경과학부
여 새 림

Global warming, low-frequency variability, and biennial oscillation contribution to the Pacific climate variability
and its decadal changes around the late-1990s
태평양 기후의 온난화, 장 주기, 2년 주기 변동 성분과
1990년대 후반에 나타난 10년 변화의 특징

지도교수 Kwang-Yul Kim

이 논문을 이학박사 학위 논문으로 제출함
2013년 6월

서울대학교 대학원
지구환경과학부
여 새 림

여새림의 이학박사 학위논문을 인준함
2013년 8월

위 원 장 (인)
부위원장 (인)
위 원 (인)
위 원 (인)
위 원 (인)
Abstract
Global warming, low-frequency variability, and biennial oscillation contribution to the Pacific climate variability
and its decadal changes around the late-1990s

Sae-Rim Yeo
School of Earth and Environmental Sciences
The Graduate School
Seoul National University

In order to understand the climate variability over the Pacific, the Pacific climate variabilities are decomposed into distinctive physical modes using a statistical method called cyclostationary empirical orthogonal function (CSEOF) analysis. In the first part of the thesis, three primary global modes of sea surface temperature (SST) variability during the period of 1871-2010 are identified, that is, the global warming mode, the low-frequency variability mode, and the biennial oscillation mode. The three-mode reconstruction captures a significant portion of the observed El Nino-Southern Oscillation (ENSO) variability. In other words, the bulk of ENSO variability is adequately explained in terms of interplay among the three modes. In particular, two key points are derived from this analysis: (1) the most extreme El Nino events occurred in 1982/83 and 1997/98 are attributed to the positive contributions of all three modes; and (2) the central Pacific (CP) El Nino events in the 1990s and 2000s have different physical mechanisms, that is, the CP El Nino events in the early 1990s originated mainly from the low-frequency mode, while those in the early 2000s derived mainly from the global warming mode.
Meanwhile, the most recent phase transition in the low-frequency variability mode is occurred in 1998/99 from positive to negative phase. In accordance with this phase transition, it is found that the relationship between the tropical Pacific and the North Pacific appears to have significantly changed since 1999. In the second part of the thesis, the characteristics of the connection between the tropical Pacific and the North Pacific during two sub-periods (1980-1998 and 1999-2010) are examined. It is found that the atmospheric anomalies induced by the ENSO alter the SST over the North Pacific through surface heat flux change during the earlier period of 1980-1998. During the later period, in contrast, ENSO-related atmospheric anomalies feature the North Pacific oscillation (NPO) pattern during winter, which is characterized by a southward shift of the atmospheric center of action from its climatological position. The NPO-related atmospheric anomalies extend to the subtropical Pacific; this extension potentially links midlatitude and tropical Pacific variability through air-sea interactions. The physical change appears to alter the El Nino characteristics into that of the CP El Nino through the wind-SST coupling mechanism.
After 1999, it is obvios that the intensified global warming signal as well as the phase transition in the low-frequency variability. In the third part of the thesis, therefore, it is investigated how the warming signal affect Pacific climate variability. In particular, the relationship between the warming in the sub-arctic Pacific (Bering and Chukchi Seas) and the Pacific climate variability is examined. Specifically, warming in the Bering and Chukchi Seas in the period of 1999-2010 involves sea ice reduction and stronger oceanic heat flux to the atmosphere in winter. The atmospheric response to the recent warming in the Bering and Chukchi Seas resembles the NPO pattern. Futher analysis reveals that the recent climate variability in the Bering and Chukchi Seas has strong covariability with large-scale climate modes in the Pacific, that is North Pacific Gyre Oscillation (NPGO) and the CP El Nino.