Editorial Type:
Article
Article Type:
Research Article

The Relative Contributions of Internal Variability and External Forcing to Pacific Walker Circulation over the Last Millennium

Shijie Wang Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
Yunnan Key Laboratory of International Rivers and Transboundary Eco-Security, Institute of International Rivers and Eco-Security, Yunnan University, Kunming, China

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Wenmin Man Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

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https://orcid.org/0000-0003-3004-5464
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Feng Chen Yunnan Key Laboratory of International Rivers and Transboundary Eco-Security, Institute of International Rivers and Eco-Security, Yunnan University, Kunming, China
Southwest United Graduate School, Kunming, China

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Meng Zuo State Key Laboratory of Severe Weather and Institute of Tibetan Plateau Meteorology, Chinese Academy of Meteorological Sciences, Beijing, China

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Wenhui Tang Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
Yunnan Key Laboratory of International Rivers and Transboundary Eco-Security, Institute of International Rivers and Eco-Security, Yunnan University, Kunming, China
Southwest United Graduate School, Kunming, China

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Online Publication:
10 Dec 2024
Print Publication:
01 Jan 2025
DOI:
https://doi.org/10.1175/JCLI-D-24-0313.1
Page(s):
219–233
Restricted access

Abstract

Pacific Walker circulation (PWC) is an important component of tropical atmospheric circulation. Current studies have mainly focused on PWC changes over recent decades and the near future, while less effort has been devoted to long-term PWC variations. In this study, we investigate PWC changes over the last millennium (LM) based on the Community Earth System Model LM Ensemble (CESM-LME). The simulated PWC variations show no significant trend but do reveal decadal fluctuations during the LM, which underestimate the strengthened Little Ice Age (LIA)–Medieval Climate Anomaly (MCA) PWC differences indicated by proxy-based reconstructions. A quantitative estimation of the contributions made to PWC variability from internal variability and external forcing is conducted by using multiple linear regression (MLR) analysis. The internal variabilities contribute approximately 80% to the changes in PWC during the LM, among which the interdecadal Pacific oscillation (IPO) has the largest contribution. In the positive phase of the IPO, the Indo-Pacific sea level pressure (SLP) gradient decreases, and anomalous westerlies occur in the tropical western Pacific, corresponding to a weakened PWC. The relationships between the IPO and PWC show multidecadal-to-centennial fluctuations, suggesting that other internal modes or external forcings may have modulated the IPO–PWC relationship. Volcanic forcing is also an important contributor to PWC variability during the LM. The simulated PWC significantly weakens and lasts for 1–2 years after large volcanic eruptions. The El Niño–like SST pattern and the corresponding zonal SLP gradient lead to the PWC weakening following large volcanic eruptions.

Significance Statement

As one of the most active components of tropical atmospheric circulation, Pacific Walker circulation (PWC) can significantly modulate global climate through atmospheric teleconnections. Previous studies have mainly focused on PWC variability over recent warm periods and future changes. However, understanding of PWC changes is currently lacking due to limited instrumental observations. The last millennium (LM) has a rich archive of proxy records that give us a longer perspective on climate variability and change, which is an excellent period for understanding the long-term mechanisms and changes in PWC. In this study, we use simulations from the CESM-LME to quantify the relative importance of internal variability and external forcings to PWC variations during the LM. The simulated PWC variations show no significant trend but decadal fluctuations, which underestimates the reconstructed PWC variations during the LM. The internal variabilities contribute approximately 80% to the changes in PWC, among which the interdecadal Pacific oscillation (IPO) has the largest contribution. External forcing, especially the contribution made by volcanic forcing, is also nonnegligible. Both the positive phase of the IPO and large tropical volcanic eruptions could trigger El Niño–like SST anomalies, accompanied by a decreased Indo-Pacific SLP gradient and a weakened PWC.

© 2024 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Wenmin Man, manwenmin@mail.iap.ac.cn

Abstract

Pacific Walker circulation (PWC) is an important component of tropical atmospheric circulation. Current studies have mainly focused on PWC changes over recent decades and the near future, while less effort has been devoted to long-term PWC variations. In this study, we investigate PWC changes over the last millennium (LM) based on the Community Earth System Model LM Ensemble (CESM-LME). The simulated PWC variations show no significant trend but do reveal decadal fluctuations during the LM, which underestimate the strengthened Little Ice Age (LIA)–Medieval Climate Anomaly (MCA) PWC differences indicated by proxy-based reconstructions. A quantitative estimation of the contributions made to PWC variability from internal variability and external forcing is conducted by using multiple linear regression (MLR) analysis. The internal variabilities contribute approximately 80% to the changes in PWC during the LM, among which the interdecadal Pacific oscillation (IPO) has the largest contribution. In the positive phase of the IPO, the Indo-Pacific sea level pressure (SLP) gradient decreases, and anomalous westerlies occur in the tropical western Pacific, corresponding to a weakened PWC. The relationships between the IPO and PWC show multidecadal-to-centennial fluctuations, suggesting that other internal modes or external forcings may have modulated the IPO–PWC relationship. Volcanic forcing is also an important contributor to PWC variability during the LM. The simulated PWC significantly weakens and lasts for 1–2 years after large volcanic eruptions. The El Niño–like SST pattern and the corresponding zonal SLP gradient lead to the PWC weakening following large volcanic eruptions.

Significance Statement

As one of the most active components of tropical atmospheric circulation, Pacific Walker circulation (PWC) can significantly modulate global climate through atmospheric teleconnections. Previous studies have mainly focused on PWC variability over recent warm periods and future changes. However, understanding of PWC changes is currently lacking due to limited instrumental observations. The last millennium (LM) has a rich archive of proxy records that give us a longer perspective on climate variability and change, which is an excellent period for understanding the long-term mechanisms and changes in PWC. In this study, we use simulations from the CESM-LME to quantify the relative importance of internal variability and external forcings to PWC variations during the LM. The simulated PWC variations show no significant trend but decadal fluctuations, which underestimates the reconstructed PWC variations during the LM. The internal variabilities contribute approximately 80% to the changes in PWC, among which the interdecadal Pacific oscillation (IPO) has the largest contribution. External forcing, especially the contribution made by volcanic forcing, is also nonnegligible. Both the positive phase of the IPO and large tropical volcanic eruptions could trigger El Niño–like SST anomalies, accompanied by a decreased Indo-Pacific SLP gradient and a weakened PWC.

© 2024 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Wenmin Man, manwenmin@mail.iap.ac.cn
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