Date of Award

12-10-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Varavut Limpasuvan

Second Advisor

Yvan Orsolini

Third Advisor

Shaowu Bao

Additional Advisors

Michael Murphy; Xiangdong Zhang

Abstract

The Arctic sea ice is a critical indicator of climate change. The extent of sea ice coverage over the Arctic Ocean has dramatically declined over the past few decades. The impact has been extensively studied through observations suggesting a linkage between the anomalously warm Arctic surface associated with the Arctic sea ice loss and the mid-latitude surface cooling in the subsequent boreal winter. This linkage could involve the wintertime stratospheric circulation by enhancing the upward planetary wave activity and weakening the polar vortex. With recent advances in climate model, more relevant studies relied on numerical simulations and some suggested that the effects of sea ice reduction on the atmospheric circulation and, in particular, on the warm Arctic-cold continent pattern at the surface are attributed to internal variability. Understanding the impact of sea ice changes on the atmospheric circulation is crucial for predicting and assessing climate changes in the coming decades as well as extreme weather. The overarching goal of this thesis is to improve our basic understanding of the physical processes that link the large-scale atmospheric circulation, particularly for Sudden Stratospheric Warming (SSW) events, and Arctic sea ice loss. In addition, the roles of internal variabilities, namely, the Quasi-biennial Oscillation (QBO) and Madden-Julian Oscillation (MJO), in modulating the atmospheric response to Arctic sea ice loss are examined. To avoid conflating the effects of sea ice loss in different sectors, this dissertation solely focuses on the Chukchi-Bering Seas (i.e., the Pacific sector) where the observed autumnal Arctic sea ice extent shows the strongest decline in recent decades. Observational data record period is too short to provide statistically convincing conclusion on how the underlying mechanism works in generating a global atmospheric response. Therefore, global climate model with well-resolved stratosphere (i.e., Whole Atmosphere Community Climate Model version 6 from the National Center for Atmospheric Research) is used. Since QBO and MJO are internally generated in the model, their roles on the responses are examined. During the easterly QBO phase (EQBO), the prescribed sea ice loss, although culminating in autumn, induces a near-surface warming that persists into winter and deepens as the SSW develops. The resulting temperature contrasts foster a deep cyclonic circulation over the North Pacific, which elicits a strong upward wavenumber-2 activity into the stratosphere, reinforcing the climatological planetary wave pattern. The induced geopotential anomalies in the lower troposphere also project onto the anomalous patterns typically observed prior to SSWs. While not affecting the SSW occurrence frequency, the amplified wave forcing in the stratosphere significantly increases the SSW duration and intensity, enhancing thereafter cold air outbreaks over the Northern Hemisphere continents. However, for the westerly QBO phase (WQBO), the induced warming does not extend vertically into the middle troposphere but instead spreads horizontally prior to SSW onset. The resulting temperature contrasts weaken the precursor of SSW over the North Pacific. The other SSW precursor over the Europe is slightly strengthened. The cancelling effect leads to insignificant change of SSW duration and wind reversal intensity. The SSW occurrence frequency is significantly increased in response to Arctic sea ice loss. To this end, despite the prescribed sea ice loss being identical, the structure of temperature response is different between EQBO and WQBO. The background state, conditioned by the QBO, influences the response of SSWs to the sea ice loss. While difficult, differentiating the QBO phases may be important in understanding the stratospheric response to sea ice loss. In the climate system, there are additional factors that might modulate the atmospheric response to sea ice loss, namely the effects of El Niño-Southern Oscillation (ENSO), the solar cycle, and global warming. In this thesis, these factors are all excluded in the experimental design. Nevertheless, the tropical variability MJO is internally generated in WACCM6. Since the SSW precursor over North Pacific could be excited by MJO phase 7, the role of MJO is also examined. In general, MJO phase 7 occurs more frequently prior to SSW onset. However, in our experiment, there is very little difference in the MJO phase 7 response during EQBO, affirming the conclusion that strengthened SSW precursor over the North Pacific is induced by sea ice loss and autumnal sea ice loss extends the SSW duration and strengthens the accompanying stratospheric wind reversal. For WQBO, the weakened SSW precursor over the North Pacific is also mainly due to sea ice loss instead of MJO. Additionally, teleconnection patterns excited by MJO are enhanced in response to sea ice loss regardless of QBO phases.

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