UC Santa Barbara

In recent decades, global warming, predominantly driven by anthropogenic forcing and arising from an energy imbalance due to increased radiative forcing from greenhouse gases, has profoundly impacted Earth’s climate systems. This warming is most significant in high northern latitudes, manifesting in various ways, such as sea ice melt, upper ocean warming in the Arctic, the slowdown of the Atlantic Meridional Overturning Circulation (AMOC), Greenland ice sheet retreat, changes in extreme weather patterns and related moisture transport near the Arctic, etc. Previous studies have extensively focused on the role of CO2 forcing in contributing to these changes, while the influence of internal atmospheric variability - large-scale atmospheric circulation - remains less explored. In particular, these phenomenon were studied separately, which hinders us from gaining a deeper insight into the large-scale climate dynamics behind recent rapid changes in the climate system of high northern latitudes. This dissertation addresses this gap by examining how large-scale atmospheric circulation influences these changes, ranging from studying high northern latitude oceans to understanding circulation-extreme weather interactions. Although these observed rapid change appear to be disconnected, actually my analyses solidly suggest that they are physically connected by recent large-scale atmospheric circulation variability with dynamical sources in the tropics.

In the following sections, consisting of three components, I focus on recent rapid changes in high northern latitudes: I. upper Arctic Ocean warming; II. subpolar North Atlantic warming hole; III. a poleward shift pattern of atmospheric rivers (ARs) in the extratropics. In the first part, I investigate the role of large-scale circulation in warming the upper Arctic Ocean over the past 40 years. Observational and modeling analyses reveal that internal atmospheric variability, characterized by a multiyear trend in summertime circulation, has significantly contributed to upper ocean warming in the Arctic. Nudging experiments in which the wind fields are constrained toward the observed state suggest that this variability accounts for up to 24% of the warming from 1979 to 2018, and up to 60% from 2000 to 2018. This internal circulation variability is known to be partially driven remotely by tropical sea surface temperature forcing. The second part of this research examines how large-scale circulation shapes ocean-atmosphere interactions in the subpolar North Atlantic (SNA), a key region for atmospheric and oceanic heat exchange between the Arctic and subarctic, as well as for the maintenance of the AMOC. Analysis of observational datasets and model simulations highlights the dominance of regional atmospheric circulation, partially initiated by remote forcing from the tropical Pacific, in driving multidecadal climate variability in the SNA by modulating local atmosphere-ocean interactions and upper ocean heat transport in the region. To further understand moisture transport from low latitudes to the extratropics due to extreme weather events such as ARs, in the last part of this dissertation, I focus on how this large-scale circulation regulates short-term extreme weather variability (e.g. ARs) in the North Pacific and Atlantic, even in a global scale. Diagnostic analysis indicates that low-frequency sea surface temperature variability in the tropical eastern Pacific, exhibiting a cooling tendency since 2000, drives this poleward shift of ARs, predominantly observed over mid-latitude oceans. Strengthened subtropical high pressures, maintained by a tropical-driven eddy-mean flow feedback, contribute to these observed AR changes.