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<p>Rapid melting of sea ice and an increased warming have been observed over the
Arctic since 1990s and is expected to continue in future climate projections. Possible linkage between the Arctic sea ice and the Northern Hemisphere mid-latitude
circulation has been studied previously but is not yet fully understood. This dissertation investigates the influence of the Arctic on the mid-latitudes and the underlying
dynamical mechanisms. Specifically, we hypothesize that the stratosphere and its
coupling with the troposphere play an important role in amplifying and extending
the mid-latitude circulation response to arctic warming.
</p><p><br></p>
<p>First, we assess the robustness of the stratospheric pathway in linking the sea
ice variability, specifically over the Barents-Kara Sea (BKS), in late autumn and
early winter to the mid-latitude circulation in the subsequent winter using an ensemble of global climate model simulations. We analyze two groups of models from
the Coupled Model Intercomparison Project phase 5 (CMIP5) archive, one with a
well-resolved stratosphere (high-top models) and the other with a poorly-resolved
stratosphere (low-top models) to distinguish the role of the stratospheric pathway.
It has been found that, collectively, high-top models are able to capture the persistent mid-latitude circulation response in the subsequent winter. The response in
low-top models is, however, weaker and not as long-lasting most likely due to lack
of stratospheric variability. Diagnosis of eddy heat flux reveals that stronger vertical
wave propagation leads to a stronger response in stratospheric polar vortex in high-
top models. The results robustly demonstrate that multi-model ensemble of CMIP5 high-top models are able to capture the prolonged impact of sea ice variability on the
mid-latitude circulation and outperforms the low-top models in this regard.</p><p><br></p></div></div></div><div><div><div>
<p>We further explore the dynamical linkage between the BKS sea ice loss and
the Siberian cold anomalies using a comprehensive Atmospheric General Circulation
Model (AGCM), with a well-resolved stratosphere, with prescribed sea ice loss over
BKS region. Decomposition of dynamic and thermodynamic components suggests a
dynamically induced warm Arctic cold Siberia pattern in the winter following sea ice
loss over the BKS in late autumn. Specifically, the results show that the meridional
component of the horizontal temperature advection, from the Arctic into the Siberia,
dominates in driving a cold temperature anomaly. Additionally, we conduct targeted
experiments in order to quantitatively measure the role of the stratospheric pathway. We find that the stratosphere plays a critical role in the tropospheric circulation
anomaly characterized by an intensified ridge-trough pattern that is attributable for
the enhanced meridional temperature advection from the Arctic into the Siberia.
</p><p><br></p>
<p>Next, we extend our study to investigate the sensitivity to geographical location
of Arctic sea ice loss and associated warming in modulating the atmospheric circulation. In particular, we assess the linear additivity of the regional Arctic sea ice loss
and Arctic Amplification (AA), using a simplified dry dynamical core model. We find
that the responses to regional AA over three key regions of the Arctic, i.e. Barents-
Kara Sea, East Siberia-Chukchi sea and Baffin Bay-Labrador Sea, separately, show
similar equatorward shift of the tropospheric jet but differences in the stratospheric
polar vortex. In addition, responses to regional Arctic Amplification are not linearly
additive and the residual resembles a positive Northern Annular Mode-like structure. Additional targeted experiments further diagnose the role of the stratosphere
in the non-linearity. It is found that the stratosphere-troposphere coupling plays an
important role in driving the non-linear circulation response to regional AA.
</p><p><br></p>
<p>The findings of our research leads to a systematic understanding of the role of the
stratospheric pathway in modulating the mid-latitude circulation response to Arctic
sea ice loss and accompanied surface warming. Our study suggests that the representation of the stratosphere in climate models plays an important role in correctly
simulating the mid-latitude circulation response and could be accountable for the
some of the discrepancies among recent studies. Additionally, the result indicates
that studying the regional sea ice loss might not provide the full picture of pan-Arctic
sea ice melting and caution the use of regional sea ice to explain the recent trend.</p></div></div></div>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/9116849 |
| Date | 02 August 2019 |
| Creators | Bithi De (7046621) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/THE_ROLE_OF_STRATOSPHERIC_PATHWAY_IN_LINKING_ARCTIC_SEA_ICE_LOSS_TO_THE_MID-LATITUDE_CIRCULATION/9116849 |
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