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第 252回 大気海洋物理学・気候力学セミナー のおしらせ

日 時: 4月 17日(木) 午前 09:30
場 所: 環境科学院 D棟2階 D201号室

発表者: 草原 和弥(北大低温研/特任助教)
Kazuya Kusahara(ILTS/Project Associate Professor)
題名:
Kelvin Waves around Antarctica

発表者: 中村 哲(地球環境科学研究院/PD)
Tetsu Nakamura(Faculty of Environmental Earth Science/PD)
題名:
A negative phase shift of the winter AO/NAO due to the recent Arctic sea ice reduction in late autumn

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Kelvin Waves around Antarctica (草原 和弥 Kazuya Kusahara)発表要旨 :

The Kelvin Wave is one of the fundamental waves in geophysical fluid dynamics, related to several phenomena on the Earth's surface. The Antarctic coastline is the longest and infinite waveguide for oceanic Kelvin Waves. Under cyclic conditions of the Southern Ocean, the horizontal wavenumbers and frequencies for circumpolarly-propagating waves are quantized, with horizontal wavenumber one, two, and three corresponding to periods of about 32, 16, and 11 hours, respectively. At these frequencies, we have detected westward propagating signals in sea level variations observed at Antarctic coastal stations. The occurrence frequency of westward propagating signals far exceeds the statistical significance and the phase speed of the observed signal agrees well with the theoretical phase speed of external Kelvin Waves. Therefore, we have concluded that the observed westward-propagating sea level variability is a signal of the external Kelvin Waves of wavenumber one, two, and three around Antarctica. A series of numerical model experiments confirms that Kelvin Waves around Antarctica are driven by surface air pressure and that these waves are excited not only by local forcing over the Southern Ocean but also by remote forcing over the Pacific Ocean. Sea level variations generated over the Pacific Ocean can travel to the western side of the South American coast and cross over Drake Passage to the Antarctic continent, constituting a part of the Kelvin Waves around Antarctica.

A negative phase shift of the winter AO/NAO due to the recent Arctic sea ice reduction in late autumn (中村 哲 Tetsu Nakamura)発表要旨 :

This study examines the possible linkage between the recent reduction in Arctic sea ice and the wintertime Arctic Oscillation (AO)/North Atlantic Oscillation (NAO). Analysis of the ERA interim reanalysis and merged Hadley/OI-SST data indicate that a reduced (increased) sea ice area in November leads to more negative (positive) phases of the AO and NAO in early and late winter, respectively. We simulate the atmospheric response to observed sea ice and sea surface temperature (SST) anomalies using an atmospheric general circulation model (AGCM for Earth Simulator, AFES version 4.1). The recent Arctic sea ice reduction gives rise to cold winters to mid-latitude continental regions, namely, Europe, eastern Siberia, and North America, linked to an anomalous circulation pattern from the negative phase of AO/NAO. The reduction in sea ice approximately doubles the frequency of large negative AO events. In comparison, SST anomalies in the tropics and middlehigh latitudes mask this continental cooling signal. Model-based analysis reveals that the stationary Rossby wave response to sea ice reduction in the Barents Sea induces an anomalous meridional circulation. We found that this anomalous meridional circulation cools the mid-latitudes and warms the Arctic, adding extra Arctic heating equivalent to about 25% of the direct surface heat release from the sea ice reduction.

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連絡先

水田 元太 (Genta Mizuta)
mail-to: mizuta@ees.hokudai.ac.jp