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Meiji Honda, Koji Yamazaki, Hisashi Nakamura, and Kensuke Takeuchi

Abstract

Influence of sea-ice extent anomalies within the Sea of Okhotsk on the large-scale atmospheric circulation is investigated through an analysis of the dynamic and thermodynamic characteristics of the response in an atmospheric general circulation model to specified anomalous sea-ice cover. Significant response appears not only around the Sea of Okhotsk, but also downstream over the Bering Sea, Alaska, and North America in the form of a stationary wave train in the troposphere. This remote response, associated with wave activity flux emanating from the Okhotsk area to the downstream, is regarded as a stationary Rossby wave generated thermally by the anomalous turbulent heat fluxes from the ocean surface as a result of the anomalous sea-ice cover. The Pacific storm track in the model that extends zonally at 35°N is located too far south of the Sea of Okhotsk to exert substantial feedback forcing on the local and remote response. Since a similar stationary wave train is identified in the composite difference fields of the observed data between heavy and light ice years, it is believed that the model appropriately reproduces the real atmospheric response to the Okhotsk sea-ice extent anomalies. Simulated seesaws in the meridional surface wind and surface air temperature anomalies between the eastern Sea of Okhotsk and eastern Bering Sea associated with the local and remote response, respectively, to the Okhotsk sea-ice anomalies seem to be consistent with the observed seesaw in the anomalous sea-ice cover between these maritime regions. There is a hint of reinforcement of the remote response around the Alaskan Pacific coast through destabilization of barotropic Rossby waves due to the thermal damping effect associated with the anomalous atmosphere–ocean heat exchange both in the model and real atmosphere.

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Carlos R. Mechoso, Max J. Suarez, Koji Yamazaki, Joseph A. Spahr, and Akio Arakawa

Abstract

The impact of an upper boundary on numerical forecasts is studied by comparing the results of a nine-layer model with a top in the lower stratosphere, to those of a 15-layer model with a top near the stratopause. A single case is considered for which initial conditions are taken from a climatologically adjusted winter simulation produced by the 15-layer model. It is found that, as a result of the lowered upper boundary, them is a marked equatorward shift of upper-level westerlies. Significant errors in the ultra-long waves appear at SW mb within the first five days. Errors at 500 mb then spread to progressively shorter waves with large errors in cyclone-scale waves by day 12. Large errors in an ultra-long, wave (wavenumber 3) after day 10 am associated with the climatological adjustment of the stationary flow to the lowered boundary.

Two different assumptions in the radiation calculation in the nine-layer model am used. Results indicate that radiative effects are of secondary importance to the predictability of waves in the middle troposphere.

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Koji Yamazaki, Masayo Ogi, Yoshihiro Tachibana, Tetsu Nakamura, and Kazuhiro Oshima

Abstract

The summer northern annular mode (NAM) and the winter North Atlantic Oscillation (NAO)/winter NAM have a positive correlation from the mid-1960s to the 1980s. Namely, when the winter NAO/NAM is in a positive phase, the following summer NAM tended to be in a positive phase. During the period from the mid-1960s to the 1980s, the NAO/NAM signals extended to the stratosphere in winter. Also, the lower-tropospheric warm anomaly over northern Eurasia in winter associated with the positive phase of NAO/NAM continued into spring. In summer, the annular anomalies in the temperature and 500-hPa height fields appeared, and the high-latitude westerly wind was enhanced following the winter positive NAO/NAM. However, after circa 1990, the seasonal linkage was broken (i.e., the winter-to-summer correlation became insignificant). The stratospheric signal in the winter NAO/NAM became weak and summer signals associated with the winter NAO/NAM almost disappeared. Seasonal evolutions of atmospheric circulation and sea surface temperature (SST) anomalies associated with the winter NAO are examined for an early good-linkage period and a recent poor-linkage period. We discuss the possible causes of the linkage breakdown such as stratospheric ozone, North Atlantic SST, and Atlantic multidecadal oscillation, besides chaotic internal variability in the climate system. Simulations with the Community Earth System Model suggest that the ocean and/or sea ice with interseasonal memories possibly cause the linkage, besides large internal variability through which the linkage can take place by chance.

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