Arctic Cryosphere Links Winter Climate to Summer Climate Drs. Yamazaki and Tachibana will explain the effect of the Arctic region to the global climate. Koji Yamazaki and Yoshihiro Tachibana(Researchers, International Arctic Research Center, FRSGC) A sea level pressure seesaw between Icelandic Low and Azores High is called the North Atlantic Oscillation (NAO) [e.g., Hurrell, 1995] and it is dominant in winter. A positive phase of the NAO means that Icelandic Low and Azores High are intensified. Recently, the Arctic Oscillation (AO) is introduced by Thompson and Wallace (1998) which has more hemispheric scale pattern, but the AO is similar to the NAO over the Atlantic region. It is well known that a positive NAO/AO causes warm winter climate to northern Eurasia. The NAO/AO index has an increasing trend that is considered to be related to global warming. We investigated the influence of winter NAO to spring and summer climate. The NAO signal can be seen in early spring (February-April) as shown in the left panel of Fig. 1. Although the NAO itself does not persist from winter to summer, the NAO signature appears even in early summer. The right panel of Fig. 1 shows a linear regression map from the winter (December-February) NAO to early summer (May-July) 500hPa geopotential height. The map indicates that summer polar vortex is deepened and heights at the midlatitudes become high when the winter NAO index is positive. In particular, a positive anomaly over northern Sea of Okhotsk is significant. The sea level pressure pattern shows a similar structure.
		Fig.1 (Left) Linear regression map between winter (December-February) NAO index and early spring (February-April) 500hPa geopotential height. Colors denote correlation coefficients for 90%, 95%, 99% significant level. (Right) Same as the left panel but for early summer (May-July).
		Since atmosphere itself does not have long memory beyond one month, the relation shown in Fig. 1 indicates that something else links winter to summer. Thus we investigated snow cover, sea-ice concentrations and sea surface temperatures which have longer memory than atmosphere (Fig. 2). When the NAO is in a positive phase, spring (February-April) snow cover in Europe and central Asia is less than normal and spring sea-ice extent over Barents Sea and Greenland Sea is also less than normal. Snow anomalies influence the atmospheric heating by anomalies in soil moisture even after snowmelts through evaporation, precipitation and sensible heat transfer processes. In early summer, snow melts over Europe and thus no signals appear over Europe. But sea-ice anomalies over Barents and Greenland Seas continue and that over East Siberian Sea emerges with snow anomalies over East Siberia. For seasonal forecast, El Niño has been considered as a primary factor so far. In Japan, El Niño tends to bring about cold summer. The present study suggests that a possibility of seasonal forecast for summer from the winter NAO through spring cryosphere such as snow and sea-ice. At the time of writing (early February), the winter NAO index in 2003 is negative. So this coming summer, Okhotsk high is expected to be weak, which bring about hot summer in Japan. Keep our fingers crossed. This study is jointly carried with Ms. M. Ogi, a graduate student at Hokkaido University.
		Fig.2 (Left) Correlation coefficients between winter NAO index and early spring (February-April) snow cover (over land), sea-ice concentration (over ocean) and sea surface temperature (contour). (Right) Same as the left panel but for early summer (May-July).
		References Hurrell, J. W., 1995: Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science, 269, 676-679. Thompson, D. W. J. and J. M. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Lett., 25, 1297-1300.