Future changes of cool northeasterly winds bringing unusual Northwest Pacific summer in CMIP3 multi-model experiments

Northeasterly winds blowing from the Okhotsk Sea and the northern Pacific, called the "Yamase" in Japanese, bring unusual cool and wet weather over northeastern Japan in summer in contrast to climatological southwesterly winds. Its future change is one of the major issues in global warming climate projection over East Asia because it highly affects agriculture and life in the region. The CMIP3 multi-model predictions in this study suggest that seasonality of the Yamase is affected clearly by the Global Warming.

The future changes in surface winds and sea level pressure (SLP) are analyzed using daily atmospheric data from fifteen CMIP3 atmosphere-ocean coupled model experiments. The changes are calculated from the present climate (1981-2000) of the 20th century climate in coupled models (20C3M) experiment and the future climate (2081-2100) of the SRES A1B scenario experiment. In addition to the change of climatological mean fields, the change of the occurrence frequency of northeasterly surface wind over the Pacific off northern Japan (40°N-45°N, 142.5°E-155°E) is also analyzed based on year-to-year 10-day mean data from May to August.

In present climate, the averaged SLP over all the fifteen models, referred to as “MME15”, generally agrees with the analysis from reanalysis data from East Asia to the northern Pacific in each month. The climatological occurrence frequency of the northeasterly wind over the Pacific off northern Japan is studied using reanalysis data. The occurrence increases from late-May and peaks at around late-June and early-July when the climatological frequency of occurrence is about 30%. Then, it decreases rapidly from mid-July and down to 10% in mid-August, which corresponds to entering midsummer season with prevailing southerly winds. These seasonal evolutions can be reproduced qualitatively in MME15 although its amplitude is small compared to the analysis and there are large model-to-model differences.

In future climate, MME15 shows that the occurrence frequency of the northeasterly winds decreases in early summer but increases in late summer. This tendency is quite consistent among the models. Total number of the occurrence during summer is changed little. The changes of seasonality are also identified in the changes of the climatological mean SLP field in each month. Many model show negative SLP anomalies around the Okhotsk Sea in June, whereas positive SLP anomalies are over the high latitudes (55°N) of the Pacific including the Okhotsk Sea and negative anomalies are found over the subtropics of the Pacific (35°N) in August. The changes projected with MME15 imply that the occurrence of the Yamase tends to delay, but tends to continue even in August. It is also interesting result that the tendency of the changes is consistent with that of rainy season around Japan pointed out by recent several studies.

What future change of global fields is related to the local future changes of the northeasterly winds in the high-latitudes northwest Pacific? We found that the models projecting stronger northeast winds in future August are also projecting weaker walker circulation measured by equatorial east-west SLP contrasts. It seems that the high-latitudes local wind changes in the northwestern Pacific are relating to the global-scale tropical circulation change. We need to analyze this point further. We also should analyze high resolution model results to detect explicit change of variables such as surface temperature over the region, considering fine topographic effects.