The summer western Pacific subtropical high (WPSH) has intensified during the past three decades. However, the underlying mechanism is not yet well understood. Here, it is shown that baiu rainband activity in midsummer, which is part of the East Asian summer monsoon, plays an important role in recent intensification in the WPSH along the baiu rainband. In contrast with the WPSH, the summer Okhotsk high, which is located to the north of the baiu rainband, has weakened during the past three decades. The north–south contrasting changes between the two highs reflect a response to northward-moved and enhanced baiu heating, which intensifies the upper-tropospheric ridge, resulting in the baroclinic intensification of the WPSH. Regional climate model experiments also support the observational analysis. Therefore, baiu convective activity in midsummer can act as a major driver for the WPSH intensification. The results here suggest that the mechanism intensifying the summer North Pacific subtropical high clearly differs between the western and eastern Pacific.

The summer climate in northern Eurasia is examined as a function of anomalous snow cover and processes associated with land-atmosphere coupling, based on a composite analysis using observational and reanalysis data. The analysis confirms that the snow-hydrological effect, which is enhanced soil moisture persisting later into the summer and contributing to cooling and precipitation recycling, is active in eastern Siberia and contributes to the formation of the subpolar jet through the thermal wind relationship in early snowmelt years.Strong anticyclonic differences (early 2 late snowmelt years) with a baroclinic structure form over eastern Siberia as a result of surface heating through the snow-hydrological effect in early snowmelt years. Surface heating contributes to the development of thermally generated stationary Rossby waves that propagate eastward to eastern Siberia. Rossby wave activity is maintained into early autumn, together with cyclonic differences over far eastern Siberia, in conjunction with the early appearance of snow cover in this region. The anticyclonic differences over eastern Siberia act like a blocking anticyclone, thereby strengthening upstream storm track activity. Furthermore, it is possible that surface anticyclonic differences over the Arctic contribute to year-to-year variability of summer Arctic sea ice concentration along the Siberian coast. The results suggest that variations in northern Eurasian snow cover and associated land-atmosphere coupling processes have important implications for the predictability of Eurasian subarctic summer climate.

Anticyclonic circulation has intensified over the Arctic Ocean in summer during recent decades. However, the underlying mechanism is, as yet, not well understood. Here, it is shown that earlier spring Eurasian snowmelt leads to anomalously negative sea level pressure (SLP) over Eurasia and positive SLP over the Arctic, which has strong projection on the negative phase of the northern annular mode (NAM) in summer through the wavemean flow interaction. Specifically, earlier spring snowmelt over Eurasia leads to a warmer land surface, because of reduced surface albedo. The warmed surface amplifies stationary Rossby waves, leading to a deceleration of the subpolar jet. As a consequence, rising motion is enhanced over the land, and compensating subsidence and adiabatic heating occur in the Arctic troposphere, forming the negative NAM. The intensified anticyclonic circulation has played a contributing role in accelerating the sea ice decline observed during the last two decades. The results here provide important information for improving seasonal prediction of summer sea ice cover.

Through extensive modeling efforts, it has been established that the ongoing global warming will increase the overall precipitation associated with the East Asian summer monsoon, but the future change of its spatial distribution has not reached a consensus. In this study, meridional shifts of the mei-yu–baiu rainband are studied in association with the subtropical jet by using outputs from atmosphere–ocean coupled climate models provided by CMIP5. The models reproduce observed associations between the jet and precipitation over wide time scales from synoptic to interannual. The same relation is found in intermodel differences in simulated climatology, so that the meridional locations of the jet and baiu precipitation are positively correlated. The multimodel-mean projection suggests that the both are shifted southward by the late twenty-first century. This shift is not inconsistent with the projected tropical expansion, not only because the change is local but also because the projected tropical expansion occurs mainly in the Southern Hemisphere. No significant future change in the continental mei-yu precipitation location is identified, which might be because the jet change is weak there. For comparison, the summertime Atlantic jet position, which shifts northward, is investigated briefly. This study suggests that the future change of the subtropical jet is an important aspect to investigate possible future changes of the baiu rainband, and it prompts further studies including the role of the ocean.

[1] An atmospheric general circulation model (AGCM) is used to investigate the effects of springtime high-latitude snow cover on the summertime climate system in the form of land-atmosphere interactions in northern Eurasia. We performed light and heavy snow runs in which the initial snow mass in northern Eurasia was varied. Significant differences in model response between the light and heavy snow runs are evident in terms of not only land surface parameters but also summertime northern atmospheric circulation. Changes in the initial snow cover have a strong effect on the simulated surface air temperature. In western Siberia, the albedo of the snow cover makes a strong contribution to the difference in surface heating between the runs, because snow mass anomalies are still present over western Siberia in June. In eastern Siberia (the Lena Basin), where the snow disappears in June in both runs, the snow-hydrological effect is prominent throughout summer. The increased soil moisture in the heavy snow run causes increased evaporation, resulting in turn in surface cooling. The initial soil moisture content is dry in eastern Siberia and wet in western Siberia, resulting in contrasting responses between the two regions. In the light snow run, the subpolar jet is strengthened and maintained along the Arctic coast in early summer, and wave activity propagates eastward over northern Eurasia. Changes in the atmospheric circulation generate an east-west dipole structure of precipitation anomalies over northern Eurasia. These results suggest that variations in the springtime Eurasian snow mass result in changes in the summertime northern atmospheric circulation and hydrological cycle via land-atmosphere interactions.Citation: Matsumura, S., K. Yamazaki, and T. Tokioka (2010), Summertime land-atmosphere interactions in response to anomalous springtime snow cover in northern Eurasia,