PAPER REVIEW R Kirsten Feng

Impact of global warming on the East Asian winter monsoon revealed by nine coupled atmosphere-ocean GCMs Masatake E. Hori and Hiroki Ueda

Outline East Asian winter monsoon Aleutian Low Tropical/local Hadley circulation Anticyclonic anomalies Pressure gradient East Asian Jet

Nine Models

I. Abstract Weakening of the EAWM accompanied by a strong anticyclonic anomaly over the North Pacific corresponding to a weakened and/or the northern shift of the Aleutian Low (AL). The weakening of both the AL and the Siberian High (SH), which gives rise to a weakened pressure gradient along the eastern coast of the Eurasian continent.

I. Tropical local Hadley circulation was found to substantially weaken the East Asian Jet (EAJ) and the resultant EAWM.

II. Introduction The EAWM is characterized by strong NW winds and cold temperatures over the northern China region. The source of the cold air outflow is the Siberian cold dome, maintained by strong radiative cooling and subsidence by the SH as well as the topological effect of the Tibetan Plateau

II. The NW cold surge itself is governed by the differential heating between the land and ocean. Also, the intensity of the cold surge has strong relations with the tropical convection and is a stage of a tropical- extratropical interaction through the existence of a local Hadley cell near the Philippines and the descent near the SH.

III. Data Nine AOGCMs used in this study is based on the contribution of the Coupled Model Inter- comparison Project. All models start their integration from the ‘20th Century Climate in Coupled Model’ (20C3M) run. Anthropogenic forcing is based on historical data of the late 19th century through the 20th century.

III. SRES-A1B conditions is imposed and integrated through year 2100, after which the concentration is fixed and integrated until year The model control period is selected as the average of 1981–2000 of the 20C3M run and compared against the SRES-A1B scenario. The ERA40 data set is used as an observational reference.

IV. Results The multi-model ensemble (MME) of the DJF averaged climatology of 850 hPa wind during the control period and the inter- model variance of the 850 hPa zonal wind. Basic features of the EAWM consisting of strong NW over northern China and westerly flow around the Tibetan plateau are well simulated.

IV. The response of EAWM under global warming, in terms of difference in DJF averaged 850 hPa wind between the 2101– 2200 mean of the SRES-A1B run and the control period is shown in Figure 3. Southeasterly anomalies in either northern China or the western North Pacific Ocean, corresponding to a weaker EAWM.

IV. Two winter monsoon indices (WMI) is defined, one based on Hu et al. [2000] (WMI1) which uses the area averaged meridional wind speed v with v < 0 over the east coast of China (in our study, 20– 40N, 120–150E) and another index based on Jhun and Lee [2004] (WMI2) which uses the difference in area averaged 300 hPa zonal wind between 27.5–37.5N, 110–170E and 50–60N, 80–140E.

IV. Some inconsistency between the lower and upper monsoon flow.

IV. The MME difference of DJF averaged SLP and meridional gradient in surface temperature between the 2101–2200 period of SRES-A1B run to the control period is given in Figure 4.

IV. Positive difference in surface temperature gradient in the north Pacific corresponds to a weakened temperature gradient in the region. This responce, which is consistent with the negative SLP anomaly in the north Pacific, may have been induced by warm advection due to an increase in cyclone activity, or an increase in oceanic heat flux possibly by a decrease in sea- ice concentration.

IV. Positive anomaly in the maritime continent and negative anomaly over the Asian continent signifies a weakening of the local Hadley circulation. Under the weaker local Hadley circulation, EAJ weakens substantially over 30–40N and contributes to the weakening of the southern flank of the AL.

V. Conclusion Response of 850 hPa wind reveals an overall weakening of the EAWM. Seven out of nine models shows an anticyclonic anomaly over the central North Pacific. The MME of SLP also confirms this feature, with weakening and/or a northward shift of the AL. Indices of the EAWM also shows a negative trend.

V. Northern shift of the AL is suggested to be partially due to the enhanced cyclonic activities in the Okhotsk and Bering Strait region the weakening of the EAJ coherent to the weakening of the local Hadley circulation the weaker Walker circulation the suppression of tropical convection the weakened local Hadley circulation

V. negative SLP anomaly over the Eurasian continent corresponds to: a weakening of the SH weakening in the AL and SH weakened pressure gradient in the eastern coast of the Eurasian continent weakening of the monsoon NW flows

V. The EAWM response against global warming. SH may have some covariability with the Arctic Oscillation (AO), which may indirectly influence the intensity of the EAWM.

VI. Future studies The influence of boundary conditions in the surrounding region The effect of snow cover, The role of the Kuroshio extension The mid-latitude air-sea coupling The long term SST distribution over the north Pacific.

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