Asian Summer Monsoon Precipitation Research Articles

Intercomparison of the climatological variations of Asian summer monsoon precipitation simulated by 10 GCMs

We assesses the overall performance of state-of-the-art atmospheric GCMs in simulating the climatological variations of summer monsoon rainfall over the Asian-Western Pacific region and the systematic errors that are common to a group of GCMs. The GCM data utilized are obtained from 10 GCM groups participated in the CLIVAR/Monsoon GCM Intercomparison Project. The model composite shows that the overall spatial pattern of summer monsoon rainfall is similar to the observed, although the western Pacific rainfall is relatively weak. For the simulated precipitation over the western Pacific, the models can be classified into two categories. The first category of models simulates the precipitation more confined to the equatorial region and weaker precipitation in the subtropical western Pacific compared to the observed. The second category of models simulates large precipitation in the subtropical western Pacific but the region is shifted to the north by 5–10°. None of the models realistically reproduce the observed Mei-yu rain band in the region from the East China Sea to the mid Pacific. Most of the models produce a rain band along the continental side of East Asia. The climatological variations of simulated summer rainfall are examined in terms of their amplitude and their principal EOF modes. All models simulate larger amplitudes of the climatological seasonal variation of Indian summer monsoon than the observed, though most models simulate smaller amplitudes in the western Pacific. The ten model composite produces four leading EOF modes over the Asian-western Pacific region, which are remarkably similar to the observed counterparts. The first and second eigenmodes, respectively, represent the smoothed seasonal march of broad-scale monsoon and the onsets of the Indian and East Asian summer monsoon. The third and fourth modes relate to the climatological intraseasonal oscillation (CISO). In contrast to the model composite, several models fail to reproduce the first principal mode, and most models do not reproduce the observed modes higher than the second. The CISO of precipitation is also examined over the Indian monsoon and the East Asia-western Pacific monsoon regions separately.

Asynchronous Holocene optimum of the East Asian monsoon

The spatial and temporal distribution of summer monsoon precipitation (or effective moisture) during the Holocene has been reconstructed on the basis of geological data, including lake levels, pollen profiles, and loess/paleosol records. In addition, the summer (July) precipitation increment, effective precipitation, and monsoon strength index have been obtained from numerical modeling experiments. Both geological data and numerical modeling indicate that the Holocene optimum, as defined by peak East Asian summer monsoon precipitation, was asynchronous in central and eastern China, reaching a maximum at different times in different regions, e.g., ca. 10,000–8000 yr ago in northeastern China, 10,000–7000 yr ago in north-central and northern east-central China, ca. 7000–5000 yr ago in the middle and lower reaches of the Yangtze River, and ca. 3000 yr ago in southern China. In southwestern China the maximum appeared ca. 11,000 yr ago, but probably was related to the maximum landward extension of the Indian summer monsoon. The regional shift in the maximum precipitation belt from northwest to southeast over the past 10,000 yr is interpreted as a response to changing seasonality related to orbital forcing of the climate. The southeastward shift of the East Asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the Holocene and the East Asian monsoon index declined, while the early maximum in southwestern China matches the maximum of the Indian monsoon index.

Evidence for Asian summer monsoon precipitation instability of the Younger Dryas phase

The loess and desert transitional zone of China responses sensitively to the Asian monsoon fluctuations. A high resolution loess-dust and aeolian sand-paleosols section records the climatic history of the Younger Dryas. Based upon AMS14C dating of pollen concentrates and proxy climate indices including magnetic susceptibilty, organic carbon content and grain size, it is demonstrated that the Asian monsoon climate manifests not only cooling as in other regions of the Northern Hemisphere, but also unusual oscillations in precipitation which were probably associated with atmospheric interactions between both the Northern and Southern Hemispheres and atmospheric pressure anomalies of topic Pacific Ocean (ENSO).

硫酸エアロゾルによる散乱を陽に表現した大気海洋結合モデルによる気候変化実験

The transient response of a coupled ocean-atmosphere model to increasing concentrations of greenhouse gases and sulfate aerosols, is investigated with an explicit representation of aerosol scattering. Experiments with an implicit representation of aerosol scattering, through the modification of surface albedo with the same parameter values as used in previous climate projections, are also conducted for comparison. The indirect effect of aerosol is yet to be included. As suggested by previous radiation computation studies, the estimated radiative forcing due to the direct effect of sulfate aerosol is significantly smaller with the explici representation, than with the implicit one. A principal source of the overestimation by the implicit method is the neglect of the dependence of aerosol scattering on near-surface humidity. The projected surface air temperature change due to the addition of sulfate aerosols is considerably smaller in magnitude especially over dry regions with the explicit method, than with the implicit one. It is also suggested that the change in the Asian summer monsoon precipitation due to an increase in sulfate aerosols is particularly sensitive to the representation of sulfate aerosol scattering.