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Based on the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis data from 1950{1999, interdecadal variability of the East Asian Summer Monsoon (EASM) and its associated atmospheric circulations are investigated. The EASM exhibits a distinct interdecadal variation, with stronger (weaker) summer monsoon maintained from 1950{1964 (1976{1997). In the former case, there is an enhanced Walker cell in the eastern Paciflc and an anti-Walker cell in the western Paciflc. The associated ascending motion resides in the central Paciflc, which ∞ows eastward and westward in the upper troposphere, descending in the eastern and western ends of the Paciflc basin. At the same time, an anomalous East Asian Hadley Cell (EAHC) is found to connect the low-latitude and mid-latitude systems in East Asia, which strengthens the EASM. The descending branch of the EAHC lies in the west part of the anti-Walker cell, ∞owing northward in the lower troposphere and then ascending at the south of Lake Baikal (40 ‐ {50 ‐ N, 95 ‐ {115 ‐ E) before returning to low latitudes in the upper troposphere, thus strengthening the EASM. The relationship between the EASM and SST in the eastern tropical Paciflc is also discussed. A possible mechanism is proposed to link interdecadal variation of the EASM with the eastern tropical Paciflc SST. A warmer sea surface temperature anomaly (SSTA) therein induces anomalous ascending motion in the eastern Paciflc, resulting in a weaker Walker cell, and at the same time inducing an anomalous Walker cell in the western Paciflc and an enhanced EAHC, leading to a weaker EASM. Furthermore, the interdecadal variation of summer precipitation over North China is found to be strongly regulated by the velocity potential over the south of Lake Baikal through enhancing and reducing the regional vertical motions.

Based on the measurements from the US National Data Buoy Center 3-m discus buoy site No. 44004 (38.5°N, 70.47°W) from January 1 to March 31 of 2003, with the COARE algorithm (Version 3.0), the results from four parameterization schemes developed recently for sea surface aerodynamic roughness length were compared with each other. Calculations of frictional speedu*, drag coefficientCd and wind stress τ indicate that the calculated frictional velocities from the four schemes (8.50%–16.20%, the normalized standard error estimate, or NSEE), the computed drag coefficients and wind stress (respectively 15.08%–28.67% and 17.26%–50.59% NSEE) are reasonable. Schemes YT96 and GW03 are consistent. The O02 scheme gives over-estimated values foru* andCd. Schemes TY01 and GW03 display discontinuous characteristics in handling young wave data.

The effect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced. The simulations adopt relatively realistic snow mass forcings based on Scanning Multi-channel Microwave Radiometer (SMMR) pentad snow depth data. The physical mechanism and spatial structure of the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Plateau are revealed. The main results are summarized as follows. The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo effect, which is compensated by weaker upward sensible heat flux associated with colder surface temperature, whereas the effects of snow melting and evaporation are relatively smaller. The anomalies of surface heat fluxes can last until June and become unobvious in July. The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May. The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June. In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May. The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this influence is only obvious in early summer and almost disappears in later stages.