Search

This study evaluates total cloud fraction (TCF) and top-of-the-atmosphere cloud radiative effects (CREs) in five widely used reanalyses (CFSR, ERA-Interim, JRA55, MERRA2, and NCEP1) using satellite-based observations for the period 2001–2014, with the emphasis on the Asian monsoon region (AMR) including East Asia (EA) and South Asia (SA). The results indicate that despite certain biases, most reanalyses (especially CFSR and ERA-Interim) broadly capture global spatial patterns of TCF and CREs, with pattern correlations with the observations being greater than 0.7, and also generally reproduce the pronounced contrast of the winter and summer means over EA and SA. In contrast, biases and differences in TCF and CREs in reanalyses are significantly larger over the AMR, particularly in summer. Over EA, the reanalyses underestimate annual and winter mean TCF, shortwave CRE (SWCRE), and net CRE (NCRE), with negative biases of 0–40%. Over SA, the reanalyses broadly reproduce the observed annual cycles of each CRE component, but most of them have considerable biases in magnitude, with an overestimation of NCRE by up to 90%, and an unrealistic ratio of longwave (LWCRE) to SWCRE. Such an unrealistic relationship between LWCRE and SWCRE in some reanalyses may produce an unrealistic annual mean state, annual cycle, and inter-annual variation of NCRE over SA. Comparatively, ERA-Interim and JRA55 give better performance in the annual cycle and inter-annual variations of CREs over EA. The inter-annual biases of TCF and NCRE in CFSR are substantially larger over SA while its TCF reproducibility over EA is the best of the reanalyses. The examination of TCF in MERRA2 with the satellite simulator shows that larger TCF biases occur over the AMR region relative to original satellite and MERRA2 products. It requires more quantitative comparison among reanalyzed and satellite-retrieved cloud properties to identify and improve cloud parameterization.

The interdecadal variability of early summer South China (SC) monsoon rainfall (SCMR) is investigated based on several long-term rainfall data sets from 1926 to 2013. Both correlation and composite analyses suggest a significant out-of-phase relationship between interdecadal fluctuations of SCMR and the preceding wintertime Pacific Decadal Oscillation (PDO), with negative (positive) PDO epochs favouring above-normal (below-normal) SCMR. The physical mechanism relating the wintertime PDO to the interdecadal variability of SCMR appears to take place via atmospheric and oceanic teleconnections. The below-normal SCMR is characterized by a meridionally arranged vortex (MAV) pattern across the East Asia–western North Pacific (WNP) sector, with an anomalous cyclone, an anomalous anticyclone and an anomalous cyclone occurring alternately over the South China Sea (SCS)–WNP, Yangtze Basin and Lake Baikal in the early summer. This may respond to the PDO-related positive sea surface temperature (SST) anomalies locally over the SCS–WNP and remotely over the tropical eastern Pacific, with the latter forcing anomalous Hadley and Walker circulations. SC is located directly under the divergent moisture environment of anomalous northeasterlies between the anomalous cyclone over the SCS–WNP and the anticyclone over the Yangtze Basin. Again, the below-normal SCMR is preceded by negative (positive) SST anomalies in the midlatitude North Pacific (tropical eastern Pacific) associated with positive PDO in wintertime. Such an anomalous SST pattern subsequently induces a strengthened and northward-shifted upper-tropospheric jet in early summer, leading to significant upper-tropospheric convergence over SC. The out-of-phase relationship between SCMR and wintertime PDO as well as related physical processes are validated by coupled model simulations.

The interannual variations of the early summer South China monsoon rainfall (SCMR) in relation to different Pacific Decadal Oscillation (PDO) backgrounds are examined using station rainfall data and the European Centre for Medium-range Weather Forecasts (ECMWF) 40-year reanalysis (ERA-40) data. The objective of this study is to investigate the atmospheric circulation patterns responsible for such variations, thereby understanding the interdecadal modulation and the cause for the extreme wet and dry SCMR. The interannual SCMR variance shows an interdecadal change before and after the late 1970s, with significant differences in sea-surface temperature (SST) and atmospheric circulation anomalies between the negative (1958–1976) and positive (1980–1998) PDO epochs. The dominant atmospheric teleconnection patterns associated with extreme wet and dry SCMR are remarkably different in these two epochs. During 1958–1976, an anomalous wet (dry) SCMR is characterized by a significantly anomalous anticyclone (cyclone) over the South China Sea (SCS) and western Pacific in the lower troposphere in the form of a meridional wave-like coupling along the coast of western North Pacific in the early summer, and preceded by a monopole circulation anomaly over the Ural Mountain in the preceding winter. During 1980–1998, however, an anomalous wet (dry) SCMR features a relatively weak anomalous anticyclone (cyclone) over the SCS in the low level without a significantly meridional juxtaposition of anomalous circulations in the early summer, and it follows a remarkable coherent long wave train pattern in middle-high latitudes in the preceding winter. Such interdecadal differences in anomalous circulation structures suggest that the relative roles of internal atmospheric variability and external forcing may be different from different epochs. But for both epochs, the circulation anomaly over the Ural Mountain shows a highly negative correlation with anomalous SCMR, suggesting that the winter circulation anomaly around the Ural Mountain is of great significance for improving the seasonal prediction of SCMR. Copyright © 2010 Royal Meteorological Society