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The Maritime Continent (MC) is characterized by a seasonal evolution of rainfall distinct from other regions, due to its unique land–sea distribution and topography. In this study, the roles of surface properties and terrains in controlling the regional climatological rainfall were investigated, based on general circulation model experiments. Results show that the existence of terrain can increase the MC land (MCL) rainfall mainly through its dynamical lifting effect, but otherwise has only moderate influence on rainfall over the MC ocean (MCO). On the other hand, the impact of MC land–sea distribution on the regional rainfall is more seasonally dependent. When replacing the MC flat-land with ocean, rainfall is significantly increased over both MCL and MCO during boreal summer-to-fall, but not in the winter-to-spring season. Further inspection showed that by eliminating the MC flat-land, there is enhanced atmospheric water vapor and convective instability in the summer-to-fall period, contributing to a dramatic increase in precipitation. On the other hand, changes in convective instability and atmospheric water vapor over the MCL act to counteract each other, leading to an only moderate change in rainfall during boreal winter-to-spring. The model-based results suggest that this seasonally dependent influence of the MC flat-land on regional climate mean rainfall is not determined by its modulation on the diurnal cycle. Our results also suggest a larger sensitivity of model bias in representing land–sea distribution/fraction over the MC region during the dry season (i.e. boreal summer) than in the wet season (i.e. boreal winter). [ABSTRACT FROM AUTHOR]

ABSTRACT The climatological intraseasonal oscillations (CISOs) of the subtropical Asian summer rain band (SASRB) are found significantly related with the CISO in the atmospheric circulation over extratropical Eurasian continent, which is dominated by an intraseasonal North Pacific oscillation (ISNAO) pattern and its associated wave trains. The anomalous trough (ridge) of the eastward propagating wave train of the ISNAO pattern affects active (break) rainfall of SASRB and modulates the CISO of SASRB, together with the anomalous active (suppressed) convection northward propagating from the tropics and the subtropics. Moreover, the anomalous trough (ridge) around the Lake Baikal bridges the CISO in the mid-high latitudes and that of SASRB. The NCEP Climate Forecast System version 2 (CFSv2) depicts the spatial-temporal features, principal modes, and propagation of the CISO of SASRB reasonably well. However, the mean state of precipitation and the variance of CISO are overestimated. In the model, the ISNAO pattern is too weak and its fluctuating frequency is too high. Moreover, the CFSv2 only shows marginal skills in simulating the ISNAO pattern and the propagation of its associated wave trains, and underestimates the relationship between the atmospheric circulation in the mid-high latitudes and the CISO of SASRB. In addition, overestimation of the intensity of CISO in the tropical-subtropical and higher-latitude atmosphere may cause overestimation of the CISO of SASRB. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, we examined the annual precipitation amounts, the seasonality over global land and their linear trends, as well as the uncertainties in two observations (precipitation reconstruction and Global Precipitation Climatology Centre), and then compared them with historical runs by multiple models. Overall, the large-scale patterns of both the climatology of the annual precipitation amount and the seasonality are consistent between the two observations. Nevertheless, some noticeable differences existed, particularly in the regions with fewer gauge observations, such as northern Africa and the Tibetan Plateau. For long-term changes, significant drying trends during 1948-2005 were observed in the tropical areas of northern Africa, accompanied by significant wetting trends in the polar region of Canada. The seasonality change during the period was dominated by a decreasing trend in precipitation, especially in the western portion of Russia. The model simulations of the Coupled Model Intercomparison Project, Phase 5 ( CMIP5) reproduced the climatological mean state of annual precipitation and its seasonality in the observations, as well as to some extent the zonal mean trends of precipitation amounts, but did not reproduce the zonal mean trends of seasonality. The two-dimensional distribution of linear trends of annual precipitation and seasonality simulated by CMIP5 models showed little consistency with their observational counterparts. One possibility for the inconsistencies was that they were largely determined by internal variations of the climate system rather than external forcings. In contrast, it might also suggest a challenge for state-of-the-art climate models to correctly simulate the spatial distribution of responses of annual precipitation amounts and seasonality to the evolution of external forcings. Our results suggest that, in addition to the precipitation amount, seasonality should be used as a metric to assess the ability of a climate model to simulate current climate conditions and project future climate change. [ABSTRACT FROM AUTHOR]

The NCEP Climate Forecast System (CFS) is an important source of information for seasonal climate prediction in many Asian countries affected by monsoon climate. The authors provide a comprehensive analysis of the prediction of the Asian summer monsoon (ASM) by the new CFS version 2 (CFSv2) using the hindcast for 1983-2010, focusing on seasonal-to-interannual time scales. Many ASM features are well predicted by the CFSv2, including heavy monsoon rainfall centers, large-scale monsoon circulation patterns, and monsoon onset and retreat features. Several commonly used dynamical monsoon indices and their associated precipitation and circulation patterns can be predicted several months in advance. The CFSv2 has better skill in predicting the Southeast Asian monsoon than predicting the South Asian monsoon. Compared to CFS version 1 (CFSv1), the CFSv2 has increased skill in predicting large-scale monsoon circulation and precipitation features but decreased skill for the South Asian monsoon, although some biases in the CFSv1 still exist in the CFSv2, especially the weaker-than-observed western Pacific subtropical high and the exaggerated strong link of the ASM to ENSO. Comparison of CFSv2 hindcast with output from Atmospheric Model Intercomparison Project (AMIP) and Coupled Model Intercomparison Project (CMIP) simulations indicates that exclusion of ocean-atmosphere coupling leads to a weaker ASM. Compared to AMIP, both hindcast and CMIP show a more realistic annual cycle of precipitation, and the interannual variability of the ASM is better in hindcast. However, CMIP does not show any advantage in depicting the processes associated with the interannual variability of major dynamical monsoon indices compared to AMIP. [ABSTRACT FROM AUTHOR]