This study investigates the projected changes in interannual variability of South Asian summer monsoon and changes of ENSO-monsoon relationships in the 21st century under the Intergovernmental Panel on Climate Change (IPCC) scenarios A1B and A2, respectively, by analyzing the simulated results of twelve Coupled Model Intercomparison Project Phase 3 (CMIP3) coupled models. The dynamical monsoon index (DMI) was adopted to describe the interannual variability of South Asian summer monsoon, and the standard deviation (SD) was used to illustrate the intensity of interannual variability. It was found that most models could project enhanced interannual variability of monsoon in the 21st century. The multi-model ensemble (MME) results showed increases in the interannual variability of DMI: 14.3% and 20.0% under scenarios A1B and A2, respectively. The MME result also showed increases in the rainfall variability are of about 10.2% and 22.0% under scenarios A1B and A2. The intensification of interannual variability tended to occur over the regions that have larger variability currently; that is, "the strong get stronger". Another finding was that ENSO-monsoon relationships are likely to be enhanced in the 21st century. The dynamical component of the monsoon will be more closely correlated to ENSO in the future under global warming, although the ENSO-summer rainfall relationship cannot be reasonably projected by current models. This suggests that the South Asian summer monsoon is more predictable in the future, at least dynamically.
The present study documents the variability of surface sensible heat flux over Northwest China using station observations for the period 1961 2000.It is found that the afternoon and nighttime sensible heat flux variations are remarkably different.The variability of the instant flux in the afternoon is much larger than in the nighttime.The afternoon and nighttime flux anomalies tend to be opposite.The diurnal and seasonal dependence of sensible heat flux variations is closely related to the diurnal cycle of mean land-air temperature difference.The relationship of sensible heat flux with land-air temperature difference based on the instant value differs from that based on the daily mean.The present study indicates the importance for the models to properly simulate mean land-air temperature difference and its diurnal and seasonal variations in order to capture surface sensible heat flux variability over Northwest China and predicts its plausible impacts on climate.
The present study compares seasonal and interdecadal variations in surface sensible heat flux over Northwest China between station observations and ERA-40 and NCEP-NCAR reanalysis data for the period 1960-2000.While the seasonal variation in sensible heat flux is found to be consistent between station observations and the two reanalysis datasets,both land-air temperatures difference and surface wind speed show remarkable systematic differences.The sensible heat flux displays obvious interdecadal variability that is season-dependent.In the ERA-40 data,the sensible heat flux in spring,fall,and winter shows interdecadal variations that are similar to observations.In the NCEP-NCAR reanalysis data,sensible heat flux variations are inconsistent with and sometimes even opposite to observations.While surface wind speeds from the NCEP-NCAR reanalysis data show interdecadal changes consistent with station observations,variations in land-air temperature difference differ greatly from the observed dataset.In terms of land-air temperature difference and surface wind speed,almost no consistency with observations can be identified in the ERA-40 data,apart from the land-air temperature difference in fall and winter.These inconsistencies pose a major obstacle to the application in climate studies of surface sensible heat flux derived from reanalysis data.
This study evaluates the spatial distributions in the quality of momentum and sensible heat fluxes,and determines the turbulent transfer characteristics with quality-controlled observations.The research is based on raw turbulence data collected over a Gobi surface in the Dunhuang area in June 2004.The results indicate that part of the momentum fluxes are of poor quality in the daytime and nighttime.The poor quality of the momentum fluxes in the daytime is mainly attributed to the development of turbulence.The footprint reveals that,in general,the momentum fluxes and sensible heat fluxes can be measured well in the east and west upwind sectors under unstable conditions.The relationship between the non-dimensional standard deviation of the wind components and atmospheric stability follow the "1/3 power law",which supports the Monin-Obukhov similarity theory.Moreover,this study identifies a clear disturbance in the measurements surrounding the Gobi surface.The momentum roughness length of z0m=0.59 mm is determined after excluding such disturbance,and the additional resistance during the daytime is proposed to be an average of 3.1,although its actual value is highly scattered.This study discusses the applicability of several thermodynamic parameterization schemes for the Gobi surface.The results show that the scheme κB-1=3.1 can represent well the summer diurnal turbulent heat transfer.
The temporal variations in storm rainfall during the first rainy season (FRS) in South China (SC) are investigated in this study. The results show that the inter-annual variations in storm rainfall during the FRS in SC seem to be mainly influenced by the frequency of storm rainfall, while both frequency and intensity affect the inter-decadal variations in the total storm rainfall. Using the definitions for the beginning and ending dates of the FRS, and the onset dates of the summer monsoon in SC, the FRS is further divided into two sub-periods, i.e., the frontal and monsoon rainfall periods. The inter-annual and inter-decadal variations in storm rainfall during these two periods are investigated here. The results reveal a significant out-of-phase correlation between the frontal and monsoon storm rainfall, especially on the inter-decadal timescale, the physical mechanism for which requires further investigation.
