Boreal summer intraseasonal variability simulated in the NCEP climate forecast system: Insights from moist static energy budget and sensitivity to convective moistening

The NCEP Climate Forecast System (CFS) with the relaxed Arakawa Schubert (RAS, hereafter referred to as CTRL) convection scheme of Moorthi and Suarez exhibits better performance in representing boreal summer tropical intraseasonal variability as compared with a simulation using simplified Arakawa-Schubert scheme. The intraseasonal moist static energy (MSE) budget is analyzed in this version of the CFS model (CTRL), which produces realistic eastward and northward propagation characteristics. The moist and thermodynamic processes involved in the maintenance and propagation of the poleward moving intraseasonal oscillation (ISO) disturbances are examined here. Budget diagnostics show that horizontal MSE advection is the principal component of the budget, contributing to the poleward movement of the convection. The injection of MSE moistens the atmosphere north of the convective area causing the poleward movement of convection by destabilization of the atmosphere. The moistening process is mainly contributed by the climatological wind acting on the anomalous moisture gradient as confirmed from the examination of moisture advection equation. While surface enthalpy fluxes (consisting of radiative and surface turbulent heat fluxes) maintain the ISO anomalies, they oppose the MSE tendency due to horizontal advection thus regulating the poleward propagation characteristics. In addition, the model results show that wind-evaporation feedback dominates over cloud-radiation feedback for ISO propagation; this is in contrast to our estimates using the newly available European Centre for Medium Range Weather Forecasts Interim reanalysis. Sensitivity experiments suggest that intraseasonal variability in the CFS model with the RAS scheme is highly sensitive to the parameterization of both the shallow convection and the convective rain evaporation and downdrafts. Removal of these components adversely affects the propagation characteristics and greatly reduces the amplitude of intraseasonal variability. Our results support the primary importance of the moisture preconditioning ahead of the ISO and the physical relationship between moisture and precipitation. For realistic ISO simulations, models need to represent these features appropriately

Global warming and the weakening of the Asian summer monsoon circulation: assessments from the CMIP5 models

The evolution of the Asian summer monsoon (ASM) in a global warming environment is a serious scientific and socio-economic concern since many recent studies have demonstrated the weakening nature of large-scale tropical circulation under anthropogenic forcing. But, how such processes affect the ASM circulation and rainfall is still a matter of debate. This study examines the climate model projections from a selected set of Coupled Model Inter-comparison Project 5 (CMIP5) models to provide a unified perspective on the future ASM response. The results indicate a robust reduction in the large-scale meridional gradient of temperature (MGT) at upper levels (200 hPa) over the ASM region, associated with enhanced ascendance and deep tropospheric heating over the equatorial Pacific in the future climate. The differential heating in the upper troposphere, with concomitant increase (decrease) in atmospheric stability (MGT), weakens the ASM circulation, promotes a northward shift of the monsoon circulation and a widening of the local Hadley cell in the eastern Indian sector. An examination of the water vapour budget indicates the competing effects of the thermodynamic (moisture convergence) and dynamics processes (monsoon circulation) on future ASM rainfall changes. The former component wins out over the later one and leads to the intensification of Indian monsoon rainfall in the CMIP5 projections. However, the diagnostics further show a considerable offset due to the dynamic component

Global warming shifts the monsoon circulation, drying South Asia

Monsoon rainfall over South Asia has decreased during the last 5 to 6 decades according to several sets of observations. Although sea surface temperature (SST) has risen across the Indo-Pacific warm pool during this period, the expected accompanying increased rainfall has occurred only in the tropical western Pacific. The above changes noted in observations are also seen in a coupled climate model, but only when the model includes the recent increase in greenhouse gas concentration. The hypothesis that the robust rise in SST over the warm pool, perhaps anchored by an increase in greenhouse gas concentrations, is instrumental in the east- west shift in monsoon rainfall (enhanced rainfall over tropical western Pacific and decreased rainfall over South Asia) is proposed.Asuite of controlled experiments with an atmospheric general circulation model has been performed to isolate the impact of regional SST warming trends on the dryness over South Asia. Model experiments support the hypothesis that the rising SST trend over the tropical western Pacific has changed the atmospheric circulation: over the Bay of Bengal more dry and cool air is advected from the northeast than previously. Moist static energy budget diagnostics on the model solutions identify the sources for this east- west shift. SST warming over the warm pool has accelerated in recent decades. Therefore, a close monitoring of that warming is important for long-term variations of monsoon rainfall. The inconsistency in the amplitude of drying over South Asia among the various land-based rainfall observations and lack of sustained rainfall observations over the open oceans, however, poses constraints in the results

Towards a realistic simulation of boreal summer tropical rainfall climatology in state-of-the-art coupled models: role of the background snow-free land albedo

State-of-the-art global coupled models used in seasonal prediction systems and climate projections still have important deficiencies in representing the boreal summer tropical rainfall climatology. These errors include prominently a severe dry bias over all the Northern Hemisphere monsoon regions, excessive rainfall over the ocean and an unrealistic double inter-tropical convergence zone (ITCZ) structure in the tropical Pacific. While these systematic errors can be partly reduced by increasing the horizontal atmospheric resolution of the models, they also illustrate our incomplete understanding of the key mechanisms controlling the position of the ITCZ during boreal summer. Using a large collection of coupled models and dedicated coupled experiments, we show that these tropical rainfall errors are partly associated with insufficient surface thermal forcing and incorrect representation of the surface albedo over the Northern Hemisphere continents. Improving the parameterization of the land albedo in two global coupled models leads to a large reduction of these systematic errors and further demonstrates that the Northern Hemisphere subtropical deserts play a seminal role in these improvements through a heat low mechanism