ALBERT

Description: Moisture budget components over a rectangular region defined by the longitudes 6.0°W–36.0°E and latitudes 30.0°N to 45.0°N, with an area of about 6.08 × 106 km2 over the Mediterranean (Med) Basin, are studied by the use of the Japan Meteorological Agency super-high-resolution (20 km) GCM monthly mean data. The research time periods are 1979–2007 for current run and 2075–2099 for future run. Six rainy months of October to March with a total of 168 months for the current run and 144 months for the future run were selected. The rain months have been categorized into five groups of months based on the mean monthly rainfall amounts where the five groups are P 〈 1.0, 1.0 ≤ P 〈 1.5, 1.5 ≤ P 〈 2.0, 2.0 ≤ P 〈 2.5, and 2.5 mm/d ≤ P. We found that generally, over the Mediterranean, the outflow-inflow is balancing the independently calculated evaporation-precipitation quite well with a correlation coefficient of about 0.89. The present seasonal (October-March) precipitation simulated from the 20 km GCM showed a quite reasonable agreement with the CRU. The seasonal area mean precipitation and evaporation are 1.85 mm/d and 2.44 mm/d, respectively. The largest two precipitation categories contribute over 50% of the total seasonal rainfall. The evaporation varies positively with the precipitation for all precipitation categories. Also, the relatively high mean recycling ratio (55%) indicates that the local Med evaporation has a central role in the local precipitation. Another important finding is that the decreasing trend of recycling ratio with the rising of the precipitation category implies that the outside moisture inflow role increases with the increase of the precipitation category. For all the precipitation categories, the total outflow is larger than the total inflow, indicating that the Med area is an important source of moisture. Individual boundary moisture flux shows that the main moisture comes from the west boundary and contributes 59% of the total inflow, while the main outflow is through east boundary and is responsible for 46% of total outflow. Analysis of monthly precipitation indicates that the October and November have the two largest amount of precipitation over the research region. The moisture budget study separated for the east and the west Med shows that the area mean precipitation for the east and the west Med are 2.14 and 2.29 mm/d, while the evaporation are 4.48 and 3.59 mm/d. The plausible reason for the differences between these two basins has been discussed. The moisture supplies to the east Med is mainly from the west boundary, while for the west Mediterranean, the north boundary inflow also plays an important role along with the west boundary. The future moisture budget components over Med suggest that the precipitation is decreasing from 1.85 to 1.62 mm/d and the evaporation is increasing from 2.44 to 2.56 mm/d between current and future. Another finding is that the largest precipitation number of months decreases from 12% to only 6% of the total number of months, while the intensity of the precipitation in this category enhances in the future.

Description: Monsoons, the most energetic tropical climate system, exert a great social and economic impact upon billions of people around the world. The global monsoon precipitation had an increasing trend over the past three decades. Whether or not this increasing trend will continue in the 21st century is investigated, based on simulations of three high-resolution atmospheric general circulation models that were forced by different future sea surface temperature (SST) warming patterns. The results show that the global monsoon area, precipitation and intensity all increase consistently among the model projections. This indicates that the strengthened global monsoon is a robust signal across the models and SST patterns explored here. The increase of the global monsoon precipitation is attributed to the increases of moisture convergence and surface evaporation. The former is caused by the increase of atmospheric water vapor and the latter is due to the increase of SST. The effect of the moisture and evaporation increase is offset to a certain extent by the weakening of the monsoon circulation.

Description: [1]  The variability of global monsoon area (GMA), global monsoon precipitation (GMP) and global monsoon intensity (GMI) in the present climate (1979–2003) and the future warmer climate (2075–2099) under RCP4.5 scenario was examined based on 19 CMIP5 simulations. In the present-day simulations, the ensemble mean precipitation reproduces the observed GMA, GMP and GMI, although the spread of individual models is large. In the RCP4.5 simulations, most (17 of 19) of the CMIP5 models project enhanced global monsoon activity, with the increases of GMA, GMP and GMI by 1.9%, 3.2%, and 1.3% respectively per 1K of surface warming. The diagnosis of a column-integrated moisture budget indicates that the increase in GMP is primarily attributed to the increases of moisture convergence and surface evaporation, whereas horizontal moisture advection has little effect. A further separation of dynamic and thermodynamic factors shows that increase of the moisture convergence comes mainly from the increase of water vapor, but is partly offset by the convergence effect. The increase of the surface evaporation is caused by the increase of sea-air specific humidity difference, while the change in surface wind speed plays a minor role. The GMP experiences a great year-to-year variation, and it is significantly negatively correlated with the Niño3.4 index averaged over a typical monsoon year (defined from May to the following April) in the pre-industrial control and present-day simulations, similar to observations. Under the RCP4.5 warming, such rainfall variability is intensified and the relationship between monsoon and El Niño strengthens. A large proportion of intensification in the year-to-year monsoon rainfall variability arises from the land monsoon region.

Description: [1]  We provide a new view of global as well as regional monsoonal rainfall and their changes in the twenty-first century under RCP4.5 and RCP8.5 scenarios as projected by 29 climate models that participated in the Coupled Model Intercomparison Project phase 5 (CMIP5). The model results show that the global monsoon area defined by the annual range in precipitation is projected to expand mainly over the central to eastern tropical Pacific, the southern Indian Ocean, and eastern Asia. The global monsoon precipitation intensity and the global monsoon total precipitation are also projected to increase. Indices of heavy precipitation are projected to increase much more than those for mean precipitation. Over the Asian monsoon domain, projected changes in extreme precipitation indices are larger than over other monsoon domains, indicating the strong sensitivity of Asian monsoon to global warming. Over the American and African monsoon regions, projected future changes in mean precipitation are rather modest, but those in precipitation extremes are large. Models project that monsoon retreat dates will delay, while onset dates will either advance or show no change, resulting in lengthening of the monsoon season. However, models' limited ability to reproduce the present monsoon climate and the large scatter among the model projections limit the confidence in the results. The projected increase of the global monsoon precipitation can be attributed to an increase of moisture convergence due to increased surface evaporation and water vapor in the air column though offset to a certain extent by the weakening of the monsoon circulation.

Description: This study focuses on projecting future changes in mean and extreme precipitation in Asia, and discusses their uncertainties. Time-slice experiments using a 20-km-mesh atmospheric general circulation (AGCM) were performed both in the present-day (1979–2003) and the future (2075–2099). To assess the uncertainty of the projections, 12 ensemble projections (i.e., combination of 3 different cumulus schemes and 4 different sea surface temperature (SST) change patterns) were conducted using 60-km-mesh AGCMs. For the present-day simulations, the models successfully reproduced the pattern and amount of mean and extreme precipitation, although the model with the Arakawa–Schubert (AS) cumulus scheme underestimated the amount of extreme precipitation. For the future climate simulations, in South Asia and Southeast Asia, mean and extreme precipitation generally increase, but their changes show marked differences among the projections, suggesting some uncertainty in their changes over these regions. In East Asia, northwestern China and Bangladesh, in contrast, mean and extreme precipitation show consistent increases among the projections, suggesting their increases are reliable for this model framework. Further investigation by analysis of variance (ANOVA) revealed that the uncertainty in the precipitation changes in South Asia and Southeast Asia are derived mainly from differences in the cumulus schemes, with an exception in the Maritime Continent where the uncertainty originates mainly from the differences in the SST pattern.

Description: Two 25 year time-slice experiments were conducted using a 20 km mesh global atmospheric model, one for the present (1979–2003) and the other for the future (2075–2099). To assess the uncertainty of climate change projections, we performed ensemble simulations with the 60 km mesh model combining 4 different sea surface temperatures and 3 atmospheric initial conditions. Horizontal resolution of these global models is higher than or comparable to that of regional climate models applied to South American climate change projections. Both the 20 km mesh model and 60 km mesh model reproduce sufficiently well the observed seasonal precipitation patterns. These models project an increase in wet-season precipitation and a decrease in dry-season precipitation over most of South America. In the future, almost all over South America, precipitation intensity will increase. In particular, precipitation intensity is largest over the southeast South America in the present-day simulation, where future change is also large, implying an increasing risk of flooding in this region including the Parana River. At the same time a large increase of consecutive dry days is projected over the western part of the Amazon, where the amplitude of the seasonal hydrograph is projected to increase in the Amazon River, implying more floods in wet season and droughts in dry season.

Description: [1]  This study investigates thermodynamic and dynamic effects on regional monsoon rainfall changes in a warmer climate, by using 20 climate model outputs in the Coupled Model Intercomparison Project (CMIP) Phase 5. In a warmer climate, rainfall is projected to increase in most monsoon regions. However, the rates of increase are not uniform among the monsoon regions. A diagnosis based on a linearized moisture budget equation reveals that the regional differences of the rainfall change are largely explained by differences in the dynamic effect. In the Asian monsoon regions, monsoon circulation slows down at a much lower rate than in the other monsoon regions, in addition, surface evaporation increases at a higher rate, resulting in a much larger increase in rainfall. These features are commonly found in both CMIP Phase 3 and Phase 5 model projections.

Description: Increases of tropical cyclone intensity with global warming have been demonstrated by historical data studies and theory. This raises great concern regarding future changes in typhoon intensity. The present study addressed the problem to what extent super-typhoons will become intense in the global warming climate of the late twenty-first century. Very high-resolution downscale experiments using a cloud-resolving model without convective parameterizations were performed for the 30 most intense typhoons obtained from the 20-km-mesh global simulation of a warmer climate. Twelve super-typhoons occurred in the downscale experiments and the most intense super-typhoon attained a central pressure of 857 hPa and a wind speed of 88 m s −1 . The maximum intensity of the super-typhoon was little affected by uncertainties that arise from experimental settings. This study indicates that the most intense future super-typhoon could attain wind speeds of 85–90 m s −1 and minimum central pressures of 860 hPa.

Description: [1]  Using both a coupled atmosphere-ocean GCM and an atmospheric GCM, we investigate the effects of the Tibetan Plateau (TP) on the onset of the South Asian summer monsoon by conducting simulations with and without the TP. In the coupled GCM, the presence of the TP causes the monsoon onset to occur approximately 15 days later in the Arabian Sea (AS) and India (ID) and approximately 10 days earlier in the Bay of Bengal (BB). These changes are attributed to different atmospheric circulation patterns and different conditions within the adjacent oceans, such as the AS and the BB. When the TP is included, lower sea surface temperatures (SSTs) in the AS contribute to a stable lower atmosphere, which suppresses convection over the AS and ID in May. In contrast, low pressure over South Asia, caused by the TP, induces a southwesterly toward the BB that transports a substantial amount of water vapor to the BB. This flow results in an earlier monsoon in the BB. Without the TP, higher SSTs that are formed in the AS in May destabilize the lower atmosphere and create a depression, resulting in an earlier onset of the monsoon over the AS and ID. Consequently, the cyclonic circulation spreads abruptly to the BB, and precipitation begins to increase over the BB. Therefore, the air-sea interaction in the adjacent ocean under the influence of the TP strongly modulates the onset of the South Asian summer monsoon. This modulation was verified by the atmospheric GCM experiments.

Description: ABSTRACT This study used the third phase of the Coupled Model Intercomparison Project (CMIP3) multi-model ensemble (MME) and atmospheric general circulation models (AGCMs) with three horizontal resolutions, 20, 60, and 180 km, to investigate climate projections of the Caribbean low-level jet (CLLJ) and accompanying moisture fluxes. Future climate simulations were also performed with 60- and 180-km mesh AGCMs forced by four lower boundary conditions both to quantify uncertainty in the CLLJ projections and to determine the physical mechanism of change in the CLLJ. Changes among the CMIP3 MME models in projected CLLJ in the future climate were inconsistent in sign and statistically insignificant, whereas consistently among the models the easterly moisture flux accompanying the CLLJ significantly intensified. The AGCM simulations with three different horizontal resolutions demonstrated that the merits of dynamical downscaling for the CLLJ and moisture flux were limited for climate projections, although the high-horizontal resolution models improved reproducibility of the CLLJ and moisture flux in the present-day climate and can provide spatially detailed projections. Different projected sea surface temperatures (SSTs) as lower boundary conditions of the 60- and 180-km mesh single-AGCM simulations clearly affected changes in the CLLJ. Both the CMIP3 MME analysis and the 60- and 180-km mesh AGCM ensembles showed that large-scale SST patterns between the eastern tropical Pacific and the region from the Caribbean Sea to the western tropical Atlantic influenced changes in the CLLJ in the future climate, as seen in the present-day climate.