ALBERT

Description: This paper addresses several hypotheses designed to explain why AOGCM simulations of future climate in the third phase of the Coupled Model Intercomparison Project (CMIP3) feature an intensified reduction of precipitation over the Meso-America (MA) region. While the drying is consistent with an amplification of the subtropical high pressure cells and an equatorward contraction of convective regions due to the “upped ante” for convection in a warmer atmosphere, the physical mechanisms behind the intensity and robustness of the MA drying signal have not been fully explored. Regional variations in sea surface temperature (SST) warming may play a role. First, SSTs over the tropical North Atlantic (TNA) do not warm as much as the surrounding ocean. The troposphere senses a TNA that is cooler than the tropical Pacific, potentially exciting a Gill-type response, increasing the strength of the North Atlantic subtropical high. Second, the warm ENSO-like state simulated in the eastern tropical Pacific could decrease precipitation over MA, as warm ENSO events are associated with drying over MA. The authors use the International Centre for Theoretical Physics (ICTP) AGCM to investigate the effects of these regional SST warming variations on the projected drying over MA. First, the change of SSTs [Special Report on Emissions Scenarios (SRES) A1B’s Twentieth-Century Climate in Coupled Model (A1B-20C)] in the ensemble average of the CMIP3 models is applied to determine if the ICTP AGCM can replicate the future drying. Then the effects of 1) removing the reduced warming over the TNA, 2) removing the warm ENSO-event-like pattern in the eastern tropical Pacific, and 3) applying uniform SST warming throughout the tropics are tested. The ICTP AGCM can reproduce the general pattern and amount of precipitation over MA. Simulations in which the CMIP3 A1B-20C ensemble-average SSTs are added to climatological SSTs show drying of more than 20% over the MA region, similar to the CMIP3 ensemble average. Replacing the relatively cooler SSTs over the TNA excites a Gill response consistent with an off-equatorial heating anomaly, showing that the TNA relative cooling is responsible for about 16% (31%) of the drying in late spring (early summer). The warm ENSO-like SST pattern over the eastern Pacific also affects precipitation over the MA region, with changes of 19% and 31% in March–June (MMJ) and June–August (JJA), respectively. This work highlights the importance of understanding even robust signals in the CMIP3 future scenario simulations, and should aid in the design and analysis of future climate change studies over the region.

Description: Rapid and broad-scale forest mortality associated with recent droughts, rising temperature, and insect outbreaks has been observed over western North America (NA). Climate models project additional future warming and increasing drought and water stress for this region. To assess future potential changes in vegetation distributions in western NA, the Community Earth System Model (CESM) coupled with its Dynamic Global Vegetation Model (DGVM) was used under the future A2 emissions scenario. To better span uncertainties in future climate, eight sea surface temperature (SST) projections provided by phase 3 of the Coupled Model Intercomparison Project (CMIP3) were employed as boundary conditions. There is a broad consensus among the simulations, despite differences in the simulated climate trajectories across the ensemble, that about half of the needleleaf evergreen tree coverage (from 24% to 11%) will disappear, coincident with a 14% (from 11% to 25%) increase in shrubs and grasses by the end of the twenty-first century in western NA, with most of the change occurring over the latter half of the twenty-first century. The net impact is a ~6 GtC or about 50% decrease in projected ecosystem carbon storage in this region. The findings suggest a potential for a widespread shift from tree-dominated landscapes to shrub and grass-dominated landscapes in western NA because of future warming and consequent increases in water deficits. These results highlight the need for improved process-based understanding of vegetation dynamics, particularly including mortality and the subsequent incorporation of these mechanisms into earth system models to better quantify the vulnerability of western NA forests under climate change.

Description: Results from aquaplanet experiments performed using the Model for Prediction across Scales (MPAS) hydrostatic dynamical core implemented within the Department of Energy (DOE)–NCAR Community Atmosphere Model (CAM) are presented. MPAS is an unstructured-grid approach to climate system modeling that supports both quasi-uniform and variable-resolution meshing of the sphere based on conforming grids. Using quasi-uniform simulations at resolutions of 30, 60, 120, and 240 km, the authors evaluate the performance of CAM-MPAS via its kinetic energy spectra, general circulation, and precipitation characteristics. By analyzing an additional variable-resolution simulation with grid spacing that varies from 30 km in a spherical, continental-sized equatorial region to 240 km elsewhere, the CAM-MPAS’s potential for use as a regional climate simulation tool is explored. Similar to other quasi-uniform aquaplanet simulations, tropical precipitation increases with resolution, indicating the resolution sensitivity of the physical parameterizations. Comparison with the finite volume (FV) dynamical core suggests a weaker tropical circulation in the CAM-MPAS simulations, which is evident in reduced tropical precipitation and a weaker Hadley circulation. In the variable-resolution simulation, the kinetic energy spectrum within the high-resolution region closely resembles the quasi-uniform 30-km simulation, indicating a robust simulation of the fluid dynamics. As suggested by the quasi-uniform simulations, the CAM4 physics behave differently in the high and low resolution regions. A positive precipitation anomaly occurs on the western edge of the high-resolution region, exciting a Gill-type response; this zonal asymmetry represents the errors incurred in a variable resolution setting. When paired with a multiresolution mesh, the aquaplanet test case offers an exceptional opportunity to examine the response of physical parameterizations to grid resolution.

Description: Aquaplanet simulations using the Community Atmosphere Model, version 4 (CAM4), with the Model for Prediction Across Scales–Atmosphere (MPAS-A) and High-Order Method Modeling Environment (HOMME) dynamical cores and using zonally symmetric sea surface temperature (SST) structure are studied to understand the dependence of the intertropical convergence zone (ITCZ) structure on resolution and dynamical core. While all resolutions in HOMME and the low-resolution MPAS-A simulations give a single equatorial peak in zonal mean precipitation, the high-resolution MPAS-A simulations give a double ITCZ with precipitation peaking around 2°–3° on either side of the equator. This study reveals that the structure of ITCZ is dependent on the feedbacks between convection and large-scale circulation. It is shown that the difference in specific humidity between HOMME and MPAS-A can lead to different latitudinal distributions of the convective available potential energy (CAPE) by influencing latent heat release by clouds and the upper-tropospheric temperature. With lower specific humidity, the high-resolution MPAS-A simulation has CAPE increasing away from the equator that enhances convection away from the equator and, through a positive feedback on the circulation, results in a double ITCZ structure. In addition, it is shown that the dominance of antisymmetric waves in the model is not enough to cause double ITCZ, and the lateral extent of equatorial waves does not play an important role in determining the width of the ITCZ but rather the latter may influence the former.

Description: Analyses of phase 5 of the Coupled Model Intercomparison Project (CMIP5) experiments show that the global monsoon is expected to increase in area, precipitation, and intensity as the climate system responds to anthropogenic forcing. Concurrently, detailed analyses for several individual monsoons indicate a redistribution of rainfall from early to late in the rainy season. This analysis examines CMIP5 projected changes in the annual cycle of precipitation in monsoon regions, using a moist static energy framework to evaluate competing mechanisms identified to be important in precipitation changes over land. In the presence of sufficient surface moisture, the local response to the increase in downwelling energy is characterized by increased evaporation, increased low-level moist static energy, and decreased stability with consequent increases in precipitation. A remote mechanism begins with warmer oceans and operates on land regions via a warmer tropical troposphere, increased stability, and decreased precipitation. The remote mechanism controls the projected changes during winter, and the local mechanism controls the switch to increased precipitation during summer in most monsoon regions. During the early summer transition, regions where boundary layer moisture availability is reduced owing to decreases in evaporation and moisture convergence experience an enhanced convective barrier. Regions characterized by adequate evaporation and moisture convergence do not experience reductions in early summer precipitation. This enhanced convective barrier leads to a redistribution of rainfall from early to late summer, and is robust in the American and African monsoons but muddled in Asia. As described here, viewing monsoons from their inherent ties to the annual cycle could help to fingerprint changes as they evolve.

Description: This study investigates the moisture budgets and resolution dependency of precipitation extremes in an aquaplanet framework based on the Community Atmosphere Model, version 4 (CAM4). Moisture budgets from simulations using two different dynamical cores, the Model for Prediction Across Scales-Atmosphere (MPAS-A) and High Order Method Modeling Environment (HOMME), but the same physics parameterizations suggest that during precipitation extremes the intensity of precipitation is approximately balanced by the vertical advective moisture transport. The resolution dependency in extremes from simulations at their native grid resolution originates from that of vertical moisture transport, which is mainly explained by changes in dynamics (related to vertical velocity ω) with resolution. When assessed at the same grid scale by area-weighted averaging the fine-resolution simulations to the coarse grids, simulations with either dynamical core still demonstrate resolution dependency in extreme precipitation with no convergence over the tropics, but convergence occurs at a wide range of latitudes over the extratropics. The use of lower temporal frequency data (i.e., daily vs 6 hourly) reduces the resolution dependency. Although thermodynamic (moisture) changes become significant in offsetting the effect of dynamics when assessed at the same grid scale, especially over the extratropics, changes in dynamics with resolution are still large and explain most of the resolution dependency during extremes. This suggests that the effects of subgrid-scale variability of ω and vertical moisture transport during extremes are not adequately parameterized by the model at coarse resolution. The aquaplanet framework and analysis described in this study provide an important metric for assessing sensitivities of cloud parameterizations to spatial resolution and dynamical cores under extreme conditions.

Description: This study compares the error characteristics associated with two grid refinement approaches including global variable resolution and nesting for high-resolution regional climate modeling. The global variable-resolution model, Model for Prediction Across Scales-Atmosphere (MPAS-A), and the limited-area model, Weather Research and Forecasting Model (WRF), are compared in an idealized aquaplanet context. For MPAS-A, simulations have been performed with a quasi-uniform-resolution global domain at coarse (1°) and high (0.25°) resolution, and a variable-resolution domain with a high-resolution region at 0.25° configured inside a coarse-resolution global domain at 1° resolution. Similarly, WRF has been configured to run on a coarse (1°) and high (0.25°) tropical channel domain as well as a nested domain with a high-resolution region at 0.25° nested two-way inside the coarse-resolution (1°) tropical channel. The variable-resolution or nested simulations are compared against the high-resolution simulations. Both models respond to increased resolution with enhanced precipitation and significant reduction in the ratio of convective to nonconvective precipitation. The limited-area grid refinement induces zonal asymmetry in precipitation (heating), accompanied by zonal anomalous Walker-like circulations and standing Rossby wave signals. Within the high-resolution limited area, the zonal distribution of precipitation is affected by advection in MPAS-A and by the nesting strategy in WRF. In both models, the propagation characteristics of equatorial waves are not significantly affected by the variations in resolution.