Aurel F. Moise

The Asian summer monsoon: an intercomparison of CMIP5 vs. CMIP3 simulations of the late 20th century

The boreal summer Asian monsoon has been evaluated in 25 Coupled Model Intercomparison Project-5 (CMIP5) and 22 CMIP3 GCM simulations of the late twentieth Century. Diagnostics and skill metrics have been calculated to assess the time-mean, climatological annual cycle, interannual variability, and intraseasonal variability. Progress has been made in modeling these aspects of the monsoon, though there is no single model that best represents all of these aspects of the monsoon. The CMIP5 multi-model mean (MMM) is more skillful than the CMIP3 MMM for all diagnostics in terms of the skill of simulating pattern correlations with respect to observations. Additionally, for rainfall/convection the MMM outperforms the individual models for the time mean, the interannual variability of the East Asian monsoon, and intraseasonal variability. The pattern correlation of the time (pentad) of monsoon peak and withdrawal is better simulated than that of monsoon onset. The onset of the monsoon over India is typically too late in the models. The extension of the monsoon over eastern China, Korea, and Japan is underestimated, while it is overestimated over the subtropical western/central Pacific Ocean. The anti-correlation between anomalies of all-India rainfall and Niño3.4 sea surface temperature is overly strong in CMIP3 and typically too weak in CMIP5. For both the ENSO-monsoon teleconnection and the East Asian zonal wind-rainfall teleconnection, the MMM interannual rainfall anomalies are weak compared to observations. Though simulation of intraseasonal variability remains problematic, several models show improved skill at representing the northward propagation of convection and the development of the tilted band of convection that extends from India to the equatorial west Pacific. The MMM also well represents the space–time evolution of intraseasonal outgoing longwave radiation anomalies. Caution is necessary when using GPCP and CMAP rainfall to validate (1) the time-mean rainfall, as there are systematic differences over ocean and land between these two data sets, and (2) the timing of monsoon withdrawal over India, where the smooth southward progression seen in India Meteorological Department data is better realized in CMAP data compared to GPCP data.

Evaluation of the South Pacific Convergence Zone in IPCC AR4 Climate Model Simulations of the Twentieth Century

Abstract Understanding how the South Pacific convergence zone (SPCZ) may change in the future requires the use of global coupled atmosphere–ocean models. It is therefore important to evaluate the ability of such models to realistically simulate the SPCZ. The simulation of the SPCZ in 24 coupled model simulations of the twentieth century is examined. The models and simulations are those used for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). The seasonal climatology and interannual variability of the SPCZ is evaluated using observed and model precipitation. Twenty models simulate a distinct SPCZ, while four models merge intertropical convergence zone and SPCZ precipitation. The majority of models simulate an SPCZ with an overly zonal orientation, rather than extending in a diagonal band into the southeast Pacific as observed. Two-thirds of models capture the observed meridional displacement of the SPCZ during El Nino and La Nina events. The four models that use ...

Consensus on Twenty-First-Century Rainfall Projections in Climate Models More Widespread than Previously Thought

AbstractUnder global warming, increases in precipitation are expected at high latitudes and near major tropical convergence zones in some seasons, while decreases are expected in many subtropical and midlatitude areas in between. In many other areas there is no consensus among models on the sign of the projected change. This is often assumed to indicate that precipitation projections in these regions are highly uncertain.Here, twenty-first century precipitation projections under the Special Report on Emissions Scenarios (SRES) A1B scenario using 24 World Climate Research Programme (WCRP)/Coupled Model Intercomparison Project phase 3 (CMIP3) climate models are examined. In areas with no consensus on the sign of projected change there are extensive subregions where the projected change is “very likely” (i.e., probability > 0.90) to be small (relative to, e.g., the size of interannual variability during the late twentieth century) or zero. The statistical significance of and interrelationships between method...

The South Pacific Convergence Zone in CMIP5 simulations of historical and future climate

The South Pacific Convergence Zone (SPCZ) is evaluated in historical simulations from 26 Coupled Model Intercomparison Project Phase 5 (CMIP5) models, and compared with previous generation CMIP3 models. A subset of 24 CMIP5 models are able to simulate a distinct SPCZ in the December to February (DJF) austral summer, although the position of the SPCZ in these models is too zonal compared with observations. The spatial pattern of SPCZ precipitation is improved in CMIP5 models relative to CMIP3 models, although the spurious double ITCZ precipitation band in the eastern Pacific is intensified in many CMIP5 models. All CMIP5 models examined capture some interannual variability of SPCZ latitude, and 19 models simulate a realistic correlation with El Niño–Southern Oscillation. In simulations of the twenty-first century under the RCP8.5 emission scenario, no consistent shift in the mean position of the DJF SPCZ is identified. Several models simulate significant shifts northward, and a similar number of models simulate significant southward shifts. The majority of CMIP5 models simulate an increase in mean DJF SPCZ precipitation, and there is an intensification of the eastern Pacific double ITCZ precipitation band in many models. Most models simulate regions of increased precipitation in the western part of the SPCZ and near the equator, and regions of decreased precipitation at the eastern edge of the SPCZ. Decomposition of SPCZ precipitation changes into dynamic and thermodynamic components reveals predominantly increased precipitation due to thermodynamic changes, while dynamic changes lead to regions of both positive and negative precipitation anomalies.

Will a Warmer World Mean a Wetter or Drier Australian Monsoon

AbstractMultimodel mean projections of the Australian summer monsoon show little change in precipitation in a future warmer climate, even under the highest emission scenario. However, there is large uncertainty in this projection, with model projections ranging from around a 40% increase to a 40% decrease in summer monsoon precipitation. To understand the source of this model uncertainty, a set of 33 climate models from the Coupled Model Intercomparison Project phase 5 (CMIP5) is divided into groups based on their future precipitation projections (DRY, MID, and WET terciles). The DRY model mean has enhanced sea surface temperature (SST) warming across the equatorial Pacific, with maximum increases in precipitation in the western equatorial Pacific. The DRY model mean also has a large cold bias in present day SSTs in this region. The WET model mean has the largest warming in the central and eastern equatorial Pacific, with precipitation increases over much of Australia. These results suggest lower confiden...

The Australian Summer Monsoon in Current and Future Climate

As the counterpart of the Asian monsoon in the Northern Hemisphere, in this chapter the Australian summer monsoon covers a spatial domain encompassing tropical Sumatra and the Java Islands, and the adjacent waters in the west, extending south and eastward into the Timor and Timor Sea region and further penetrating into the tropical Australian continent. This chapter documents its observed features at a range of temporal and spatial scales, evaluates how well the current climate models can reproduce these fundamental features, and finally summarizes current projections of its potential changes in future and primary processes leading to such changes. The monsoon system shows pronounced seasonal variations of rainfall and prevailing wind, with its austral summer season rainfall being supported by the reversal of easterly trade winds into deep and moist westerlies. Its onsets are influenced by a number of factors including the Madden-Julian Oscillation (MJO), land-sea thermal contrast, the influence of middle latitude systems, and the inherent atmospheric instability. The inter-annual variations of the monsoon onset are correlated to El Nino-Southern Oscillation (ENSO), but the total summer monsoon rainfall is not. This is partially attributed to the seasonally varying air-sea interactions over the waters north of the Australian continent. The Asian aerosol and tropical SSTs near the continent have been used in explaining observed rainfall increases northwest of the tropical Australian continent. In fully coupled global climate model simulations, the broad features of the monsoon mean climate, its seasonal and inter-annual variations of rainfall, temperature and circulation, can be reasonably reproduced by a majority of the models, but there are very large variations in individual model skills. The relevant importance of primary large-scale drivers governing the monsoon variations differs significantly across current climate models, with some models having too strong an ENSO influence. There is great uncertainty in model projections of the changes in the monsoon rainfall under global warming. A weak change in mean rainfall from multi-model ensemble averages is largely produced accompanied by large model discrepancies, with the number of the models showing likely increases in rainfall being matched by roughly the same number of the models showing decreased rainfall. There is a lack of consensus regarding the changes in the Australian monsoon onset/retreat, with studies using rainfall in defining the monsoon onset suggesting the onset comes earlier. However, for the studies using circulation as one of the criteria, they showed a possible delay due to the weakening and shifting of the atmospheric circulation. Future progress to improve the modelling skill and increase our confidence of the projection of the Australian monsoon relies on a number of key aspects, including the improved model physics and dynamics with increased model resolutions; improved representations of key drivers of the monsoon system, such as realistic MJO, ENSO and Indian Ocean Dipole (IOD) in climate models; and improved understanding of the model discrepancies.

Constraints on Southern Australian Rainfall Change Based on Atmospheric Circulation in CMIP5 Simulations

AbstractAtmospheric circulation change is likely to be the dominant driver of multidecadal rainfall trends in the midlatitudes with climate change this century. This study examines circulation features relevant to southern Australian rainfall in January and July and explores emergent constraints suggested by the intermodel spread and their impact on the resulting rainfall projection in the CMIP5 ensemble. The authors find relationships between models’ bias and projected change for four features in July, each with suggestions for constraining forced change. The features are the strength of the subtropical jet over Australia, the frequency of blocked days in eastern Australia, the longitude of the peak blocking frequency east of Australia, and the latitude of the storm track within the polar front branch of the split jet. Rejecting models where the bias suggests either the direction or magnitude of change in the features is implausible produces a constraint on the projected rainfall reduction for southern A...

Projected increases in daily to decadal variability of Asian-Australian monsoon rainfall

Changes in rainfall variability in future climate will pose challenges for adaptation. To evaluate changes in Asian-Australian monsoon wet season rainfall, daily data from historical and future (Representative Concentration Pathway “RCP8.5”) climate simulations are band pass-filtered to isolate variability on near-daily, weekly, monthly, intraseasonal, annual, interannual, and decadal time scales. This method is used to quantify changes in variability from 35 coupled climate models for each time scale over the Australian, South Asian, and East Asian monsoon domains. In nearly all cases, the median model change is positive, indicating increased rainfall variability, although with large model spread. The role of increased atmospheric moisture is examined by estimating the change due to an idealized thermodynamic enhancement. This enhancement produces increases in variability that are within the range of the simulated changes under the RCP8.5 scenario, indicating that thermodynamic responses provide a first-order explanation for the increased daily to decadal monsoon rainfall variability.

On the influence of simulated SST warming on rainfall projections in the Indo-Pacific domain: an AGCM study

Significant uncertainty exists in regional climate change projections, particularly for rainfall and other hydro-climate variables. In this study, we conduct a series of Atmospheric General Circulation Model (AGCM) experiments with different future sea surface temperature (SST) warming simulated by a range of coupled climate models. They allow us to assess the extent to which uncertainty from current coupled climate model rainfall projections can be attributed to their simulated SST warming. Nine CMIP5 model-simulated global SST warming anomalies have been super-imposed onto the current SSTs simulated by the Australian climate model ACCESS1.3. The ACCESS1.3 SST-forced experiments closely reproduce rainfall means and interannual variations as in its own fully coupled experiments. Although different global SST warming intensities explain well the inter-model difference in global mean precipitation changes, at regional scales the SST influence vary significantly. SST warming explains about 20–25% of the patterns of precipitation changes in each of the four/five models in its rainfall projections over the oceans in the Indo-Pacific domain, but there are also a couple of models in which different SST warming explains little of their precipitation pattern changes. The influence is weaker again for rainfall changes over land. Roughly similar levels of contribution can be attributed to different atmospheric responses to SST warming in these models. The weak SST influence in our study could be due to the experimental setup applied: superimposing different SST warming anomalies onto the same SSTs simulated for current climate by ACCESS1.3 rather than directly using model-simulated past and future SSTs. Similar modelling and analysis from other modelling groups with more carefully designed experiments are needed to tease out uncertainties caused by different SST warming patterns, different SST mean biases and different model physical/dynamical responses to the same underlying SST forcing.