Graziano Giuliani

Changes in extremes and hydroclimatic regimes in the CREMA ensemble projections

We analyze changes of four extreme hydroclimatic indices in the RCP8.5 projections of the Phase I CREMA experiment, which includes 21st century projections over 5 CORDEX domains (Africa, Central America, South America, South Asia, Mediterranean) with the ICTP regional model RegCM4 driven by three CMIP5 global models. The indices are: Heat Wave Day Index (HWD), Maximum Consecutive Dry Day index (CDD), fraction of precipitation above the 95th intensity percentile (R95) and Hydroclimatic Intensity index (HY-INT). Comparison with coarse (GPCP) and high (TRMM) resolution daily precipitation data for the present day conditions shows that the precipitation intensity distributions from the GCMs are close to the GPCP data, while the RegCM4 ones are closer to TRMM, illustrating the added value of the increased resolution of the regional model. All global and regional model simulations project predominant increases in HWD, CDD, R95 and HY-INT, implying a regime shift towards more intense, less frequent rain events and increasing risk of heat wave, drought and flood with global warming. However, the magnitudes of the changes are generally larger in the global than the regional models, likely because of the relatively low “climate sensitivity” of the RegCM4, especially when using the CLM land surface scheme. In addition, pronounced regional differences in the change signals are found. The data from these simulations are available for use in impact assessment studies.

Atlantic forcing of Pacific decadal variability

This paper investigates the Atlantic Ocean influence on equatorial Pacific decadal variability. Using an ensemble of simulations, where the ICTPAGCM (“SPEEDY”) is coupled to the NEMO/OPA ocean model in the Indo-Pacific region and forced by observed sea surface temperatures in the Atlantic region, it is shown that the Atlantic Multidecadal Oscillation (AMO) has had a substantial influence on the equatorial Pacific decadal variability. According to AMO phases we have identified three periods with strong Atlantic forcing of equatorial Pacific changes, namely (1) 1931–1950 minus 1910–1929, (2) 1970–1989 minus 1931–1950 and (3) 1994–2013 minus 1970–1989. Both observations and the model show easterly surface wind anomalies in the central Pacific, cooling in the central-eastern Pacific and warming in the western Pacific/Indian Ocean region in events (1) and (3) and the opposite signals in event (2). The physical mechanism for these responses is related to a modification of the Walker circulation because a positive (negative) AMO leads to an overall warmer (cooler) tropical Atlantic. The warmer (cooler) tropical Atlantic modifies the Walker circulation, leading to rising (sinking) and upper-level divergence (convergence) motion in the Atlantic region and sinking (rising) motion and upper-level convergence (divergence) in the central Pacific region.

Present and future climatologies in the phase I CREMA experiment

We provide an overall assessment of the surface air temperature and precipitation present day (1976–2005) and future (2070–2099) ensemble climatologies in the Phase I CREMA experiment. This consists of simulations performed with different configurations (physics schemes) of the ICTP regional model RegCM4 over five CORDEX domains (Africa, Mediterranean, Central America, South America, South Asia), driven by different combinations of three global climate models (GCMs) and two greenhouse gas (GHG) representative concentration pathways (RCP8.5 and RCP4.5). The biases (1976–2005) in the driving and nested model ensembles compared to observations show a high degree of spatial variability and, when comparing GCMs and RegCM4, similar magnitudes and more similarity for precipitation than for temperature. The large scale patterns of change (2070–2099 minus 1976–2005) are broadly consistent across the GCM and RegCM4 ensembles and with previous analyses of GCM projections, indicating that the GCMs selected in the CREMA experiment are representative of the more general behavior of current GCMs. The RegCM4, however, shows a lower climate sensitivity (reduced warming) than the driving GCMs, especially when using the CLM land surface scheme. While the broad patterns of precipitation change are consistent across the GCM and RegCM4 ensembles, greater differences are found at sub-regional scales over the various domains, evidently tied to the representation of local processes. This paper serves to provide a reference view of the behavior of the CREMA ensemble, while more detailed and process-based analysis of individual domains is left to companion papers of this special issue.

Tropical cyclones in a regional climate change projection with RegCM4 over the CORDEX Central America domain

The characteristics of tropical cyclones (TCs) over the Central America Coordinated Regional Downscaling Experiment (CORDEX) domain are examined for present and future climate conditions using the regional climate model RegCM4. RegCM4 is first tested in a 22 year (1982–2003) simulation with boundary forcing from the ERA-Interim reanalysis, showing a generally good performance in reproducing the observed TC climatology and over the Atlantic in reproducing the interannual variations of TC counts. Four scenario simulations (1970-2100) are generated using two model configurations and two driving global models (MPI and HadGEM). The simulations employing the Grell convection scheme produce too few TCs, while those using the Emanuel convection scheme reproduce the observed climatology, especially when driven by the MPI global model. The simulation of TCs is thus sensitive to both the model convection scheme and the forcing GCM. Comparison of future and present day TC statistics indicates that the frequency of future TCs decreases over the tropical Atlantic and the East Pacific coastal areas while it increases over the western areas of the East Pacific and the northern areas of the Atlantic. We also find an increase in the frequency of intense TCs and long lasting TCs, along with a northward shift of TC tracks over the Atlantic. Conclusions on the changes in TC activity are not found to be sensitive to the inclusion of SST thresholds in the detection procedure. These findings

Description and evaluation of the Earth System Regional Climate Model (Reg CM-ES)

We describe a new, state-of-the-art, Earth System Regional Climate Model (RegCM-ES), which includes the coupling between the atmosphere, ocean and land surface, as well as an hydrological and ocean biogeochemistry model, with the capability of using a variety of physical parameterizations. The regional coupled model has been implemented and tested over some of the COordinated Regional climate Downscaling Experiment (CORDEX) domains and more regional settings featuring climatically important coupled phenomena. Regional coupled ocean-atmosphere models can be especially useful tools to provide information on the mechanisms of air-sea interactions and feedbacks occurring at fine spatial and temporal scales. RegCM-ES shows a good representation of precipitation and SST fields over the domains tested, as well as realistic simulations of coupled air-sea processes and interactions. The RegCM-ES model, which can be easily implemented over any regional domain of interest, is open source, making it suitable for usage by the broad scientific community.