Andrea F. Carril

Differences in El Niño Response over the Southern Hemisphere

Southern Hemisphere (SH) circulation conditions during austral springs of ENSO events are examined. Based on previous knowledge that SST variations over the subtropical south-central Pacific (SSCP) region are linked to differences among El Nino (EN) events, a stratification of the springs associated with EN events was performed according to SST conditions over the El Nino-3.4 sector and over the SSCP region. The EN events associated with cold conditions in the SSCP (WC) exhibit enhanced convection not only in the intertropical convergence zone over the central equatorial Pacific but also in the South Pacific convergence zone (SPCZ) extended south- eastward into the subtropical regions. The resulting heating forcing intensifies a localized overturning cell, which is associated with an anomalous Rossby wave source in the central South Pacific extratropical region. Neither the Rossby wave source nor the associated wave pattern is evident during EN events associated with warm conditions in the SSCP and inactive SPCZ (WW). The basic features that characterize the differences in the EN response over the South Pacific can also be identified through the analysis of the SPCZ activity over the central South Pacific. The fact that variations in SPCZ activity lead SST anomaly changes in the SSCP would indicate that the differences in the EN response over the SH might be mainly driven by atmospheric changes, which induces extratropical SST anomalies. The differences in the circulation anomalies that characterize both types of EN response over the SH were further explored through the analysis of the activity of the three leading modes of circulation variability. The combined effect of the three leading patterns describes in some extent the intensification (weakening) of the cyclonic circulation anomaly observed over the southeastern Pacific in WC (WW), associated with an active (inactive) SPCZ. In particular, the interdecadal variability observed in the Pacific by many previous studies influences the circulation response to ENSO over the SH, mainly through changes in the activity of the SH annular mode.

The INGV-CMCC Seasonal Prediction System: improved ocean initial conditions

Abstract The development of the Istituto Nazionale di Geofisica e Vulcanologia (INGV)–Centro Euro-Mediterraneo per i Cambiamenti Climatici (CMCC) Seasonal Prediction System (SPS) is documented. In this SPS the ocean initial-conditions estimation includes a reduced-order optimal interpolation procedure for the assimilation of temperature and salinity profiles at the global scale. Nine-member ensemble forecasts have been produced for the period 1991–2003 for two starting dates per year in order to assess the impact of the subsurface assimilation in the ocean for initialization. Comparing the results with control simulations (i.e., without assimilation of subsurface profiles during ocean initialization), it is shown that the improved ocean initialization increases the skill in the prediction of tropical Pacific sea surface temperatures of the system for boreal winter forecasts. Considering the forecast of the 1997/98 El Nino, the data assimilation in the ocean initial conditions leads to a considerable impro...

Performance of a multi-RCM ensemble for South Eastern South America

The ability of four regional climate models to reproduce the present-day South American climate is examined with emphasis on La Plata Basin. Models were integrated for the period 1991–2000 with initial and lateral boundary conditions from ERA-40 Reanalysis. The ensemble sea level pressure, maximum and minimum temperatures and precipitation are evaluated in terms of seasonal means and extreme indices based on a percentile approach. Dispersion among the individual models and uncertainties when comparing the ensemble mean with different climatologies are also discussed. The ensemble mean is warmer than the observations in South Eastern South America (SESA), especially for minimum winter temperatures with errors increasing in magnitude towards the tails of the distributions. The ensemble mean reproduces the broad spatial pattern of precipitation, but overestimates the convective precipitation in the tropics and the orographic precipitation along the Andes and over the Brazilian Highlands, and underestimates the precipitation near the monsoon core region. The models overestimate the number of wet days and underestimate the daily intensity of rainfall for both seasons suggesting a premature triggering of convection. The skill of models to simulate the intensity of convective precipitation in summer in SESA and the variability associated with heavy precipitation events (the upper quartile daily precipitation) is far from satisfactory. Owing to the sparseness of the observing network, ensemble and observations uncertainties in seasonal means are comparable for some regions and seasons.

Low‐frequency variability of the Antarctic Circumpolar Wave

This study evaluates the Antarctic Circumpolar Wave (ACW) variability in the Southern Ocean using a 27-year series of sea surface temperature (SST) and sea level pressure (SLP). From a cross-covariance analysis we find that the strongest covariation between the extratropical wavenumber-2 in SST and in SLP is during the austral summer in the South Pacific Ocean. Examination of the time series reveals two timescales of variability. A wavenumber-2 structure in SST and SLP anomalies correlates well at interannual (IA) timescales, when ENSO-driven teleconnections seem to energize the ACW. Moreover, in the 1973-1999 epoch, the ACW has been modulated by interdecadal (ID) variability possibly related to the Southern Oscillation.

Climate change scenarios over the South American region: an intercomparison of coupled general atmosphere-ocean circulation models

Results of four different coupled atmosphere-ocean general circulation model runs for South America and surrounding oceans are examined. The mean response of the simulated climate system to a gradual increase of greenhouse gases is presented. The transient coupled experiment data provided for the Intergovernmental Panel on Climate Change (IPCC) were the most recent available to us at the time of this study, from the following research centres in Europe and USA: the UK Meteorological Office (UKMO), the Max-Planck Institute for Meteorology (MPI), the Geophysical Fluid Dynamic Laboratory (GFDL) and the National Center for Atmospheric Research (NCAR). The regional performance of the control simulations (with fixed CO2 concentration) is determined by comparing sealevel pressure, near-surface zonal wind, precipitation and surface air temperature from the models, against observed climatological fields. The response of the climate system to the enhanced emission scenarios is established comparing the control experiments with the respective transient experiments around the time of doubling CO2. To assist in estimating likely future climate change in the South American region, only the results of those model experiments with the best control simulation of current climate in the region are considered: the UKMO and MPI models. A maximum warming over the continent and a minimum warming in the extratropical oceans, a slight intensification and southward migration of the subtropical ridge, a deepening of the sub-Antarctic trough, an intensification of the westerly winds, and increasing precipitation in the tropics and in the mid-latitude oceans are the most important potential changes simulated by both models for the increased CO2 scenarios. # 1997 Royal Meteorological Society. Int. J. Climatol., Vol. 17, 1613‐1633

La Plata basin precipitation variability in spring: role of remote SST forcing as simulated by GCM experiments

An ensemble of nine experiments with the same interannually varying sea surface temperature (SST), as boundary forcing, and different initial conditions is used to investigate the role of tropical oceans in modulating precipitation variability in the region of La Plata Basin (LPB). The results from the ensemble are compared with a twentieth-century experiment performed with a coupled ocean-atmosphere model, sharing the same atmospheric component. A rotated empirical orthogonal functions analysis of South America precipitation shows that the dominant mode of variability in spring is realistically captured in both experiments. Its principal component (RPC1) correlated with global SST and atmospheric fields identifies the pattern related to El Niño Southern Oscillation and its large-scale teleconnections. Overall the pattern is well simulated in the tropical southern Pacific Ocean, mainly in the ensemble, but it is absent or too weak in other oceanic areas. The coupled model experiment shows a more realistic correlation in the subtropical South Atlantic where air-sea interactions contribute to the relationship between LPB precipitation and SST. The correspondence between model and data is much improved when the composite analysis of SST and atmospheric fields is done over the ensemble members having an RPC1 in agreement with the observations: the improvement relies on avoiding climate noise by averaging only over members that are statistically similar. Furthermore, the result suggests the presence of a high level of uncertainty due to internal atmospheric variability. The analysis of some individual years selected from the model and data RPC1 comparison reveals interesting differences among rainy springs in LPB. For example, 1982, which corresponds to a strong El Niño year, represents a clean case with a distinct wave train propagating from the central Pacific and merging with another one from the eastern tropical south Indian Ocean. The year 2003 is an example of a rainy spring in LPB not directly driven by remote SST forcing. In this case the internal variability has a dominant role, as the model is not able to reproduce the correct local precipitation pattern.

Ocean, sea‐ice, atmosphere oscillations in the Southern Ocean as simulated by the SINTEX coupled model

This research was supported by the PREDICATE EU project (EVK2 – CT-1999 – 0020,). First author was supported by the PRISM EU project (EVR1 – CT-2001 – 40012).

Future hydroclimatological changes in South America based on an ensemble of regional climate models

Changes between two time slices (1961–1990 and 2071–2100) in hydroclimatological conditions for South America have been examined using an ensemble of regional climate models. Annual mean precipitation (P), evapotranspiration (E) and potential evapotranspiration (EP) are jointly considered through the balances of land water and energy. Drying or wetting conditions, associated with changes in land water availability and atmospheric demand, are analysed in the Budyko space. The water supply limit (E limited by P) is exceeded at about 2% of the grid points, while the energy limit to evapotranspiration (E = EP) is overall valid. Most of the continent, except for the southeast and some coastal areas, presents a shift toward drier conditions related to a decrease in water availability (the evaporation rate E/P increases) and, mostly over much of Brazil, to an increase in the aridity index (Ф = EP/P). These changes suggest less humid conditions with decreasing surface runoff over Amazonia and the Brazilian Highlands. In contrast, Argentina and the coasts of Ecuador and Peru are characterized by a tendency toward wetter conditions associated with an increase of water availability and a decrease of aridity index, primarily due to P increasing faster than both E and EP. This trend towards wetter soil conditions suggest that the chances of having larger periods of flooding and enhanced river discharges would increase over parts of southeastern South America. Interannual variability increases with Ф (for a given time slice) and with climate change (for a given aridity regimen). There are opposite interannual variability responses to the cliamte change in Argentina and Brazil by which the variability increases over the Brazilian Highlands and decreases in central-eastern Argentina.

Maps of wind hazard over South Eastern South America considering climate change

Wind is one of the most dangerous natural phenomena for the built environment in South Eastern South America. The hazard posed by wind depends on the extreme wind speeds on the surface and can be quantified by calculating the Average Recurrence Interval—more commonly known as return period—of these winds. Maps of return period of extreme wind speeds are used by planning authorities to enforce appropriate standards for infrastructure construction in most countries of the world. These maps are usually built up from wind speeds recorded at a network of weather stations. In some countries, however, the quality of the records is poor or the stations have not been in operation for long enough to give appropriate data for wind hazard studies. In this paper, we discuss an alternative approach based on wind speeds calculated by climate models. The approach provides longer datasets and facilitates assessment of the impact of climate change on wind hazard, a matter of great of importance for planning and emergency authorities. Map quality is evaluated by comparing results from the climate simulations with results from reanalysis. The comparison shows that the climate simulations produce results close enough to the reanalysis and hence they can be used for wind hazard assessment. The results also show that we could expect little variation in wind hazard in South Eastern South America during most of this century.

Assessment of CORDEX simulations over South America: added value on seasonal climatology and resolution considerations

A new set of CORDEX simulations over South America, together with their coarser-resolution driving Global Climate Models (GCMs) are used to investigate added value of Regional Climate Models (RCMs) in reproducing mean climate conditions over the continent. There are two types of simulations with different lateral boundary conditions: five hindcast simulations use re-analysis as boundary conditions, and five other historical simulations use GCMs outputs. Multi-model ensemble means and individual simulations are evaluated against two or three observation-based gridded datasets for 2-m surface air temperature and total precipitation. The analysis is performed for summer and winter, over a common period from 1990 to 2004. Results indicate that added value of RCMs is dependent on driving fields, surface properties of the area, season and variable considered. A robust added value for RCMs driven by ERA-Interim is obtained in reproducing the summer climatology of surface air temperature over tropical and subtropical latitudes. Mixed results can be seen, however, for summer precipitation climatology in both hindcast and historical experiments. For winter, there is no noticeable improvement by the RCMs for the large-scale precipitation and surface air temperature climatology. To further understand the added value of RCMs, models deviations from observation are decomposed according to different terms that reflect the observational uncertainty, the representativeness error, the interpolation error, and the actual performance of the model. Regions where these errors are not negligible, such as in complex terrain regions, among others, can be identified. There is a clear need for complementary assessment to understand better the real value added by RCMs.