Hadley Centre for Climate Prediction and Research, Meteorological Office, Bracknell, England

A multi-model set of atmospheric simulations forced by historical sea surface temperature (SST) or SSTs plus Greenhouse gases and aerosol forcing agents for the period of 1950–1999 is studied to identify and understand which components of the Asian–Australian monsoon (A–AM) variability are forced and reproducible. The analysis focuses on the summertime monsoon circulations, comparing model results against the observations. The priority of different components of the A–AM circulations in terms of reproducibility is evaluated. Among the subsystems of the wide A–AM, the South Asian monsoon and the Australian monsoon circulations are better reproduced than the others, indicating they are forced and well modeled. The primary driving mechanism comes from the tropical Pacific. The western North Pacific monsoon circulation is also forced and well modeled except with a slightly lower reproducibility due to its delayed response to the eastern tropical Pacific forcing. The simultaneous driving comes from the western Pacific surrounding the maritime continent region. The Indian monsoon circulation has a moderate reproducibility, partly due to its weakened connection to June–July–August SSTs in the equatorial eastern Pacific in recent decades. Among the A–AM subsystems, the East Asian summer monsoon has the lowest reproducibility and is poorly modeled. This is mainly due to the failure of specifying historical SST in capturing the zonal land-sea thermal contrast change across the East Asia. The prescribed tropical Indian Ocean SST changes partly reproduce the meridional wind change over East Asia in several models. For all the A–AM subsystem circulation indices, generally the MME is always the best except for the Indian monsoon and East Asian monsoon circulation indices.

The ECHAM 3.2 (T21), ECHAM 4 (T30) and LMD (version 6, grid-point resolution with 96 longitudes × 72 latitudes) atmospheric general circulation models were integrated through the period 1961 to 1993 forced with the same observed Sea Surface Temperatures (SSTs) as compiled at the Hadley Centre. Three runs were made for each model starting from different initial conditions. The large-scale tropical inter-annual variability is analysed to give a picture of the skill of each model and of some sort of combination of the three models. To analyse the similarity of model response averaged over the same key regions, several widely-used indices are calculated: Southern Oscillation Index (SOI), large-scale wind shear indices of the boreal summer monsoon in Asia and West Africa and rainfall indices for NE Brazil, Sahel and India. Even for the indices where internal noise is large, some years are consistent amongst all the runs, suggesting inter-annual variability of the strength of SST forcing. Averaging the ensemble mean of the three models (the super-ensemble mean) yields improved skill. When each run is weighted according to its skill, taking three runs from different models instead of three runs of the same model improves the mean skill. There is also some indication that one run of a given model could be better than another, suggesting that persistent anomalies could change its sensitivity to SST. The index approach lacks flexibility to assess whether a model’s response to SST has been geographically displaced. We focus on the first mode in the global tropics, found through singular value decomposition analysis, which is clearly related to El Niño/Southern Oscillation (ENSO) in all seasons. The Observed-Model and Model-Model analyses lead to almost the same patterns, suggesting that the dominant pattern of model response is also the most skilful mode. Seasonal modulation of both skill and spatial patterns (both model and observed) clearly exists with highest skill (between tropical Pacific SST and tropical rainfall) and reproducibility amongst the runs in December-February, and least skill/reproducibility in March-May and June-August. The differences between each model suggest that a simple linear regression combination of each GCM’s prediction indices will be improved upon by combination methods that take account of the errors in the spatial teleconnection structures generated by the GCM.

An analysis of observations from 1948-1998 suggests that the atmosphere in the North Atlantic region does respond to North Atlantic Sea-Surface Temperatures (SSTs) throughout the annual cycle. In the subtropics, high geopotential heights are seen to be a local response to warm SSTs. In winter, the North Atlantic Oscillation responds to a «tripole» pattern in North Atlantic SSTs. In summer, anticyclonicity over the U.K. is seen downstream of warm SST anomalies off Newfoundland and is possibly also related to warm subtropical SSTs. Such responses imply a degree of seasonal predictability and help quantify the strength of natural ocean-atmosphere coupled modes of variability. The average of an ensemble of 10 simulations of the HadAM3 atmospheric model forced with observed SSTs for the same period produces robust ocean-forced responses which agree well with those identifi ed in the observations and with a previous model. The agreement is encouraging as it confi rms the physical signifi cance of the observational results and suggests that the model responds with the correct patterns to SST forcing. In the subtropics, the magnitude of the ensemble mean response is comparable with the observational response. In the extratropics, the magnitude of the model response is about half that of the observations. Although atmospheric internal variability may have affected the observed atmospheric patterns and there are considerations regarding the lack of two-way air-sea interaction with an atmospheric model, it is suggested that the model’s extratropical response may be too weak. The 10 individual simulations of HadAM3 and 28 50-year periods of the ocean-atmosphere model, HadCM3, display similar results to each other with generally weaker ocean-forced links than observed. Seasonal predictability may, therefore, be too low in HadCM3 and low-frequency coupled modes under-represented. A moderate increase in the extratropics in the sensitivity of surface heat fl uxes to surface temperatures is one possibility for improving these model deficiencies.

The realistic simulation of the summer Mediterraneanclimaterequires not only refined spatial scales, but also an adequate representation of land-atmosphere interactions andteleconnections. Addressing all of these issues remains a challenge for most of the CMIP3/CMIP5 generation models. In this study we analyze high-24resolution (~0.5° lat x lon) RCP8.5 future projections of the Geophysical Fluid Dynamics Laboratory CM2.5 model with anew incorporatedland model (LM3). The simulated regional future changes suggest pronounced warming and drying over mostparts of the Mediterranean. However the changes aredistinctivelyless radical when compared with the CMIP5 multimodel ensemble. Moreover, changes over the Southeast (off the coast area of the Balkans) and Central Europe indicate not only a very modest warming, compared to the CMIP5 projections, but also wetting tendencies. The difference of CM2.5 projections of future changes over previous-generation modelshighlights the mportance of a) a correctlyprojectedmagnitude of changes of the North Atlantic Oscillation and its regional impacts, which have thecapacity to partly offset the anthropogenicwarming and drying over the western andcentral Mediterranean; b) a refined representation of land surface-atmospheric interactions, whichare a governing factor for thermal-and hydro-climate over Central and SoutheasternEurope. The CM2.5 projections also indicate a maximum of warming (Levant) and drying (Asia Minor) over the eastern Mediterranean. The changes derived in this region indicate a decreasing influence of atmospheric dynamics in maintaining the regional temperature and precipitation balance and instead an increasing influence of local surface temperature on the local surface atmospheric circulation.

The ECHAM 3.2 (T21), ECHAM 4 (T30) and LMD (version 6, grid-point resolution with 96 longitudes × 72 latitudes) atmospheric general circulation models were integrated through the period 1961 to 1993 forced with the same observed Sea Surface Temperatures (SSTs) as compiled at the Hadley Centre. Three runs were made for each model starting from different initial conditions. The mid-latitude circulation pattern which maximises the covariance between the simulation and the observations, i.e. the most skilful mode, and the one which maximises the covariance amongst the runs, i.e. the most reproducible mode, is calculated as the leading mode of a Singular Value Decomposition (SVD) analysis of observed and simulated Sea Level Pressure (SLP) and geopotential height at 500 hPa (Z500) seasonal anomalies. A common response amongst the different models, having different resolution and parametrization should be considered as a more robust atmospheric response to SST than the same response obtained with only one model. A robust skilful mode is found mainly in December-February (DJF), and in June-August (JJA). In DJF, this mode is close to the SST-forced pattern found by Straus and Shukla (2000) over the North Pacific and North America with a wavy out-of-phase between the NE Pacific and the SE US on the one hand and the NE North America on the other. This pattern evolves in a NAO-like pattern over the North Atlantic and Europe (SLP) and in a more N-S tripole on the Atlantic and European sector with an out-of-phase between the middle Europe on the one hand and the northern and southern parts on the other (Z500). There are almost no spatial shifts between either field around North America (just a slight eastward shift of the highest absolute heterogeneous correlations for SLP relative to the Z500 ones). The time evolution of the SST-forced mode is moderatly to strongly related to the ENSO/LNSO events but the spread amongst the ensemble of runs is not systematically related at all to the intensity of Niño3.4 SST anomalies. The leading reproducible mode in JJA is clearer and more skilful for SLP than for Z500 and also seems related to the SST time evolution of tropical Pacific. It is characterised by an out-of-phase between the whole North Pacific and a horseshoe shaped area from Eastern Siberia and Gulf of Mexico. The leading OM mode found in MAM and SON, are quite close to the DJF one (at least for the modelled anomalies), but they are less skilful than in DJF. The most skilful mode (i.e. SLP-Z500 mode in DJF and SLP mode in JJA) is almost similar to the most reproducible one during these particular seasons. In MAM and SON, the SST-forced pattern is very close to the wintertime one. The warm episodes in the central and eastern tropical Pacific are then associated with negative pressure anomalies at the sea level and also at 500 hPa over the whole North Pacifkc and from SE US Coast to Western Europe (from SE US Coast to Scandinavia for Z500) and positive pressure anomalies on Central Canada, north of 55°-60°N across the North Atlantic and also over Northern Siberia (in MAM). The variance forced by SST are lower in MAM and SON than in DJF and, as suggested above, the skill of this SST-forced mode is weak in MAM and almost close to zero in SON.