ALBERT

Description: Statistical downscaling (SD) is commonly used to provide information for the assessment of climate change impacts. Using as input the output from large-scale dynamical climate models and observation-based data products, SD aims to provide a finer grain of detail and to mitigate systematic biases. It is generally recognized as providing added value. However, one of the key assumptions of SD is that the relationships used to train the method during a historical period are unchanged in the future, in the face of climate change. The validity of this assumption is typically quite difficult to assess in the normal course of analysis, as observations of future climate are lacking. We approach this problem using a “perfect model” experimental design in which high-resolution dynamical climate model output is used as a surrogate for both past and future observations. We find that while SD in general adds considerable value, in certain well-defined circumstances it can produce highly erroneous results. Furthermore, the breakdown of SD in these contexts could not be foreshadowed during the typical course of evaluation based on only available historical data. We diagnose and explain the reasons for these failures in terms of physical, statistical, and methodological causes. These findings highlight the need for caution in the use of statistically downscaled products and the need for further research to consider other hitherto unknown pitfalls, perhaps utilizing more advanced perfect model designs than the one we have employed.

Description: The causes for the observed occurrence of anomalous zonally symmetric upper-level pressure ridges in the midlatitude belts of both hemispheres during the year after warm El Niño–Southern Oscillation (ENSO) events have been investigated. Sea surface temperature (SST) anomalies in the Indo–western Pacific (IWP) sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed ENSO episodes in the eastern/central tropical Pacific. The spatiotemporal evolution of these SST conditions through a composite ENSO cycle was then inserted as lower boundary conditions within the IWP domain in an ensemble of atmospheric general circulation model (GCM) integrations. This experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of ENSO. The model evidence hence supports the notion that these global-scale atmospheric changes are primarily responses to SST perturbations in IWP, which are in turn linked to ENSO variability in the equatorial Pacific by the “atmospheric bridge” mechanism. Experimentation with a stationary wave model indicates that the Eastern Hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated with precipitation anomalies in the IWP region, which are closely related to the underlying SST changes. Diagnosis of the output from the GCM integrations reveals that these circulation changes due to diabatic heating are accompanied by alterations of the propagation path and intensity of the high-frequency eddies at locations farther downstream. The geopotential tendencies associated with the latter disturbances bear some resemblance to the anomalous height pattern in the Western Hemisphere. Such local eddy–mean flow feedbacks hence contribute to the zonal symmetry of the atmospheric response pattern to forcing in the IWP region. Analysis of zonally averaged circulation statistics indicates that the mean meridional circulation induced by divergence of anomalous transient eddy momentum fluxes in ENSO events could also generate zonally symmetric perturbations in midlatitudes. The model-simulated precipitation and surface temperature anomalies in the North American sector in response to SST changes in IWP suggest an increased frequency of droughts and heat waves in that region during the summer season after warm ENSO events.

Description: The individual impacts of sea surface temperature (SST) anomalies in the deep tropical eastern–central Pacific (DTEP) and Indo-western–central Pacific (IWP) on the evolution of the observed global atmospheric circulation during the 1997–2003 period have been investigated using a new general circulation model. Ensemble integrations were conducted with monthly varying SST conditions being prescribed separately in the DTEP sector, the IWP sector, and throughout the World Ocean. During the 1998–2002 subperiod, when prolonged La Niña conditions occurred in DTEP and the SST in IWP was above normal, the simulated midlatitude atmospheric responses to SST forcing in the DTEP and IWP sectors reinforced each other. The anomalous geopotential height ridges at 200 mb in the extratropics of both hemispheres exhibited a distinct zonal symmetry. This circulation change was accompanied by extensive dry and warm anomalies in many regions, including North America. During the 1997–98 and 2002–03 El Niño events, the SST conditions in both DTEP and IWP were above normal, and considerable cancellations were simulated between the midlatitude responses to the oceanic forcing from these two sectors. The above findings are contrasted with those for the 1953–58 and 1972–77 periods, which were characterized by analogous SST developments in DTEP, but by cold conditions in IWP. It is concluded that a warm anomaly in IWP and a cold anomaly in DTEP constitute the optimal SST configuration for generating zonally elongated ridges in the midlatitudes. Local diagnoses indicate that the imposed SST anomaly alters the strength of the zonal flow in certain longitudinal sectors, which influences the behavior of synoptic-scale transient eddies farther downstream. The modified eddy momentum transports in the regions of eddy activity in turn feed back on the local mean flow, thus contributing to its zonal elongation. These results are consistent with the inferences drawn from zonal mean analyses, which accentuate the role of the eddy-induced circulation on the meridional plane.

Description: The modulation of El Niño and La Niña responses by the long-term sea surface temperature (SST) warming trend in the Indian–Western Pacific (IWP) Oceans has been investigated using a large suite of sensitivity integrations with an atmospheric general circulation model. These model runs entail the prescription of anomalous SST conditions corresponding to composite El Niño or La Niña episodes, to SST increases associated with secular warming in IWP, and to combinations of IWP warming and El Niño/La Niña. These SST forcings are derived from the output of coupled model experiments for climate settings of the 1951–2000 and 2001–50 epochs. Emphasis is placed on the wintertime responses in 200-mb height and various indicators of surface climate in the North American sector. The model responses to El Niño and La Niña forcings are in agreement with the observed interannual anomalies associated with warm and cold episodes. The wintertime model responses in North America to IWP warming bear a distinct positive (negative) spatial correlation with the corresponding responses to La Niña (El Niño). Hence, the amplitude of the combined responses to IWP warming and La Niña is notably higher than that to IWP warming and El Niño. The model projections indicate that, as the SST continues to rise in the IWP sector during the twenty-first century, the strength of various meteorological anomalies accompanying La Niña (El Niño) will increase (decrease) with time. The response of the North American climate and the zonal mean circulation to the combined effects of IWP forcing and La Niña (El Niño) is approximately equal to the linear sum of the separate effects of IWP warming and La Niña (El Niño). The summertime responses to IWP warming bear some similarity to the meteorological anomalies accompanying extended droughts and heat waves over the continental United States.

Description: The impacts of ENSO on the evolution of the East Asian monsoon have been studied using output from a general circulation model experiment. Observed monthly variations of the sea surface temperature (SST) field have been prescribed in the tropical eastern and central Pacific, whereas the atmosphere has been coupled to an oceanic mixed layer model beyond this forcing region. During the boreal summer of typical El Niño events, a low-level cyclonic anomaly is simulated over the North Pacific in response to enhanced condensational heating over the equatorial central Pacific. Advective processes associated with the cyclone anomaly lead to temperature tendencies that set the stage for the abrupt establishment of a strong Philippine Sea anticyclone (PSAC) anomaly in the autumn. The synoptic development during the onset of the PSAC anomaly is similar to that accompanying cold-air surges over East Asia. The air–sea interactions accompanying the intraseasonal variations (ISV) in the model atmosphere exhibit a strong seasonal dependence. During the summer, the climatological monsoon trough over the subtropical western Pacific facilitates positive feedbacks between the atmospheric and oceanic fluctuations. Conversely, the prevalent northeasterly monsoon over this region in the winter leads to negative feedbacks. The onset of the PSAC anomaly is seen to be coincident with a prominent episode of the leading ISV mode. The ENSO events could influence the amplitude of the ISV by modulating the large-scale flow environment in which the ISV are embedded. Amplification of the summer monsoon trough over the western Pacific during El Niño enhances air–sea feedbacks on intraseasonal time scales, thereby raising the amplitudes of the ISV. A weakening of the northeasterly monsoon in El Niño winters suppresses the frequency and strength of the cold-air surges associated with the leading ISV mode in that season. Many aspects of the model simulation of the relationships between ENSO and the East Asian monsoon are in agreement with observational findings.

Description: The summertime northeastward march of the climatological maritime monsoon over the South China Sea (SCS) and subtropical western North Pacific (WNP) is examined using the output from a 200-yr integration of a coupled atmosphere–ocean general circulation model (GCM). Increased cloud cover and surface wind speed during monsoon onset over the SCS in May–June reduce the incoming shortwave flux and enhance the upward latent heat flux at the ocean surface, thereby cooling the local sea surface temperature (SST). The resulting east–west gradient in the SST pattern, with lower temperature in the SCS and higher temperature in the WNP, is conducive to eastward migration of the monsoon precipitation over this region. Upon arrival of the precipitation center in the WNP in July–August, the local circulation changes lead to weakening of the mei-yu–baiu rainband near 30°N. The subsequent increases in local shortwave flux and SST impart a northward tendency to the evolution of the WNP monsoon. Many of these model inferences are supported by a parallel analysis of various observational datasets. The modulation of the above climatological scenario by El Niño–Southern Oscillation (ENSO) events is investigated by diagnosing the output from the coupled GCM and from experiments based on the atmospheric component of this GCM with SST forcings being prescribed separately in the equatorial Pacific, Indian Ocean, and SCS/WNP domains. During the May period after the peak phase of ENSO, the simulated monsoon onset over the SCS occurs later (earlier) than normal in El Niño (La Niña) events. These changes are primarily remote responses to the anomalous SST forcing in the equatorial Pacific and Indian Ocean. The ENSO-related changes in the SCS/WNP are associated with above-normal (below normal) mei-yu–baiu activity during warm (cold) events. In the ensuing July period of the warm events, the simulated precipitation response over the SCS to the local warm SST anomaly tends to oppose the remote response to SST forcing in the northern Indian Ocean. In the July period of cold events, the equatorial Pacific SST anomaly retains its strength and moves still farther westward. This forcing cooperates with the cold SST anomaly in the SCS in influencing the precipitation pattern in the SCS/WNP sector.

Description: The climatological characteristics and interannual variations of the development of the South Asian summer monsoon (SASM) in early summer are studied using output from a 200-yr simulation of a coupled atmosphere–ocean general circulation model (CM2.1). Some of the model results are compared with corresponding observations. Climatological charts of the model and observational data at pentadal intervals indicate that both the precipitation and SST signals exhibit a tendency to migrate northward. Enhanced monsoonal precipitation at a given site is accompanied by a reduction in incoming shortwave radiation and intensification of upward latent heat flux, and by oceanic cooling. An extended empirical orthogonal function analysis is used to identify the dates for initiation of the northward march of SASM in individual summers. It is noted that early monsoon development prevails after the mature phase of La Niña events, whereas delayed development occurs after El Niño. Sensitivity experiments based on the atmospheric component of CM2.1 indicate that the effects of SST forcings in the tropical Pacific (TPAC) and Indian Ocean (IO) on monsoon development are opposite to each other. During El Niño events, the atmospheric response to remote TPAC forcing tends to suppress or postpone monsoon development over South Asia. Conversely, the warm SST anomalies in IO, which are generated by the “atmospheric bridge” mechanism in El Niño episodes, lead to accelerated monsoon development. The net result of these two competing effects is an evolution scenario with a timing that is intermediate between the response to TPAC forcing only and the response to IO forcing only.

Description: The synoptic behavior of present-day heat waves (HW) over Europe is studied using the GFDL high-resolution atmospheric model (HiRAM) with 50-km grid spacing. Three regions of enhanced and coherent temperature variability are identified over western Russia, eastern Europe, and western Europe. The simulated HW characteristics are compared with those derived from Climate Forecast System Reanalysis products. Composite charts for outstanding HW episodes resemble well-known recurrent circulation types. The HW region is overlain by a prominent upper-level anticyclone, which blocks the passage of synoptic-scale transients. The altered eddy vorticity transports in turn reinforce the anticyclone. The anticyclone is part of a planetary-scale wave train. The successive downstream development of this wave train is indicative of Rossby wave dispersion. Additional runs of HiRAM are conducted for the “time slices” of 2026–35 and 2086–95 in the climate scenario corresponding to representative concentration pathway 4.5 (RCP4.5). By the end of the twenty-first century, the average duration and frequency of HW in the three European sites are projected to increase by a factor of 1.4–2.0 and 2.2–4.5, respectively, from the present-day values. These changes can be reproduced by adding the mean shift between the present and future climatological temperatures to the daily fluctuations in the present-day simulation. The output from a continuous integration of a coupled general circulation model through the 1901–2100 period indicates a monotonic increase in severity, duration, and HW days during the twenty-first century.