Coupled Atmosphere Ocean General Circulation Model Research Articles

Southern Hemisphere winter cyclone activity under recent and future climate conditions in multi‐model AOGCM simulations

ABSTRACTThis article investigates extratropical winter cyclone activity in the Southern Hemisphere (SH) in a multi‐model ensemble (MME) of coupled atmosphere–ocean general circulation model (AOGCM) simulations of recent and potential future climate conditions. Most individual models and also the ensemble mean yield good reproductions of the typical cyclone characteristics found in reanalysis data, although some individual models show peculiarities, and a large inter‐model spread in terms of quantity of identified cyclone tracks is found. We use a scaling approach to combine the cyclone statistics from different models into a MME. In the future climate simulations, the total number of SH cyclones is reduced, whereas an increased number of strong cyclones is found in most models and in the ensemble mean. The long term trend with respect to all cyclones is a robust feature throughout the simulations. It is associated with a general poleward shift, shown to be related to both tropical upper tropospheric warming and shifting meridional sea surface temperature gradients in the Southern Ocean. The magnitude of increased strong cyclone activity has a focus on the Eastern Hemisphere. It is clearly influenced by natural variability and thus depends on specific time periods considered.

Summertime land–sea thermal contrast and atmospheric circulation over East Asia in a warming climate—Part I: Past changes and future projections

Land–sea surface air temperature (SAT) contrast, an index of tropospheric thermodynamic structure and dynamical circulation, has shown a significant increase in recent decades over East Asia during the boreal summer. In Part I of this two-part paper, observational data and the results of transient warming experiments conducted using coupled atmosphere–ocean general circulation models (GCMs) are analyzed to examine changes in land–sea thermal contrast and the associated atmospheric circulation over East Asia from the past to the future. The interannual variability of the land–sea SAT contrast over the Far East for 1950–2012 was found to be tightly coupled with a characteristic tripolar pattern of tropospheric circulation over East Asia, which manifests as anticyclonic anomalies over the Okhotsk Sea and around the Philippines, and a cyclonic anomaly over Japan during a positive phase, and vice versa. In response to CO2 increase, the cold northeasterly winds off the east coast of northern Japan and the East Asian rainband were strengthened with the circulation pattern well projected on the observed interannual variability. These results are commonly found in GCMs regardless of future forcing scenarios, indicating the robustness of the East Asian climate response to global warming. The physical mechanisms responsible for the increase of the land–sea contrast are examined in Part II.

Comparison of the SST-Forced Responses between Coupled and Uncoupled Climate Simulations

Abstract It is commonly assumed that a reasonable estimate of the SST-forced component of the observed atmospheric circulation is given by an atmospheric GCM (AGCM) forced with the observed SST. However, there are results that find different SST-forced responses from the observed, for example for the ENSO–monsoon relationship, and suggest that these differences are due to lack of coupling to the ocean rather than atmospheric model bias unrelated to coupling. Here, the coupling issue is isolated and examined through perfect model experiments. A coupled atmosphere–ocean GCM (CGCM) simulation and an AGCM simulation forced by the SST from the CGCM are compared to examine whether the SST-forced responses are the same. This question cannot be addressed directly, since the SST-forced response of the CGCM is a priori unknown. Therefore, two indirect tests are applied, based on the assumption that the noise decorrelation time scale is short compared to a month. The first test is to compare the time-lagged linear regressions of the atmospheric fields onto several SST indices (defined as the area-averaged SST anomalies in the tropics or extratropics), with SST leading the atmosphere by a month. The second test is to compare the time lagged linear covariances of several atmospheric indices (including two monsoon indices and a North Atlantic Oscillation index) and SST, with the SST leading the atmosphere by a month. Both tests find that the SST-forced responses are the same in the CGCM and SST-forced AGCM. These tests can be extended to compare the SST-forced responses between different AGCMs, CGCMs, and observations.

A COMBINED DYNAMICAL/STATISTICAL DOWNSCALING APPROACH FOR ASSESSING FUTURE OF WATER RESOURCES IN THE TONE RIVER BASIN, JAPAN

Dynamical downscaling is a promising tool to assess the future fate of water in a catchment. To overcome strong biases in Coupled Ocean-Atmosphere General Circulation Models (CGCMs), the Pseudo Global Warming Downscaling (PGW-DS), which combines climatology differences (future-past) of CGCM ensembles and Reanalysis Dataset (RD) was proposed as a reliable and efficient method, but biases originated from downscaling of RD was not addressed. Hence, this study evaluated the results from a long-term, high-resolution Downscaled Precipitation (DP) derived from a RD over the Tone river basin. DP showed strong bias over mountainous regions owing to resolution enhancement. DP and CGCM precipitations were merged with Statistical Bias Correction methods (SBC) to obtain a comprehensive outlook on biases and the most appropriate SBC. Characteristics of biases were quite different in DP and CGCM, which performed very poor for extreme rainfall intensities and thus required an explicit treatment. Seasonal inconsistency in extreme events of CGCMs affected the corrected rainfall and discharges significantly. Conversely, a simple cumulative gamma method was successful for DP to represent climatology, extreme statistics of rainfall and discharges owing to the use of RD, which generated lesser bias compared to CGCMs. The proposed method will be applied to PGW-DS to obtain reliable information of future changes of water in the basin.

Anthropogenic agent implicated as a prime driver of shift in precipitation in eastern China in the late 1970s

Abstract. Observation shows that eastern China experienced an interdecadal shift in the summer precipitation during the second half of the 20th century. The summer precipitation increased in the middle and lower reaches of the Yangtze River valley, whereas it decreased in northern China. Here we use a coupled ocean–atmosphere general circulation model and multi-ensemble simulations to show that the interdecadal shift is mainly caused by the anthropogenic forcing. The rapidly increasing greenhouse gases induce a notable Indian Ocean warming, causing a westward shift of the western Pacific subtropical high (WPSH) and a southward displacement of the East Asia westerly jet (EAJ) on an interdecadal timescale, leading to more precipitation in Yangtze River valley. At the same time the surface cooling effects from the stronger convection, higher precipitation and rapidly increasing anthropogenic aerosols contribute to a reduced summer land–sea thermal contrast. Due to the changes in the WPSH, the EAJ and the land–sea thermal contrast, the East Asian summer monsoon weakened resulting in drought in northern China. Consequently, an anomalous precipitation pattern started to emerge over eastern China in the late 1970s. According to the model, the natural forcing played an opposite role in regulating the changes in WPSH and EAJ, and postponed the anthropogenically forced climate changes in eastern China. The Indian Ocean sea surface temperature is crucial to the response, and acts as a bridge to link the external forcings and East Asian summer climate together on a decadal and longer timescales. Our results further highlight the dominant roles of anthropogenic forcing agents in shaping interdecadal changes of the East Asian climate during the second half of the 20th century.

The ability of stratospheric climate engineering in stabilizing global mean temperatures and an assessment of possible side effects

AbstractA coupled atmosphere–ocean general circulation model is used to assess the ability of stratospheric climate engineering (CE) in stabilizing global mean temperatures at the threshold level +2 °C compared to pre‐industrial conditions. CE impact by solar radiation management (SRM) method is applied, when temperature threshold (+2 °C) is reached. Injection intensity is adjusted by the model so that calculated global temperature is remained close to the threshold value. We demonstrate that CE temperature stabilization during the 21st century is possible within the range of +2 ± 0.11 °C. Changes of precipitation intensity as well as effect of sharp termination of CE are investigated.

Evaluation of CMIP5 coupled atmosphere-ocean general circulation models and projection of the Southeast Asian winter monsoon in the 21st century

The Southeast Asian (SEA) winter monsoon (December–February) precipitation simulation and the potential future climate changes may bring about under different representative concentration pathways (RCPs) were evaluated using ten coupled atmosphere–ocean general circulation models (AOGCMs) in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Although all the models simulated the broad features of winter monsoon precipitation spatial patterns, the spread of the bias magnitudes was very large across the AOGCMs. All the models simulated the relationship between the regional rainfall and circulation associated with El Niño-Southern Oscillation (ENSO). However, the strength of the association tended to be weaker in the simulations, ensuing a generally weaker winter monsoon rainfall interannual variability. Three AOGCMs, namely CNRM-CM5, NorESM1-M and IPSL-CM5A-MR, that simulated the most realistic present-day climate, were used for examination of future climate projection based on the RCP 2.5, RCP 4.5 and RCP 8.5 scenarios. The projections showed a gradual increment of SEA winter monsoon rainfall under all the scenarios with the highest changes in the RCP 8.5 scenario. The increment was manifested as a zonal band of positive anomalies located at ∼10°N and ∼5°S, indicating northward and southward expansion of the intertropical convergence belt. Between these bands of increasing rainfall, the equatorial South China Sea (SCS) area was projected to be slightly drier. The projected increase of rainfall appeared to be associated with the prevailing of cyclonic anomalies over the centre SCS, which promote enhance moisture convergence over this region in the warmer climate.

The Climate of the MIS-13 Interglacial according to HadCM3

The climate of the Marine Isotopic Stage 13 (MIS-13) is explored in the fully coupled atmosphere–ocean general circulation model the Hadley Centre Coupled Model, version 3 (HadCM3). It is found that the strong insolation forcing at the time imposed a strengthened land–ocean thermal contrast, resulting in an intensified summer monsoon over Asia. The addition of land ice over North America and Eurasia results in a stationary wave feature across the Eurasian continent. This leads to a high pressure anomaly over the Sea of Japan with increased advection of warm moist air onto the Chinese landmasses. This in turn reinforces the East Asian summer monsoon (EASM), highlighting the counterintuitive notion that, depending on the background insolation and its size, ice can indeed contribute to strengthening the EASM. The modeling results support the geological record indication of a strong EASM 500 000 years ago. Furthermore, Arctic Oscillation, El Niño–Southern Oscillation, and Indian Ocean dipole–like teleconnection features are discussed in the MIS-13 environment. It is shown that the change in the tropical Pacific sea surface temperature has the potential to impact the North Atlantic climate through an atmospheric “bridge.”

Indian summer monsoon rainfall predictability and variability associated with Northwest Pacific circulation in a suit of coupled model hindcasts

The Northwest Pacific (NWP) circulation (subtropical high) is an important component of the East Asian summer monsoon system. During summer (June–August), anomalous lower tropospheric anticyclonic (cyclonic) circulation appears over NWP in some years, which is an indicative of stronger (weaker) than normal subtropical high. The anomalous NWP cyclonic (anticyclonic) circulation years are associated with negative (positive) precipitation anomalies over most of Indian summer monsoon rainfall (ISMR) region. This indicates concurrent relationship between NWP circulation and convection over the ISMR region. Dry wind advection from subtropical land regions and moisture divergence over the southern peninsular India during the NWP cyclonic circulation years are mainly responsible for the negative rainfall anomalies over the ISMR region. In contrast, during anticyclonic years, warm north Indian Ocean and moisture divergence over the head Bay of Bengal-Gangetic Plain region support moisture instability and convergence in the southern flank of ridge region, which favors positive rainfall over most of the ISMR region. The interaction between NWP circulation (anticyclonic or cyclonic) and ISMR and their predictability during these anomalous years are examined in the present study. Seven coupled ocean–atmosphere general circulation models from the Asia-Pacific Economic Cooperation Climate Center and their multimodel ensemble mean skills in predicting the seasonal rainfall and circulation anomalies over the ISMR region and NWP for the period 1982–2004 are assessed. Analysis reveals that three (two) out of seven models are unable to predict negative (positive) precipitation anomalies over the Indian subcontinent during the NWP cyclonic (anticyclonic) circulation years at 1-month lead (model is initialized on 1 May). The limited westward extension of the NWP circulation and misrepresentation of SST anomalies over the north Indian Ocean are found to be the main reasons for the poor skill (of some models) in rainfall prediction over the Indian subcontinent. This study demonstrates the importance of the NWP circulation variability in predicting summer monsoon precipitation over South Asia. Considering the predictability of the NWP circulation, the current study provides an insight into the predictability of ISMR. Long lead prediction of the ISMR associated with anomalous NWP circulation is also discussed.

Impact of Climate Drift on Twenty-First-Century Projection in a Coupled Atmospheric–Ocean General Circulation Model

Abstract Reducing climate drift in coupled atmosphere–ocean general circulation models (AOGCMs) usually requires 1000–2000 years of spinup, which has not been practical for every modeling group to do. For the purpose of evaluating the impact of climate drift, the authors have performed a multimillennium-long control run of the Goddard Institute for Space Studies model (GISS-EH) AOGCM and produced different twentieth-century historical simulations and subsequent twenty-first-century projections by branching off the control run at various stages of equilibration. The control run for this model is considered at quasi equilibration after a 1200-yr spinup from a cold start. The simulations that branched off different points after 1200 years are robust, in the sense that their ensemble means all produce the same future projection of warming, both in the global mean and in spatial detail. These robust projections differ from the one that was originally submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), which branched off a not-yet-equilibrated control run. The authors test various common postprocessing schemes in removing climate drift caused by a not-yet-equilibrated ocean initial state and find them to be ineffective, judging by the fact that they differ from each other and from the robust results that branched off an equilibrated control. The authors' results suggest that robust twenty-first-century projections of the forced response can be achieved by running climate simulations from an equilibrated ocean state, because memory of the different initial ocean state is lost in about 40 years if the forced run is started from a quasi-equilibrated state.

A Comparison of Statistical Approaches for Seasonal Precipitation Prediction in Pakistan

Abstract The present study focuses on two statistical approaches for improving seasonal precipitation prediction skills for Pakistan. Precipitation over Pakistan is concentrated in July–August (JA), when droughts and floods occur recurrently and cause disasters. Empirical orthogonal function (EOF) analysis is used to assess spatial patterns of precipitation, and two precipitation patterns are identified: a consistent pattern and a north–south dipole pattern. Two statistical approaches, the statistical regression method using prewinter predictors and statistical downscaling, are employed to perform rainfall predictions for JA in Pakistan. Linear regression (LR) and optimal subset regression (OSR) are used for each approach, and the regression forecast methods are compared with the raw model outputs. Historical data for large-scale variables from the NCEP–NCAR reanalysis and version 1.0 of the coupled atmosphere–ocean general circulation model from the Beijing Climate Center (CGCM1.0/BCC) outputs in 1986–2011 are used as predictors for the statistical prewinter method and statistical downscaling, respectively. In the majority of the years, the statistical prewinter method and statistical downscaling are able to correct the erroneous signs of the raw dynamical model output for the consistent pattern. The statistical prewinter method is found to provide more skillful predictions than the statistical downscaling on the prediction of the dipolelike pattern. The best prediction skills for the consistent pattern and dipolelike pattern are provided by NCEP-OSR and NCEP-LR, which have significant correlations of 0.39 and 0.40, respectively. For all the forecast methods in this study, prewinter prediction and downscaled prediction show considerable improvements when compared with model output. These statistical methods provide valuable approaches for studying local climates.

Decadal Prediction of the Sahelian Precipitation in CMIP5 Simulations

Abstract In this study the capability of eight state-of-the-art ocean–atmosphere coupled models in predicting the monsoonal precipitation in the Sahel on a decadal time scale is assessed. To estimate the importance of the initialization, the predictive skills of two different CMIP5 experiments are compared, a set of 10 decadal hindcasts initialized every 5 years in the period 1961–2009 and the historical simulations in the period 1961–2005. Results indicate that predictive skills are highly model dependent: the Fourth Generation Canadian Coupled Global Climate Model (CanCM4), Centre National de Recherches Météorologiques Coupled Global Climate Model, version 5 (CNRM-CM5), and Max Planck Institute Earth System Model, low resolution (MPI-ESM-LR) models show improved skill in the decadal hindcasts, while the Model for Interdisciplinary Research on Climate, version 5 (MIROC5) is skillful in both the decadal and historical experiments. The Beijing Climate Center, Climate System Model, version 1.1 (BCC-CSM1.1), Hadley Centre Coupled Model, version 3 (HadCM3), L'Institut Pierre-Simon Laplace Coupled Model, version 5, coupled with NEMO, low resolution (IPSL-CM5A-LR), and Meteorological Research Institute Coupled Atmosphere–Ocean General Circulation Model, version 3 (MRI-CGCM3) models show insignificant or no skill in predicting the Sahelian precipitation. Skillful predictions are produced by models properly describing the SST multidecadal variability and the initialization appears to play an important role in this respect.

Megadroughts in Southwestern North America in ECHO-G Millennial Simulations and Their Comparison to Proxy Drought Reconstructions*

Abstract Simulated hydroclimate variability in millennium-length forced transient and control simulations from the ECHAM and the global Hamburg Ocean Primitive Equation (ECHO-G) coupled atmosphere–ocean general circulation model (AOGCM) is analyzed and compared to 1000 years of reconstructed Palmer drought severity index (PDSI) variability from the North American Drought Atlas (NADA). The ability of the model to simulate megadroughts in the North American southwest is evaluated. (NASW: 25°–42.5°N, 125°–105°W). Megadroughts in the ECHO-G AOGCM are found to be similar in duration and magnitude to those estimated from the NADA. The droughts in the forced simulation are not, however, temporally synchronous with those in the paleoclimate record, nor are there significant differences between the drought features simulated in the forced and control runs. These results indicate that model-simulated megadroughts can result from internal variability of the modeled climate system rather than as a response to changes in exogenous forcings. Although the ECHO-G AOGCM is capable of simulating megadroughts through persistent La Niña–like conditions in the tropical Pacific, other mechanisms can produce similarly extreme NASW moisture anomalies in the model. In particular, the lack of low-frequency coherence between NASW soil moisture and simulated modes of climate variability like the El Niño–Southern Oscillation, Pacific decadal oscillation, and Atlantic multidecadal oscillation during identified drought periods suggests that stochastic atmospheric variability can contribute significantly to the occurrence of simulated megadroughts in the NASW. These findings indicate that either an expanded paradigm is needed to understand multidecadal hydroclimate variability in the NASW or AOGCMs may incorrectly simulate the strength and/or dynamics of the connection between NASW hydroclimate variability and the tropical Pacific.

Sensitivity of Arctic Climate Variability to Mean State: Insights from the Cretaceous

Abstract This study investigates Arctic climate variability during a period of extreme warmth using the Community Climate System Model, version 3 (CCSM3) coupled ocean–atmosphere general circulation model. Four mid-Cretaceous simulations were completed with different CO2 levels (1, 10, and 16 times preindustrial levels with dynamic vegetation) and vegetation treatments (10 times with specified uniform bare ground). The magnitude and frequency of Arctic temperature variability is highly sensitive to the mean state and high-latitude upper-ocean static stability. As stability increases with a rise in CO2 levels from 1 to 10 times preindustrial levels, the frequency of temperature variability increases from decades (1x) to centuries (10x with bare ground) and longer (10x) and the peak-to-peak magnitude increases from ~1° (for 1x) to ~2°C (for 10x). In the 16x simulation with a highly stratified ocean, Arctic temperature variability is low with peak-to-peak magnitudes <0.5°C. Under low CO2, Arctic climate variability is tied to sensible heat release from the ocean during movement of the sea ice margin. In absence of substantial sea ice, variability is driven by mass transport and upper-ocean salinity advection into the Arctic. In both cases, destruction of low-level clouds acts as an important feedback on low-level warming. The authors also report a link between unforced Arctic climate variability and North Pacific meridional overturning with warming events leading intensification. These results suggest that the nature of Arctic climate variability was likely much different in past climates and is likely to be so in the future.

Seasonal Forecasts of the Pan-Arctic Sea Ice Extent Using a GCM-Based Seasonal Prediction System

Abstract An ocean–sea ice model reconstruction spanning the period 1990–2009 is used to initialize ensemble seasonal forecasts with the Centre National de Recherches Météorologiques Coupled Global Climate Model version 5.1 (CNRM-CM5.1) coupled atmosphere–ocean general circulation model. The aim of this study is to assess the skill of fully initialized September and March pan-Arctic sea ice forecasts in terms of climatology and interannual anomalies. The predictions are initialized using “full field initialization” of each component of the system. In spite of a drift due to radiative biases in the coupled model during the melt season, the full initialization of the sea ice cover on 1 May leads to skillful forecasts of the September sea ice extent (SIE) anomalies. The skill of the prediction is also significantly high when considering anomalies of the SIE relative to the long-term linear trend. It confirms that the anomaly of spring sea ice cover in itself plays a role in preconditioning a September SIE anomaly. The skill of predictions for March SIE initialized on 1 November is also encouraging, and it can be partly attributed to persistent features of the fall sea ice cover. The present study gives insight into the current ability of state-of-the-art coupled climate systems to perform operational seasonal forecasts of the Arctic sea ice cover up to 5 months in advance.

A model–data comparison of the Holocene global sea surface temperature evolution

Abstract. We compare the ocean temperature evolution of the Holocene as simulated by climate models and reconstructed from marine temperature proxies. We use transient simulations from a coupled atmosphere–ocean general circulation model, as well as an ensemble of time slice simulations from the Paleoclimate Modelling Intercomparison Project. The general pattern of sea surface temperature (SST) in the models shows a high-latitude cooling and a low-latitude warming. The proxy dataset comprises a global compilation of marine alkenone- and Mg/Ca-derived SST estimates. Independently of the choice of the climate model, we observe significant mismatches between modelled and estimated SST amplitudes in the trends for the last 6000 yr. Alkenone-based SST records show a similar pattern as the simulated annual mean SSTs, but the simulated SST trends underestimate the alkenone-based SST trends by a factor of two to five. For Mg/Ca, no significant relationship between model simulations and proxy reconstructions can be detected. We test if such discrepancies can be caused by too simplistic interpretations of the proxy data. We explore whether consideration of different growing seasons and depth habitats of the planktonic organisms used for temperature reconstruction could lead to a better agreement of model results with proxy data on a regional scale. The extent to which temporal shifts in growing season or vertical shifts in depth habitat can reduce model–data misfits is determined. We find that invoking shifts in the living season and habitat depth can remove some of the model–data discrepancies in SST trends. Regardless whether such adjustments in the environmental parameters during the Holocene are realistic, they indicate that when modelled temperature trends are set up to allow drastic shifts in the ecological behaviour of planktonic organisms, they do not capture the full range of reconstructed SST trends. Results indicate that modelled and reconstructed temperature trends are to a large degree only qualitatively comparable, thus providing a challenge for the interpretation of proxy data as well as the model sensitivity to orbital forcing.

Evaluating Low-Cloud Simulation from an Upgraded Multiscale Modeling Framework Model. Part II: Seasonal Variations over the Eastern Pacific

Abstract The eastern Pacific is a climatologically important region. Conventional coupled atmosphere–ocean general circulation models produce positive sea surface temperature biases of 2–5 K in this region because of insufficient stratocumulus clouds. In this study, a global multiscale modeling framework (MMF), which replaces traditional cloud parameterizations with a 2D cloud-resolving model (CRM) in each atmospheric column, is used to examine the seasonal variations of this Pacific region. The CRM component contains an advanced third-order turbulence closure, helping it to better simulate boundary layer turbulence and low-level clouds. Compared to available satellite observations of cloud amount, liquid water path, cloud radiative effects, and precipitation, this MMF produces realistic seasonal variations of the eastern Pacific region, although there are some disagreements in the exact location of maximum cloudiness centers in the Peruvian region and the intensity of ITCZ precipitation. Analyses of profile- and subcloud-based decoupling measures reveal very small amplitudes of seasonal variations in the decoupling strength in the subtropics except for those regions off the subtropical coasts where the decoupling measures suggest that the boundary layers should be well coupled in all four seasons. In the Peruvian and Californian regions, the seasonal variations of low clouds are related to those in the boundary layer height and the strength of inversion. Factors that influence the boundary layer and the inversion, such as solar incident radiation, subcloud-layer turbulent mixing, and large-scale subsidence, can collectively explain the seasonal variations of low clouds rather than the deepening–warming mechanism of Bretherton and Wyant cited in earlier studies.

Regional hydrological cycle changes in response to an ambitious mitigation scenario

Climate change impacts on the regional hydrological cycle are compared for model projections following an ambitious emissions-reduction scenario (E1) and a medium-high emissions scenario with no mitigation policy (A1B). The E1 scenario is designed to limit global annual mean warming to 2 °C or less above pre-industrial levels. A multi-model ensemble consisting of ten coupled atmosphere–ocean general circulation models is analyzed, which includes five Earth System Models containing interactive carbon cycles. The aim of the study is to assess the changes that could be mitigated under the E1 scenario and to identify regions where even small climate change may lead to strong changes in precipitation, cloud cover and evapotranspiration. In these regions the hydrological cycle is considered particularly vulnerable to climate change, highlighting the need for adaptation measures even if strong mitigation of climate change would be achieved. In the A1B projections, there are significant drying trends in sub-tropical regions, precipitation increases in high latitudes and some monsoon regions, as well as changes in cloudiness and evapotranspiration. These signals are reduced in E1 scenario projections. However, even under the E1 scenario, significant precipitation decrease in the subtropics and increase in high latitudes are projected. Particularly the Amazon region shows strong drying tendencies in some models, most probably related to vegetation interaction. Where climate change is relatively small, the E1 scenario tends to keep the average magnitude of potential changes at a level comparable to current intra-seasonal to inter-annual variability at that location. Such regions are mainly located in the mid-latitudes.

Consistency of 20th century sea level pressure trends as simulated by a coupled and uncoupled GCM

[1] A coupled (ocean-atmosphere) general circulation model (CGCM) and an uncoupled atmospheric general circulation model forced with the SST and external forcing of the coupled model simulate similar 2 m air temperature (TS) trends and also similar sea level pressure (SLP) trends for the latter half of the 20th century. This suggests that the inability of atmospheric models forced by observed SST and external forcing to reproduce observed SLP trends in the Indian Ocean could be due to model bias rather than lack of coupling. The internally generated TS trend in the CGCM is found to be small in comparison to the externally forced component. Intrinsic atmospheric noise explains most of the CGCM's internally generated high-latitude SLP trend, while in low latitudes, the response of the SLP trend to the internally generated SST trend is important.

Evaluation of Weather Noise and Its Role in Climate Model Simulations*

Abstract The relationship between coupled atmosphere–ocean general circulation model simulations and uncoupled simulations made with specified SST and sea ice is investigated using the Community Climate System Model, version 3. Experiments are carried out in a perfect model framework. Two closely related questions are investigated: 1) whether the statistics of the atmospheric weather noise in the atmospheric model are the same as in the coupled model, and 2) whether the atmospheric model reproduces the SST-forced response of the coupled model. The weather noise in both the coupled and uncoupled simulations is found by removing the forced response, as determined from the uncoupled ensemble, from the atmospheric field. The weather-noise variance is generally not distinguishable between the coupled and uncoupled simulations. However, variances of the total fields differ between the coupled and uncoupled simulations, since there is constructive or destructive interference between the SST-forced response and weather noise in the coupled model but no correlation between the SST-forced and weather-noise components in the uncoupled model simulations. Direct regression estimates of the forced response show little difference between the coupled and uncoupled simulations. Differences in local correlations are explained by weather noise because weather noise forces SST in the coupled simulation only. The results demonstrate and explain an important intrinsic difference in precipitation statistics between the coupled and uncoupled simulations. This difference could have consequences for the design of dynamical downscaling experiments and for tuning general circulation models.