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Seasonal climate predictions are formulated from known, present conditions, and simulate the near-term climate for approximately a year in the future. Recent efforts in seasonal climate prediction include coupled general circulation model (CGCM) ensemble predictions, but other efforts have included atmospheric general circulation model (AGCM) ensemble predictions that are forced by time-varying sea surface temperatures (SSTs). CGCMs and AGCMs have differences in the way surface energy fluxes are simulated, which may lead to differences in skill and predictability. Concerning model biases, forecasted SSTs have errors compared to observed SSTs, which may also affect skill and predictability. This manuscript focuses on the role of the ocean in climate predictions, and includes the influences of ocean-atmosphere coupling and SST errors on skill and predictability. We perform a series of prediction experiments comparing coupled and uncoupled Community Climate System Model version 4.0 (CCSM4) predictions, and forecasted versus observed SSTs to determine which is the leading cause for differences in skill and predictability. Overall, prediction skill and predictability are only weakly influenced by ocean-atmosphere coupling, with the exception of the western Pacific, while errors in forecasted SSTs significantly impact skill and predictability. Comparatively, SST errors lead to more significant and robust differences in prediction skill and predictability versus inconsistencies in ocean-atmosphere coupling.

Under previous reconstructions of late Pliocene boundary conditions, climate models have failed to reproduce the warm sea surface temperatures reconstructed in the North Atlantic. Using a reconstruction of mid-Piacenzian paleogeography that has the Bering Strait and Canadian Arctic Archipelago Straits closed, however, improves the simulation of the proxy-indicated warm sea surface temperatures in the North Atlantic in the Community Climate System Model. We find that the closure of these small Arctic gateways strengthens the Atlantic Meridional Overturning Circulation, by inhibiting freshwater transport from the Pacific to the Arctic Ocean and from the Arctic Ocean to the Labrador Sea, leading to warmer sea surface temperatures in the North Atlantic. This indicates that the state of the Arctic gateways may influence the sensitivity of the North Atlantic climate in complex ways, and better understanding of the state of these Arctic gateways for past time periods are needed.

Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between deoxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore water redox conditions in the South Atlantic and Southern Indian Oceans and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5–2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low-oxygen sites, than at well-oxygenated modern sites, indicating higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (Community Climate System Model Version 3), but the simulated absolute dissolved O2 values are low (

Continental dust input into the ocean-atmosphere system has significant ramifications for biogeochemical cycles and global climate, yet direct observations of dust deposition in the ocean remain scarce. The long-lived isotope thorium-232 (232Th) is greatly enriched in upper continental crust compared to oceanic crust and mid-ocean ridge basalt-like volcanogenic material. In open ocean sediments, away from fluvial and ice-rafted sources of continental material, 232Th is often assumed to be of predominantly eolian origin. In conjunction with flux normalization based on the particle reactive radioisotope thorium-230 (230Th), 232Th measurements in marine sediments are a promising proxy for dust accumulation in the modern and past ocean. Here we present ThoroMap, a new global data compilation of 230Th-normalized fluxes of 232Th. After careful screening, we derive dust deposition estimates in the global ocean averaged for the late Holocene (0–4 ka) and the Last Glacial Maximum (LGM, 19–23 ka). ThoroMap is compared with dust deposition estimates derived from the Community Climate System Model (CCSM3) and CCSM4, two coupled atmosphere, land, ocean, and sea ice models. Model-data correlation factors are 0.63 (CCSM3) and 0.59 (CCSM4) in the late Holocene and 0.82 (CCSM3) and 0.83 (CCSM4) in the LGM. ThoroMap is the first compilation that is built on a single, specific proxy for dust and that exclusively uses flux-normalization to derive dust deposition rates.

The latitude of landfall for atmospheric rivers (ARs) is examined in the fully coupled half-degree version of the Community Climate System Model, version 4 (CCSM4) for warm future climate simulations. Two regions are examined: U.S. West Coast/North Pacific ARs, and United Kingdom/North Atlantic ARs. Changes in AR landfall-latitude reflect changes in the atmospheric steering flow. West coast U.S. ARs are projected to push equatorward in response to the subtropical jet climate change. UK AR response is dominated by eddy-driven jets and is seasonally dependent. UK simulated AR response is modest in the winter with the largest relative changes occurring in the seasonal transition months. Precipitation associated with ARs is also projected to increase in intensity under global warming. CCSM4 projects a marked shift to higher rainfall rates for Southern California. Small to modest rainfall rates may increase for all UK latitudes, for the Pacific Northwest, and central and northern California.

Atmospheric rivers are recognized as major contributors to the poleward transport of water vapor. Upon reaching land, these phenomena also play a critical role in extreme precipitation and flooding events. The Pineapple Express (PE) is defined as an atmospheric river extending out of the deep tropics and reaching the west coast of North America. Community Climate System Model (CCSM4) high-resolution ensemble simulations for the twentieth and 21st centuries are diagnosed to identify the PE. Analysis of the twentieth century simulations indicated that the CCSM4 accurately captures the spatial and temporal climatology of the PE. Analysis of the end 21st century simulations indicates a significant increase in storm duration and intensity of precipitation associated with landfall of the PE. Only a modest increase in the number of atmospheric rivers of a few percent is projected for the end of 21st century.

The global fully coupled half-degree Community Climate System Model Version 4 (CCSM4) was integrated for a suite of climate change ensemble simulations including five historical runs, five Representative Concentration Pathway 8.5 [RCP8.5) runs, and a long Pre-Industrial control run. This study focuses on precipitation at regional scales and its sensitivity to horizontal resolution. The half-degree historical CCSM4 simulations are compared to observations, where relevant, and to the standard 1° CCSM4. Both the halfdegree and 1° resolutions are coupled to a nominal 1° ocean. North American and South Asian/Indian monsoon regimes are highlighted because these regimes demonstrate improvements due to higher resolution, primarily because of better-resolved topography. Agriculturally sensitive areas are analyzed and include Southwest, Central, and Southeast U.S., Southern Europe, and Australia. Both mean and extreme precipitation is discussed for convective and large-scale precipitation processes. Convective precipitation tends to decrease with increasing resolution and large-scale precipitation tends to increase. Improvements for the half-degree agricultural regions can be found for mean and extreme precipitation in the Southeast U.S., Southern Europe, and Australian regions. Climate change responses differ between the model resolutions for the U.S. Southwest/Central regions and are seasonally dependent in the Southeast and Australian regions. Both resolutions project a clearmore » drying signal across Southern Europe due to increased greenhouse warming. As a result, differences between resolutions tied to the representation of convective and large-scale precipitation play an important role in the character of the climate change and depend on regional influences.« less

The question of whether ocean coupling matters for the extratropical Northern Hemisphere atmospheric response to projected late 21st century Arctic sea ice loss is addressed using a series of experiments with Community Climate System Model version 4 at 1° spatial resolution under different configurations of the ocean model component: no interactive ocean, thermodynamic slab ocean, and full-depth (dynamic plus thermodynamic) ocean. Ocean-atmosphere coupling magnifies the response to Arctic sea ice loss but does not change its overall structure; however, a slab ocean is inadequate for inferring the role of oceanic feedbacks. The westerly winds along the poleward flank of the eddy-driven jet weaken in response to Arctic sea ice loss, accompanied by a smaller-magnitude strengthening on the equatorward side, with largest amplitudes in winter. Dynamical and thermodynamic oceanic feedbacks amplify this response by approximately 50%. Air temperature, precipitation, and sea level pressure responses also show sensitivity to the degree of ocean coupling.

This study performs high-spatial-resolution (12 km) Weather Research and Forecasting (WRF) simulations over a very large domain (7200 km × 6180 km, covering much of North America) to explore changes in mean and extreme precipitation in the mid and late 21st century under Representative Concentration Pathways 4.5 (RCP 4.5) and 8.5 (RCP 8.5). We evaluate WRF model performance for a historical simulation and future projections, applying the Community Climate System Model version 4 (CCSM4) as initial and boundary conditions with and without a bias correction. WRF simulations using boundary and initial conditions from both versions of CCSM4 show smaller biases versus evaluation data sets than does CCSM4 over western North America. WRF simulations also improve spatial details of precipitation over much of North America. However, driving the WRF with the bias-corrected CCSM4 does not always reduce the bias. WRF-projected changes in precipitation include decreasing intensity over the southwestern United States, increasing intensity over the eastern United Sates and most of Canada, and an increase in the number of days with heavy precipitation over much of North America. Projected precipitation changes are more evident in the late 21st century than the mid 21st century, and they are more evident under RCP 8.5 than under RCP 4.5 in the late 21st century. Uncertainties in the projected changes in precipitation due to different warming scenarios are non-negligible. Differences in summer precipitation changes between WRF and CCSM4 are significant over most of the United States.

The response of El Nino–Southern Oscillation (ENSO) to mid-Holocene boundary conditions remains an open question: paleoclimate proxies and climate model simulations do not agree in the magnitude of the reduction of ENSO variability, while recent proxy evidence from fossil corals from the central Pacific show that the reduction in mid-Holocene ENSO variability compared to the end of the twentieth century is not different from the reduction during other Holocene time intervals. This is inconsistent with the interpretation of lake and ocean sediment records from the eastern Pacific, which show a significant reduction compared to all other Holocene periods. In order to reconcile the seemingly conflicting proxy evidence from the eastern and central Pacific, we hypothesize that ENSO remained active during the mid-Holocene; however, there was a change in the spatial pattern of the sea surface temperature anomalies, also known as ENSO flavors. Using National Center for Atmospheric Research's Community Climate System Model version 4 forced with mid-Holocene orbital conditions, we find that the frequency of occurrence of the strongest eastern Pacific (EP) events decreases in the mid-Holocene and their variance is reduced by ∼30%, while the frequency of central Pacific (CP) events slightly increases and their variance does not change. We also find a shift in the seasonality of EP events, but not in that of CP events. Lastly, mid-Holocene EP events develop more slowly and decay faster. The differential response of ENSO flavors to mid-Holocene forcing is remotely forced by the West Pacific, where a weakening of the trade winds in early boreal spring in the mid-Holocene initiates an anomalous downwelling annual Kelvin wave, which reaches the eastern Pacific during the ENSO development season, weakens the upper ocean stratification, and results in reduced ENSO upwelling feedback. The simulated reduction in the EP flavor versus the CP flavor in the mid-Holocene is consistent with proxy evidence: The teleconnection patterns of the two flavors with temperature, precipitation, and salinity are distinct, and proxies from different regions of the Pacific might be recording variability associated with only one of the flavors, or some combination of their relative effects.

Pronounced millennial-scale climate variability during marine isotope stage 3 (MIS3) is considered to be linked to changes in the state of the Atlantic meridional overturning circulation (AMOC), i.e., a warm interstadial/cold stadial state corresponds to a strong/weak AMOC. Based on a series of freshwater hosing/extraction experiments with the state-of-the-art Community Climate System Model version 3, we construct a global spatial fingerprint of oceanic temperature anomalies in response to AMOC changes under MIS3 boundary conditions. Highest sensitivity to AMOC changes, especially in summer, is found in northeastern North Atlantic sea surface temperature, but a characteristic temperature fingerprint is also found at subsurface levels. After testing significance of the linear sea surface temperature (SST)-AMOC regressions, the model results are combined with paleo-SST records to estimate the magnitude of millennial-scale Dansgaard-Oeschger AMOC variations during MIS3. The results suggest a mean difference in AMOC strength between interstadial and (non-Heinrich) stadial states of 9.21.2Sv (1 sigma).

An analysis of the recently reconstructed gridded May–September total precipitation in the Indian monsoon region for the past half millennium discloses significant variations at multidecadal timescales. Meanwhile, paleo-climate modeling outputs from the National Center for Atmospheric Research Community Climate System Model 4.0 show similar multidecadal variations in the monsoon precipitation. One of those variations at the frequency of 40–50 years per cycle is examined in this study. Major results show that this variation is a product of the processes in that the meridional gradient of the atmospheric enthalpy is strengthened by radiation loss in the high-latitude and polar region. Driven by this gradient and associated baroclinicity in the atmosphere, more heat/energy is generated in the tropical and subtropical (monsoon) region and transported poleward. This transport relaxes the meridional enthalpy gradient and, subsequently, the need for heat production in the monsoon region. The multidecadal timescale of these processes results from atmospheric circulation-radiation interactions and the inefficiency in generation of kinetic energy from the potential energy in the atmosphere to drive the eddies that transport heat poleward. This inefficiency creates a time delay between the meridional gradient of the enthalpy and the poleward transport. The monsoon precipitation variation lags that in the meridional gradient of enthalpy but leads that of the poleward heat transport. This phase relationship, and underlining chasing process by the transport of heat to the need for it driven by the meridional enthalpy gradient, sustains this multidecadal variation. This mechanism suggests that atmospheric circulation processes can contribute to multidecadal timescale variations. Interactions of these processes with other forcing, such as sea surface temperature or solar irradiance anomalies, can result in resonant or suppressed variations in the Indian monsoon precipitation.

Total water levels (TWLs) within estuaries are influenced by tides, wind, offshore waves, and streamflow, all of which are uniquely affected by climate change. The magnitude of TWL associated with various return periods is relevant to understanding how the hydrodynamics of a bay or estuary may evolve under distinct climate scenarios. A methodology for assessing the hydrodynamic response of a small estuary under major boundary condition perturbations is presented in this study. The coupled Advanced Circulation (ADCIRC) and Simulating Waves Nearshore (SWAN) model was used to simulate wave and water elevation conditions within Tillamook Bay, OR, USA for two long-term scenarios; 1979–1998 and 2041–2060. The model output provided multidecadal time series of TWLs for statistical analysis. Regional climate data from the North American Regional Climate Change Assessment Program (NARCCAP) were used to drive streamflow modeling (MicroMet/SnowModel/HydroFlow) and meteorological forcing within ADCIRC-SWAN. WAVEWATCH III, which was forced with global climate data from the Community Climate Science Model (CCSM, a contributing model to NARCCAP), was used to produce open boundary wave forcing. Latitudinal and seasonal gradients were found in TWLs associated with varying return periods for both the hindcast and forecast. Changes in TWLs from hindcast to forecast included the sea level rise component and were also modulated by changes in boundary conditions.

The 8.2ka event is a key test case for simulating the coupled climate response to changes in the Atlantic Meridional Overturning Circulation (AMOC). Recent advances in quantifying freshwater fluxes at 8.2ka from the proxy record have improved the realism of the forcing magnitude in model simulations, yet this forcing is still generally applied in an unrealistic geographic manner, across most of the Labrador Sea rather than just along the Labrador coast. Previous simulations with eddy- resolving ocean models have come to conflicting conclusions regarding the ability of such a coastally confined flow to impact the AMOC. These simulations have also not incorporated full atmosphere models nor have they used the new meltwater forcing values for 8.2ka. We use the Community Climate System Model, version 3, with an ocean model resolution only slightly coarser than that used in previous eddy-resolving simulations, to test the sensitivity to freshwater forcing location. When revised freshwater forcing is applied across the Labrador Sea, the AMOC is reduced by similar to 40% and climate anomalies compare well with proxy records of the 8.2ka event in terms of magnitude and duration. When the forcing is added just along the Labrador coast, however, most meltwater joins the subtropical gyre and travels to the subtropics with minor impact to the AMOC (similar to 10% decrease). It is likely that model biases in the placement of the North Atlantic Current remain an important limitation for correctly simulating the 8.2ka event.

Late Cenozoic marine proxy data record a long-term transition in the tropical Pacific from El Nino-like conditions with reduced zonal sea surface temperature (SST) gradient, deepened thermocline, and reduced upwelling in the eastern equatorial Pacific (EEP) to conditions similar to modern. This transition coincides with kilometer-scale uplift of the central Andes. To understand whether the rise of the Andes contributed to tropical Pacific climate evolution, we performed experiments with the National Center for Atmospheric Research's Community Climate System Model version 4 to quantify changes in tropical Pacific climate and El Nino–Southern Oscillation as a function of Andean elevations. Our results demonstrate that uplift increases the equatorial east-west SST gradient and Walker circulation. The rise of the Andes from 1 to 3 km increases the SST gradient by 0.8°C and Walker circulation by 60% due to strengthened radiative cooling by enhanced low-cloud formation in the EEP. This cooling effect is largest in the southeastern tropical Pacific and accounts for about one half of the reconstructed SST cooling along the Peru coast. The uplift also strengthens upwelling north of the EEP, consistent with documented increases in biological productivity in this region, and decreases the frequency of El Nino–Southern Oscillation and the number of strong El Nino events. Simulated responses to Andean uplift are generally consistent with the late Cenozoic proxy records, but too small in magnitude. Taken together, our results indicate that Andean uplift was likely one of the multiple factors that contributed to the long-term evolution of both the mean climate state and the interannual variability in the tropical Pacific.

Using transient accelerated simulations of the Community Climate System Model version 3 and an Earth System Model of Intermediate Complexity as well as equilibrium experiments of the Community Earth System Model, we identified a response of the extratropical air-ocean coupled system to the precessional insolation changes at orbital timescales and named this extratropical response pattern as the North Pacific mode (NPM). Corresponding to the increased/decreased boreal winter/summer insolation at 22 ka (relative to 10–8 ka), the NPM is characterized by a western warm-eastern cold seesaw pattern of sea surface temperature (SST) over the extratropical North Pacific from November to April, a weakened winter Aleutian low and an anomalous anticyclonic circulation throughout the troposphere. This feature forms a barotropic warm-ridge response of tropospheric temperature and geopotential height to the precessional insolation. At the surface, rainfall increases over East Asia and the Northwest Pacific, which indicates a weakened East Asian winter monsoon, while drier conditions appear over the Northeast Pacific and the western coasts of North America. Associated with a negative phase of NPM is a weaker warming over the equatorial Pacific during winter. The increased winter insolation at precessional band not only induces the in-phase SST warming over the Northwest Pacific and the tropical Pacific, but also explains those extratropical atmospheric changes associated with NPM. The latter might be associated with the warm SST-induced tropospheric downstream ridge response through transient eddy activities. Besides the vital role of air-ocean interactions, the decreased summer insolation is also essential to the zonal SST seesaw of NPM at precessional band.

In this study, an atmospheric river (AR) detection algorithm is developed to investigate the downstream modulation of the eastern North Pacific ARs by another weather extreme, known as the East Asian cold surge (EACS), in both reanalysis data and high-resolution global model simulations. It is shown that following the peak of an EACS, atmospheric disturbances of intermediate frequency (IF; 10–30 day period) are excited downstream. This leads to the formation of a persistent cyclonic circulation anomaly over the eastern North Pacific that dramatically enhances the AR occurrence probability and the surface precipitation over the western U.S. between 30°N and 50°N. A diagnosis of the local geopotential height tendency further confirms the essential role of IF disturbances in establishing the observed persistent anomaly. This downstream modulation effect is then examined in the two simulations of the National Center for Atmospheric Research Community Climate System Model version 4 with different horizontal resolutions (T85 and T341) for the same period (1979–2005). The connection between EACS and AR is much better captured by the T341 version of the model, mainly due to a better representation of the scale interaction and the characteristics of IF atmospheric disturbances in the higher-resolution model. The findings here suggest that faithful representations of scale interaction in a global model are critical for modeling and predicting the occurrences of hydrological extremes in the western U.S. and for understanding their potential future changes.

Surface albedo changes from anthropogenic land cover change (ALCC) represent the second largest negative radiative forcing behind aerosol during the industrial era. Using a new reconstruction of ALCC during the Holocene era by Kaplan et al. (2011), we quantify the local and global temperature response induced by Holocene ALCC in the Community Climate System Model, version 4. We find that Holocene ALCC causes a global cooling of 0.17°C due to the biogeophysical effects of land-atmosphere exchange of momentum, moisture, and radiative and heat fluxes. On the global scale, the biogeochemical effects of Holocene ALCC from carbon emissions dominate the biogeophysical effects by causing 0.9°C global warming. The net effects of Holocene ALCC amount to a global warming of 0.73°C during the preindustrial era, which is comparable to the ~0.8°C warming during industrial times. On local to regional scales, such as parts of Europe, North America, and Asia, the biogeophysical effects of Holocene ALCC are significant and comparable to the biogeochemical effect.

Average formation rates for Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the South Pacific are calculated from the National Center for Atmospheric Research Community Climate System Model version 4 (NCAR-CCSM4), using chlorofluorocarbon inventories (CFC-12). When compared to observations, CCSM4 accurately simulates the southeast Pacific as the main formation region for SAMW and AAIW. Formation rates for SAMW in CCSM4 are 3.4 sverdrup (Sv), about half of the observational rate, due in part to shallow mixed layers, a thinner SAMW layer, and insufficient meridional transport. A formation rate of 8.1 Sv for AAIW in CCSM4 is higher than observations due to higher inventories in the southwest and central Pacific and surface concentrations within CCSM4. Also, a lack of data in the southwest Pacific may bias the observational rate low. This model-observation comparison is useful for understanding the uptake and transport of other gases, e.g., CO2 by the model.

We introduce a new perspective of climate change by revealing the changing characteristics of atmospheric information flow in a warming climate. The key idea is to interpret large-scale atmospheric dynamical processes as information flow around the globe and to identify the pathways of this information flow using a climate network based on causal discovery and graphical models. We construct such networks using the daily geopotential height data from the Community Climate System Model Version 4.0 (CCSM4.0)'s 20th century climate simulation and 21st century climate projection. We show that in the CCSM4.0 model under enhanced greenhouse gases (GHGs) forcing, prominent midlatitude information pathways in the midtroposphere weaken and shift poleward, while major tropical information pathways start diminishing. Averaged over the entire Northern Hemisphere, the atmospheric information flow weakens. The implications of this weakening for the interconnectivity among different geographical locations and for the intrinsic predictability of the atmosphere are discussed.

[1] In this study we examine ENSO diversity in a 500 year control simulation of the National Center for Atmospheric Research (NCAR) Community Climate System Model version 4 (CCSM4), focusing on warm events. Standard and modified Nino3 and Nino4 indices are used to identify different event types. CCSM4 shows a rich diversity of El Nino flavors with characteristics that are comparable to what was found in observations, the SODA 2.0.2/3 ocean reanalysis, and the GFDL CM2.1 model, a climate model whose ENSO characteristics have been extensively analyzed. In agreement with previous studies available in the literature, warm events peaking in the central/western Pacific are characterized by wind stress and precipitation fields confined to the western side of the basin, and show weak or absent recharge/discharge thermocline processes. A heat budget analysis of four different El Nino flavors, peaking at different longitudes, confirms the leading role of the thermocline and zonal advective feedbacks in the Nino3 and Nino4 regions, respectively. However, the growth of events centered further west appears to be controlled by nonlinear zonal advection, a result that differs from what was found in the GFDL CM2.1 model, but that is consistent with some observational evidence.

[1] This study investigates the effect of parameterized westerly wind bursts (WWBs) on the diversity of El Nino–Southern Oscillation (ENSO) warm events, namely, eastern Pacific (EP) and central Pacific (CP) ENSO in Community Climate System Model version 3 (CCSM3) and version 4 (CCSM4). The parameterization includes the possibility of state-independent (SI) and state-dependent (SD) WWBs. The inclusion of WWBs enhances the overall tropical Pacific variability. This is accompanied by an increased in the so-called thermocline and advective feedback mechanisms. The SI WWB results present some model discrepancies, with CCSM3 (CCSM4) having enhanced CP (EP) ENSO variability. For the SD WWB case, both models show enhanced EP variability relative to CP variability.

[1] In this article, the relationship between the Antarctic Oscillation (AAO) and the tropical cyclone (TC) activity in the southern Indian Ocean (SIO) was examined. It was found that on the interannual time scale, the AAO is well linked with the TC activity in the SIO during TC season (December–March). The rank correlation coefficient between the AAO index and the TC frequency (TCF) in the SIO is 0.37, which is significant at the 95% confidence level. When the AAO is in a positive phase, TC passage in the northwestern coast of Australia (100E°–120°E and 10°S–30°S) increases by approximately 50%–100% from the climatology. The increase in the TC passage is primarily the result of more frequent TCs originating in this region due to enhanced water vapor convergence and ascending motions, which are caused by a cyclonic height anomaly over the western coast of Australia associated with the positive AAO phases. In addition, the AAO-height covariations, which are essential to the formation of the AAO-TC links in the SIO, were investigated through a historical climate simulation using the Community Climate System Model 4 from the Coupled Model Intercomparison Project Phase 5. The AAO-height links were well reproduced in the simulation. The similarity in the AAO-height links between the observation and the simulation supports the physical robustness of the AAO-TC links in the SIO.

[1] A new framework for simulating the El Nino/Southern Oscillation (ENSO) using a generalized linear model (GLM) is provided. The GLM provides a versatile and computationally inexpensive method for investigating ENSO dynamics, by conditioning an ENSO index on an arbitrary set of input variables. Here the system state (El Nino/neutral/La Nina) at previous times is combined with the first few principal components of sea surface temperature (SST) and thermocline depth. Despite having relatively few degrees of freedom, the model accurately reproduces 20th century SST time series, seasonal variance, power spectra, and autocorrelation functions for both the eastern and western Pacific. The GLM also has good overall forecast skill, especially at subyearly lead times; performance is competitive with models currently used for operational ENSO forecasting. The model is then used to examine changes to El Nino/La Nina statistics under CO2 increases, by using the GLM to represent simulations run with the National Center for Atmospheric Research (NCAR) Community Climate System Model (CCSM), versions 3.5 and 4. GLM simulations of 21st century CCSM4 changes to El Nino/La Nina magnitudes show insignificant results, despite a slight increase in El Nino persistence. GLM fits conditioned on millennial stabilized CCSM3.5 simulations with varying CO2 levels, however, show a weakening and shortening of El Nino events as CO2 concentration increases, whereas La Nina events become markedly stronger and do not change significantly in length. The reduction in El Nino persistence in CCSM3.5 is consistent with previous results showing that at higher CO2 levels, a stronger seasonal cycle creates a Southern Hemisphere “seasonal footprint” leading to more efficient El Nino termination.

[1] The magnitude of sea surface temperature variability in the NINO3.4 region of the equatorial Pacific on decadal and longer timescales is assessed in observational data, state-of-the-art (Coupled Model Intercomparison Project 5) climate model simulations, and a new ensemble of paleoclimate reconstructions. On decadal to multidecadal timescales, variability in these records is consistent with the null hypothesis that it arises from “multivariate red noise” (a multivariate Ornstein-Uhlenbeck process) generated from a linear inverse model of tropical ocean-atmosphere dynamics. On centennial and longer timescales, both a last millennium simulation performed using the Community Climate System Model 4 (CCSM4) and the paleoclimate reconstructions have variability that is significantly stronger than the null hypothesis. However, the time series of the model and the reconstruction do not agree with each other. In the model, variability primarily reflects a thermodynamic response to reconstructed solar and volcanic activity, whereas in the reconstruction, variability arises from either internal climate processes, forced responses that differ from those in CCSM4, or nonclimatic proxy processes that are not yet understood. These findings imply that the response of the tropical Pacific to future forcings may be even more uncertain than portrayed by state-of-the-art models because there are potentially important sources of century-scale variability that these models do not simulate.

[1] Sea level rise (SLR) is an inescapable consequence of increasing greenhouse gas concentrations, with potentially harmful effects on human populations in coastal and island regions. Observational evidence indicates that global sea level has risen in the 20th century, and climate models project an acceleration of this trend in the coming decades. Here we analyze rates of future SLR on regional scales in a 40-member ensemble of climate change projections with the Community Climate System Model Version 3. This unique ensemble allows us to assess uncertainty in the magnitude of 21st century SLR due to internal climate variability alone. We find that simulated regional SLR at mid-century can vary by a factor of 2 depending on location, with the North Atlantic and Pacific showing the greatest range. This uncertainty in regional SLR results primarily from internal variations in the wind-driven and buoyancy-driven ocean circulations.

[1] We explore annual and seasonal changes in moist static energy transport (MSE) into the Arctic over the 21st century as projected by the National Center for Atmospheric Research Community Climate System Model, version 3. Self-organizing maps are used to assess changes in MSE and its components—the latent heat flux and dry static energy flux (DSE)—across 70°N. These are computed from multilevel fields of specific humidity, meridional wind, geopotential, and temperature spanning periods in the 20th century (1960 to 1999) and the 21st century (2070 to 2089). The 21st century simulation incorporates the Special Report on Emission Scenarios A2 scenario. A strong decrease in tropospheric DSE of about 9% by the end of 21st century offsets an increase in latent heat flux of about 20% relative to its 20th century average. The combined changes result in a total annual decrease in tropospheric MSE of about 3% by the late 21st century. The difference, while statistically not significant, represents a weak negative feedback on Arctic amplification. Self-organizing maps also allow an attribution of changes in MSE to factors related to varying atmospheric dynamics and/or thermodynamics. A positive contribution to the MSE related to more frequent low pressure systems in high latitudes (dynamic factor) occurs in all seasons, particularly in the summer. The decrease in DSE is mainly due to a weakened poleward temperature gradient (thermodynamic factor) during all seasons except summer, which in turn is caused by amplified warming at high latitudes as a result of increased greenhouse gases.

Subseasonal-to-seasonal prediction is influenced by slowly varying surface fields such as sea surface temperature (SST) and soil moisture. Fully coupled hindcasts were recently completed in the Community Climate System Model version 4.0 (CCSM4) as part of the North American Multi-Model Ensemble project. Using similar land and atmosphere initialization strategies, but with prescribed climatological SSTs, we attempt to determine the isolated impact of combined observed atmosphere and land initialization and of observed atmosphere initialization on monthly precipitation and 2 m temperature prediction-estimated skill (i.e., skill assessed without SST variability) and predictability on monthly time scales. CCSM4 has been cited as having low land-atmosphere coupling, and while combined land and atmosphere initialization significantly increases the estimated skill of precipitation and temperature in the first month after initialization (lead 0), land initialization influence is weak, consistent with low land-atmosphere coupling in CCSM4. In contrast, atmosphere initialization is a stronger contributor to prediction skill and predictability. We find stronger influence of land and atmosphere initialization on precipitation in CCSM4 versus results from CCSM3. Predictability results show that there is potential skill to be gained for both precipitation and temperature should model errors, atmosphere or land initial state errors, and/or land-atmosphere coupling improve.

The west-east sea surface temperature (SST) gradient in the equatorial Pacific Ocean is a key feature of Earth's climate. How this gradient responds to varying climatic forcing is a challenge to both climate theory and climate modeling. Using the coupled atmosphere-ocean general circulation models, Community Climate System Model version 3 (CCSM3) and version 4 (CCSM4), we show that the zonal SST gradient is an almost monotonically decreasing function of atmospheric CO2 concentration (pCO2) across a wide range from 17.5 to 4576 ppmv. As pCO2 is increased, the optical depth of clouds over the western and central Pacific increases significantly, reflecting more insolation back to space, suppressing surface warming in this region and thereby reducing the zonal SST gradient. Ocean adjustment to a weakening of surface zonal winds is characterized by relaxations of the equatorial thermocline tilt, zonal surface currents, and ocean upwelling.

Preliminary results of a short climate simulation with a 2-D cloud resolving model (CRM) installed into each grid column of an NCAR Community Climate System Model (CCSM) are presented. The CRM replaces the conventional convective and stratiform cloud parameterizations, and allows for explicit computation of the global cloud fraction distribution for radiation computations. The extreme computational cost of the combined CCSM/CRM model has thus far limited us to a two-month long climate simulation (December-January) using 2.8° × 2.8° resolution. The simulated geographical distributions of the total rainfall, precipitable water, cloud cover, and Earth radiation budget, for the month of January, look very reasonable.

This paper provides an introduction to the special issue of the Journal of Geophysical Research on “New Developments and Applications With the NCAR Regional Climate Model (RegCM).” In the first part of the paper we revisit and discuss outstanding issues in regional climate modeling in view of the progress achieved in this area of research during the last decade. We discuss issues of simulation length, spin-up, model physics, domain and resolution, lateral boundary conditions, multiple and two way nesting, and variable resolution approaches. In the second part we introduce the papers included in this issue. Among the primary model developments that occurred in the last few years are inclusions of the radiative transfer package and cumulus convection scheme from the National Center for Atmospheric Research (NCAR) global model CCM3, a simplified explicit moisture scheme including direct interaction with cloud radiation, testing of a variable resolution model configuration, improvements in the coupled lake model, and interactive coupling with radiatively active atmospheric aerosols. The papers in the issue illustrate a wide range of applications over different regions, such as the United States, East Asia, central Asia, eastern Africa. The main model limitations and areas in need of improvement are indicated.

Fully coupled global climate model experiments are performed using the Community Climate System Model version 4.0 (CCSM4) for preindustrial, present, and future climate to study the effects of realistic land surface initializations on subseasonal to seasonal climate forecasts. Model forecasts are verified against model control simulations (perfect model experiments), thus overcoming to some extent issues of uncertainties in the observations and/or model parameterizations. Findings suggest that realistic land surface initialization is important for climate predictability at subseasonal to seasonal time scales. We found the highest predictability for soil moisture, followed by evapotranspiration, temperature, and precipitation. The predictability is highest for the 16 to 30 days forecast period, and it progressively decreases for the second and third month forecasts. We found significant changes in the spatial distributions of temperature predictability in the present and future climate compared to the preindustrial climate, although the spatial average changes for North America were rather small (

[1] We study limits of interannual to decadal predictability of sea surface temperature (SST) in the North Pacific sector of the Community Climate System Model version 3 (CCSM3). Using a set of low-frequency and intermittent spatiotemporal SST modes acquired through nonlinear Laplacian spectral analysis (a nonlinear data manifold generalization of singular spectrum analysis), we build a hierarchy of regression models with external factors to determine which modes govern the dynamic evolution and predictability of prominent large-scale patterns, namely the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO). Retaining key triple correlations between prognostic variables and external factors, as well as the seasonality of the data, we find that the PDO and NPGO modes of CCSM3 can be described with remarkably high fidelity as an outcome of forcing by the intermittent modes (with phase demodulation by the seasonal cycle) and cubic interactions between the low-frequency modes. Our results differ from the classical picture of ENSO-driven autoregressive models for North Pacific SST variability, providing evidence that intermittent processes, such as variability of the Kuroshio current, limit long-range predictability in this climate model.

Deep soil profiles containing permafrost (Gelisols) were characterized for organic carbon (C) and total nitrogen (N) stocks to 3 m depths. Using the Community Climate System Model (CCSM4) we calcul ...

[1] Plausible predictions of future climate require realistic representations of past and current climate. Simulations of the distribution of permafrost in the 21st century made with the Community Climate System Model (CCSM4) indicate that substantial decreases in permafrost extent can be expected, especially under high emissions scenarios. One of the implications of permafrost loss is the potential release of carbon from newly thawed soils into the atmosphere, thus raising its concentration of greenhouse gases and amplifying the initial warming trend. However, the biogeochemical cycle simulated by CCSM4 presents significant biases in carbon fluxes such as gross primary production, net primary production, and vegetation carbon storage in permafrost regions. The biases in the carbon cycle simulated \by CCSM4 are in part due to excessively dry soils in permafrost regions. In this study, we show that the CCSM4 dry soil bias results from the model's formulation of soil hydraulic permeability when soil ice is present. The calculation of the hydraulic properties of frozen soils is first modified by replacing their dependence on total water content with liquid water content only. Then an ice impedance function having a power-law form is incorporated. When the parameterization of the hydraulic properties of frozen soil is corrected, the model simulates significantly higher moisture contents in near-surface soils in permafrost regions, especially during spring. This result is validated qualitatively by comparing soil moisture profiles to descriptions based on field studies, and quantitatively by comparing simulated hydrographs of two large Siberian rivers to observed hydrographs. After the dry soil bias is reduced, the vegetation productivity simulated by the model is improved, which is manifested in leaf area indices that at some locations are twice as large as in the original model.

[1] This study compares interdecadal changes in tropospheric temperature and geopotential height over eastern China in four reanalyses and upper air radiosonde (UAR) observations. It is found that the tropospheric temperature and geopotential height over eastern China during the period 1992–2000 were systematically lower in the National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) reanalysis-1 and the NCEP-Department of Energy reanalysis-2 compared to the UAR observations. Accordingly, the tropospheric temperature and geopotential height over eastern China in the NCEP reanalyses showed a pronounced interdecadal decrease around 1992, while this decrease was not significant in the UAR observations, the 40 year European Centre for Medium-Range Weather Forecasts (ERA-40) reanalysis, and the 25 year Japanese Meteorological Agency (JRA-25) reanalysis. The overestimated interdecadal change in the NCEP reanalyses may lead to an inconsistent variation between tropospheric and surface climates. When summer surface air temperature increases (decreases) over southern China, summer tropospheric temperature over eastern China generally increases (decreases) in the ERA-40, JRA-25, and UAR data sets, which is physically supported by the NCAR Community Climate System Model version 3. However, this link between the troposphere and surface is not observed in the two NCEP reanalyses. The interdecadal bias in tropospheric temperature after 1992 in the NCEP reanalyses is possibly related to an operational change in the bias correction tables since 1992.

[1] How much and why precipitation changes as the climate warms is uncertain, even for the global mean. In the 21st Century of the IPCC AR4 A1b forcing scenario, global-mean precipitation increase per degree warming varies among models by over a factor of three. Clear-sky atmospheric shortwave absorption change explains over half of the intermodel spread (r2= 0.61) in precipitation increase. Removing the part of clear-sky atmospheric shortwave absorption change due to water vapor increase reveals that shortwave absorption forcing decreases in NCAR CCSM 3.0 but increases in GFDL CM 2.1, which we attribute to differences in black carbon forcing reported by these modeling groups. The range of applied forcing causes a range in atmospheric shortwave absorption increase, which leads to spread in precipitation increase. In contrast, in the CO2-doubling forcing scenario, clear-sky atmospheric shortwave absorption explains an insignificant amount of spread (r2 = 0.04).

[1] Climate forcing by greenhouse gases and aerosols and climate feedbacks from snow, sea-ice, and clouds all significantly impact the future evolution of the climate system's shortwave energy budget. We examine prospects for tracking changes in these forcings and feedbacks using top-of-atmosphere measurements of shortwave reflected radiation. We quantify the extent to which spectral measurements may reduce the time required to detect changes in the climate the climate system with high statistical confidence relative to conventional broadband measurements. We have developed an Observing System Simulation Experiment (OSSE) based on the Community Climate System Model 3.0 for the NASA CLARREO mission and have analyzed forced and unforced simulations of the 21st Century from the Intergovernmental Panel on Climate Change assessments. We find that changes in the simulated nadir spectral reflectance measurements in the visible window and between near-infrared water-vapor overtone channels under clear-sky conditions are detectible faster than the corresponding changes in broadband albedo, with many trends detectible within a five-year satellite mission lifetime. Under all-sky conditions, the superposition of unforced cloud variability on the secular climate trends lengthens the times required for climate-change detection in both the spectral and broadband data. However, migration of the ITCZ and stratus regions can be detected after 16–18 years of observation while broadband albedo measurements require 33–61 years of observation. We find that measurement uncertainty and instrument drift significantly lengthen detection times for broadband albedo and spectral reflectances in window channels but do not have the same effect for spectral measurements in water vapor bands.

[1] This paper investigates the impact of climate change on drought by addressing two questions: (1) How reliable is the assessment of climate change impact on drought based on state-of-the-art climate change projections and downscaling techniques? and (2) Will the impact be at the same level from meteorological, agricultural, and hydrologic perspectives? Regional climate change projections based on dynamical downscaling through regional climate models (RCMs) are used to assess drought frequency, intensity, and duration, and the impact propagation from meteorological to agricultural to hydrological systems. The impact on a meteorological drought index (standardized precipitation index, SPI) is first assessed on the basis of daily climate inputs from RCMs driven by three general circulation models (GCMs). Two periods and two emission scenarios, i.e., 1991–2000 and 2091–2100 under B1 and A1Fi for Parallel Climate Model (PCM), 1990–1999 and 2090–2099 under A1B and A1Fi for Community Climate System Model, version 3.0 (CCSM3), 1980–1989 and 2090–2099 under B2 and A2 for Hadley Centre CGCM (HadCM3), are undertaken and dynamically downscaled through the RCMs. The climate projections are fed to a calibrated hydro-agronomic model at the watershed scale in Central Illinois, and agricultural drought indexed by the standardized soil water index (SSWI) and hydrological drought by the standardized runoff index (SRI) and crop yield impacts are assessed. SSWI, in particular with extreme droughts, is more sensitive to climate change than either SPI or SRI. The climate change impact on drought in terms of intensity, frequency, and duration grows from meteorological to agricultural to hydrological drought, especially for CCSM3-RCM. Significant changes of SSWI and SRI are found because of the temperature increase and precipitation decrease during the crop season, as well as the nonlinear hydrological response to precipitation and temperature change.

[1] Aboriginal vegetation burning practices and their role in the Australian environment remains a central theme of Australian environmental history. Previous studies have identified a decline in the Australian summer monsoon during the late Quaternary and attributed it to land surface-atmosphere feedbacks, related to Aboriginal burning practices. Here we undertake a comprehensive, ensemble model evaluation of the effects of a decrease in vegetation cover over the summer monsoon region of northern Australia. Our results show that the climate response, while relatively muted during the full monsoon, was significant for the pre-monsoon season (austral spring), with decreases in precipitation, higher surface and ground temperatures, and enhanced atmospheric stability. These early monsoon season changes can invoke far-reaching ecological impacts and set-up land surface-atmosphere feedbacks that further accentuate atmospheric stability.

[1] Projected changes in the Earth system will likely be manifested in changes in reflected solar radiation. This paper introduces an operational Observational System Simulation Experiment (OSSE) to calculate the signals of future climate forcings and feedbacks in top-of-atmosphere reflectance spectra. The OSSE combines simulations from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report for the NCAR Community Climate System Model (CCSM) with the MODTRAN radiative transfer code to calculate reflectance spectra for simulations of current and future climatic conditions over the 21st century. The OSSE produces narrowband reflectances and broadband fluxes, the latter of which have been extensively validated against archived CCSM results. The shortwave reflectance spectra contain atmospheric features including signals from water vapor, liquid and ice clouds, and aerosols. The spectra are also strongly influenced by the surface bidirectional reflectance properties of predicted snow and sea ice and the climatological seasonal cycles of vegetation. By comparing and contrasting simulated reflectance spectra based on emissions scenarios with increasing projected and fixed present-day greenhouse gas and aerosol concentrations, we find that prescribed forcings from increases in anthropogenic sulfate and carbonaceous aerosols are detectable and are spatially confined to lower latitudes. Also, changes in the intertropical convergence zone and poleward shifts in the subsidence zones and the storm tracks are all detectable along with large changes in snow cover and sea ice fraction. These findings suggest that the proposed NASA Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission to measure shortwave reflectance spectra may help elucidate climate forcings, responses, and feedbacks.

[1] The Community Land Model is the land component of the Community Climate System Model. Here, we describe a broad set of model improvements and additions that have been provided through the CLM development community to create CLM4. The model is extended with a carbon-nitrogen (CN) biogeochemical model that is prognostic with respect to vegetation, litter, and soil carbon and nitrogen states and vegetation phenology. An urban canyon model is added and a transient land cover and land use change (LCLUC) capability, including wood harvest, is introduced, enabling study of historic and future LCLUC on energy, water, momentum, carbon, and nitrogen fluxes. The hydrology scheme is modified with a revised numerical solution of the Richards equation and a revised ground evaporation parameterization that accounts for litter and within-canopy stability. The new snow model incorporates the SNow and Ice Aerosol Radiation model (SNICAR) - which includes aerosol deposition, grain-size dependent snow aging, and vertically-resolved snowpack heating – as well as new snow cover and snow burial fraction parameterizations. The thermal and hydrologic properties of organic soil are accounted for and the ground column is extended to ∼50-m depth. Several other minor modifications to the land surface types dataset, grass and crop optical properties, surface layer thickness, roughness length and displacement height, and the disposition of snow-capped runoff are also incorporated. The new model exhibits higher snow cover, cooler soil temperatures in organic-rich soils, greater global river discharge, and lower albedos over forests and grasslands, all of which are improvements compared to CLM3.5. When CLM4 is run with CN, the mean biogeophysical simulation is degraded because the vegetation structure is prognostic rather than prescribed, though running in this mode also allows more complex terrestrial interactions with climate and climate change.

This study examines how semi-stochastic Westerly Wind Bursts (WWBs) affect El Nino Southern Oscillation (ENSO) predictability. An ensemble ENSO prediction experiment is presented in which the Community Climate System Model version 3 (CCSM3) and CCSM3 with a state-dependent WWB parameterization are used as both “truth” and as predictor systems. Inclusion of WWBs has little effect on ENSO predictability if the “truth” lacks WWBs. If the “truth” includes WWBs, the limit of ENSO predictability is larger for a forecast system that captures the correct statistics of WWBs. Predictability drops considerably if a forecast system that lacks WWB events is used to predict a “truth” that includes WWBs. At longer lead times, predictability is more dependent on the dynamical properties of the truth; that is, the importance of capturing the WWB statistics becomes less important and the statistics (e.g., signal-to-noise ratio) of the truth determine the limit of predictability. At short leads, ENSO predictability depends on the prediction system and the “truth.” ENSO prediction skill is model and phase dependent. Predictability of extreme warm events remains a challenge as the number of ensemble members required to capture these events is on the order of 100 members. Finally, we examine real ENSO predictions with and without the WWB parameterization. It is found that including WWBs in the prediction system significantly increases ENSO prediction skill compared with a prediction system that lacks WWBs. Also, it is found that the so-called forecast spring prediction barrier is, at least partially, caused by the lack of WWB representation in the forecast system.

[1] A new overflow parameterization (OFP) of density-driven flows through ocean ridges via narrow, unresolved channels has been developed and implemented in the ocean component of the Community Climate System Model version 4. It represents exchanges from the Nordic Seas and the Antarctic shelves, associated entrainment, and subsequent injection of overflow product waters into the abyssal basins. We investigate the effects of the parameterized Denmark Strait (DS) and Faroe Bank Channel (FBC) overflows on the ocean circulation, showing their impacts on the Atlantic Meridional Overturning Circulation and the North Atlantic climate. The OFP is based on the Marginal Sea Boundary Condition scheme of Price and Yang (1998), but there are significant differences that are described in detail. Two uncoupled (ocean-only) and two fully coupled simulations are analyzed. Each pair consists of one case with the OFP and a control case without this parameterization. In both uncoupled and coupled experiments, the parameterized DS and FBC source volume transports are within the range of observed estimates. The entrainment volume transports remain lower than observational estimates, leading to lower than observed product volume transports. Due to low entrainment, the product and source water properties are too similar. The DS and FBC overflow temperature and salinity properties are in better agreement with observations in the uncoupled case than in the coupled simulation, likely reflecting surface flux differences. The most significant impact of the OFP is the improved North Atlantic Deep Water penetration depth, leading to a much better comparison with the observational data and significantly reducing the chronic, shallow penetration depth bias in level coordinate models. This improvement is due to the deeper penetration of the southward flowing Deep Western Boundary Current. In comparison with control experiments without the OFP, the abyssal ventilation rates increase in the North Atlantic. In the uncoupled simulation with the OFP, the warm bias of the control simulation in the deep North Atlantic is substantially reduced along with salinity bias reductions in the northern North Atlantic. There are similar but more modest bias reductions in the deep temperature and salinity distributions especially in the northern North Atlantic in the coupled OFP case. In coupled simulations, there are noticeable impacts of the OFP on climate. The sea surface temperatures (SSTs) are warmer by more than 5°C off the North American coast and by more than 1°C in the Nordic Sea with the OFP. The surface heat fluxes mostly act to diminish these SST changes. There are related changes in the sea level pressure, leading to about 15% weaker westerly wind stress in the northern North Atlantic. In response to the warmer Nordic Sea SSTs, there are reductions in the sea ice extent, improving comparisons with observations. Although the OFP cases improve many aspects of the simulations compared to observations, some significant biases remain, more in coupled than in uncoupled simulations.

[1] We present an analysis of the variability of the liquid Arctic freshwater (FW) export, using a simulation from the Community Climate System Model Version 3 (CCSM3) that includes passive tracers for FW from different sources. It is shown that the FW exported through the western Canadian Arctic Archipelago (CAA) comes mainly from the Pacific and from North American runoff. The variability of the FW export from both of these sources is generally in phase, due to the strong influence of variations of the velocity anomaly on the CAA FW export variability. The velocity anomaly in the CAA is in turn mainly governed by variations in the large-scale atmospheric circulation (i.e., the Arctic Oscillation). In Fram Strait, the FW export is mainly composed of Eurasian runoff and FW of Pacific origin. The variability of the Fram Strait FW export is governed both by changes in the velocity and in the FW concentration, and the variability of the FW concentration from the two largest sources is not in phase. The Eurasian runoff export through Fram Strait depends strongly on the release of FW from the Eurasian shelf, which occurs during years with an anticyclonic circulation anomaly (negative Vorticity index) and takes 3 years to reach Fram Strait after leaving the shelf. In contrast, the variability of the Pacific FW export through Fram Strait is mainly controlled by changes in the Pacific FW storage in the Beaufort Gyre, with an increased export during years with a cyclonic circulation anomaly (positive Vorticity index).

[1] Community Climate System Model (CCSM3) sensitivity experiments are performed to investigate the impact of past wetland drainage on the hydroclimatology of midwestern United States (MW USA). Coupled land surface and atmospheric components of CCSM3 are used at T85 (∼140 km) horizontal grid mesh size to create four control model experiments. These include: (1) 355 ppm CO2 for year 1990 excluding wetland (present condition), (2) 355 ppm CO2 for 1990 including wetland, (3) 289 ppm CO2 for year 1870 excluding wetland, and (4) 289 ppm CO2 for year 1870 including wetland. The CCSM3 control run for the present condition is validated with high-resolution North American Regional Reanalysis (NARR) data for the Midwest region. Validation results show that CCSM3 is reasonable in simulating at-surface incoming solar radiation, net short-wave radiation, and 2 m air temperature. However, partitioning of net radiation into sensible and latent heat fluxes is imprecise, and summer precipitation is largely underestimated in CCSM3. To remove any biases in CCSM3 output, results from sensitivity experiments are analyzed in terms of difference in monthly climatology. Sensitivity experiment results show significant changes in summer sensible and latent heat fluxes due to wetland drainage. Near-surface (2 m) air temperature has significantly increased, and convective precipitation has decreased by a small amount (∼50 mm) during the summer. Except for 2 m air temperature which is affected by both greenhouse gas emission–based climate change and wetland drainage over the last century, all other climatic variables are primarily affected by wetland drainage in the region.

[1] A global general circulation model coupled to an ocean ecosystem model is used to quantify the response of carbon fluxes and climate to changes in orbital forcing. Compared to the present-day simulation, the simulation with the Earth's orbital parameters from 115,000 years ago features significantly cooler northern high latitudes but only moderately cooler southern high latitudes. This asymmetry is explained by a 30% reduction of the strength of the Atlantic Meridional Overturning Circulation that is caused by an increased Arctic sea ice export and a resulting freshening of the North Atlantic. The strong northern high-latitude cooling and the direct insolation induced tropical warming lead to global shifts in precipitation and winds to the order of 10%–20%. These climate shifts lead to regional differences in air-sea carbon fluxes of the same order. However, the differences in global net air-sea carbon fluxes are small, which is due to several effects, two of which stand out: first, colder sea surface temperature leads to a more effective solubility pump but also to increased sea ice concentration which blocks air-sea exchange, and second, the weakening of Southern Ocean winds that is predicted by some idealized studies occurs only in part of the basin, and is compensated by stronger winds in other parts.

[1] Using a version of NCAR's Community Climate System Model (CCSM3), we apply lowered greenhouse gas (GHG) concentrations (240 ppm CO2 and 450 ppb CH4) based on typical values reached during the latter stages of previous interglacials, which are not influenced by anthropogenic GHG emissions. Using these lowered GHG levels and including feedbacks both from vegetation changes produced by the dynamic vegetation model and from the plant physiological response to lowered CO2, the climate is 3.14 K cooler than in the modern control simulation. The plant physiology response alone is a small component of the total cooling globally (0.16 K), but it contributes an additional cooling of about half that caused by changes in vegetation distribution alone (0.29 K). The inclusion of plant physiology also amplifies the loss of high-latitude boreal and tropical vegetation and enhances permanent snow cover area.

[1] Mean climate and intraseasonal to interannual variability of two versions of the Community Climate System Model (CCSM) coupled atmosphere-ocean general circulation model (CGCM) are analyzed. The first version is the standard CCSM, in which cloud effects on the large-scale circulation are represented via parameterizations. The second version includes “super-parameterization” (SP) of convective processes by replacing parameterized cloud processes with a two-dimensional (2D) cloud-process resolving model (CRM) at each CGCM grid column. The SP-CCSM improves several shortcomings of the CCSM simulation, including mean precipitation patterns, equatorial SST cold tongue structure and associated double intertropical convergence zone (ITCZ), the Asian monsoon, periodicity of the El Nino–Southern Oscillation, and the intraseasonal Madden-Julian Oscillation. These improvements were obtained without the retuning of the coupled model, which is surprising in view of previous experience with other coupled models.