ALBERT

Description: The Dansgaard–Oeschger (D-O) relaxation oscillation that governed glacial climate variability during marine isotope stage 3 has been accurately simulated using a high-resolution coupled climate model. Here the authors present additional detailed analyses of both the slow physics transition between warm and cold states and the fast physics transition between cold and warm states of the D-O cycle. First, the authors demonstrate that the mechanisms active during the slow transition from interstadial to stadial conditions involves the continuous flux of thick and old sea ice from the Arctic basin into the North Atlantic subpolar gyre region along the East Greenland Current. During this slow physical process, the freshwater input from sea ice melting as it moves over the surface of the warm ocean restratifies the high-latitude North Atlantic and leads to a significant reduction in the rate of North Atlantic Deep Water formation. A detailed freshwater budget and hydrography analysis is also presented to demonstrate that the D-O cycle is a low-latitude–high-latitude salt oscillator as the authors have previously argued. Second, the authors provide a more detailed analysis than previously of the fast-time-scale processes that govern the extremely rapid transition from cold stadial conditions back to the warm interstadial state. These are associated with the onset of a sub-sea ice thermohaline convective instability, which opens a massive polynya to the north of the southern boundary of the extensive North Atlantic sea ice lid that is characteristic of stadial conditions. This instability is enabled by the continuous increase of salinity above the sub-sea ice pycnocline, which eliminates the vertical salinity gradient that prevents convective destabilization of the water column under full stadial conditions. This reduction in the vertical salinity gradient beneath the sea ice lid results from the continuing northward salt transport by the North Atlantic gyre circulation once the expansion of the stadial sea ice lid has ceased. The onset of instability occurs in the Irminger basin to the south of Denmark Strait, and the authors discuss the reason for this localization of instability onset.

Description: An analysis of changes in precipitation extremes in western Canada is presented, based upon an ensemble of high-resolution regional climate projections. The ensemble is composed of four independent, identically configured Community Earth System Model (CESM) integrations that were dynamically downscaled to 10-km resolution, using the WRF Model in two different configurations. Only the representative concentration pathway 8.5 (RCP8.5) scenario is considered. Changes in extremes are found to generally follow changes in the (seasonal) mean, but changes in mean and extreme precipitation differ strongly between seasons and regions (where extremes are defined as the seasonal maximum of daily precipitation). At the end of the twenty-first century, the highest projected increase in precipitation extremes is approximately 30% in winter away from the coast and in fall at the coast. Changes in winter are consistent between models; however, changes in summer are not: CESM is characterized by a decrease in summer precipitation (and extremes), while one WRF configuration shows a significant increase and another no statistically significant change. Nevertheless, the fraction of convective precipitation (extremes) in summer increases by 20%–30% in all models. There is also evidence that the climate change signal in summer is sensitive to the choice of the convection scheme. A comparison of CESM and WRF shows that higher resolution clearly improves the representation of winter precipitation (extremes), while summer precipitation does not appear to be sensitive to resolution (convection is parameterized in both models). To increase the statistical power of the extreme value analysis that has been performed, a novel method for combining data from climatologically similar stations was employed.

Description: The impact of anthropogenic climate change on water resources and flood and drought risk is of great interest for impact modeling and to inform adaptation strategies. Here an analysis of hydroclimatic changes in the Fraser and Athabasca River basins in western Canada is presented, based on an ensemble of climate projections, which have been dynamically downscaled to 10-km resolution using the Weather Research and Forecasting Model in two configurations. The GCM ensemble comprises four independent integrations of the Community Earth System Model under the representative concentration pathway 8.5. Basin-integrated changes in the seasonal cycle of hydroclimatic variables, and the variability of water supply and flood and drought risk, are considered. It is found that fall and winter precipitation generally increase by 20%–30% toward the end of the century, while changes in summer precipitation are smaller and associated with high model uncertainty. Furthermore, a reduction in snowfall and an increase in evapotranspiration are projected. However, projected impacts on water resources east and west of the Rocky Mountains are quite different: in basins closer to the coast (west of the Rocky Mountains) higher temperatures lead to a transition from predominantly solid to liquid precipitation and a significantly weaker spring freshet, followed by drier summers. In the lee of the Rocky Mountains the spring freshet remains largely unaffected and in summer the increase in evapotranspiration (ET) is compensated by increasing precipitation, so that water balance changes appear to be small. It is further found that a shift in runoff seasonality near the coast may lead to significantly increased flood risk in fall.

Description: The complex orography of South Asia, including both the Himalayas and the Tibetan Plateau, renders the regional climate complex. How this climate, especially the monsoon circulations, will respond to the global warming process is important given the large population of the region. In a first step toward a contribution to the understanding of the expected impacts, a series of dynamically downscaled instrumental-era climate simulations for the Indian subcontinent are described and will serve as a basis for comparison against global warming simulations. Global simulations based upon the Community Earth System Model (CESM) are employed to drive a dynamical downscaling pipeline in which the Weather Research and Forecasting (WRF) Model is employed as regional climate model, in a nested configuration with two domains at 30- and 10-km resolution, respectively. The entire ensemble was integrated for 15 years (1980–94), with the global model representing a complete integration from the onset of Northern Hemisphere industrialization. Compared to CESM, WRF significantly improves the representation of orographic precipitation. Precipitation extremes are also characterized using extreme value analysis. To investigate the sensitivity of the South Asian summer monsoon simulation to different parameterization schemes, a small physics ensemble is employed. The Noah multiphysics (Noah-MP) land surface scheme reduces the summer warm bias compared to the Noah land surface scheme. Compared with the Kain–Fritsch cumulus scheme, the Grell-3 scheme produces an increased moisture bias at the first western rain barrier, whereas the Tiedtke scheme produces less precipitation over the subcontinent than observed. Otherwise the improvement of fit to the observations derived from applying the downscaling methodology is highly significant.

Description: Diurnal and semidiurnal ocean tides are calculated for both the present day and the Last Glacial Maximum. A numerical model with complete global coverage and enhanced resolution at high latitudes is used including the physics of self-attraction and loading and internal tide drag. Modeled present-day tidal amplitudes are overestimated at the standard resolution, but the error decreases as the resolution increases. It is argued that such results, which can be improved in the future using higher-resolution simulations, are preferable to those obtained by artificial enhancement of dissipative processes. For simulations at the Last Glacial Maximum a new version of the ICE-5G topographic reconstruction is used along with density stratification determined from coupled atmosphere–ocean climate simulations. The model predicts a significant amplification of tides around the Arctic and Antarctic coastlines, and these changes are interpreted in terms of Kelvin wave dynamics with the aid of an exact analytical solution for propagation around a polar continent or basin. These polar tides are shown to be highly sensitive to the assumed location of the grounding lines of coastal ice sheets, and the way in which this may contribute to an interaction between tides and climate change is discussed. Globally, the picture is one of energized semidiurnal tides at the Last Glacial Maximum, with an increase in tidal dissipation from present-day values, the dominant energy sink being the conversion to internal waves.

Description: The “Snowball Earth” hypothesis, proposed to explain the Neoproterozoic glacial episodes in the period 750–580 million years ago, suggested that the earth was globally covered by ice/snow during these events. This study addresses the problem of the forcings required for the earth to enter such a state of complete glaciation using the Community Climate System Model, version 3 (CCSM3). All of the simulations performed to address this issue employ the geography and topography of the present-day earth and are employed to explore the combination of factors consisting of total solar luminosity, CO2 concentration, and sea ice/snow albedo parameterization that would be required for such an event to occur. The analyses demonstrate that the critical conditions beyond which runaway ice–albedo feedback will lead to global freezing include 1) a 10%–10.5% reduction in solar radiation with preindustrial greenhouse gas concentrations; 2) a 6% reduction in solar radiation with 17.5 ppmv CO2; or 3) 6% less solar radiation and 286 ppmv CO2 if sea ice albedo is equal to or greater than 0.60 with a snow albedo of 0.78, or if sea ice albedo is 0.58 with a snow albedo equal to or greater than 0.80. These bifurcation points are very sensitive to the sea ice and snow albedo parameterizations. Moreover, “soft Snowball” solutions are found in which tropical open water oceans stably coexist with year-round snow-covered low-latitude continents, implying that tropical continental ice sheets would actually be present. The authors conclude that a “soft Snowball” is entirely plausible, in which the global sea ice fraction may reach as high as 76% and sea ice margins may extend to 10°S(N) latitudes.

Description: Accurate identification of the impact of global warming on water resources in major river systems represents a significant challenge to the understanding of climate change on the regional scale. Here, dynamically downscaled climate projections for western Canada are presented, and impacts on hydrological variables in two major river basins, the Fraser and Athabasca, are discussed. These regions are both challenging because of the complexity of the topography and important because of the economic activity occurring within them. To obtain robust projections of future conditions, and to adequately characterize the impact of natural variability, a small initial condition ensemble of independently downscaled climate projections is employed. The Community Earth System Model, version 1 (CESM1), is used to generate the ensemble, which consists of four members. Downscaling is performed using the Weather Research and Forecasting Model, version 3.4.1 (WRF V3.4.1), in a nested configuration with two domains at 30- and 10-km resolution, respectively. The entire ensemble was integrated for a historical validation period and for a mid-twenty-first-century projection period [assuming representative concentration pathway 8.5 (RCP8.5) for the future trajectory of greenhouse gases]. The projections herein are characterized by an increase in winter precipitation for the mid-twenty-first-century period, whereas net precipitation in summer is projected to decrease, due to increased evapotranspiration. In the Fraser River basin, a shift to more liquid precipitation and earlier snowmelt will likely reduce the seasonal variability of runoff, in particular the spring freshet. In the Athabasca River basin, winter precipitation and snowmelt may increase somewhat, but increasing evapotranspiration may lead to reduced streamflow in late summer.

Description: The nature of the multidecadal variability in the North Atlantic basin is investigated through detailed analysis of multicentury integrations performed using the low-resolution version of the Community Climate System Model, version 3 (CCSM3), a modern atmosphere–ocean coupled general circulation model. Specifically, the results of control simulations under both preindustrial and present-day perpetual seasonal cycle conditions are compared to each other and also to the results of five simulations with increasing CO2 concentration scenarios. In the absence of greenhouse gas–induced warming, the meridional overturning circulation (MOC) variability is shown to be dependent on the details of the simulation. In the present-day control simulation, the MOC is characterized by a broad spectrum of low frequencies, whereas, in preindustrial control simulations, MOC variability is characterized either by a well-defined periodicity of 60 yr or by a broad spectrum of low frequencies. In all the control simulations, the MOC appears to respond with a delay of 10 yr to synchronous temperature and salinity anomalies in the deep water formation sites located in the subpolar gyre, but salinity dominates the density anomalies. The explanation of the modeled MOC periodicity is therefore sought in the creation of these density anomalies. The influence of increased sea ice coverage under cold/preindustrial conditions is shown to modify the salinity variability, but it is not a sufficient condition for the support of the MOC periodicity. Instead, its source appears to be a modified subpolar gyre circulation resulting from interaction with the bottom bathymetry, which is able to sustain strong coupling between the horizontal and overturning circulations. Based on the global warming analyses, for the simulations initialized from the cold/preindustrial statistical equilibrium run, the North Atlantic variability continues to be dominated by strong coupling between the horizontal and overturning circulations if the imposed forcing is weak. More generally, the delayed response of the MOC to surface density anomalies in the deep water formation regions is preserved under weak forcing.