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Marc d’Orgeville and W. Richard Peltier

Abstract

In the low-resolution version of the Community Climate System Model, version 3 (CCSM3), the modeled North Pacific decadal variability is demonstrated to be independent of the epoch for which a statistically steady control simulation is constructed, either preindustrial or modern; however, it is demonstrated to be significantly affected by the different global warming scenarios investigated.

In the control simulations, the North Pacific basin is shown to be dominated by sea surface temperature (SST) variability with a time scale of approximately 20 yr. This mode of variability is in close accord with the observed characteristics of the Pacific decadal oscillation (PDO). A detailed analysis of the statistical equilibrium runs is performed based on other model variables as well [sea surface salinity (SSS), barotropic circulation, freshwater and heat fluxes, wind stress curl, sea ice, and snow coverage]. These analyses confirm that the underlying mechanism of the PDO involves a basin-scale mode of ocean adjustment to changes of the atmospheric forcing associated with the Aleutian low pressure system. However, they also suggest that the observed sign reversal of the PDO arises from a feedback in the northern part of the basin. In this novel hypothesis, the advection to the Bering Sea of “spice” anomalies formed in the central and western Pacific sets up a typical 10-yr time scale for the triggering of the PDO reversal.

In all of the global warming simulations described in this paper, the signal represented by the detrended SST variability in the North Pacific displays significant power at multidecadal frequencies. In these simulations, the natural North Pacific decadal variability, as characterized in the control simulations (the PDO), remains the leading mode of variability only for moderate forcing. If the warming is too strong, then the typical 20-yr time scale of the canonical PDO can no longer be detected, except in terms of SSS variability and only prior to a significant change that occurs in the Bering Strait Throughflow.

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Marc d’Orgeville and W. Richard Peltier

Abstract

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.

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Andre R. Erler, W. Richard Peltier, and Marc D’Orgeville

Abstract

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.

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Guido Vettoretti, Marc d’Orgeville, William R. Peltier, and Marek Stastna

Abstract

It is generally accepted that the ocean thermohaline circulation plays a key role in polar climate stability and rapid climate change. Recently reported analyses of the impact of anomalous freshwater outflows from the North American continent onto either the North Atlantic or Arctic Oceans demonstrate that, in either case, a clear reduction in the Atlantic meridional overturning circulation, accompanied by an increase in sea ice extent, is predicted. The results also reconcile proxy-inferred Younger Dryas Greenland temperature variations. The aim of the present work is to provide a detailed investigation of the pathways along which the signal associated with overturning circulation anomalies propagates into both the midlatitudes and the tropics and the effect such teleconnections have on the tropical ocean–atmosphere system. The authors consider both the impact of substantial slowing of the overturning circulation due to freshwater forcing of the North Atlantic as well as its recovery after the anomalous forcing has ceased. The changes in tropical climate variability are shown to manifest themselves in shifts of both the typical time scale and intensity of ENSO events in the model. Evidence is presented for mechanisms that involve both atmospheric and oceanic pathways through which such Northern Hemisphere high-latitude events are communicated into both the midlatitudes and the tropics and thereafter transformed into changes in the nature of tropical variability.

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W. Richard Peltier, Marc d’Orgeville, Andre R. Erler, and Fengyi Xie

Abstract

Physics-based miniensembles of Weather Research and Forecasting (WRF) Model configurations have been employed to investigate future precipitation changes over the Great Lakes basin of eastern North America. All physics configurations have been employed to downscale multiple distinct Community Earth System Model, version 1 (CESM1), simulations driven by the representative concentration pathway 8.5 (RCP8.5) radiative forcing scenario, spanning a range from moderate (2045–60) to considerable (2085–2100) climate change. Independent of the physics configuration employed, all projected future precipitation changes are characterized by a general increase and a fattening of the tail of the daily rainfall distribution by the end of the century. The fattening of the tail can however be masked by natural variability in the case of the moderate warming expected by midcentury. The heavy-rainfall-derived precipitation increase is projected to be larger than or equal to the Clausius–Clapeyron thermodynamic reference of 7% increase per degree Celsius of surface warming, whereas the increase of average-rainfall-based precipitation becomes limited only for the largest global warming projections. This limitation is dramatically illustrated in one physics configuration at the end of the century. By downscaling the results obtained from the initial-condition ensemble, it is demonstrated that the extreme drying of the Great Lakes basin region characteristic of the most extreme end member of the CESM1 ensemble is significantly modified by downscaling with the version of WRF coupled to the Freshwater Lake model (FLake) of lake processes. This result does, however, depend upon the physics configuration employed in WRF for the parameterization of processes that cannot be explicitly resolved.

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Lucia Bunge, Christine Provost, Jonathan M. Lilly, Marc D’Orgeville, Annie Kartavtseff, and Jean-Luc Melice

Abstract

This paper presents initial results from new velocity observations in the eastern part of the equatorial Atlantic Ocean from a moored current-meter array. During the “EQUALANT” program (1999–2000), a mooring array was deployed around the equator near 10°W that recorded one year of measurements at various depths. Horizontal velocities were obtained in the upper 60 m from an upward-looking acoustic Doppler current profiler (ADCP) and at 13 deeper levels from current meters between 745 and 1525 m. To analyze the quasiperiodic variability observed in these records, a wavelet-based technique was used. Quasiperiodic oscillations having periods between 5 and 100 days were separated into four bands: 5–10, 10–20, 20–40, and 40–100 days. The variability shows (i) a strong seasonality (the first half of the series is dominated by larger periods than the second one) and (ii) a strong dependence with depth (some oscillations are present in the entire water column while others are only present at certain depths). For the oscillations that are present in the entire water column the origin of the forcing can be traced to the surface, while for the others the question of their origin remains open. Phase shifts at different depths generate vertical shears in the horizontal velocity component with relatively short vertical scales. This is especially visible in long-duration events (>100 days) of the zonal velocity component. Comparison with a simultaneous lowered acoustic Doppler current profiler (LADCP) section suggests that some of these flows may be identified with equatorial deep jets. A striking feature is a strong vertical shear lasting about 7 months between 745 and 1000 m. These deep current-meter observations would then imply a few months of duration for the jets in this region.

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