Search Results

You are looking at 1 - 3 of 3 items for

  • Author or Editor: Marilyn N. Raphael x
  • Refine by Access: All Content x
Clear All Modify Search
Marilyn N. Raphael

Abstract

The three primary quasi-stationary waves in the geopotential height field of the Southern Hemisphere, as simulated by the National Center for Atmospheric Research (NCAR) Climate System Model (CSM1) and the Community Climate Model, version 3 (CCM3), are examined and compared with the NCAR–National Centers for Environmental Prediction reanalyses. Fourier analysis is used to decompose the geopotential heights into its zonal harmonic components. Both models are able to simulate the mean and zonal asymmetry of the geopotential heights; however, the CSM1 simulates the interannual variability considerably better than the CCM3. The amplitude and phase of wave 1 are well simulated by the models, particularly in the subantarctic region. The models are also able to reproduce the interannual variation in phase and amplitude of wave 1. The success of the simulation is attributed to the models’ ability to simulate well the important features of the geopotential height and temperature distributions. The models vary in their ability to simulate waves 2 and 3. Reasons for these variations are discussed.

Full access
Jeffrey C. Rogers and Marilyn N. Raphael

Abstract

The geographical distribution of meridional eddy sensible heat transport in the extremes of the Pacific/North American (PNA) teleconnection pattern is examined and compared to heat transport occurring in conjunction with other regional teleconnections. The heat fluxes are estimated using 700-mb air temperatures and geostrophic winds during 12 winter months when the PNA index reaches its highest values (large-amplitude standing ridge and trough pattern over North America) and during 12 months when it is lowest (relatively zonal flow across the continent). The standing wave fluxes are generally largest in the positive PNA phase, especially across latitudes 45°–55°N, although the flux between 60°–75°N is not as great as in the negative phase, when poleward heat transport is strong over northern Canada and near Iceland. The largest spatial heat flux variations in the extremes of the PNA occur in areas with long-term climatological flux maxima and relatively large long-term standard deviations. These include eastern Asia, the northeastern Pacific, western North America, and over the Atlantic Ocean, although in the latter region the maxima are split between Newfoundland during positive PNA index months and Iceland in negative PNA index months. In the extremes of the PNA, there is a strong tendency for the relative magnitudes of the standing and transient eddy fluxes to be out of phase in many areas of the hemisphere. This characteristic is not predominant in other regional teleconnections although it occurs in the western Pacific pattern. In other teleconnections the eddy fluxes are generally in phase, contributing directly to the total eddy flux, and centers of flux maxima do not generally correspond to those appearing in the long-term means.

Full access
William Richard Hobbs, Nathaniel L. Bindoff, and Marilyn N. Raphael

Abstract

Using optimal fingerprinting techniques, a detection analysis is performed to determine whether observed trends in Southern Ocean sea ice extent since 1979 are outside the expected range of natural variability. Consistent with previous studies, it is found that for the seasons of maximum sea ice cover (i.e., winter and early spring), the observed trends are not outside the range of natural variability and in some West Antarctic sectors they may be partially due to tropical variability. However, when information about the spatial pattern of trends is included in the analysis, the summer and autumn trends fall outside the range of internal variability. The detectable signal is dominated by strong and opposing trends in the Ross Sea and the Amundsen–Bellingshausen Sea regions. In contrast to the observed pattern, an ensemble of 20 CMIP5 coupled climate models shows that a decrease in Ross Sea ice cover would be expected in response to external forcings. The simulated decreases in the Ross, Bellingshausen, and Amundsen Seas for the autumn season are significantly different from unforced internal variability at the 95% confidence level. Unlike earlier work, the authors formally show that the simulated sea ice response to external forcing is different from both the observed trends and simulated internal variability and conclude that in general the CMIP5 models do not adequately represent the forced response of the Antarctic climate system.

Full access