Marilyn N. Raphael

The Santa Ana Winds of California

Abstract A 33-yr, numerical dataset of the occurrence of Santa Ana winds for the period 1968–2000 has been created and validated. Daily Weather Maps were examined to identify the days when a surface high pressure system existed over the Great Basin simultaneously with a surface low pressure system offshore of southern California, and the prevailing wind over southern California was from the northeast quadrant. The dates of these occurrences, as well as the wind speed, temperature, and dewpoint temperature among other variables, were extracted and tabulated. The frequency of Santa Ana events derived from the weather maps was compared to events defined by wind direction only and there is agreement between the two. Preliminary results show that the Santa Ana event is limited to the period September–April and that the month of peak occurrence is December. The average frequency of events is 20 yr–1 and the average duration of an event is 1.5 days. Humidity levels are not uniform across Santa Ana events; the dr...

The Amundsen Sea Low: Variability, Change, and Impact on Antarctic Climate

The Amundsen Sea Low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, we summarize current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, and ice-core chemistry and climate model projections to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the 21st century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases.

New Perspectives on Observed and Simulated Antarctic Sea Ice Extent Trends Using Optimal Fingerprinting Techniques

AbstractUsing 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, Bellingsha...

The influence of the large-scale atmospheric circulation on Antarctic sea ice during ice advance and retreat seasons

Antarctic sea ice, a key component of the Southern Hemisphere climate system, is influenced by several large-scale modes of the atmospheric circulation. Antarctic sea ice variability is spatially heterogeneous, and links between the atmospheric circulation modes and the sea ice variability are unclear. Using the observed sea ice concentration data, this research isolates distinct regions of sea ice variability around Antarctica and determines the advance and retreat periods for each of them. The latter are then statistically linked with the observed geopotential height data to determine the atmospheric circulation pattern associated with the variability in the sea ice for each period and region. The results clarify which circulation mechanism is of primary importance to sea ice variability during critical periods of the ice lifecycle in the different regions around Antarctica and have potential for making estimates of past sea ice extent using the observed geopotential height data.

Meridional Eddy Sensible Heat Fluxes in the Extremes of the Pacific/North American Teleconnection Pattern

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 ...

The Pacific zonal asymmetry and its influence on Southern Hemisphere sea ice variability

Abstract At monthly and annual timescales, the zonally asymmetric circulation over the Southern Ocean is dominated by two quasi-stationary anticyclones: a western anticyclone approximately located south of New Zealand, and an eastern anticyclone located over the Drake Passage region. In this research their influence on late 20th century Antarctic sea ice is explored. During early winter, sea ice in the Weddell, Amundsen and Bellingshausen seas is influenced by the location of the east anticyclone. During late winter, the strength and location of the west anticyclone influences sea ice primarily in the Ross and Amundsen seas. The anticyclones have some effect on wind-driven sea ice motion, but the primary mechanism explaining their link to sea ice appears to be meridional atmospheric thermal advection. A western shift in the west anticyclone may be partly responsible for observed increases in ice cover in the Ross Sea over the late 20th century, but there is little evidence in existing data to support a link between the east anticyclone and observed sea ice trends.

Recent, Large-Scale Changes in the Extratropical Southern Hemisphere Atmospheric Circulation

Abstract The zonal anomalies in the 500-hPa geopotential height field of the Southern Hemisphere are examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalyses, 1958–96. The zonal anomalies were constructed by removing the zonal mean from each grid point, and principal components analysis was performed on the resulting spatial anomalies. The principal components of the leading EOF of the seasons March–April–May (MAM) to September–October–November (SON) indicate that a change has occurred in the extratropical, large-scale, atmospheric circulation of the Southern Hemisphere. It is estimated to have begun after 1975 and is limited to the southern late fall, winter, and early spring. The elements of this change are a deeper Pacific trough and higher Indian Ocean ridge at subtropical latitudes. At higher latitudes the ridge over the Pacific strengthened and shifted east while troughing occurred in the southwest Atlantic–Weddell Sea region. Consistent wi...

Quasi‐stationary wave variability in NSCAT winds

Planetary and interseasonal properties of the NASA scatterometer (NSCAT) winds are explored in the context of the quasi-stationary waves (QSWs) of the southern hemisphere. The QSWs are examined by means of zonal asymmetries of 3-month running averages of the meridional velocity derived from NSCAT. The study period spans the entire NSCAT record from September 15, 1996, through June 29, 1997. Meridional winds from the European Space Agency ERS 2 scatterometer are used to augment 3-month averages centered on August 1996 through May 1997. The time period corresponds to the transition from year −1 to 0 of the 1997 warm event in the Southern Oscillation. Comparisons are made with QSW signals in geopotential height anomalies from the National Centers for Environmental Prediction/National Center for Atmospheric Research Climate Data Assimilation System. The zonal anomalies of meridional wind from NSCAT are shown to be in approximate geostrophic balance with zonal gradients in the zonal anomalies of geopotential height at 500 and 1000 hPa.

Quasi-Stationary Waves in the Southern Hemisphere: An Examination of Their Simulation by the NCAR Climate System Model, with and without an Interactive Ocean*

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 ...

THE UNUSUAL STORMS OF FEBRUARY 1992 IN SOUTHERN CALIFORNIA

During February 1992, a series of relatively warm storms passed eastward across southern California, yielding intense precipitation that triggered widespread mass movement, flooding, property damage, and loss of life. These storms were triggered by an intense low pressure system (976 mb) off northern California which deepened as its eastward progress was initially blocked by a high pressure ridge (1040 mb) across western North America. Between February 10 and 13, large areas of Ventura and Los Angeles counties experienced cumulative precipitation of 200–400 mm with intensities reaching 40–50 mm hr-1. Mass movement, mainly as soil slips that transformed downslope into debris flows, occurred where cumulative precipitation exceeded 300 mm and when sustained intensities exceeded 25 mm hr-1. Stream response was rapid, particularly in urban areas where impermeable surfaces and storm drains fed concrete stream channels. The canalized upper Los Angeles River and Arroyo Simi exceeded all previous discharges for ov...