Linda M. Keller

Atmosphere-Ocean Interaction in the North Atlantic: Near-Surface Climate Variability*

The impact of an interactive ocean on the midlatitude atmosphere is examined using a 31-yr integration of a variable depth mixed layer ocean model of the North Atlantic (between 208 and 608N) coupled to the NCAR Community Climate model (CCM1). Coupled model results are compared with a 31-yr control simulation where the annual cycle of sea surface temperatures is prescribed. The analysis focuses on the northern fall and winter months. Coupling does not change the mean wintertime model climatology (December‐February); however, it does have a significant impact on model variance. Air temperature and mixing ratio variance increase while total surface heat flux variance decreases. In addition, it is found that air‐sea interaction has a greater impact on seasonally averaged variance than monthly variance. There is an enhancement in the persistence of air temperature anomalies on interannual timescales as a result of coupling. In the North Atlantic sector, surface air and ocean temperature anomalies during late winter are uncorrelated with the following summer but are significantly correlated (0.4‐0.6) with anomalies during the following winter. These autocorrelations are consistent with the ‘‘re-emergence’’ mechanism, where late winter ocean temperature anomalies are sequestered beneath the shallow summer mixed layer and are reincorporated into the deepening fall mixed layer. The elimination of temperature anomalies from below the mixed layer in a series of uncoupled sensitivity experiments notably reduces the persistence of year-to-year anomalies. The persistence of air temperature anomalies on monthly timescales also increases with coupling and is likely associated with ‘‘decreased thermal damping.’’ When coupled to the atmosphere, the ocean is able to adjust to the overlying atmosphere so that the negative feedback associated with anomalous heat fluxes decreases, and air temperature anomalies decay more slowly.

Antarctic Automatic Weather Station Program: 30 Years of Polar Observation

Antarctica boasts one of the worlds harshest environments. Since the earliest expeditions, a major challenge has been to characterize the surface meteorology around the continent. In 1980, the University of Wisconsin—Madison (UW-Madison) took over the U.S. Antarctic Program (USAP) Automatic Weather Station (AWS) program. Since then, the UW-Madison AWS network has aided in the understanding of unique Antarctic weather and climate. This paper summarizes the development of the UW-Madison AWS network, issues related to instrumentation and data quality, and some of the ways these observations have and continue to benefit scientific investigations and operational meteorology.

Atmospheric Temperature Measurement Biases on the Antarctic Plateau

AbstractObservations of atmospheric temperature made on the Antarctic Plateau with thermistors housed in naturally (wind) ventilated radiation shields are shown to be significantly warm biased by solar radiation. High incoming solar flux and high surface albedo result in radiation biases in Gill (multiplate)-styled shields that can occasionally exceed 10°C in summer in cases with low wind speed. Although stronger and more frequent when incoming solar radiation is high, biases exceeding 8°C are found even when solar radiation is less than 200 W m−2. Compared with sonic thermometers, which are not affected by radiation but are too complex to be routinely used for mean temperature monitoring, commercially available aspirated shields are shown to efficiently protect thermistor measurements from solar radiation biases. Most of the available in situ reports of atmospheric temperature on the Antarctic Plateau are from automatic weather stations that use passive shields and are thus likely warm biased in the summ...

Case Study of a Barrier Wind Corner Jet off the Coast of the Prince Olav Mountains, Antarctica

AbstractThe Ross Ice Shelf airstream (RAS) is a barrier parallel flow along the base of the Transantarctic Mountains. Previous research has hypothesized that a combination of katabatic flow, barrier winds, and mesoscale and synoptic-scale cyclones drive the RAS. Within the RAS, an area of maximum wind speed is located to the northwest of the protruding Prince Olav Mountains. In this region, the Sabrina automatic weather station (AWS) observed a September 2009 high wind event with wind speeds in excess of 20 m s−1 for nearly 35 h. The following case study uses in situ AWS observations and output from the Antarctic Mesoscale Prediction System to demonstrate that the strong wind speeds during this event were caused by a combination of various forcing mechanisms, including katabatic winds, barrier winds, a surface mesocyclone over the Ross Ice Shelf, an upper-level ridge over the southern tip of the Ross Ice Shelf, and topographic influences from the Prince Olav Mountains. These forcing mechanisms induced a b...

Composite Analysis of the Effects of ENSO Events on Antarctica

AbstractPrevious investigations of the relationship between El Nino–Southern Oscillation (ENSO) and the Antarctic climate have focused on regions that are impacted by both El Nino and La Nina, which favors analysis over the Amundsen and Bellingshausen Seas (ABS). Here, 35 yr (1979–2013) of European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) data are analyzed to investigate the relationship between ENSO and Antarctica for each season using a compositing method that includes nine El Nino and nine La Nina periods. Composites of 2-m temperature (T2m), sea level pressure (SLP), 500-hPa geopotential height, sea surface temperatures (SST), and 300-hPa geopotential height anomalies were calculated separately for El Nino minus neutral and La Nina minus neutral conditions, to provide an analysis of features associated with each phase of ENSO. These anomaly patterns can differ in important ways from El Nino minus La Nina composites, which may be expected from the geographical shift in...

Utilization of Automatic Weather Station Data for Forecasting High Wind Speeds at Pegasus Runway, Antarctica

Abstract Reduced visibility due to blowing snow can severely hinder aircraft operations in the Antarctic. Wind speeds in excess of approximately 7–13 m s−1 can result in blowing snow. The ability to forecast high wind speed events can improve the safety and efficiency of aircraft activities. The placement of automatic weather stations to the south (upstream) of the Pegasus Runway, and other airfields near McMurdo Station, Antarctica, can provide the forecaster the information needed to make short-term (3–6 h) forecasts of high wind speeds, defined in this study to be greater than 15 m s−1. Automatic weather station (AWS) data were investigated for the period of 1 January 1991 through 31 December 1996, and 109 events were found that had high wind speeds at the Pegasus North AWS site. Data from other selected AWS sites were examined for precursors to these high wind speed events. A temperature increase was generally observed at most sites before such an event commenced. Increases in the temperature differen...

Atmospheric circulation around the Greenland Crest

The Greenland Ice Sheet Program 2 (GISP2) required meteorological support for the layout of the field camp, ice core site, landing strip, snow sampling sites, and air sampling sites. An automatic weather station installed in May 1987 provided the initial data for the support of several components of the program using the 2 year wind speed and direction distributions. The automatic weather station (AWS) network was expanded to six sites with four sites located approximately 100 km from the highest point in Greenland in the cardinal directions and sites located at GISP2 and at the air sampling site. The wind direction and speed wind roses for August and September 1994 show that there is considerable variation in the wind field around the Greenland Crest. The constancy (ratio of the vector wind speed to the scalar wind speed) of the monthly wind vectors was lowest at the southern site and increased as one moved clockwise around the crest to the eastern site. Vorticity and divergence were determined from triangles formed by three AWS sites. The mean horizontal divergence for August and September 1994 was 1.4 x 10 -5 s -1 , and the vorticity was -3.7 x 10 -5 s -1 . Apparently, the flow around the Greenland Crest is most influenced by topography when the 500 hPa contours indicate a pressure ridge over central Greenland. The surface winds showed anticyclonic flow, positive horizontal divergence, and negative vertical vorticity. In the mean, this configuration indicates the presence of an inversion wind, especially at the sites located on the slopes to the west and east of the crest.

Stability and variability in a coupled ocean-atmosphere climate model : results of 100-year simulations

Abstract Two 100-year seasonal simulators, one performed with a low resolution atmospheric general circulation model (GCM) coupled to a mixed-layer ocean formulation and the other made with the GCM forced by prescribed ocean conditions, are compared to assess the effects of an interactive ocean and sea-ice component on the stability and interannual variability of a climate system. Characteristics of the time variation of surface temperature, 700 mb temperature and sea-ice coverage are analyzed for selected land and ocean areas. Both simulations showed stable seasonal cycles of basic variables, although small trends were found. These trends were roughly linear in nature and quite distinct from all other components of variability. Detrended time series were used to describe the other aspects of variability. There was pronounced interannual variability in the simulations from both models as seen in the time series for temperature and sea ice over the entire 100-year time period. Consistent with observations,...

Antarctic Satellite Meteorology: Applications for Weather Forecasting

For over 30 years, weather forecasting for the Antarctic continent and adjacent Southern Ocean has relied on weather satellites. Significant advancements in forecasting skill have come via the weather satellite. The advent of the high-resolution picture transmission (HRPT) system in the 1980s and 1990s allowed real-time weather forecasting to become a reality. Small-scale features such as mesocyclones and polar lows could be tracked and larger-scale features such as katabatic winds could be detected using the infrared channel. Currently, HRPT is received at most of the manned Antarctic stations. In the late 1990s the University of Wisconsin composites, which combined all available polar and geostationary satellite imagery, allowed a near-real-time hemispheric view of the Southern Ocean and Antarctic continent. The newest generation of satellites carries improved vertical sounders, special sensors for microwave imaging, and the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. In spite of the advances in sensors, shortcomings still impede the forecaster. Gaps in satellite data coverage hinder operations at certain times of the day. The development and implementation of software to derive products and visualize information quickly has lagged. The lack of high-performance communications links at many of the manned stations reduces the amount of information that is available to the forecasters. Future applications of weather satellite data for Antarctic forecasting include better retrievals of temperature and moisture and more derived products for fog, cloud detection, and cloud drift winds. Upgrades in technology at Antarctic stations would allow regional numerical prediction models to be run on station and use all the current and future satellite data that may be available.

The future of medium-extended-range weather prediction user perspectives

AMERICAN METEOROLOGICAL SOCIETY | MAY 2007 | 635 634 A long with the increased skill of the short-term (1–3 day) weather forecasts, the prospects for accurate 3–14 day forecasts have improved, thereby opening up new possibilities for forecast users. In the United States, the National Weather Service (NWS)’s Hydrometeorological Prediction Center (HPC) currently provides forecast products and guidance for the 3–7-day range, including maximum and minimum temperature, surface circulation and fronts, and precipitation probabilities, while the NWS Climate Prediction Center is responsible for longer-range forecasts such as the 6–10and 8–14-day probability outlooks for temperature and precipitation. The potential and current uses of these products for agriculture, aviation, utilities, and commodities are numerous, but can only be fully and productively realized if the economic, educational, and decision-making needs of specific users are taken into account. Discussions of user needs with our colleagues have yielded an outline of some of these opportunities and challenges of mediumand extended-range forecasts.1