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William E. Johns

Abstract

During November 1986, a 6-day record was collected from a 150 kHz Acoustic Doppler Current Profiler (ADCP) mounted in the upward-looking mode on a subsurface mooring in the Gulf Stream near Cape Hatteras. The flotation unit used for the ADCP was a newly developed streamlined float, designed to minimize the effects of drag-induced tilt and high-frequency buoy motion on the range and precision of the Doppler measurements. The overall performance of the float was found to be excellent, with a mean tilt of less than 2° in up to 2 kt of current and a high apparent stability to vortex-induced oscillations. As a result, good velocity data were obtained to within 30 m of the surface from a mean depth of 375 m. A comparison of the near-field ADCP velocity data with a conventional Aanderra current meter moored 20 m below the ADCP yielded mean and root-mean-square speed and direction differences of 1.0 ± 3.7 cm s−1 and 0.5 ± 2.9°, respectively. Also, a comparison with Pegasus velocity profiles taken within 1 n mi of the mooring site showed qualitatively good agreement, with the ADCP reproducing well the small-scale vertical structure. Significant fluctuations in the vertical component were also observed, related to diurnal migration of biological scatterers, with vertical “speeds” often in excess of 3–4 cm s−1.

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William H. Klein
and
John E. Walsh

Abstract

A comparison is made between two types of specification of monthly wintertime surface temperatures over the United States. The specifications are obtained by multiple regression of station temperature anomaly at each of 37 stations onto 700 mb height anomalies represented by 1) grid-point values selected by a forward stepwise screening procedure, and 2) coefficients of the dominant empirical orthogonal functions (EOF's). Various measures of skill show that specifications derived from the pointwise screening are superior in both developmental (dependent) and independent samples. The differences in the skill levels are interpreted as a disadvantage of the spatial generality inherent in the EOF representations.

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John R. Gerhardt
and
William E. Gordon

Abstract

Selected portions of microtemperature data obtained continuously and with near simultaneity at several levels up to six feet over a desert surface are plotted on expanded height-time coordinates. The resulting isotherm patterns are shown to be strikingly consistent at all levels and are qualitatively analyzed in relation to the turbulence field present. Correlation coefficients between temperature fluctuations simultaneously at two levels and at a point for various time intervals are evaluated and their variation with separation, time, wind speed, and thermal stability is discussed. Tentative intensity and scale measures of turbulence derived from iemperature data are presented.

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William Bluman
and
John E. Hart

Abstract

Airbone Doppler lidar wind measurements were obtained in the lee of Mount Shasta in northern California on 28 August 1984. These data consist of line of sight wind vectors at flight level (3000 m) and along planes tilted at 1, 2 and 3 degree below the 3000 m level. The observed field is confined to a rectangular box, encompassing the mountain, that extends about 40 km downwind and about 20 km crosswind. The spatial resolution of the measured wind field is approximately 330 m.

The upstream southwesterly flow tended to circumvent the mountain although some air did rise over the peak (at 4317 m) to initiate three-dimensional internal gravity waves in the lee. These waves are delineated in the two-dimensional divergence field D, determined from the downwind velocity components on each of the tilted planes with line of sight wind vector measurements. The observed field of D exhibits a peak in its power spectrum, determined along the downstream direction, at a wavelength of about 8 km with a secondary peaks at about 17 km. Data from upper air soundings at Medford, Oregon and from onboard sensors establish that the 8 km wavelength represents the free wave response, which is determined by the airstream characteristics. Comparison with the power spectrum of the mountain slope indicates that the longer wavelength is a forced response.

Qualitative aspect of the lee-wave pattern are reproduce in a linear model with uniform airstream characteristics. However, the amplitude of the free wave response is underestimated by a factor of two, and the forced wave amplitude is about three time that of the free wave. In addition, the wave disturbance produced by the linear model decays more rapidly in the downstream direction than the observed wave. These discrepancies are interpreted in relation to physical features that are contained in the linear model.

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William L. Chapman
and
John E. Walsh

Abstract

Simulations of Arctic surface air temperature and sea level pressure by 14 global climate models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change are synthesized in an analysis of biases and trends. Simulated composite GCM surface air temperatures for 1981–2000 are generally 1°–2°C colder than corresponding observations with the exception of a cold bias maximum of 6°–8°C in the Barents Sea. The Barents Sea bias, most prominent in winter and spring, occurs in 12 of the 14 GCMs and corresponds to a region of oversimulated sea ice. All models project a twenty-first-century warming that is largest in the autumn and winter, although the rates of the projected warming vary considerably among the models. The across-model and across-scenario uncertainties in the projected temperatures are comparable through the first half of the twenty-first century, but increases in variability associated with the choice of scenario begin to outpace increases in across-model variability by about the year 2070. By the end of the twenty-first century, the cross-scenario variability is about 50% greater than the across-model variability. The biases of sea level pressure are smaller than in the previous generation of global climate models, although the models still show a positive bias of sea level pressure in the Eurasian sector of the Arctic Ocean, surrounded by an area of negative pressure biases. This bias is consistent with an inability of the North Atlantic storm track to penetrate the Eurasian portion of the Arctic Ocean. The changes of sea level pressure projected for the twenty-first century are negative over essentially the entire Arctic. The most significant decreases of pressure are projected for the Bering Strait region, primarily in autumn and winter.

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John E. Walsh
and
William L. Chapman

Abstract

Associations between cloudiness, radiative fluxes, and surface air temperature in the central Arctic are evaluated from 1) measurements made at Russian drifting ice stations, and 2) atmospheric reanalyses of the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF). In the ice station data, cloudiness is associated with an increase of downward longwave radiation in all months and an increase of net (downward minus upward) total radiation from September through March. The surface air temperatures under overcast skies are 6°–9°C higher than under clear skies during September–March, and the differences are even larger when the observations are stratified by wind as well as cloudiness. The warming by the radiative flux enhancement after a transition from clear skies to overcast has a 1–2-day timescale, while the cooling after the transition to clear skies has a somewhat shorter timescale. The NCEP reanalysis exaggerates slightly the association between cloudiness and surface air temperature, while the ECMWF reanalysis shows a considerably weaker association.

The maximum cloud-radiative forcing (MCRF), defined as the difference between the ice station measurements of net surface radiation under cloudy and clear skies, ranges from −59 W m−2 in June to positive values of 20–30 W m−2 in September–March. The annual mean is small but positive, 3 W m−2, despite the approximately three-month summer period of substantially negative MCRF. These findings are consistent with the conventional cloud-radiative forcing obtained in earlier studies using satellite data and one-dimensional models of the Arctic atmosphere and sea ice. Neither reanalysis captures the seasonality of the observationally deduced effects of clouds on surface radiation. The NCEP reanalysis does not capture the seasonality of the actual cloudiness (as defined by the reported cloud fractions), while the ECMWF reanalysis does not show an impact of clouds on the surface solar flux.

Issues needing further attention in the model–data comparison are the effects of surface heterogeneities, the characterization of Arctic clouds, the formulational reasons for the discrepancies between the model-derived reanalyses and the observational data, and the implications for model-derived projections of climate change in the Arctic.

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John R. Gerhardt
and
William E. Gordon

The propagation of radio waves above about 30 megacycles is seriously affected by certain weather phenomena. The meteorological aspects of this effect for a particular case are considered and a forecasting technique proposed.

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William L. Chapman
and
John E. Walsh

Feedbacks resulting from the retreat of sea ice and snow contribute to the polar amplification of the greenhouse warming projected by global climate models. A gridded sea-ice database, for which the record length is now approaching four decades for the Arctic and two decades for the Antarctic, is summarized here. The sea-ice fluctuations derived from the dataset are characterized by 1) temporal scales of several seasons to several years and 2) spatial scales of 30°–180° of longitude. The ice data are examined in conjunction with air temperature data for evidence of recent climate change in the polar regions. The arctic sea-ice variations over the past several decades are compatible with the corresponding air temperatures, which show a distinct warming that is strongest over northern land areas during the winter and spring. The temperature trends over the subarctic seas are smaller and even negative in the southern Greenland region. Statistically significant decreases of the summer extent of arctic ice are apparent in the sea-ice data, and new summer minima have been achieved three times in the past 15 years. There is no significant trend of ice extent in the Arctic during winter or in the Antarctic during any season. The seasonal and geographical changes of sea-ice coverage are consistent with the more recent greenhouse experiments performed with coupled atmosphere–ocean models.

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William L. Chapman
and
John E. Walsh

Abstract

Monthly surface air temperatures from land surface stations, automatic weather stations, and ship/buoy observations from the high-latitude Southern Hemisphere are synthesized into gridded analyses at a resolution appropriate for applications ranging from spatial trend analyses to climate change impact assessments. Correlation length scales are used to enhance information content while limiting the spatial extent of influence of the sparse data in the Antarctic region. The correlation length scales are generally largest in summer and over the Antarctic continent, while they are shortest over the winter sea ice. Gridded analyses of temperature anomalies, limited to regions within a correlation length scale of at least one observation, are constructed and validated against observed temperature anomalies in single-station-out experiments. Trends calculated for the 1958–2002 period suggest modest warming over much of the 60°–90°S domain. All seasons show warming, with winter trends being the largest at +0.172°C decade−1 while summer warming rates are only +0.045°C decade−1. The 45-yr temperature trend for the annual means is +0.082°C decade−1 corresponding to a +0.371°C temperature change over the 1958–2002 period of record. Trends computed using these analyses show considerable sensitivity to start and end dates, with trends calculated using start dates prior to 1965 showing overall warming, while those using start dates from 1966 to 1982 show net cooling over the region. Because of the large interannual variability of temperatures over the continental Antarctic, most of the continental trends are not statistically significant. However, the statistically significant warming over the Antarctic Peninsula is the strongest and most seasonally robust in the spatial patterns of temperature change.

Composite (11-model) global climate model (GCM) simulations for 1958–2002 with forcing from historic aerosol and greenhouse gas concentrations show warming patterns and magnitudes similar to the corresponding observed trends for the 45-yr period. GCM projections for the rest of the twenty-first century, however, discontinue the pattern of strongest warming over the Antarctic Peninsula, but instead show the strongest warming over the Antarctic continent.

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John E. Walsh
and
William L. Chapman

Abstract

Because much of the deep water of the world's oceans forms in the high-latitude North Atlantic, the potential climatic leverage of salinity and temperature anomalies in this region is large. Substantial variations of sea ice have accompanied North Atlantic salinity and temperature anomalies, especially the extreme and long-lived “Great Salinity Anomaly” of the late 1960s and early 1970s. Atmospheric pressure data are used hem to show that the local forcing of high-latitude North Atlantic Ocean fluctuations is augmented by antecedent atmospheric circulation anomalies over the central Arctic. These circulation anomalies are consistent with enhanced wind-forcing of thicker, older ice into the Transpolar Drift Stream and an enhanced export of sea ice (fresh water) from the Arctic into the Greenland Sea prior to major episodes of ice severity in the Greenland and Iceland seas. An index of the pressure difference between southern Greenland and the Arctic-Asian coast reached its highest value of the twentieth century during the middle-to-late 1960s, the approximate time of the earliest observation documentation of the Great Salinity Anomaly.

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