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Haig Iskenderian and Richard D. Rosen

Abstract

Low-frequency signals in the daily variability of temperature in the midtroposphere are investigated, thereby complementing published studies of changes in day-to-day temperature variability and in extreme weather events at the surface. The results are based upon approximately four decades of upper-air data from radiosondes and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses. The annual mean field of 500-hPa submonthly temperature variance, var(T), is oriented zonally across most of the globe, with maxima in the midlatitudes over the major landmasses of North America and Asia and over the oceans of the Southern Hemisphere. Seasonally, var(T) shifts equatorward from the warm to cool season in both hemispheres. Therefore, var(T) reflects day-to-day changes in temperature about the jet stream associated with baroclinic synoptic-scale systems.

Year-to-year changes in var(T) over the Northern Hemisphere are greatest over the major landmasses of North America, northern Europe, and Asia. There is also evidence of an influence of ENSO upon the interannual variability of var(T) over the northern portion of North America during winter, where there is a westward displaced maximum in cold events relative to warm events. Trend analysis over the Northern Hemisphere shows that there has been a significant increase in submonthly temperature variance over the northeastern portion of North America, the North Atlantic, and Scandinavia, representing as much as 30% of the climatological values of var(T) in these regions. These regional trends are most apparent during the Northern Hemisphere winter and spring seasons.

The zonally averaged var(T) has generally decreased over polar latitudes and increased over the midlatitudes of the Northern Hemisphere, although there are considerable differences from season to season. Averaged over the entire Northern Hemisphere, var(T) exhibits a slight upward trend since the late 1950s in the NCEP–NCAR reanalysis, although this trend is significant in the spring season only. The robustness of this springtime trend, however, is in doubt, because the trend found from a radiosonde-only dataset is negative. For the conterminous United States, the two datasets do agree by showing mostly small positive trends in most seasons. These positive trends, however, are not statistically significant, and therefore the authors cannot state with confidence that there has been a change in synoptic-scale temperature variance in the midtroposphere over the United States since 1958.

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Richard D. Rosen and David A. Salstein

Abstract

The sensitivity of various zonal mean general circulation statistics to the choice of the averaging period used to define them is tested with upper-air data for the Northern Hemisphere taken from the NMC global analysis for the winter of 1976–77. We find that averaging periods of less than about 10 days do not permit a clear separation of total eddy momentum and heat fluxes into their transient and standing eddy components. Between 10 and 30 days, the definition of these components is less sensitive to the specific averaging period chosen during the winter. We illustrate one use of monitoring general circulation statistics on short time scales by studying the evolution of 10-day mean eddy fluxes of sensible heat and their relation to changes in the meridional temperature gradient during this winter. It appears, for this one season at least, that the standing waves regulated the temperature structure of midlatitudes, whereas the transient waves merely responded to the temperature gradient that was imposed.

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Richard D. Rosen and David A. Salstein

Abstract

General circulation fields and statistics are derived for the months of January and July 1983 from different sets of daily global analyses produced by NMC. The two sets differ only in that one has been archived prior to an initialization step, whereas the other has been archived after initialization. To place our comparisons of these pre- and post-initialized NMC fields in perspective, we have also calculated circulation statistics from analyses produced by the ECMWF for the same months and from analyses of available rawinsonde data.

As suggested by earlier studies, the adiabatic normal mode initialization procedure used until recently at NMC is found to reduce the strength of the Hadley cell. However, the impact of this initialization is not large. In fact, the difference between the pre- and post-initialized NMC Hadley cells is generally smaller than that between the NMC and ECMWF Hadley cells. Other zonal mean circulation fields appear even less affected by NMC's initialization. Locally, the initialization does noticeably dampen the maxima in the vertical motion field, and it modifies the velocity potential field over certain regions of the globe, particularly in the western Pacific. For the most part, however, the post-initialized NMC fields, which have been the only NMC fields normally available to the scientific community, appear suitable for use in diagnostic studies of the circulation. More effort is needed to understand and reduce the differences between the NMC and ECMWF fields, if possible.

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Rui M. Ponte and Richard D. Rosen

Abstract

Variability in surface winds at subseasonal time scales can affect the estimates of the mean and seasonal stress over the ocean, owing to the nonlinear dependence of stress on wind speed. A global ocean wind product that merges the European Centre for Medium-Range Weather Forecasts (ECMWF) fields with satellite and in situ data is used to assess the nonlinear effects of wind variability, in particular synoptic signals (taken here to include all periods of <6 days), on estimates of the mean and the seasonal cycle in zonal and meridional stress. Climatologies based on the period March 1988–February 1999 are considered. Synoptic effects are most pronounced at mid- and high latitudes, where they can amount up to 20% of the mean or seasonal stress. Uncertainties in stress values associated with synoptic wind errors are assessed by comparing estimates from merged-data winds to those from original ECMWF winds. Differences in synoptic winds contribute noticeably to the stress differences. Outside the Tropics, uncertainties related to synoptic wind terms can have amplitudes of more than 20% of the total estimated uncertainty for mean and seasonal zonal stress, with much higher values for the meridional stress. Implications of these findings for studies of the atmospheric and oceanic circulations are discussed. Results point to the importance of accurately determining wind variability at subweekly periods, thereby placing constraints on sampling strategies for observing winds over the ocean.

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Rui M. Ponte and Richard D. Rosen

Abstract

Atmospheric angular momentum (AAM) reached extremely high values during the large 1982–83 El Niño event. The mechanisms responsible for the anomalously high AAM are examined using mountain torque (τ m) and friction torque (τ f) time series computed from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalyses. AAM anomalies, defined with respect to a 29-yr climatology (1968–96), are mostly positive from mid-1982 onward, but notably they double in amplitude over a 2-week period in early 1983. Analysis of the torque series reveals that this sharp AAM increase is mostly related to anomalies in τ m, primarily associated with American and Eurasian orography. After reaching its peak value in January, AAM anomalies decay slowly to near-normal values over the next three months, with anomalies in τ f, especially over the subtropical North Pacific, playing a dominant role in this downturn. The relevant anomalies in τ m and τ f are discussed in relation to rapid synoptic-scale variability and longer-term, large-scale anomalous patterns in surface pressure and winds that characterized this El Niño event.

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Richard D. Rosen and David A. Salstein

Abstract

Statistics concerning the budgets of angular momentum heat and water vapor over the Northern Hemisphere are computed by two different methods for the winter of 1976–77. The first method employs an objective analysis scheme applied to the set of conventional upper air sounding obtained from the hemispheric network of rawinsonde stations. The second method uses grid-point values produced daily by the NMC global Hough analysis based on data from several sources. Our results show that the gridded Hough data do not contain mean meridional circulations, thus seriously limiting their usefulness for studies in which these cells play a major role. In addition, the gridded data appear to yield unreasonably large values of water vapor. On the other hand, they produce a realistic temperature structure and seem quite adequate for use in studies of midlatitude waves and their transports. They have also proven much easier to work with than the conventional station data. We find, too, that these station data have their own deficiencies caused largely by gaps in the rawinsonde network, such as those resulting from the loss of several ocean weather ship stations since 1973.

Our study also provides an added appreciation for the highly amplified nature of atmospheric waves during the 1976–77 winter. A strong conversion of kinetic energy from its eddy to zonal mean state and a large standing eddy heat flux are both evident. Additionally, transient eddy momentum fluxes were found to peak at 230 mb, a level not usually included in previous general circulation statistics.

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Rui M. Ponte and Richard D. Rosen

Abstract

The ocean's angular momentum (M) and torques about the Polar axis are analyzed using output from the global, eddy-resolving model of Semtner and Chervin. Seasonal variability in M is dominated by the annual cycle, whose magnitude appears capable of helping explain the residual in the solid earth-atmosphere annual momentum budget. Planetary (M Ω) and relative (Mr) ocean angular momentum components have comparable seasonal amplitudes. Most of the mean signal in Mr, results from flows in the Antarctic Circumpolar Current region, but flows as far north as approximately 30°S am needed to explain the seasonal cycle. Local1y, the strongest variability in relative angular momentum is found in the Tropics at all depths, a manifestation of the zonal, recirculating character of the tropical circulation. The time rate of change of M is very small compared to the applied wind torque. Calculation of bottom pressure torques using the geostrophic relation reveals a dominant balance between them and the surface wind torques in the model, implying a rapid transfer of angular momentum between the atmosphere and the solid earth through the ocean. The torque balance holds for latitudes totally blocked by continental boundaries as well as for latitudes that are only partially blocked (e.g., Drake Passage), suggesting the same angular momentum transfer mechanism for closed basin and Antarctic Circumpolar Current regions. Implications of the results for future ocean modeling efforts are discussed.

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Judah Cohen, Allan Frei, and Richard D. Rosen

Abstract

The simulated North Atlantic Oscillation (NAO) teleconnection patterns and their interannual variability are evaluated from a suite of atmospheric models participating in the second phase of the Atmospheric Model Intercomparison Project (AMIP-2). In general the models simulate the observed spatial pattern well, although there are important differences among models. The NAO response to interannual variations in sea surface temperature (SST) and snow-cover boundary forcings are also evaluated. The simulated NAO indices are not correlated with the observed NAO index, despite being forced with observed SSTs, indicating that SSTs are not driving NAO variability in the models. Similarly, although a number of studies have identified a link between Eurasian snow extent and the phase of the NAO, no such link is apparent in the AMIP-2 results. It appears that, within the framework of the AMIP-2 experiments, the NAO is an internal mode of atmospheric variability and that impacts of SSTs and Eurasian snow cover on the phase of the NAO are not discernable. However, these conclusions do not necessarily apply to decadal-scale and longer variability or to coupled atmosphere–ocean models.

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David S. Gutzler and Richard D. Rosen

Abstract

Digitized maps of Northern Hemisphere snow cover derived from visible satellite imagery are examined to assess the interannual variability of snow cover in winter months for years 1972–90. The secular trend of winter snow cover over the landmasses of Eurasia and North America during this period is extremely small in December and January. A decreasing trend of somewhat larger magnitude is observed in Eurasian snow cover in February. Fluctuations of detrended interannual snow-cover anomalies averaged over the Eurasian and North American continents are positively correlated. By subdividing the continents into longitudinal sectors it is determined that this intercontinental relationship is due to high correlations between European and North American sectors. The relationship of snow-cover fluctuations to large-scale circulation anomalies is described using lime series of teleconnection pattern indices derived from monthly mean geopotential height fields. A pattern of height anomalies resembling the North Atlantic Oscillation is correlated with snow-cover anomalies in North America and Europe. The Pacific-North American teleconnection pattern is highly correlated with snow-cover anomalies in western North America but has limited influence on intercontinental snow-cover fluctuations.

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Richard D. Rosen and William J. Gutowski Jr.

Abstract

The possible impact of doubling C02 on the zonal-mean zonal winds and the angular momentum of the atmosphere is examined using general circulation model output archived by the Goddard Institute for Space Studies, the National Center for Atmospheric Research, and the Geophysical Fluid Dynamics Laboratory. Whereas the emphasis in most previous studies with these models has been placed on the temperature and precipitation changes expected from a doubled-CO2 scenario, the intent here is to investigate some of the dynamical consequences predicted by them models, especially within the tropics where the zonal-wind and temperature changes are less tightly coupled than elsewhere.

Comparisons among the three models of the difference in zonal-mean zonal winds between 2×C02 and 1×C02 simulations indicate a common tendency when C02 is doubled for winds to become more easterly in much of the tropics during June-July-August. Less of a consensus for the tropics emerges for December-January-February, perhaps as a result of differences among the models' basic climatologies for the zonal-wind field. In general, however, changes predicted for the zonal winds in the tropics and elsewhere are comparable to the interannual variability currently observed, suggesting that these changes ought to become detectable eventually.

Largely because of the tropical wind changes, decreases in the troposphere's relative angular momentum accompany a doubling of C02 in all the model runs. The amplitude of the decrease is typically a considerable fraction of a model's seasonal cycle and, in some cases, is large enough that a measurable change in the length of day could result. Although the possibility of an anthropogenic effect on earth's rotation is noteworthy, such a prediction must be regarded as tentative in light of the shortcomings found in the models’ zonal-wind climatologies and the differences in their zonal-mean responses.

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