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Haig Iskenderian and David A. Salstein

Abstract

The sources of high-frequency (⩽14 day) fluctuations in global atmospheric angular momentum (AAM) are investigated using several years of surface torque and AAM data. The midlatitude mountain torque associated with the Rockies, Himalayas, and Andes is found to be responsible for much of the high-frequency fluctuations in AAM, whereas the mountain torque in the Tropics and polar regions as well as the friction torque play a much lesser role on these timescales. A maximum in the high-frequency mountain torque variance occurs during the cool season of each hemisphere, though the Northern Hemisphere maximum substantially exceeds that of the Southern. This relationship indicates the seasonal role played by each hemisphere in the high-frequency fluctuations of global AAM.

A case study reveals that surface pressure changes near the Rockies and Himalayas produced by mobile synoptic-scale systems as they traversed these mountains contributed to a large fluctuation in mountain torque and a notable high-frequency change in global AAM in mid-March 1996. This event was also marked by a rapid fluctuation in length of day (LOD), independently verifying the direct transfer of angular momentum from the atmosphere to solid earth below. A composite study of the surface pressure patterns present during episodes of high-frequency fluctuations in AAM reveals considerable meridional elongation of the surface pressure systems along the mountain ranges, thus establishing an extensive cross-mountain surface pressure gradient and producing a large torque. The considerable along-mountain extent of these surface pressure systems may help to explain the ability of individual synoptic-scale systems to affect the global AAM. Furthermore, midlatitude synoptic-scale systems tend to be most frequent in the cool season of each hemisphere, consistent with the contemporary maximum in hemispheric high-frequency mountain torque variance.

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David A. Salstein and Richard D. Rosen

Abstract

Modern atmospheric and geodetic datasets have demonstrated that changes in the axial component of the atmosphere's angular momentum and in the rotation rate of the solid earth are closely coupled on time scales of up to several years. We therefore examine the feasibility of using a historical record of the earth's rotation as a proxy for year-to-year changes in the zonal wind held over the globe. The bulk of the earth rotation series acquired for this purpose is based on telescopic observations of the occulation of starts by the moon; semiannual values of changes in the length of day derived from these observations have acceptably small errors from about 1860 onwards. We filter these values to remove decade-scale fluctuations, which are driven primarily by non-atmospheric processes, and we examine the resulting proxy series to see if it contains a signal associated with one of the major modes of interannual variability in the atmosphere, namely that due to the El Nino/Southern Oscillation (ENSO). According to tests of statistical significance, such a signal is present in the historical earth rotation series, in that the day is typically 1onger during the year following an ENSO oceanic warm event than otherwise. We therefore proceed to consider other signals of interannual variability in the proxy series. In particular, we infer that noteworthy trends in atmospheric interannual variability have occurred over the last century, for example, the decade of the 1920s was marked by much year-to-year changes in the zonal circulation over the globe than that of the 1940s. Based on modern atmospheric data, we tentatively suggest that most of these circulation changes have resulted from anomalies in the region between 30°N and 30°S.

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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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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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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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Robert X. Black, David A. Salstein, and Richard D. Rosen

Abstract

The interannual variability of atmospheric angular momentum over a 26-yr period is studied regionally using monthly analyses of zonal winds derived from the global rawinsonde network. Variations in zonal-mean momentum, filtered to emphasize interannual timescales, exhibit a coherent propagating signal emanating from low latitudes, as identified in other studies using shorter records. Applying extended empirical orthogonal function (EEOF) analyses to zonally varying data, the authors isolate a dominant pair of eigenvectors whose principal component time series and spatial patterns are in quadrature with one another, indicating oscillatory behavior. The oscillation described by the two EEOFs has a period of about 36 months and is linked a posteriori to the time evolution of the El Niño-Southern Oscillation phenomenon. Beginning as an anomaly over the Tropics that extends from the Indian Ocean into the Pacific, the signal is observed to progress eastward and poleward into both hemispheres, leading to a bipolar structure straddling the central tropical Pacific Ocean. A lagged teleconnection analysis between the Pacific centers and remote sectors corroborates the EEOF results. The first pair of eigenvectors contributes substantially to the interannual variance in global angular momentum and to the variability of the zonal-mean momentum field at low latitudes. A second pair of eigenvectors, also in quadrature with one another, describes a biennial oscillation related to zonal momentum variability at higher latitudes.

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David A. Salstein, Richard D. Rosen, and Jose P. Peixoto

Abstract

The variability of annually averaged water vapor and water vapor transport fields over the Northern Hemisphere during the fifteen year period 1958–1973 is studied by means of empirical orthogonal function (EOF) analysis. Examined are vertically integrated values of the moisture quantities at a set of 91 stations chosen to cover the hemisphere as uniformly as possible. Analyses of the zonal and meridional transports are performed separately, but in addition a version of the EOF analysis is performed in which the two are treated as components of a single vector. All resulting modes are examined in light of statistical significance criteria established through a form of Monte Carlo testing.

The first mode in moisture variability appears to be highly significant, and it is dominated by opposite behavior in the held over Africa and over the western equatorial Pacific. Its time series reflects a sharp change in regime during the period studied. The significant mode of variability in the zonal transport field mirrors that of water vapor content, whereas the first two modes of meridional transport variability point to the Afro-Indian region as being important in explaining changes in the zonal mean Hadley cell during the period.

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Richard D. Rosen, John M. Henderson, and David A. Salstein

Abstract

As part of its mandate to oversee the design of measurement networks for future weather and climate observing needs, the North American Atmospheric Observing System (NAOS) program hypothesized that replacing some of the existing radiosonde stations in the continental United States (CONUS) with another observing system would have little impact on weather forecast accuracy. The consequences of this hypothesis for climate monitoring over North America (NA) are considered here by comparing estimates of multidecadal trends in seasonal mean 500-mb temperature (T) integrated regionally over CONUS or NA, made with and without the 14 upper-air stations initially targeted for replacement. The trend estimates are obtained by subsampling gridded reanalysis fields at points nearest the 78 (126) existing CONUS (NA) radiosonde stations and at these points less the 14 stations. Trends in T for CONUS and NA during each season are also estimated based on the full reanalysis grid, but regardless of the sampling strategy, differences in trends are small and statistically insignificant. A more extreme reduction of the existing radiosonde network is also considered here, namely, one associated with the Global Climate Observing System (GCOS), which includes only 6 (14) stations in CONUS (NA). Again, however, trends for CONUS or NA based on the GCOS sampling strategy are not significantly different from those based on the current network, despite the large difference in station coverage. Estimates of continental-scale trends in 500-mb temperature therefore appear to be robust, whether based on the existing North American radiosonde network or on a range of potential changes thereto. This result depends on the large spatial scale of the underlying tropospheric temperature trend field; other quantities of interest for climate monitoring may be considerably more sensitive to the number and distribution of upper-air stations.

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Richard D. Rosen, David A. Salstein, and JoséP. Peixoto

Abstract

Maps of the streamfunction field for the vertically integrated flux of water vapor over the Northern Hemisphere are presented for each of six annual periods. This approach reveals the existence of a broad-scale cellular pattern in the flow that is not so readily apparent from the more conventional analyses of its zonal and meridional components. In addition, changes in the moisture flow from one year to the next are clearly pictured, a particular good example of which is provided by the behavior of the streamfunction over the Pacific during the period considered.

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Richard D. Rosen, David A. Salstein, and Thomas Nehrkorn

Abstract

Prompted by the need for forecasts of changes in the length of day on 1-10 day time scales, this paper examines the quality of predictions of a proxy variable, namely the atmosphere's relative angular momentum (M) made by the medium-range forecast model (MRF)of the Nationa Meteorological Center during December 1985-November 1989. Skillful forecasts of M relative to persistence am produced by the MRF over its entire 1-10 day range, as found previously. Errors in the MRF are smaller than those of a damped persistence of anomaly empirical model only out to 8 dan however. Moreover, beyond about 3-4 dam MRF tommts of M for day N + 1 show no more skill than forecasts made by simply persisting the MRF prediction for day N, suggesting that significant room for improvement in dynamical forecasts of M still exists.

Errors in the MRF foments of M are separated into their bias and nonsystematic components. Bias errors became especially prominent with the introduction of the most recent version of the MRF examined here, MRF88, whereas random errors in the M forecasts appear not to have been affected by model changes. Both types of errors in the M forecasts can be traced to problems with forecasts of the zonal mean zonal wind, [u], in the tropics. Bias errors in MRF88 forecasts of the globally integrated quantity M are large despite notable reductions in biases in [u] forecasts locally since the MRF was fim introduced. Evidence is offered that the pattern of bias in 10-day forecasts of [u] develops much earlier in the forecast cycle.

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