Richard D. Rosen

The Sub-bureau for Atmospheric Angular Momentum of the International Earth Rotation Service - A meteorological data center with geodetic applications

By exchanging angular momentum with the solid portion of the earth, the atmosphere plays a vital role in exciting small but measurable changes in the rotation of our planet. Recognizing this relationship, the International Earth Rotation Service invited the U.S. National Meteorological Center to organize a Sub-bureau for Atmospheric Angular Momentum (SBAAM) for the purpose of collecting, distributing, archiving, and analyzing atmospheric parameters relevant to earth rotation/polar motion. These functions of wind and surface pressure are being computed with data from several of the worlds weather services, and they are being widely applied to the research and operations of the geodetic community. The SBAAM began operating formally in October 1989, and this article highlights its development, operations, and significance.

Interannual Variability of Wintertime Snow Cover across the Northern Hemisphere

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 N...

The Latitude-Height Structure of 40–50 Day Variations in Atmospheric Angular Momentum

Abstract Quasi-periodic variations in the relative angular momentum of the atmosphere on time scales of around 40–50 days have been observed by Langley et al. (1981). A description of the two-dimensional (latitude-height) structure of the winds responsible for these changes is constructed here from five years of NMC twice-daily global analyses. Using cross-spectral and amplitude-phase eigenvector techniques, we find these variations are associated with wavelike motions in the tropical upper troposphere that propagate and downward in phase within the tropics. A coherently connected midlatitude Northern Hemisphere component is also present whose phase is essentially independent of height. We believe the tropical component to be the zonally averaged part of the motions described by Madden and Julian (1971, 1972). The Northern Hemisphere midlatitude component may be a direct response to the tropical motions or both motions may be the common response to an as yet unidentified tropical forcing.

Multiple Indices of Northern Hemisphere Cyclone Activity, Winters 1949–99

The National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) reanalysis is used to estimate time trends of, and analyze the relationships among, six indices of cyclone activity or forcing for the winters of 1949‐99, over the region 208‐708N. The indices are Eady growth rate and temperature variance, both at 500 hPa; surface meridional temperature gradient; the 95th percentile of near-surface wind speed; and counts of cyclones and intense cyclones. With multiple indices, one can examine different aspects of storm activity and forcing and assess the robustness of the results to various definitions of a cyclone index. Results are reported both as averages over broad spatial regions and at the resolution of the NCEP‐NCAR reanalysis grid, for which the false discovery rate methodology is used to assess statistical significance. The Eady growth rate, temperature variance, and extreme wind indices are reasonably well correlated over the two major storm track regions of the Northern Hemisphere as well as over northern North America and Eurasia, but weakly correlated elsewhere. These indices show moderately strong correlations with each of the two cyclone count indices over much of the storm tracks when the count indices are offset 7.58 to the north. Regional averages over the Atlantic, the Pacific, and Eurasia show either no long-term change or a decrease in the total number of cyclones; however, all regions show an increase in intense cyclones. The Eady growth rate, temperature variance, and wind indices generally increase in these regions. On a finer spatial scale, these three indices increase significantly over the storm tracks and parts of Eurasia. The intense cyclone count index also increases locally, but insignificantly, over the storm tracks. The wind and intense cyclone indices suggest an increase in impacts from cyclones, primarily over the oceans.

Sea Level Response to Pressure Forcing in a Barotropic Numerical Model

Abstract A barotropic shallow-water model is used to study the large-scale sea level response to realistic barometric forcing at periods ranging from 1 day to 1 year. Results are presented from coarse resolution “open” ocean experiments (i.e., no shallow continental shelf regions or marginal seas) with coastal geometries and bottom topography representative of the North Atlantic and Pacific basins. The validity of the inverted barometer (IB) approximation is examined in detail, including nonlocal effects which result from taking into account the constant volume of the ocean. These effects are found to be important at low latitudes, where a considerable part of the sea level variability is related to pressure forcing over higher latitudes. Root-mean-square deviations from an IB response in the range of 1–3 cm are typical, with most of the variance occurring at high frequencies. Basin-averaged estimates yield IB deviations of only a few percent at time scales longer than 1 week increasing to 5%–20% over the...

An El Nino signal in atmospheric angular momentum and earth rotation

Anomalously high values of atmospheric angular momentum and length of day were observed in late January 1983. This signal in the time series of these two coupled quantities appears to have been a consequence of the equatorial Pacific Ocean warming event of 1982-1983.

Variability in the Annual Fields of Large-Scale Atmospheric Water Vapor Transport

Abstract Annual mean holds of the zonal and meridional components of the vertically integrated water vapor flux in the atmosphere have been computed objectively from six years of Northern Hemispheric upper air data. Emphasis is placed on the longitudinal structure of these fields in order to identify regional contributions to the annual variations of the zonally averaged moisture fluxes. These temporal changes, particularly those associated with the meridional transport of latent heat. appear large enough to impact significantly on the maintenance of the atmospheric energy balance. While the distribution and quality of some upper air data make it difficult to estimate the degree to which the analyses precisely duplicate atmospheric behavior everywhere, this study provides documentation of a wide range of interannual variability. In this regard, the behavior of one of the years chosen for study contrasts sharply with that of the rest of the sample, as was similarly noted for other quantities evaluated by R...

Earth rotation as a proxy for interannual variability in atmospheric circulation, 1860―present

Abstract Modern atmospheric and geodetic datasets have demonstrated that changes in the axial component of the atmospheres 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 earths 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)....

Accuracy of Atmospheric Angular Momentum Estimates from Operational Analyses

Abstract Modern, high resolution atmospheric and geodetic data are finding increasing use in geophysical studies of the angular momentum balance of the earth-atmosphere system. Such studies require knowledge of the level of uncertainty associated with each data set. Here we attempt to assess the accuracy of global atmospheric angular momentum determinations by comparing time series of this quantity derived from both the NMC and ECMWF zonal wind analyses for 1981–85. We find that over this period the rms difference between the two atmospheric series is comparable to the difference between an atmospheric momentum series and the series that is consistent with observed (nontidal) changes in earth rotation. Most of the difference between the NMC and ECMWF global momentum values comes from the region 33°S–16°N. Differences between NMC and ECMWF regional momentum values become vanishingly small in the high latitudes of the Northern Hemisphere, but they remain large throughout the entire Southern Hemisphere. An a...

Trends in the surface meridional temperature gradient

Given the presence of a meridional temperature gradient (MTG) across midlatitudes, large-scale eddies transport heat poleward, thereby shaping Earths climate. Defining an MTG index here as the difference in surface temperature between the 30°–35°N belt and the 50°–55°N belt, we use a temperature record compiled from observations over a 110-year period to determine a trend in the MTG in the last century. We find a significant decreasing trend in the MTG over this period of 0.4±0.1°C per 100 years, along with indications of substantial multidecadal variability.