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  • Author or Editor: Richard D. Rosen x
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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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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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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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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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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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Rui M. Ponte
,
Amala Mahadevan
,
Jayendran Rajamony
, and
Richard D. Rosen

Abstract

Changes in axial atmospheric angular momentum M are related to zonal torques on the atmosphere, but studies reveal large imbalances between the estimated torques and M variations on seasonal timescales. The observed imbalances are commonly attributed to uncertainties in the torque estimates. One particularly important torque component at the seasonal period is that due to zonal wind stresses over the ocean T O . The uncertainties in T O are explored by calculating different multiyear time series based on surface wind products derived from passive and active microwave satellite data. The satellite-based T O are compared to available reanalysis products. Results indicate that there are indeed substantial uncertainties in the seasonal T O , and that these uncertainties are related mostly to the wind fields rather than to the particular parameterizations of the surface stress in the boundary layer. Regional analyses point to the need to improve knowledge of the wind fields over extensive areas of the ocean, particularly in many tropical and southern latitude regions. Resolving subweekly variability in surface winds is also found to be important when determining the seasonal cycle in T O . The current satellite-based T O estimates can lead to a better seasonal momentum budget, but results are tempered by the uncertain effects of gravity wave torque in that budget.

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Huei-Ping Huang
,
Klaus M. Weickmann
, and
Richard D. Rosen

Abstract

The global atmospheric angular momentum (AAM) is known to increase with tropical eastern Pacific sea surface temperature (SST) anomalies during El Niño events. Using a reanalysis dataset, the ratio of the monthly AAM anomaly to El Niño SST anomaly (based on the Niño-3.4 index) is found to be approximately 1 angular momentum unit (=1025 kg m2 s−1) per degree Celsius for most post-1975 El Niños. This ratio is much smaller, however, during the 1965/66 and 1972/73 El Niños, raising the possibilities that either the early reanalysis data are in error due to sparse observations, or the atmospheric response to the two early El Niños was unusual. The possibility of a severe data problem in the reanalysis is ruled out by cross-validating the AAM time series with independent measurements of length of day. The latitudinal structures of the zonal wind anomalies in 1965/66 and 1972/73 are examined for both the reanalysis and a set of general circulation model (GCM) simulations. Multiple GCM runs with specified SST produce a more positive ensemble-mean AAM anomaly in 1965 than its counterpart in the reanalysis. The GCM-simulated ensemble-mean zonal wind anomaly resembles the canonical El Niño response with accelerations of subtropical zonal jets in both hemispheres, a pattern that is almost absent in the reanalysis. On the other hand, a large spread exists among the individual ensemble members in the 1965/66 GCM simulations. Although the majority of the individual ensemble members shows the canonical El Niño response, two outliers (out of 12 runs) exhibit very small zonal wind responses in the Northern Hemisphere similar to the reanalysis. Thus, the observed AAM anomaly during 1965/66 is interpreted as an outlier with atmospheric noise being strong enough to overwhelm the canonical El Niño response. The low AAM in the 1972/73 event is related in the reanalysis to a significantly negative zonal wind response on the equator. This signal is robustly reproduced, although with a slightly smaller amplitude, in the ensemble mean and all individual ensemble members in the GCM simulations. The small ensemble standard deviation and large ensemble-mean response on the equator indicate that the negative response is due to the lower-boundary forcing related to the SST anomaly. The fact that the AAM anomaly in 1972/73 is not well correlated with the Niño-3.4 index, then, indicates that SST anomalies outside the conventional El Niño region may be responsible for the low AAM. The uncharacteristically low values of global AAM in 1965/66 and 1972/73 contribute to a low mean for the decade before 1975, which, combined with high AAM in the post-1980 era, produces a significant upward trend in AAM in the second half of the twentieth century. If the weak AAM anomalies during the two pre-1975 El Niños are due to random noise or incidental non-El Niño influences, taking them at face value would result in an overestimate of about 15%–20% in the multidecadal trend of AAM due to boundary forcing alone. Notably, a multidecadal trend in AAM is also simulated in the ensemble mean of the multiple GCM runs, but its magnitude is smaller than the observed counterpart and more consistent with the multidecadal trend of the Niño-3.4 index. The implications of these findings for climate change detection are discussed.

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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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Rui M. Ponte
,
Richard D. Rosen
, and
George J. Boer

Abstract

Anomalies in the angular momentum of the atmosphere (M) during the 1982-83 El Niño event and the torques responsible for these anomalies are investigated using output from the Canadian Climate Centre general circulation model. Model values of M during the year of the event are generally larger than those for the model climatology, thereby capturing the observed tendency toward higher values of M during El Niñto. Differences exist between the model and observations in the timing and amplitude of the largest anomalies, but these differences may he due to natural variability and not necessarily directly associated with the 1982-83 El Niño conditions.

In late September and October 1982, the model atmosphere acquires momentum more rapidly than usual, leading to the development of the largest deviations from mean conditions at the end of this period, mostly associated with strong westerly momentum signals centered at 25°N. Large, sustained positive anomalies in tangential stress torques over the northern tropics are the major mechanism responsible for the modeled increase in M, but mountain torque anomalies centered at 35°N are also important at the end of October. A secondary maximum in the departure from mean M values occurs in January 1983 and is related to a general strengthening of westerly momentum anomalies over the model's tropical and midlatitude regions. Both mountain and tangential stress torques are involved in this episode, but no particular mechanism or region dominates the anomalous exchange of momentum.

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Christopher J. Paciorek
,
James S. Risbey
,
Valérie Ventura
, and
Richard D. Rosen

Abstract

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 20°–70°N. 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 7.5° 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.

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