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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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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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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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Judah L. Cohen
,
David A. Salstein
, and
Richard D. Rosen

Abstract

The zonal-mean meridional transport of water vapor across the globe is evaluated using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis for 1948–97. The shape of the meridional profile of the climatological mean transport closely resembles that of previous mean climate descriptions, but values tend to be notably larger than in climatologies derived from radiosonde-only-based analyses. The unprecedented length of the NCEP–NCAR dataset invites a focus on interannual variations in the zonal-mean moisture transport, and these results for northern winter are highlighted here. Although interannual variability in the transport is typically small at most latitudes, a significant ENSO signal is present, marked by a strengthening of water vapor transports over much of the winter hemisphere during warm events. Because of an increase in tropical sea surface temperatures and in the frequency of warm events relative to cold events in the latter half of the 50-yr record, this interannual signal projects onto an overall trend toward enhanced meridional moisture transports in the global hydrological cycle.

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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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