Search Results

You are looking at 11 - 20 of 31 items for :

  • Author or Editor: Robert E. Livezey x
  • Refine by Access: All Content x
Clear All Modify Search
Anthony G. Barnston and Robert E. Livezey

Abstract

A recently discovered association between the 11-year solar cycle and the Northern Hemispheric low-frequency atmospheric circulation structure, which is most easily delectable when the two phases of the Quasi-biennial Oscillation (QBO) are considered individually, is described and subjected to global statistical significance tests.

Highly significant relationships are found during the January–February period. This is especially true for the west QBO phase, in which the solar flux is positively correlated with 700 mb heights and surface temperatures over central and northern Canada, and negatively correlated with heights in the western Atlantic along 30°N and with temperature in the southern and much of the eastern portions of the United States. The pattern of the flux-height correlation field resembles primarily the Tropical/Northern Hemisphere (TNH) long-wave circulation pattern and secondarily the North Atlantic Oscillation (NAO) pattern. For east QBO phase years a different structure is found, and for all years pooled a weaker but quite Characterizable pattern emerges.

January–February correlations are studied for sensitivity to lead time in the QBO phase definition and for shorter period means for the west QBO phase. The latter inquiry reveals a concentration of the west phase relationship during the latter half of January.

The climate of the October–November period also appears to participate, to a lesser but significant degree, in a solar–QBO relationship for west phase QBO years.

For the west QBO phase, the January–February solar flux versus 700 mb height (and United States–Canada surface temperature) correlation pattern contains sufficient amplitude and field significance to be exploited for operational forecasting purposes at the Climate Analysis Center. However, in the absence of a verifiable physical basis of the solar–QBO–atmosphere association, and because the 45 mb stratospheric winds were selected to characterize the QBO in an a posteriors manner, the relationships are accepted with caution and will be regularly reevaluated.

Full access
Daniel S. Wilks and Robert E. Livezey

Abstract

Eleven alternatives to the annually updated 30-yr average for specifying climate “normals” are considered for the purpose of projecting nonstationarity in the mean U.S. temperature climate during 2006–12. Comparisons are made for homogenized U.S. Historical Climatology Network station data, corresponding nonhomogenized station data, and spatially aggregated (“megadivision”) data. The use of homogenized station data shows clear improvement over nonhomogenized station data and spatially aggregated data in terms of mean-squared specification errors on independent data. The best single method overall was the most recent 15-yr average as implemented by the Climate Prediction Center (CPC15), consistent with previous work using nonhomogenized and spatially aggregated data, although “hinge” functions with the change point fixed at 1975 performed well for the spring and summer seasons. A hybrid normals-specification method, using one of these piecewise continuous functions when the regressions are sufficiently strong and the CPC15 otherwise, exhibits a favorable trade-off between squared error and bias that may make it an optimal choice for some users.

Full access
Robert E. Livezey and Kingtse C. Mo

Abstract

Monthly mean teleconnections during the northern winter between proxies for tropical heating (OLR and SST data) and Northern Hemisphere 700 mb circulation patterns (PNA, TNH, and WPO) are examined, principally with correlation analysis. In particular it is found that positive projections on all three patterns are highly probable during certain strong ENSO winters but the means to predict their relative strengths was not discovered, although the absolute strength of the TNH pattern is directly related to SST anomalies in the central Pacific. Other ENSO winters also have a tendency for positive PNA and WPO projections, but for a negative TNH projection. For other winters the importance of an area north of the equator and 25 degrees to the west of the date line is confirmed as a probable source region for the PNA pattern. Another area about 25 degrees to the east of the date line is singled out as a possible tropical response to the PNA pattern. Implications for current and future GCM experiments and long-range prediction are discussed.

Full access
Thomas M. Smith and Robert E. Livezey

Abstract

Specifications of 1- and 3-month mean Pacific–North America region 700-hPa heights and U.S. surface temperatures and precipitation, from global sea surface temperatures (SSTs) and the ensemble average output of multiple runs of a general circulation model with the same SSTs prescribed, were explored with canonical correlation analysis. In addition to considerable specification skill, the authors found that 1) systematic errors in SST-forced model variability had substantial linear parts, 2) use of both predictor fields usually enhanced specification performance for the U.S. fields over that for just one of the predictor fields, and 3) skillful specification and model correction of the heights and temperatures were also possible for nonactive or transitional El Niño–Southern Oscillation situations.

Full access
Robert E. Livezey and Thomas M. Smith

Abstract

Rotated canonical correlation analysis between seasonal- and longer-mean global SSTs and either U.S. surface temperatures or 700-hPa heights in the Pacific–North America region have led to decompositions into three distinct signals. One of these represents the interannual variability of ENSO and a second is related to the North Atlantic oscillation and exhibits considerable variability on interdecadal timescales. In contrast the temporal behavior of the third, which is referred to here as the global signal, is mostly characterized by a steady trend since the late 1960s. The robustness of this time series to variations in the analyses, as well as the robustness of the spatial structure of the SST pattern accompanying it, suggests that the decomposition represents a successful separation of the climate signal from the climate noise. When viewed in the context of other recent work, the global signal cannot be discounted as a “fingerprint” of global warming. Finally, calculations that exploit ensemble mean output from prescribed-SST GCM runs reveal notable systematic errors in the simulation of the features of all three signals.

Full access
Robert E. Livezey and Sherwin W. Jamison

Abstract

Operational long-range weather prediction in the Soviet Union is reviewed. Methods for producing forecasts at the 5- and 10-day, monthly and seasonal range are described in terms of the synoptic, statistical and hydrodynamic tools available to Soviet forecasters. Skill scores for these forecasts published by the Soviets are summarized and examined.

Skill scores for Soviet operational forecasts of mean seasonal (about two months) temperature anomaly and precipitation category are computed separately for regions, seasons and years and compared to persistence skill scores. In addition, forecast-observation sets for the sign of the mean temperature anomaly are tested for “no skill.” The forecasts for the sign of the mean temperature anomaly are found to be best by region for the Arctic and by season for March through April, but generally do not outperform persistence, exhibit demonstrable skill, or show an improvement trend over the verification period. Forecasts of the mean precipitation category are shown to be consistently better than persistence, but to have quite modest skill scores.

Full access
Anthony G. Barnston and Robert E. Livezey

Abstract

An expanded version of the multifield analog prediction system developed by Barnett and Preisendorfer (1978) was described and applied to the winter season in Part I of this two-part series (Livezey and Barnston 1988). This second part reviews briefly the major design features detailed in Part I, and then describes the predictive skills in spring, summer, fall and all other intermediate 3-month seasons.

In none of the 11 nonwinter seasons is the United States surface temperature predicted with as much skill as in winter. The major winter skill peak (16%) extends partially into the following two seasons (January–February–March and February–March–April), and a secondary maximum in summer (13%) similarly includes the two following seasons (July–August–September and August–September–October). In both skillful periods of the year the skill tends to be greatest over the eastern third of the United States and the immediate Pacific Coast and lowest over the Rockies and Plateau. More predictor variables are used as criteria for analog selection during the skillful times of the year, while fewer are found to contribute to skill levels at other times. The annual cycle of skill of simple persistence forecasts has its primary maximum in August–September–October, when it slightly exceeds the skill of the analog method, and a secondary maximum in winter when it is outperformed by the analog method by a substantial margin. The analog-forecasted temperature patterns are found to be statistically largely independent of the temperature patterns of persistence forecasts at all times of the year.

In an exercise aimed at determining which part of the predicted period within the season is most skilfully forecast, it is found that from fall through winter the later month(s) of the season are better predicted than the earlier months, suggesting a potential for useful long-lead forecasting of subseasonal periods in much of the coldest part of the year.

Full access
Robert E. Livezey and Thomas M. Smith

Abstract

No abstract available.

Full access
Anthony G. Barnston and Robert E. Livezey

Abstract

The association between the 11-year solar cycle and the tropospheric Northern Hemisphere climate in January-February for the 21 west QBO phase years in the 1951–88 period failed strongly in 1989. This failure is explained in part by the high Southern Oscillation (SO) episode of 1988/89, whose influence on the climate conflicted with that hypothesized from the solar flux/QBO in much of North America. The occurrence of high flux during the west QBO phase along with a high SO (i.e., mid tropical Pacific SST event) was unprecedented before 1989. Bivariate multiple linear regression is used to predict Northern Hemisphere 700 mb heights and United States surface temperatures on the basis of the solar flux and the SO for each QBO phase. Exploratory analyses are carried out to describe more generally the associations among flux, QBO, SO, and the climate.

Major findings are: 1) Interactions among the four phenomena include primarily the SO-climate and solar flux-climate relationships, with the QBO phase as a required stratifier for the latter. Within this stratified framework the solar flux and the SO are essentially independent influences on the tropospheric climate, but act with moderate strength in overlapping regions such that their effects may offset or enhance one another; and 2) the true strength and nature of the solar flux-QBO-climate association still has large uncertainty because of the small samples resulting from QBO stratification and sharing of predictive power with the SO.

Full access
Kingtse C. Mo and Robert E. Livezey

Abstract

Simultaneous and lagged correlation statistics have been calculated between time series of seasonal height anomalies at selected stations and extratropical grid-point anomalies in both hemispheres. The tropical stations in two major tropical precipitation zones, the Indo-China maritime continent and Africa, are well correlated with each other. These stations are also correlated with stations in the North Pacific and Australia, but the coefficients are smaller. The correlations between height anomalies at any of these stations and Northern Hemisphere height anomalies show a well-defined global pattern. Depending upon the location of the stations, the pattern is either a Pacific North American (PNA), a Tropical Northern Hemisphere (TNH) pattern or a mixed pattern having both elements. All three patterns, PNA, TNH and WPO (Western Pacific Oscillation), have been linked to tropical variations. The correlations between height anomalies at these well-correlated stations and the Southern Hemisphere height anomalies at the 500 mb level give the summer teleconnection pattern of Mo and White (1985). The vertical structure of patterns indicate that they are approximately equivalent barotropic.

The TNH pattern tends to be associated more with tropical variability for time scales longer than one season, while the PNA pattern is present in both high- and low-pass filtered analyses, although weakly in the former. Moreover, its low frequency connection to the tropics appears to be confined to ENSO years.

During ENSO years both patterns appear in both simultaneous and lagged maps, but in non-ENSO years, the TNH is weak in simultaneous charts.

Full access