Robert E. Livezey

Classification, Seasonality and Persistence of Low-Frequency Atmospheric Circulation Patterns

Abstract Orthogonally rotated principle component analysis (RPCA) of Northern Hemisphere 1-month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability. The analysis is detailed and comprehensive, in that 1) a high resolution, approximately equal-area 358-point grid is used for the virtually maximum possible 35-year period of record, 2) a positive bias in the NMC data base in the early 1950s in the subtropics is largely eliminated for the first time, and 3) homogeneous, separate analyses of each month of the year are carried out, detailing the mouth-to-month changes in the dominant circulation patterns. Winter results are similar to those of other recent RPCA and teleconnection studies except that some less obvious patterns are identified and further detail of the better-known patterns is provided. Two north-south dipole patterns are found over the Pacific Ocean (West Pacific Oscillation and East Pacific pattern) and over the Atla...

Long-Lead Seasonal Forecasts—Where Do We Stand?

Abstract The National Weather Service intends to begin routinely issuing long-lead forecasts of 3-month mean U.S. temperature and precipitation by the beginning of 1995. The ability to produce useful forecasts for certain seasons and regions at projection times of upto 1 yr is attributed to advances in data observing and processing, computer capability, and physical understanding-particularly, for tropical ocean-atmosphere phenomena. Because much of the skill of the forecasts comes from anomalies of tropical SST related to ENSO, we highlight here long-lead forecasts of the tropical Pacific SST itself, which have higher skill than the U.S forecasts that are made largely on their basis. The performance of five ENSO prediction systems is examined: Two are dynamical [the Cane-Zebiak simple coupled model of Lamont-Doherty Earth Observatory and the nonsimpie coupled model of the National Centers for Environmental Prediction (NCEP)]; one is a hybrid coupled model (the Scripps Institution for Oceanography-Max Pla...

Tropical-extratropical geopotential height teleconnections during the Northern Hemisphere winter

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 heig...

Teleconnective Response of the Pacific–North American Region Atmosphere to Large Central Equatorial Pacific SST Anomalies

Abstract A prominent year-round ensemble response to a global sea surface temperature (SST) anomaly field fixed to that for January 1992 (near the peak of a major warm El Nino–Southern Oscillation episode) was observed in a 20-yr integration of the general circulation model used for operational seasonal prediction by the U.S. National Weather Service. This motivated a detailed observational reassessment of the teleconnections between strong SST anomalies in the central equatorial Pacific Ocean and Pacific–North America region 700-hPa heights and U.S. surface temperatures and precipitation. The approach used consisted of formation of monthly mean composites formed separately from cases in which the SST anomaly in a key area of the central equatorial Pacific Ocean was either large and positive or large and negative. Extensive permutation tests were conducted to test null hypotheses of no signal in these composites. The results provided a substantial case for the presence of teleconnections to either the pos...

A Comparison of the NCEP–NCAR Reanalysis Precipitation and the GPCP Rain Gauge–Satellite Combined Dataset with Observational Error Considerations

Abstract The Global Precipitation Climatology Project (GPCP) has released monthly mean estimates of precipitation that comprise gauge observations and satellite-derived precipitation estimates. Estimates of standard random error for each month at each grid location are also provided in this data release. One of the primary intended uses of this dataset is the validation of climatic-scale precipitation fields that are produced by numerical models. Nearly coincident with this dataset development, the National Centers for Environmental Prediction and the National Center for Atmospheric Research have joined in a cooperative effort to reanalyze meteorological fields from the present back to the 1940s using a fixed state-of-the-art data assimilation system and large input database. In this paper, monthly accumulations of reanalysis precipitation are compared with the GPCP combined rain gauge–satellite dataset over the period 1988–95. A unique feature of this comparison is the use of standard error estimates tha...

Practical Considerations in the Use of Rotated Principal Component Analysis (RPCA)in Diagnostic Studies of Upper-Air Height Fields

Abstract Rotated principal component analysis (RPCA) is a powerful tool for studying upper air height data because of its ability to distill information about the variance existing in a large number of maps to a much smaller set of physically meaningful maps which together explain a large fraction of the variance of she input dataset. However, in order to achieve this, one faces the problem of deciding how many eigenmodes to rotate. A discussion of the dangers of incorrectly choosing the rotation point and a quasi-objective technique that leads to a good compromise between over- and underrotation are presented. Finally, the use of RPCA for detecting errors and inconsistencies in upper air data along with two examples is discussed.

NCEP Forecasts of the El Niño of 1997–98 and Its U.S. Impacts

Abstract The strong El Nino of 1997-98 provided a unique opportunity for National Weather Service, National Centers for Environmental Prediction, Climate Prediction Center (CPC) forecasters to apply several years of accumulated new knowledge of the U.S. impacts of El Nino to their long-lead seasonal forecasts with more clarity and confidence than ever previously. This paper examines the performance of CPCs official forecasts, and its individual component forecast tools, during this event. Heavy winter precipitation across California and the southern plains-Gulf coast region was accurately forecast with at least six months of lead time. Dryness was also correctly forecast in Montana and in the southwestern Ohio Valley. The warmth across the northern half of the country was correctly forecast, but extended farther south and east than predicted. As the winter approached, forecaster confidence in the forecast pattern increased, and the probability anomalies that were assigned reached unprecedented levels in ...

Tropical-extratropical teleconnections during the northern hemisphere winter. II: Relationships between monthly mean northern hemisphere circulation patterns and proxies for tropical convection

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 an...

Variability of Skill of Long-Range Forecasts and Implications for their Use and Value

Abstract The widely held belief that monthly and seasonal predictions, while containing some information, are not skillful enough to be of economic benefit, is reexamined through an extended example of winter seasonal temperature skill as it relates to the natural gas industry. A case is made that forecasts of mean conditions for periods as long as a season can be made with appreciable reliability for certain parameters, places, seasons, and situations, thereby making them potentially beneficial to certain users. Several opportunities to improve the reliability of these forecasts over the next several years are described in the context of operational systems currently used to make predictions for the United States. Finally, the levels and types of research necessary to realize the potential benefits to users of skillful long-range forecasts are outlined. It is argued that it makes little sense, from a scientific or societal point of view, to neglect research on prediction of intraseasonal to interannual t...

Estimation and Extrapolation of Climate Normals and Climatic Trends

WMO-recommended 30-yr normals are no longer generally useful for the design, planning, and decisionmaking purposes for which they were intended. They not only have little relevance to the future climate, but are often unrepresentative of the current climate. The reason for this is rapid global climate change over the last 30 yr that is likely to continue into the future. It is demonstrated that simple empirical alternatives already are available that not only produce reasonably accurate normals for the current climate but also often justify their extrapolation to several years into the future. This result is tied to the condition that recent trends in the climate are approximately linear or have a substantial linear component. This condition is generally satisfied for the U.S. climate-division data. One alternative [the optimal climate normal (OCN)] is multiyear averages that are not fixed at 30 yr like WMO normals are but rather are adapted climate record by climate record based on easily estimated characteristics of the records. The OCN works well except with very strong trends or longer extrapolations with more moderate trends. In these cases least squares linear trend fits to the period since the mid-1970s are viable alternatives. An even better alternative is the use of “hinge fit” normals, based on modeling the time dependence of large-scale climate change. Here, longer records can be exploited to stabilize estimates of modern trends. Related issues are the need to avoid arbitrary trend fitting and to account for trends in studies of ENSO impacts. Given these results, the authors recommend that (a) the WMO and national climate services address new policies for changing climate normals using the results here as a starting point and (b) NOAA initiate a program for improved estimates and forecasts of official U.S. normals, including operational implementation of a simple hybrid system that combines the advantages of both the OCN and the hinge fit.