Greenland ice-core data have revealed large decadal climate variations over the North Atlantic that can be related to a major source of low-frequency variability, the North Atlantic Oscillation. Over the past decade, the Oscillation has remained in one extreme phase during the winters, contributing significantly to the recent wintertime warmth across Europe and to cold conditions in the northwest Atlantic. An evaluation of the atmospheric moisture budget reveals coherent large-scale changes since 1980 that are linked to recent dry conditions over southern Europe and the Mediterranean, whereas northern Europe and parts of Scandinavia have generally experienced wetter than normal conditions.

http://science.sciencemag.org/content/sci/269/5224/676.full.pdf

Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation

A number of recent studies have reported an ENSO-like EOF mode in the global sea surface temperature (SST) field, whose time variability is marked by an abrupt change toward a warmer tropical eastern Pacific and a colder extratropical central North Pacific in 1976‐77. The present study compares this pattern with the structure of the interannual variability associated with the ENSO cycle and documents its time history back to 1900. The analysis is primarily based on the leading EOFs of the SST anomaly and ‘‘anomaly deviation’’ fields in various domains and the associated expansion coefficient (or principal component) time series, which are used to construct global regression maps of SST, sea level pressure (SLP), and a number of related variables. The use of ‘‘anomaly deviations’’ (i.e., departures of local SST anomalies from the concurrent global-mean SST anomaly) reduces the influence of global-mean SST trends upon the structure of the EOFs and their expansion coefficient time series. An important auxiliary time series used in this study is a ‘‘Southern Oscillation index’’ based on marine surface observations. By means of several different analysis techniques, the time variability of the leading EOF of the global SST field is separated into two components: one identified with the ‘‘ENSO cycle-related’’ variability on the interannual timescale, and the other a linearly independent ‘‘residual’’ comprising all the interdecadal variability in the record. The two components exhibit rather similar spatial signatures in the global SST, SLP, and wind stress fields. The SST signature in the residual variability is less equatorially confined in the eastern Pacific and it is relatively more prominent over the extratropical North Pacific. The corresponding SLP signature is also stronger over the extratropical North Pacific, and its counterpart in the cold season 500-mb height field more closely resembles the PNA pattern. The amplitude time series of the ENSO-like pattern in the residual variability reflects the above-mentioned shift in 1976‐77, as well as a number of other prominent features, including a shift of opposite polarity during the 1940s.

/pdf/enso-like-interdecadal-variability-1900-93-1od1ppsaks.pdf

ENSO-like Interdecadal Variability: 1900–93

Large changes in the wintertime atmospheric circulation have occurred over the past two decades over the ocean basins of the Northern Hemisphere, and these changes have had a profound effect on regional distributions of surface temperature and precipitation. The changes over the North Pacific have been well documented and have contributed to increases in temperatures across Alaska and much of western North America and to decreases in sea surface temperatures over the central North Pacific. The variations over the North Atlantic are related to changes in the North Atlantic Oscillation (NAO). Over the past 130 years, the NAO has exhibited considerable variability at quasi-biennial and quasi-decadal time scales, and the latter have become especially pronounced the second half of this century. Since 1980, the NAO has tended to remain in one extreme phase and has accounted for a substantial part of the observed wintertime surface warming over Europe and downstream over Eurasia and cooling in the northwest Atlantic. Anomalies in precipitation, including dry wintertime conditions over southern Europe and the Mediterranean and wetter-than-normal conditions over northern Europe and Scandinavia since 1980, are also linked to the behavior of the NAO. Changes in the monthly mean flow over the Atlantic are accompanied by a northward shift in the storm tracks and associated synoptic eddy activity, and these changes help to reinforce and maintain the anomalous mean circulation in the upper troposphere. It is important that studies of trends in local climate records, such as those from high elevation sites, recognize the presence of strong regional patterns of change associated with phenomena like the NAO.

Decadal Variations in Climate Associated with the North Atlantic Oscillation

Early instrumental pressure measurements from Gibraltar and the Reykjavik area of Iceland have been used to extend to 1821 the homogeneous pressure series at the two locations In winter the two sites are located close to the centres of action that comprise the North Atlantic Oscillation (NAO). The extended 'winter half-year' record of the NAO enables recent changes in the record to be placed in the context of the period 1823-1996 The period since the early 1970s is the most prolonged positive phase of the oscillation and the late 1980s and early 1990s is the period with the highest values (strongest westerlies) The winter of 1995-1996 marked a dramatic switch in the index, with the change from 1994-1995 being the greatest change recorded from one year to the next since the series began in 1923. (The extended Gibraltar and Reykjavik monthly pressures and the NAO series can be found on the Climatic Research Unit home page. www.cru.uea-ac.uk.

Extension to the North Atlantic oscillation using early instrumental pressure observations from Gibraltar and south-west Iceland

The North Atlantic Oscillation (NAO) is one of the most prominent and recurrent patterns of atmospheric circulation variability. It dictates climate variability from the eastern seaboard of the United States to Siberia and from the Arctic to the subtropical Atlantic, especially during boreal winter, so variations in the NAO are important to society and for the environment. Understanding the processes that govern this variability is, therefore, of high priority, especially in the context of global climate change. This review, aimed at a scientifically diverse audience, provides general background material for the other chapters in the monograph, and it synthesizes some of their central points. It begins with a description of the spatial structure of climate and climate variability, including how the NAO relates to other prominent patterns of atmospheric circulation variability. There is no unique way to define the spatial structure of the NAO, or thus its temporal evolution, but several common approaches are illustrated. The relationship between the NAO and variations in surface temperature, storms and precipitation, and thus the economy, as well as the ocean and ecosystem responses to NAO variability, are described. Although the NAO is a mode of variability internal to the atmosphere, indices of it exhibit decadal variability and trends. That not all of its variability can be attributed to intraseasonal stochastic atmospheric processes points to a role for external forcings and, perhaps, a small but useful amount of predictability. The surface, stratospheric and anthropogenic processes that may influence the phase and amplitude of the NAO are reviewed.

/pdf/an-overview-of-the-north-atlantic-oscillation-3uug0y0py4.pdf

An overview of the North Atlantic Oscillation

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...

/pdf/classification-seasonality-and-persistence-of-low-frequency-49ti4r0jg4.pdf

Classification, seasonality and persistence of low-frequency atmospheric circulation patterns

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...

Tropical-Extratropical Geopotential Height Teleconnections during the Northern Hemisphere Winter

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...

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

The proposal of Harnack et al. (1986) that monthly 700-mb height anomalies can be predicted using simulated medium-range numerical forecasts is tested with actual numerical forecasts produced under strict operational conditions. These conditions included nonavailability of long model forecast histories from which to develop estimates of drift, frequent model changes over the period of the experiment, a requirement that each monthly forecast be made four days before the beginning of the month, and the use of only data which are available on the day of the forecast. The overall results and month-by-month results confirm the hypothesis of the Harnack group.

The Relative Utility of Persistence and Medium-Range Dynamical Forecasts of Monthly Mean 700 mb Heights

This paper examines four statistical procedures for correcting the NMC medium-range forecast (MRF) errors in the 700-mb heights over the Northern Hemisphere. The tests were designed to include normal operational constraints, such as frequent changes in the forecast model and the lack of long-term forecast histories on which to base estimates of systematic errors and climate drifts. It was found that the two statistical procedures which provided a substantial reduction in mean square error were a procedure based on simple regression correction at each grid point and one based on lagged-average correction with one day lag at each grid point.

Robert E. Livezey

Papers

Classification, seasonality and persistence of low-frequency atmospheric circulation patterns

Tropical-Extratropical Geopotential Height Teleconnections during the Northern Hemisphere Winter

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

The Relative Utility of Persistence and Medium-Range Dynamical Forecasts of Monthly Mean 700 mb Heights

Statistical corrections to the NMC medium range 700 mb height forecasts