Jeffery C. Rogers

The Seesaw in Winter Temperatures between Greenland and Northern Europe. Part I: General Description

Abstract We have investigated the well-known tendency for winter temperatures to be low over northern Europe when they are high over Greenland and the Canadian Arctic, and conversely. Well-defined pressure anomalies over most of the Northern Hemisphere are associated with this regional seesaw in temperature, and these pressure anomalies are so distributed that the pressure in the region of the Icelandic low is negatively correlated with the pressure over the North Pacific Ocean and over the area south of 50°N in the North Atlantic Ocean, Mediterranean and Middle East, but positively correlated with the pressure over the Rocky Mountains. The composite patterns of pressure anomalies in the seesaw are almost identical to the fist eigenvector in the monthly mean pressure, but the standard deviations of pressure anomalies in seesaw mouths are as large as the standard deviations of monthly means in general. Since 1840 the seesaw, as defined by temperatures in Scandinavia and Greenland, occurred in more than 40%...

The Association between the North Atlantic Oscillation and the Southern Oscillation in the Northern Hemisphere

Abstract The North Atlantic Oscillation (NAO) and Southern Oscillation (SO) are compared from the standpoint of their association with Northern Hemisphere winter mean distributions of sea-level pressure (SLP) and 500 mb height. The NAO and SO are associated with significant SLP differences over much of the hemisphere except for Siberia and western North America. Significant SLP and 500 mb height differences occur in the NAO over the Atlantic Ocean and near Baja California, while in the SO they occur over the Pacific Ocean, India and the western Atlantic. Only over the latter region do large pressure and height variations consistently occur in the extremes of both oscillations; these are also associated with winter temperature variability over the southeastern United States. For example, during winter 1982–83, when the two oscillations simultaneously reached extremes, the NAO was associated with record December warmth east of the Mississippi River, but during January and February the SO dominated the heigh...

Spatial Variability of Sea Level Pressure and 500 mb Height Anomalies over the Southern Hemisphere

Abstract The spatial variability of mean sea level pressure (SLP) and 500 mb height anomalies over the Southern Hemisphere during summer (DJF) and winter (JJA) is determined using eigenvector analysis based on daily synoptic maps from 1972 to 1979. The patterns of spatial distribution of pressure and height anomalies are further verified and examined by means of station data, and the eigenvectors are compared between the seasons and to those found for the Northern Hemisphere. The first eigenvector shows that midlatitude anomalies of SLP and 500 mb height are of an opposite sign to those found over and around Antarctica. The pattern is highly barotropic and suggests strengthening and weakening of the zonal wind in alternating latitude belts. The 500 mb height differences are calculated for five midlatitude to Antarctic station pairs using data from the late 1950s onward. These latitudinal height differences are also used to describe the association between the hemispheric westerlies and the Southern Oscil...

The Southern Oscillation. Part II: Associations with Changes in the Middle Troposphere in the Northern Winter

Abstract We have investigated the relationship between the extremes of the Southern Oscillation and the following quantities at 700 mb in winter, 1948/1949 to 1978/1979: eddy transfer of sensible heat, temperature, geopotential height and geostrophic wind. In the phase of the Southern Oscillation when pressures are high over the tropical South Indian Ocean and low over the tropical South Pacific Ocean, in contrast with the opposite pressure distribution, the zonal mean poleward flux of sensible heat in the quasistationary waves tends to be higher in middle latitudes; the temperatures and heights tend to be lower between 30 and 60°N with the maximum difference at 45°N; the geostrophic wind tends to be stronger south of 45°N and weaker to the north; and the transfer of sensible heat by the transient waves tends to be stronger south of 45°S, and weaker to the north. In this extreme of the Southern Oscillation the zonal mean geostrophic wind on both hemispheres is stronger in the subtropics and weaker at high...

Spatial Variability of Seasonal Sea Level Pressure and 500 mb Height Anomalies

Abstract The spatial variability of seasonal mean sea level pressure (SLP) and 500 mb height anomalies are determined using eigenvector analysis. Previously, spatial variability had only been analyzed in such a manner during winter months, and the time coefficients of the eigenvectors of SLP and 500 mb heights were not compared. The eigenvectors of SLP and 500 mb heights obtained here are compared during each season, and the vectors are compared between seasons. Surface and 500 mb level eigenvector coefficients are found to be correlated and they are correlated in some seasons to air temperatures at surface stations. The data used were National Meteorological Center analyses of daily SLP and 500 mb heights between 1946 and 1977. The eigenvectors are determined for the covariance matrices of seasonal mean pressure and height departures from the 31 or 32 year long term normal over a 132 grid-point network. The coefficients of the first eigenvector of 500 mb heights are most highly correlated to the coeffici...

Remarks on the Circulation over the Southern Hemisphere in FGGE and on Its Relation to the Phases of the Southern Oscillation

Abstract The circulation anomalies over the whole Southern Hemisphere in the First GARP Global Experiment (FGGE) were not those that one would expect in either extreme of the Southern Oscillation; examples of the anomalies in such extremes are given. The zonally averaged pressure gradients between 50 and 65°S in FGGE when compared with those of several other years turned out to be abnormally strong in winter (2.5σ above the mean), and moderately strong in summer (1.4σ. above the mean). The 500 mb heights were above normal in middle latitudes and below normal at high latitudes when compared with station data from series 16–29 years long. As the computations are based on operational analyses they are not final, although the conclusions are unlikely to be changed by the use of the complete FGGE data set.

ARE STRONGER NORTH-ATLANTIC SOUTHWESTERLIES THE FORCING TO THE LATE-WINTER WARMING IN EUROPE?

We examine a possible mechanism leading to late-winter warming, and thus to an early spring in Europe. From the National Centers for Environmental Prediction reanalysis, we extract for the years 1948–99 ocean-surface winds over the eastern North Atlantic, and air temperatures at the surface Ts, and at the 500 hPa level T500 in late-winter and spring. Ts is extracted at six European locations, all at 50.5 °N, ranging in longitude from 1.9 °E (northeastern France) to 26.2 °E (Ukraine). To quantify the advection of maritime air into Europe, we evaluate for three-pentad groups the index Ina of the southwesterlies at 45 °N, 20 °W; Ina is the average wind speed at this point if the direction is from the quadrant 180–270° (when the direction is different, the contribution counts as zero). In late winter, correlations Cit between Ina and Ts are substantial, up to the 0.6 level in western Europe (but weaker correlations for Poland and Ukraine). Cit drops sharply by mid-March, occasionally taking negative values subsequently. This drop in Cit indicates that maritime air advection is no longer associated closely with the surface-air warming; the role of insolation becomes important, and thus

ATMOSPHERIC TELECONNECTIONS AND CITRUS FREEZES IN THE SOUTHERN UNITED STATES

The relationship between tropospheric teleconnections and Florida citrus freezes is examined using 70 kPa heights and sea-level pressure data from 1949 to 1987. Rotated principal component analysis (RPCA) is used in determining the main patterns of five-day averaged pressure and height variability. The mean RPCA scores are obtained during five-day periods when polar high pressure cells migrate toward the southern United States. Three upper air teleconnections, including the Pacific/North American pattern, and one sea-level pressure pattern have significantly large mean scores when they are averaged over the time periods when cold outbreaks and citrus freezes occur in the South. Plots of the tracks of all strong anticyclones that move southward over North America during the extreme phases of the teleconnections provide further indication that the highs moving in conjunction with the pronounced ridge/trough configuration of the Pacific/North American pattern are especially likely to reach the southern Unite...

PROBLEMS ASSOCIATED WITH SMOOTHING AND FILTERING OF GEOPHYSICAL TIME-SERIES DATA

This study presents guidelines for the construction of simple time-series filters that facilitate the analysis of geophysical data. Emphasis is placed on underlying assumptions, filter design, and interpretative restrictions. Examples of filter design and associated problems are presented utilizing precipitation data from South America. [Key words: time-series analysis, filtering, simple moving average, band-pass filters, smoothing operation.]

THE MERIDIONAL FLUX OF EDDY SENSIBLE HEAT AT 700 MB IN THE NORTHERN HEMISPHERE WINTER

The long-term geographical distribution of the Northern Hemisphere winter, meridional eddy sensible heat flux has been studied for the period 1948–1987 using 700 mb grid-averaged temperatures and geopotential heights. The flux is calculated after the manner of van Loon (1976) and the long term average is compared to the variation over the winters of the period 1976–1987. Results indicate that while the winter mean total eddy flux possesses the same general characteristics associated with the long term climatological averages, significant changes have occurred over the regions extending from western North America to east Asia. These changes appear to reflect variations in both the transient and quasi-stationary fluxes, and are associated with the Pacific North American teleconnection pattern in its positive phase.