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John W. Kidson

Abstract

An index cycle occurring in the Northern Hemisphere during the late winter and spring of 1979 has been examined in detail using the ECMWF Level IIIb data set.

The cycle has an average period of around 25 days and is most prominent over the Atlantic sector at middle and high latitudes.

It is seen as an alternation between zonal flow with weak poleward heat transport and a blocking phase where the increased meridional flow strengthens the poleward transport at midlatitudes. The negative correlation between the eddy heat flux and the meridional temperature gradient suggests that the cycle may play some part in the short term regulation of the temperature gradient between 52 and 70°N.

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John W. Kidson

Abstract

Analysis of the zonal wind variations during the Southern Hemisphere winter of 1979 has shown the existence of an irregular cycle with an average period of 27 days. Opposing variations in the 500 hPa zonal wind are found in bands centered on 48.75° and 67.5°S and an index based on their difference has been used to depict the changes in circulation patterns.

At higher latitudes the changes at the 500 hPa level and at mean sea level resemble the changes in circulation between the mean winter and equinoctial seasons. In particular the wavenumber 3 which is closely associated with the semiannual oscillation duplicates its movement between winter and the equinoctial seasons. During low index situations blocking is favored near and to the east of New Zealand while in high index situations the South Pacific High is well developed.

A preliminary comparison of the zonal index variations with those in other years suggests that they were larger and more regular than usual during the winter of 1979.

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John W. Kidson

Abstract

The time and spatial variation of the Southern Hemisphere 500 hPa zonal wind has been examined using spectral analysis, bandpass filtering, and empirical orthogonal function (EOF) analysis applied to a 15-year sample of Australian hemispheric analyses.

Spectral analysis of the entire 15-year record of the 500 hPa zonal mean westerly wind shows significant peaks only on the seasonal and interannual time scales. The mean segmented spectra for each season failed to show any additional significant peaks at higher frequencies or any significant differences between seasons.

The primary pattern of variation revealed by EOF analysis shows similar behavior on synoptic to seasonal time scales, with compensating departures occurring at intervals of 20° latitude. The first EOF has opposing peaks near 40° and 60°S and shows an equivalent barotropic pattern when correlated with individual grid point values at 500 hPa and mean sea level pressure. Correlations of the zonal pattern with regional anomalies reveal that at midlatitudes, the activity is concentrated in the Indian and western Pacific Oceans while the pattern is more zonally symmetric south of 60°S. The second EOF has opposing peaks at 30° and 50°S with differing correlation patterns at 500 hPa and mean sea level.

The variance of the daily 500 hPa zonal wind anomalies is greatest for three “seasons” centered on April, August and December. Small but significant differences are observed in the corresponding hemispheric patterns for each season, and the July–September pattern has some feature related to those obtained in an earlier study for the Southern Hemisphere winter of 1979.

Indices obtained from differencing the zonal wind anomalies at 40° and 60°S and at 30° and 50°S should be useful in characterizing variations in the Southern Hemisphere zonal circulation.

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John W. Kidson

Abstract

Intraseasonal variations in the Southern Hemisphere atmospheric circulation have been examined through the application of a 10–50–day bandpass filter to daily ECMWF analyses for 1980–88.

Variations on this time scale contribute more than 40% of the daily variance in 500 hPa geopotential over much of the middle and high latitudes of the Southern Hemisphere. EOF analysis of the unnormalized variance for all seasons shows that 49% of this variance can be explained by zonal wave trains centered on the South Pacific and southern Atlantic/Indian oceans, a high-latitude mode of global extent, and a wavenumber 3 pattern at midlatitudes. These modes are essentially equivalent barotropic but slope westward with height so that the patterns at 100 hPa typically lag those at 1000 hPa by around 1o° longitude. There appears to be little interaction with the low-latitude circulation.

All of the leading modes propagate eastward but the most consistent movement is shown by the South Pacific wave train represented by EOFs 1 and 2. This wavenumber 4 pattern moves eastward at 4°–7° longitude per day near 55°S. SpectM analysis indicates the main contribution to the variance of the intraseasonal modes comes from specc2 bands peaking at 13–14 and 22–24 days.

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John W. Kidson

Abstract

Eigenvector or principal component analysis has been carried out on monthly means of surface pressure, temperature, and rainfall defined on grids extending over both hemispheres and the tropical belt. In each case, 10 out of a possible 120 components were sufficient to account for more than 80% of the observed variance regardless of whether the annual cycle was first removed. The major pressure components can be interpreted in terms of meridional and land-sea temperature gradients, and reflect a basic difference in the seasonal cycle in the Northern and Southern Hemispheres. While both pressure and temperature show coherent departures on a hemispheric scale, rainfall components are regional in character, and the variance reduction is more evenly distributed over the major components. The use of principal components as climatological indices merits greater attention than has so far been given.

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John W. Kidson

Abstract

Analysis of the 300-hPa streamfunction from the 40-year NCEP–NCAR dataset has identified the principal modes of Southern Hemisphere variability on intraseasonal (IS), interannual (IA), and intradecadal (ID) timescales. The variance of streamfunction departures from the annual cycle is zonally symmetric in the IS and IA range with the largest values at midlatitudes. The ID variance is concentrated in the Pacific sector, where it extends to lower latitudes.

For the IS band, obtained by applying a 10–50-day bandpass filter to the twice-daily streamfunction fields, three pairs of EOF patterns were obtained. These show eastward-propagating wavenumber-4 and -5 patterns largely confined to middle and higher latitudes, and two interleaved wavenumber-4 patterns expressing intensification of the zonal wind near 30°S. The wavenumber-5 patterns are more prominent in summer (December–February).

On the interannual timescale, including all periods beyond 50 days, the leading EOF with 26% of the variance expresses fluctuations in the strength of the subtropical jet. EOFs 2 and 3 contain mixed representations of the high-latitude mode and an ENSO (El Niño–Southern Oscillation) related pattern largely confined to the Pacific. EOFs 4 and 5 depict a wave train extending from Australia across the South Pacific to the east of South America. These appear to be stable throughout the period of analysis and contribute around 52% of the >50-day variance.

Variations on ID timescales were isolated by forming 11-month running means of the streamfunction anomalies, and five leading EOFs were found to be significant. The first of these, with 37% of the variance, captured variations forced by ENSO that extend from the equator to higher latitudes in the South Pacific. The second EOF with 18% of the variance accounted for changes in the circulation occurring in the early part of the record, and its amplitude since 1970 has been small. The third was identifiable as the high-latitude mode.

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John W. Kidson

Abstract

An analysis has been made of variations in the atmospheric circulation with periods of more than 50 days occurring in the Southern Hemisphere between 1980 and 1986, using analyses produced by the European Centre for Medium Range Weather Forecasts (ECMWF), mainly at the 500 hPa level. A smoothed mean annual cycle was removed from the data to define anomalies which were then filtered to retain variations with periods greater than 50 days. The >50 day period variations account for 20%–30% of the variance of daily 500 hPa geopotential anomalies at midlatitudes, up to 50% over Antarctica and more than 70% in the tropics. The total variance was found to be more uniform at high latitudes over the southern oceans than reported in previous studies, possibly reflecting the more comprehensive dataset used in the ECMWF analyses.

The first three empirical orthogonal functions (EOFs) of the geopotential height anomalies amounted for 40% of the low-pass filtered fields. The first is a zonal field with the sign of the anomalies reversing near 60°S and a three-wave pattern superimposed at midlatitudes. The second and third have nearly equal eigenvalues and resemble two interleaved wave trains at middle and high latitudes. The first EOF is seen to be related to barotropic variations in the zonally averaged westerly wind which are preceded a few days earlier by changes in the poleward eddy momentum flux. The concomitant changes in energy integrals and conversions averaged over the hemisphere are small. While time series of the EOF coefficients show persistent anomalies of up to 4 months duration, the correlations between the series are small and add nothing to the predictability resulting from persistence over the 3 week period in which their autocorrelations remain positive.

Plots of both the trajectories of the daily analyses and the distribution of persistent events in phase space, projected on to the planes of the principal axes of variation, showed little or no indication of concentration about multiple equilibria. Cluster analysis also failed to compress the original distribution to a small number of clusters unless some totally uncorrelated states were grouped together.

Taken overall, these results are not encouraging to those seeking to develop long-range forecasting schemes for the Southern Hemisphere. Even with synoptic scale variations removed, the resulting anomaly patterns show great variety and little evidence for preferred paths between them. Apparently, on this time scale at least, the atmosphere does not conform to the predictions of nonlinear theory or to the behavior of simple nonlinear models.

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John W. Kidson

Abstract

Principal component analysis of tropical surface data over the period 1951–1960 suggests the Southern Oscillation can be represented by the first components of pressure, temperature, and rainfall. The patterns obtained are generally consistent with previous definitions of the oscillation but show another center of action over northeast Brazil in phase with the Indian Ocean. This center is associated with a modulated Walker cell over Brazil and the Atlantic which appears to interact with the sea surface in a similar way to that which Bjerknes (1969) has described for the Pacific.

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John W. Kidson and Kevin E. Trenberth

Abstract

To assess the impact of missing data on general circulation statistics, an analysis has been carried out on the effect of systematically decreasing the amount of available data. Thus the accuracy of monthly mean analyses of a variety of parameters including means, variances, and covariances is determined from ECMWF data as a function of the number of twice-daily analyses included in the monthly mean. Because the standard error of a monthly mean depends on the standard deviation of the daily values and the effective number of independent observations during each month, results have been expressed as the ratio of the root-mean-square error (RMSE) in the monthly mean to the daily standard deviation, thus allowing fairly universal relationships to be developed for application to many parameters, and to different season latitudes and longitudes. Results are indeed dependent upon the numbers of observations and the autocorrelation within each series, but could be modeled sufficiently well, for this purpose, with a first order autoregressive (Markov) process to allow simulated data to be used for further tests. Experiments have been carried out varying the number of observations missing that were 1) evenly spaced, 2) randomly spaced, or 3) occurring in single blocks. If missing observations am randomly spaced, the RMSE increases by factors of 2–3 over equally spaced data and there is virtually no advantage due to autocorrelation in the data. If the missing data occur in one block, another increase in RMSE occurs by a factor up to 2.

The zonal mean daily standard deviations for the horizontal wind components, vertical p-velocity ω geopotential height, and temperature are presented for January. Tables are given of the ratio of the RMSE of the monthly mean to the daily standard deviation, and along with the autocorrelation, these allow estimates of either the relative or absolute errors to be expected in monthly mean statistics. For more persistent variables, fewer observations per month are needed to accurately define a monthly mean. Consequently, for variables such as ω, in which the persistence in the series is weak up to twice as many observations are needed to bring about the same RMSE ratio as for other linear variables. The RMSE ratios for variances and covariances are only slightly greater than those obtained for highly correlated linear variables such as the wind components u and v.

As an example, for the zonal wind component at 300 hPa, 11 observations equally distributed in time would produce a RMSE in the monthly mean wind of ∼2.4 m s−1. If the same number of observations were randomly distributed, the RMSE increases to 4.1 m s−1; or if 20% of the observations were missing in one block, the RMSE would be 2.4 m s−1. For the monthly mean poleward momentum flux by the transient eddies the expected RMSE would be up to 27 m2 s−2 for 11 randomly distributed observations.

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John W. Kidson and Ian G. Watterson

Abstract

The CSIRO9 general circulation model shows a zonally symmetric mode of variability, which closely resembles the high-latitude mode (HLM) in middle and high latitudes of the Southern Hemisphere. The leading EOF of the zonal mean zonal wind between 30° and 68°S, whose amplitude has been taken as an index of the HLM, shows opposing variations centered near 40° and 60°S accounting for 43% of the daily variance. Analysis has concentrated on composites for periods when the index changed quickly between significant peaks of the opposite sign or persisted with a large amplitude for an extended period. The momentum flux variations are small at the northern and southern boundaries and the principal variations are centered near 49°S between the maxima in the zonal wind. The changes in angular momentum content are around 30% smaller in the southern band. Eddy heat fluxes are less coherent but help in maintaining the zonal wind anomalies against friction.

A simple model of the zonal wind index with stochastic forcing and linear damping reproduces its short period variations well but is less successful in simulating the observed continuity over 10- to 20-day lags.

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