Abstract

The Eliassen-Palm (E-P) flux, applied to zonal men flows, is an indicator of both the flux of eddy activity and the eddy forcing of the zonal mean flow. For time mean flows, a localized E-P flux is derived and used diagnostically to assess the impact of transient eddies on a major blocking episode that occurred over the South Pacific during the Southern Hemisphere winter of 1979. In contrast to previous studies that have focused on the mean quasi-geostrophic potential vorticity equation, the focus here is on the mean momentum equations. Eddy transports and the associated induced meridional circulation and other internal adjustments necessary to maintain the thermal wind balance, are gathered together allowing the residual circulation and the effects of the eddies to be determined. The time-mean equations of motion are thus transformed to consist of mean terms, the residual circulation and the divergence of a localized E-P flux vector. The latter is a measure of the eddy forcing of the mean flow, and the east-west component is shown to be related to the flux of wave activity. For the zonal mean case it is identical to the E-P flux. The local E-P flux is closely related to, but differs from, the E-vector of Hoskins et al. and Plumb's radiative wave activity flux, but has several advantages over both.

For the blocking episode, defined as 20 July-31 August 1979, transient eddies were steered around the location of the blocking anticyclones following the two branches of the split westerly jet. However, the transient eddies in each branch differed in character, both from each other and from those in the main Southern Hemisphere storm track that extends across the southern Indian Ocean near 50°S. In the latter, the high frequency synoptic-scale baroclinic eddies are barotropically damped. The eddies have similar character to the south of the block but consist mainly of zonal wavenumbers 3 and 4 with periods shorter than a week. In contrast, the transient eddies in the subtropical branch of the jet are higher wavenumber (mostly waves 5 and 6) with periods longer than a week and, although primarily baroclinic, they are also maintained by barotropic processes. Most transient wave energy propagates eastward and wave packets can be followed around the entire hemisphere, mostly following the split westerly jet, with a period of about six days.

The local E-P flux divergence is divided into barotropic and baroclinic components. The former is coherent in the vertical but strongest at 300 mb near the tropopause. The transient eddies barotropically accelerate the westerlies in the main storm track and branch south of the block, and this is partially balanced by the baroclinic component. Thus a large part of the momentum balance is between transient eddy momentum convergence and the Coriolis torque arising from the poleward heat transport induced Ferrel cell, in combination with Surface friction.

Where the main westerly jet splits as part of the blocking flow configuration, both the barotropic and baroclinic local E-P flux components are acting to decelerate the westerlies and thus the transient eddies are helping to maintain the blocking episode. The main differences between the storm track and blocking regions arise in the barotropic component of the local E-P flux. It appears that the configuration of the split westerly jet acts to systematically deform the transient eddies in such a way that they feed back to help maintain the split structure.

The concept of dividing the year into four seasons is reexamined to appraise critically the relative merit of two commonly used definitions of the seasons: 1) the astronomical definition; and 2) the meteorological breakdown into four three-month periods. These are compared with the definition of winter as the coldest season, summer as the warmest season, and spring and autumn as the transition seasons. Observational data on surface temperatures over the entire globe and, in particular, over the United States, are used to determine what the seasons should be. Presented here is an analysis of the amplitude, and phase of and percentage variance explained by the first harmonic of solar radiation at the top of the atmosphere and surface temperatures.

Annual changes in surface temperature associated with the seasons are much larger over land than over the oceans. Surface temperatures lag the solar cycle by 27½ days over the United States, compared with 32½ days in mid-latitudes over the Northern Hemisphere as a whole, and 44 days in mid-latitudes of the Southern Hemisphere.

The astronomical definition of seasons is appropriate only over the oceanic regions of the Southern Hemisphere. Over the continental regions of the Northern Hemisphere, the “meteorological” seasons in which winter is December, January, and February, etc., agree reasonably well with observed events and are recommended for general usage.

Abstract

The extent to which mass is conserved in European Centre for Medium Range Weather Forecasts (ECMWF) analyses archived on pressure surfaces is examined from two perspectives and with two different datasets. The data used come from the WMO archive of 7-level, twice daily initialized analyses and from the WCRP archive of 14-level, four-times daily uninitialized analyses. The first perspective, which considers the equation of continuity in pressure coordinates locally in three dimensions, reveals spurious residuals in the equation of up to 100% of the size of the divergence term, with largest errors of 60% to 100% in the tropics. In this case the horizontal velocity and vertical p-velodty (ω) fields are checked for consistency. Modest improvements occur when 14 versus 7 levels are used. The second perspective considers the vertical integral in which the surface pressure tendency should balance the total mass convergence into a column, and thus does not involve the omega fields. The latter reveals that the dominant residual is due to the failure to resolve the semidiurnal tide with only twice-daily data, so that a large-scale wavenumber 2 pattern with maxima in the tropics results as a residual. However, there are also many smaller scale and smaller amplitude features in the residual that cannot be explained simply. The residuals from the uninitialized analyses are much larger so that while four-times daily data removes much of the wavenumber 2 semidiurnal tide structure, the total residuals are just as large as with the twice-daily initialized data. Improvements in using 14 levels versus 7 are marginal for the vertical integral. The moisture budget, which is an important part of the overall mass budget, produces plausible estimates of vertically integrated evaporation minus precipitation, but needs to be validated.

The primary source of the mass imbalance arises from the methods of postprocessing the variables onto pressure surfaces. Because sophisticated four-dimensional data assimilation now performs analyses of fields on model (sigma) surfaces, the postprocessing performs interpolations (using splines under tension) from model coordinates to pressure coordinates, with the result that the archived variables are really representative of individual levels rather than finite-sized layers. The implications are that residuals computed in other budgets, such as diabatic heating from the thermodynamic equation, will also include large errors, most of which will, however, be systematic.

The effects of several common approximations in the treatment of the lower boundary on mass conservation are examined, and proper accounting for the full variations in both space and time of surface pressure on vertical mass integrals lead to the computation of kinematic orographic forcing from the analyses as a residual. The diagnosed lower boundary-induced vertical motions are weak and do not reveal the strong dipole patterns normally expected, which may indicate that the planetary waves are not adequately forced in the forecast model used in the four-dimensional data assimilation.

Abstract

An analysis has been made of the interseasonal and interannual variability of mean circulation and eddy statistics for both summer and winter in the Southern Hemisphere. Total variance fields of geopotential height, the noith-wuth and east-west wind components and poleward transient eddy momentum fluxes at 500 mb are analyzed along with their contributions from two broad frequency bands covering 2-8 day and 8-64 day period fluctuations. Largest interannual variability occurs between 40-60°S in association with the main jet stream in summer or the polar jet stream in winter and the main belt of eddy activity within each season.

The circulation and eddy statistics during the year of the Global Weather Experiment (GWE) are compared with the means and standard deviations over all years from 1972–80, and contrasted with individual years The GWE summer of 1978–79 is contrasted with 1976–77, and the 1979 winter is contrasted with 1980. The year of the GWE was charactrized by an exceptionally deep circumpolar trough, an increase in westerlies between 45–70°S and a decrease to the north, with a southward shift in the main westerly jet during summer 1978–79 and a considerably enhanced and southward shifted polar jet but weaker subtropical jet in winter 1979. Associated with these changes was a southward shift in storm tracks and high frequency eddy activity throughout the year. In both seasons anomalous convergence of momentum by the eddies into the jets was such that it would have helped sustain the abnormal distribution of westerlies against surface friction.

Many of the anomalies in the circulation statistics during the GWE are statistically significant. most notably in winter, and their reality is supported by station data and the dynamical consistency of the relationships between the anomalous mean flow and storm tracks. In addition, the deficit of mass over the Southern Hemisphere revealed by sea level pressures in April-July 1979 is compensated by the surfeit that occurred in the North Hemisphere. Although the vastly improved observations during the GWE may have contributed to the size of the anomaly, they cannot account for the systematic change in location of the features of the flow. The circulation during the GWE appears to have been at one extreme of the large natural interannual variability that is so much a feature of the Southern Hemisphere flow. The atypical nature of the circulation should be borne in mind in analyses based solely on the GWE over the Southern Hemisphere.

Abstract

An approximate formulation of how much moisture that precipitates out comes from local evaporation versus horizontal transport, referred to as “recycling,” has allowed new estimates of recycling to be mapped globally as a function of length scale. The recycling is formulated in terms of the “intensity of the hydrological cycle” I, which is alternatively referred to as a “precipitation efficiency” as it denotes the fraction of moisture flowing through a region that is precipitated out, and a “moistening efficiency,” M, which is defined as the fraction of moisture evaporated from a region to that flowing through. While datasets of the pertinent quantities have improved, they still contain uncertainties. Results show that often the intensity is not greatest at times of greatest precipitation because moisture transport into the region is also a maximum, especially in the monsoonal regions. The annual cycle variations of I are fairly small over North America and Europe while large seasonal variations in M occur in most places. Seasonal mean maps of precipitation, evaporation (E), and atmospheric moisture transport are presented and discussed along with the seasonal and annual means of derived precipitation and moisture efficiencies and the recycling fraction. The recycling results depend greatly on the scale of the domain under consideration and global maps of the recycling for seasonal and annual means are produced for 500- and 1000-km scales that therefore allow the heterogeneity of the fields across river basins to be captured. Global annual mean recycling for 500-km scales is 9.6%, consisting of 8.9% over land and 9.9% over the oceans. Even for 1000-km scales, less than 20% of the annual precipitation typically comes from evaporation within that domain. Over the Amazon, strong advection of moisture dominates the supply of atmospheric moisture over much of the river basin but local evaporation is much more prominent over the southern parts, and, for the annual cycle as a whole, about 34% of the moisture is recycled. Over the Mississippi Basin, the recycling is about 21%. The smaller number mostly reflects the smaller domain size. Relatively high annual values of recycling (>20%) occur in the subtropical highs, where E is high and the advective moisture flux is small, and in convergence zones where, again, the advective moisture flux is small. Low annual values occur over the southern oceans, the North Pacific, and the eastern equatorial Pacific, where the moisture flux is at a maximum.

Abstract

The utility of a simple index for monitoring the Southern Oscillation signal is explored in detail. Based upon sea level pressure data at the two stations Tahiti (T) and Darwin (D), an optimal index, in the sense that it combines the Southern Oscillation variance into one series is the combination [T _n + D _n ] where the subscript n denotes normalization by the overall standard deviation of each series. A direct measure of the noise due to small-scale or transient phenomena that are not a part of the large-scale coherent Southern Oscillation fluctuations is the index [T _n + D _n ]. It is recommended that this index of noise also should be monitored in order to determine the representativeness of the Southern Oscillation index.

The signal-to-noise ratio is shown to depend upon the cross correlation between Darwin and Tahiti, and can be increased by applying weighted moving average low-pass filters to the data. Monthly data exhibit a signal-to-noise ratio, defined as the ratio of the standard deviations, of 1.44 and this increases to 1.97 for seasonal data. An 11-term low-pass filter is designed that increases the signal-to-noise ratio to 2.70 without adversely reducing the variance in frequencies that are important in the Southern Oscillation. Resulting time series plots are presented.

Abstract

An analysis has been made of the means and variability of the 500 mb field in the Southern Hemisphere, with accent on the zonal means of geopotential height and westerly wind. Long-term means for May 1972-January 1978 are significantly different from previous analyses and reveal very large and significant trends in geopotential height, especially over Antarctica. Standard deviations of zonal winds and heights are larger than in the Northern Hemisphere and, in contrast to the Northern Hemisphere, are lowest in winter. Temporal variations in the zonal mean component of the flow are very pronounced and some aspects of the very anomalous flow in December 1976 are documented. Interannual variations show a remarkable quasi-biennial fluctuation in the zonal mean fields with a systematic progression of the anomalies from low to high latitudes. These are correlated with the quasi-biennial oscillation in the equatorial stratosphere.

Abstract

Numerical time integrations of a nine-layer, quasi-geostrophic, highly truncated spectral model of the atmosphere are used to study tropospheric-stratospheric interaction with particular regard to sudden stratospheric warmings. The model is global, extends to 0.05 mb (71 km) with roughly 10-km resolution in the stratosphere, and includes an annual heating cycle.

Model integrations simulating the months of December and January were made (1) without nonzonal forcing and (2) with nonzonal heating and orography included, to represent Southern Hemisphere and Northern Hemisphere winters, respectively. The presence of nonzonal heating in the winter hemisphere brought about an increase in circulation intensity and produced a stationary perturbation having a strong westward slope with height extending high into the stratosphere. This feature is similar to the Aleutian system. It was accompanied by considerably warmer temperatures in the polar night stratosphere and a weaker stratospheric westerly jet.

Sudden stratospheric warmings occurred as a result of large increases in the intensity of planetary scale waves in the troposphere, which in turn produced surges of upward propagating energy. The energetics of the warming occurred in two phases. A change from a baroclinically direct to a driven circulation occurred as the stratospheric temperature gradient reversed. This coincided with a change from enhancement to absorption of the vertical energy flux. The mechanism of the warming was similar to that described by Matsuno.

Nonlinear interactions between the progressive long wave and the nonzonal heating were primarily responsible for the tropospheric events that produced the transient upward flux of energy and thus the warmings. A seasonally coupled index cycle in the long waves was also of significance, while interactions with other waves and orographic forcing were of secondary importance in the long-wave energetics of sudden warmings.

Abstract

Nearly eight years of daily Southern Hemisphere analyses at 500 mb have been used to define the spatial dependence of the variance fields of geopotential height and the two geostrophic wind components, the corresponding covariance fields, and the transient kinetic energy. The fields are further examined in the frequency domain by using Lorenz' (1979) “poor man's spectral analysis” technique. In view of the small variation in eddy statistics as a function of the time of the year in the SH, this study removes the first four harmonies of the annual cycle and then considers all data together, so that contributions from all time scales from 2 to 4096 days (∼11 years) can be resolved. The main results are based on analyses from May 1972–January 1978 but are verified with analyses from the relatively data-rich FGGE period.

Results for the zonal mean statistics are compared with those from previous studies. The zonal means of the geopotential height and westerly wind component have spectra which roughly follow that of red noise with an autocorrelation of about 0.5, whereas the northward wind component spectra closely resembles red noise with autocorretation of 0.2, resulting in considerable anisotropy in the wind fields. The northward component of transient kinetic energy is larger than the eastward component at high frequencies in middle latitudes but the reverse is true for periods of greater than two months. The westerly momentum flux by the transient eddies has a broad spectral peak at 8–32 days and is dominated by contributions from fluctuations of less than about two weeks period.

The geographical dependence of the eddy statistics is mapped for four broad frequency bands covering periods of roughly less than one week, one week to two months, two months to two years, and greater than two years, thereby separating out contributions from transient baroclinic eddies, episodes of blocking, and intermonthly and interannual variability. The spatial patterns of the statistics are interpreted in the light of synoptic behavior of systems and storm tracks as defined by synoptic studies and satellite observations in the Southern Hemisphere. For periods less than a week, variances are largest in the southern Indian Ocean and relationships between the storm tracks and eddy statistics are similar to those found in the Northern Hemisphere by Blackmon, Lau, Wallace and others. However, there also are differences associated with the differences in the mean flow in each hemisphere and these are discussed in the context of baroclinic theory. At periods longer than a week geopotential height variances are largest near southern New Zealand and, to a lesser extent, southeast of South America and appear to be related to the incidence of blocking in the Southern Hemisphere. The corresponding transient kinetic energy has a maximum further north in association with cutoff cold-centered lows. In general, the high-frequency transient eddies play a much larger role in the circulation of the Southern Hemisphere than is true for the winter circulation of the Northern Hemisphere, and the eddy statistics are more zonally symmetric.

Abstract

A brief review and evaluation of various analyses of the Southern Hemisphere westerlies is given along with the presentation of some relent results. Several features characterize the westerlies of the Southern Hemisphere as quite different from those in the Northern Hemisphere and, in the past, thew have typically been difficult to reproduce well in general circulation modes. They are the double jet structure in winter, the stronger midlatitude tropospheric winds in summer than in winter, and the ensuing much smaller amplitude of the annual cycle which is associated with a maximum of global atmospheric angular momentum in January. New values for the hemispheric angular momentum integrals are than previously reported.

Two estimates of the distribution and strength of the southern westerlies that have been widely used are considered to be seriously biased. Factors contributing to discrepancies among different results am large natural variability, missing data and biases in observing systems, and methods of analysis. Over the sparsely observed Southern Hemisphere, the latter is the main reason why biases exist in analyses based only on mean station data, and the absence of imposed dynamical constraints has led to internally inconsistent fields. Even recent estimates of the southern westerlies from global operational analyses should be used judiciously with proper consideration given to reliability and biases.