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Kevin E. Trenberth

A review is given of the meaning of the term “El Niño” and how it has changed in time, so there is no universal single definition. This needs to be recognized for scientific uses, and precision can only be achieved if the particular definition is identified in each use to reduce the possibility of misunderstanding. For quantitative purposes, possible definitions are explored that match the El Niños identified historically after 1950, and it is suggested that an El Niño can be said to occur if 5-month running means of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N–5°S, 120°–170°W) exceed 0.4°C for 6 months or more. With this definition, El Niños occur 31% of the time and La Niñas (with an equivalent definition) occur 23% of the time. The histogram of Niño 3.4 SST anomalies reveals a bimodal character. An advantage of such a definition is that it allows the beginning, end, duration, and magnitude of each event to be quantified. Most El Niños begin in the northern spring or perhaps summer and peak from November to January in sea surface temperatures.

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Kevin E. Trenberth

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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.

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Kevin E. Trenberth

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Studies of the interannual variability of the 500 mb flow in the Southern Hemisphere for 1972–78 have revealed 1) a quasi-biennial fluctuation in the zonal mean geopotential height and westerly wind fields where there is a systematic poleward progression of anomalies, and 2) a longer period fluctuation, primarily in Wave 1, which has been identified with the Southern Oscillation. These phenomena are further studied regionally by considering four 90° sectors. Results show the dominant contributions to the fluctuations in the zonal mean fields come from the Australasian and South Pacific sectors. Only in the Australasian region is the quasi-biennial poleward progression of anomalies strongly evident. The South Pacific is dominated by lower frequency fluctuations associated with the Southern Oscillation. However, the sequences in all four sectors are correlated with those in the zonal mean fields. When the flow is stronger than normal in mid-latitudes it tends to be weaker than normal in low and high latitudes, and vice versa, in all sectors and in the zonal mean.

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Kevin E. Trenberth

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Time series of meteorological variables typically exhibit a pronounced annual cycle and persistence and samples are of finite size. This paper analyses the impact of these complicating features on certain statistics computed from the time series. The presence of an annual cycle means that statistics are nonstationary unless computed from multiyear samples of limited duration. Persistence leads to lack of independence of observations. Large amplitude weather (high frequency) events induce natural variability at low frequencies, known as climatic noise, that is enhanced by the presence of persistence. This natural variability should be, taken into account when estimating population statistics from a finite sample, but generally this has not been done in meteorology.

A number of studies in meteorology have computed statistics from daily data by 1) removing the annual cycle; 2) computing second moment statistics over each individual season; and 3) averaging the second moment statistics over all years. This procedure fails to take into account the natural interannual variability that should be present and results in biased estimates of certain statistics. In particular, autocorrelations that lag are systematically negatively biased. It is shown for first order autoregressive (AR) time series that theautocorrelations computed in this way become negative after just a few days lag. Consequently, several studies have drawn doubtful conclusions about the stochastic character of meteorological time series, and a few reported results are questionable. A brief discussion of some papers adversely affected by the methodology is given.

The effects of the statistical methodology are illustrated with simulated, and thus known, time series. It is shown that the best possible estimate of autocorrelations for stationary time series is obtained by subtraction of the mean of all available data, rather than subtracting a different mean for each subsample (season) in order to compute the anomalies. Appropriate methods for computing the statistics are discussed.

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Kevin E. Trenberth

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.

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KEVIN E. TRENBERTH

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The development of a nine-layer, quasi-geostrophic, highly truncated spectral model of the atmosphere is described. 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. Preliminary integrations without eddies reveal the seasonal variation of a thermally driven circulation.

A model integration was made to simulate the months of December and January without nonzonal forcing, thus being more representative of a Southern Hemisphere winter. The overall features of the atmosphere were well simulated. A midlatitude temperature maximum was produced in the winter mesosphere of the model, which was driven in the manner of the lower stratosphere. With the inclusion of the annual heating cycle, the model successfully reproduced a more intense circulation in January than existed in December. This caused the maximum tropospheric meridional temperature gradient in the winter hemisphere to occur weeks prior to the maximum in the external heating field. A seasonally coupled index cycle in the very long waves was of significance in producing transient upward energy propagation and, as such, may be the source of sudden stratospheric warming events.

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Kevin E. Trenberth

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An analysis of variance approach is used to estimate the potential predictability of interannual fluctuations of the seasonal mean flow over the Southern Hemisphere. The potential predictability of the 1000 and 500 mb geopotential height fields for both summer and winter is assessed. Daily variances at 500 mb are roughly double those at 1000 mb, but otherwise the patterns are quite similar in both seasons. The level of climatic noise is estimated from the daily variances and compared with the actual interannual variances under a null hypothesis that no signal exists. Results reveal that interannual or longer-term variability clearly exceeds the noise level at both levels and in both seasons over Antarctica and in the tropics No clear statement can be made one way or the other in midlatitudes. The methodology appears to fail over the Australia-New Zealand region where clear interannual signals associated with the Southern Oscillation and a quasi-biennial oscillation have previously been found. However, the noise in this region appears to include the 40–50 day oscillation which may be organized by or systematically contribute to the interannual variability. The implied correlation between signal and noise voids a basic assumption of the methodology.

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Kevin E. Trenberth

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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.

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Kevin E. Trenberth

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.

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Kevin E. Trenberth

Abstract

An analysis has been made of the persistence of geopotential heights at 1000 and 500 mb over the Southern Hemisphere for both summer and winter seasons. The focus is on lagged autocorrelation T 0 the effective time between independent samples and low order autoregressive (AR) models fit to the data.

A red noise AR(1) model provides the best fit to the data over most of the domain, especially in midlatitudes. However, there are substantial areas where AR(2) and AR(3) processes provide the best fit and these areas are much the same at both levels and for both seasons. Over Antarctica low-frequency trends appear to provide persistence exceeding that of the red noise process and contribute to the higher order AR processes there. However, over Austalasia the AR(2) processes fit to the data imply a damped quasi-periodicity and T 0 values less than for the corresponding red noise process. Such behavior may be linked to the 40–50 day oscillation of Madden and Julian.

Smallest T 0 values are found over the southern oceans and especially in the main storm track regions where developing synoptic systems and rapid advection contribute to reduce the autocorrelations and T 0 values. Largest persistence is found over Antarctica, in low latitudes and across Australasia Persistence in these regions is greatly enhanced by interannual variability. In part, such low frequency variability over Australasia and in the tropics is associated with the Southern Oscillation, and is thus linked to low frequency coupling between the atmospheric circulation and sea surface temperatures. The overall meridional profile of autocorrelations is similar to that in the Northern Hemisphere but persistence is less in the Southern Hemisphere in both seasons. Factors contributing to the differences are discussed.

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