Paul R. Julian

Description of Global-Scale Circulation Cells in the Tropics with a 40–50 Day Period

Abstract Long time series (5–10 years) of station pressure and upper air data from stations located in the tropics are subjected to spectral and cross-spectral analysis to investigate the spatial extent of a previously detected oscillation in various variables with a period range of 40–50 days. In addition, time series of station pressure from two tropical stations for the 1890s are examined and indicate that the oscillation is a stationary feature. The cross-spectral analysis suggests that the oscillation is of global scale but restricted to the tropics: it possesses features of an eastward-moving wave whose characteristics change with time. A mean wave disturbance, constructed with data from the IGY, provides additional descriptive material on the spatial and temporal behavior of the oscillation. The manifestation in station pressure consists of anomalies which appear between 10N and 10S in the Indian Ocean region and propagate eastward to the Eastern Pacific. Zonal winds participate in the oscillation...

Observations of the 40–50-Day Tropical Oscillation—A Review

Abstract Observational aspects of the 40–50-day oscillation are reviewed. The oscillation is the result of large-scale circulation cells oriented in the equatorial plane that move eastward from at least the Indian Ocean to the central Pacific. Anomalies in zonal winds and the velocity potential in the upper troposphere often propagate the full circumference of the globe. Related, complex convective regions also show an eastward movement. There is a zonally symmetric component to the oscillation. It is manifest in changes in surface pressure and in the relative atmospheric angular momentum. The oscillation is an important factor in the timing of active and break phases of the Indian and Australian monsoons. It affects ocean waves, currents, and air-sea interaction. The oscillation was particularly active during the First GARP (Global Atmospheric Research Program) Global Experiment year, and some features that were evident during the Monsoon Experiment are described.

The Tropical Ocean‐Global Atmosphere observing system: A decade of progress

A major accomplishment of the recently completed Tropical Ocean-Global Atmosphere (TOGA) Program was the development of an ocean observing system to support seasonal-to-interannual climate studies. This paper reviews the scientific motivations for the development of that observing system, the technological advances that made it possible, and the scientific advances that resulted from the availability of a significantly expanded observational database. A primary phenomenological focus of TOGA was interannual variability of the coupled ocean-atmosphere system associated with El Nino and the Southern Oscillation (ENSO).Prior to the start of TOGA, our understanding of the physical processes responsible for the ENSO cycle was limited, our ability to monitor variability in the tropical oceans was primitive, and the capability to predict ENSO was nonexistent. TOGA therefore initiated and/or supported efforts to provide real-time measurements of the following key oceanographic variables: surface winds, sea surface temperature, subsurface temperature, sea level and ocean velocity. Specific in situ observational programs developed to provide these data sets included the Tropical Atmosphere-Ocean (TAO) array of moored buoys in the Pacific, a surface drifting buoy program, an island and coastal tide gauge network, and a volunteer observing ship network of expendable bathythermograph measurements. Complementing these in situ efforts were satellite missions which provided near-global coverage of surface winds, sea surface temperature, and sea level. These new TOGA data sets led to fundamental progress in our understanding of the physical processes responsible for ENSO and to the development of coupled ocean-atmosphere models for ENSO prediction.

A Study of the Southern Oscillation and Walker Circulation Phenomenon

Abstract A survey of the literature dating back to the early 1920’s along with some appropriate statistical studies delineate an atmospheric-oceanic phenomenon of considerable interest. The Southern Oscillation—an oscillatory exchange of atmospheric mass between the eastern south Pacific and Indonesia—and the Walker Circulation—its counterpart in wind circulation—have a time-scale of years and are manifestations of a near-global variation in circulation, clouds and precipitation, centered in the equatorial eastern Pacific. Ocean surface temperatures in this region are intimately involved; in their warmest phase these variations are known as El Nino events. Some evidence that the strength of the Northern Hemisphere subtropical jet stream varies in conjunction with this phenomenon is given. Since a fully coupled atmosphere-ocean model is presently impractical, a set of general circulation model experiments using altered ocean boundary temperatures has been performed with the NCAR 5° global atmospheric model...

A Study of Atmospheric Energetics During the January–February 1963 Stratospheric Warming

Abstract The spectral energy equations for zonal and eddy kinetic and available potential energies are used to investigate the energetics of the lowest 30 km of the atmosphere during the months of January and February 1963. A major stratospheric warming, manifested in the reversal of the stratospheric meridional temperature gradient and destruction of the polar-night stratospheric vortex, began in mid-January. Data for eight standard pressure levels, from 850 to 10 mb, for every fifth day were utilized. The horizontal wind field was estimated by using both the geostrophic approximation and a modified stream function. The vertical motion field (dp/dt) was estimated both by the adiabatic technique and by the solution of the omega equation. The variation of the energy exchanges with height (pressure) indicates that the lower and middle troposphere and the middle stratosphere were baroclinically active regions before and during the warming. The principal path of energy flow in these regions was from zonal to ...

On Some Properties of Correlation Functions Used in Optimum Interpolation Schemes

Abstract Objective analyses using the so-called method of optimum interpolation incorporates statistical information on the variable(s) by means of the covariance or correlation functions. The concern in this contribution is with some properties of the analytic forms of the correlation functions that are used to model the statistical structure. First, some attention is directed to the question of fitting the various analytic forms (containing adjustable constants) to samples of actual correlations. All but one of the candidate forms were indistinguishable on the basis of the residuals of the statistical fitting procedure. Second, the criterion of positive-definiteness of the correlation function is extended to stipulate that the transform (or spectrum) of the function should possess some features of the spectra of actual variables—the most important one being the spectral decay rate at high wavenumber. Again, all but one of the candidate forms (the same one as above) had transforms that were acceptable. T...

On the Spectral Distribution of Large-Scale Atmospheric Kinetic Energy

Abstract Current interest in the limits of deterministic prediction in the atmosphere has focused attention on the longitudinal spectrum of kinetic energy at synoptic wavelengths. A number of additions and improvements on past determinations of kinetic energy are presented. We define the kinetic energy spectrum of interest as the portion of the spectrum arising from stochastic transient motion on all scales. This spectrum is estimated from four sets of data: the first two are balanced winds derived from objective analysis routines, the third streamline-isotach analyses for the tropics and sub-tropics, and the fourth untreated observed winds. For comparison of the effect of computing kinetic energy spectra in data-rich and data-poor regions, the first data act comprises balanced winds from 360° of longitude and the second from a 180° segment including North America and Europe. Spectra computed by using observed winds allow a comparison between the distribution of actual kinetic energy and that assumed by o...

Temporal Variations of the Tropical 40-50 Day Oscillation

Abstract In recent years there has been a great deal of interest in a quasi-periodic tropical oscillation of zonal winds, which was first reported by Madden and Julian. An attempt to determine the temporal variation of the oscillation parameters is presented here. Using a 4-year duration global time series and a 25-year station time series, we find that although the nonseasonal variations are large, any seasonal cycle in the oscillation amplitude and frequency must be very small. The small seasonal signal in the oscillation frequency seems to argue against explanations for the time scale based on Doppler-shifted traveling waves.

Objective Analysis in the Tropics: A Proposed Scheme

Abstract The tropical wind field presents some unique problems to an assimilation scheme designed for extratropical latitudes. Foremost among these is the relative increased importance of the divergent component of the upper and lower troposphere. Examination of tropical wind field analyses from conventional assimilation schemes points to some unsatisfactory results. Because of these problems an algorithm is proposed which is intended to handle the rotational and divergent components separately. The divergent component is estimated by transforming satellite-observed outgoing longwave radiation to a velocity potential field. The full analysis is then obtained by statistical optimum interpolation (OI) using a guess field which is an addition of a rotational forecast field with the divergent field estimated above. The scheme is tested against a conventional (single-level) OI scheme for a number of cases selected from the FGGE year. The proposed scheme is shown to perform comparably to the conventional OI sch...

The Direct Estimation of Spatial Wavenumber Spectra of Atmospheric Variables

Abstract We attempt to determine the zonal kinetic energy and temperature spectra of the large-scale flow in the atmosphere by utilizing observations, unequally spaced, rather than objectively-analyzed grid point values of the wind and temperature variables. Sample correlation functions, are obtained by forming correlations from all possible pair-wise combinations of stations oriented along a parallel of latitude (∼52N). Formally, the estimated line spectrum is given by a Fourier transform of this sample correlation function, Here, owing to the unequal spacing of this sample function, the Fourier transform takes the form of a set of simultaneous, coupled, linear equations, which are poorly conditioned. Four different methods of effecting the solution are tried, all of which are directed at mitigating the ill-conditioning of the system of equations. Spectra of kinetic energy and temperature for the winter and summer seasons are obtained at 850, 500 and 200 mb. There is no significant difference in the shap...