Roland A. Madden

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.

Intraseasonal oscillations in 15 atmospheric general circulation models: results from an AMIP diagnostic subproject

The ability of 15 atmospheric general circulation models (AGCM) to simulate the tropical intraseasonal oscillation has been studied as part of the Atmospheric Model Intercomparison Project (AMIP). Time series of the daily upper tropospheric velocity poential and zonal wind, averaged over the equatorial belt, were provided from each AGCM simulation. These data were analyzed using a variety of techniques such as time filtering and space-time spectral analysis to identify eastward and westward moving waves. The results have been compared with an identical assessment of the European Centre for Medium-range Weather Forecasts (ECMWF) analyses for the period 1982–1991. The models display a wide range of skill in simulating the intraseasonal oscillation. Most models show evidence of an eastward propagating anomaly in the velocity potential field, although in some models there is a greater tendency for a standing oscillation, and in one or two the field is rather chaotic with no preferred direction of propagation. Where a model has a clear eastward propagating signal, typical periodicities seem quite reasonable although there is a tendency for the models to simulate shorter periods than in the ECMWF analyses, where it is near 50 days. The results of the space-time spectral analysis have shown that no model has captured the dominance of the intraseasonal oscillation found in the analyses. Several models have peaks at intraseasonal time scales, but nearly all have relatively more power at higher frequencies (< 30 days) than the analyses. Most models underestimate the strength of the intraseasonal variability. The observed intraseasonal oscillation shows a marked seasonality in its occurrence with greatest activity during northern winter and spring. Most models failed to capture this seasonality. The interannual variability in the activity of the intraseasonal oscillation has also been assessed, although the AMIP decade is too short to provide any conclusive results. There is a suggestion that the observed oscillation was suppressed during the strong El Niño of 1982/83, and this relationship has also been reproduced by some models. The relationship between a models intraseasonal activity, its seasonal cycle and characteristics of its basic climate has been examined. It is clear that those models with weak intraseasonal activity tend also to have a weak seasonal cycle. It is becoming increasingly evident that an accurate description of the basic climate may be a prerequisite for producing a realistic intraseasonal oscillation. In particular, models with the most realistic intraseasonal oscillations appear to have precipitation distributions which are better correlated with warm sea surface temperatures. These models predominantly employ convective parameterizations which are closed on buoyancy rather than moisture convergence.

The Southern Oscillation. Part I: Global Associations with Pressure and Temperature in Northern Winter

Abstract We describe the global correlations between a measure of the Southern Oscillation and sea level pressure and surface air temperature in the northern winter. The stability of these correlations were tested on the Northern Hemisphere for an 80-year period, and it turned out that most stable correlation coefficients were found over India, the North Pacific Ocean, the Rocky Mountains, and the central and western North Atlantic Ocean. On the Southern Hemisphere most records are too short for a similar test, but the following may tentatively be said about the Southern Oscillation in middle and high southern latitudes: when pressure is low in lower latitudes over the South Pacific Ocean it tends to be high at higher latitudes of that ocean, high over East Antarctica and low in the belt of westerlies in the Indian and South Atlantic oceans. In the zonal average on both hemispheres the pressure gradients in this extreme of the oscillation tend to be steeper at lower latitudes and flatter at higher latitud...

Estimates of the Natural Variability of Time-Averaged Sea-Level Pressure

Abstract Estimates of the natural variability of monthly-mean sea-level pressure are made based on a 74-year, grid-point data set. The natural variability of monthly means is defined as those interannual fluctuations that can be ascribed to the effects of statistical sampling alone. That is, the natural variability of monthly means is that variability resulting from the variance and autocorrelation associated with daily weather fluctuations. The natural variability does not reflect a “climate change,” but rather it is the variability within an “unchanging climate.” As such it is a measure of the “climatic noise.” Comparisons between natural and actual interannual variability are discussed in the context of potential long-range predictability. A characteristic time between independent estimates is determined.

Seasonal Variations of the 40-50 Day Oscillation in the Tropics

Abstract Daily rawinsonde data from 19 near-equatorial stations are examined to learn more about annual variations of the 40–50 day oscillations. Lengths of the available time series range from 5 to 28 years. A technique is devised to isolate spectral and cross-spectral quantities as a function of season. It is determined that a variance of the zonal wind in a relatively broad band centered on 47-day periods generally exceeds that in adjacent lower and higher frequency bands by the largest amount during December January and February (DJF) and at stations in the Indian and western Pacific Oceans during all seasons. The coherence between lower-and upper-tropospheric zonal winds tends to be largest in the summer hemisphere for stations in the Indian and western Pacific Oceans. Upper tropospheric zonal and meridional winds are coherent and out of (in) phase at several stations there during DJF [June, July and August (JJA) These results. coupled with composited wind and outgoing longwave radiation data, lead u...

Detecting Climate Change due to Increasing Carbon Dioxide

The observed interannual variability of temperature at 60�N has been investigated. The results indicate that the surface warming due to increased carbon dioxide which is predicted by three-dimensional climate models should be detectable now. It is not, possibly because the predicted warming is being delayed more than a decade by ocean thermal inertia, or because there is a compensating cooling due to other factors. Further consideration of the uncertainties in model predictions and of the likely delays introduced by ocean thermal inertia extends the range of time for the detection of warming, if it occurs, to the year 2000. The effects of increasing carbon dioxide should be looked for in several variables simultaneously in order to minimize the ambiguities that could result from unrecognized compensating cooling.

Further Evidence of Global-Scale 5-Day Pressure Waves

Abstract Cross spectra between long time series (5–10 years) of pressure data from 27 stations and that from Canton Island show peaks in the coherence squares near 5-day periods. Phase angles indicate that these peaks are manifestations of a westward propagating, zonal wavenumber 1 disturbance. Sea-level pressures and 500-mb heights recorded during the IGY are filtered in time and harmonically analyzed in space revealing the westward propagating 5-day waves. A composite wave based on the IGY sea-level pressure data suggests that the amplitude of the disturbance increases with increasing latitude. The observed characteristics of the 5-day pressure wave are shown to he not inconsistent with those of a planetary wave, or wave mode of the second class, theoretically predicted by Laplaces tidal equations.

The Correlation between Temperature and Precipitation in the United States and Europe

Abstract The correlation between seasonal mean temperatures and precipitation totals is computed at some 98 North American and European stations. Negative correlation is most frequent in summers, while negative and positive correlation appear about equally in other seasons. Normalized cospectra show that these correlation do not, in general, reflect a relationship common to a single time scale but rather one that is prevalent at all time scales.

Further Evidence of Traveling Planetary Waves

Abstract Evidence of regularly propagating, large-scale waves is found in a 73-year record of Northern Hemisphere sea-level pressure data and in a nine-year record of upper air data. Cross-spectrum analyses indicate that south of 50°N, in all seasons, a zonal wavenumber 1 disturbance moves westward around the world in 5 days. In addition, north of 50°N a zonal wavenumber 1 disturbance moves westward around the world in one to three weeks with an average period near 16 days. This disturbance appears to be strongest in winter and spring. The structure of the 16-day wave during winter is studied in detail, and it is shown to be consistent, in many respects, with that of a theoretically predicted free planetary wave, or wave of the second class. A similar conclusion can be made concerning the 5-day wave.