Abstract

The effect of tropical latent heat release in accelerations of the Southern Hemisphere subtropical jet is discussed based on a case study for 6–8 August 1979 and general circulation model simulations. This jet is a main feature of the winter time circulation extending over Australia and the western Pacific Ocean. An intensification of this jet was observed for 8 August, with peak values of 93 m s⁻¹. Satellite pictures and daily precipitation amounts revealed that the 6–8 August period was characterized by extensive rainfall in the tropics, with precipitation diminishing from that day on. Level III-b data for the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory for these dates was projected into the normal modes of a primitive equation model linearized about a basic state at rest. The analysis shows that the jet accelerations were due both to external and internal Rossby modes, with somewhat stronger contributions from the external mode. An enhancement of the local meridional circulation is due to these extensive precipitation areas which project mostly in internal gravity modes with maximum values upstream of the jet accelerations.

Conjecture that the jet stream is reacting in this short time scale to tropical latent heat release is tested with the Goddard Laboratory for the Atmospheres fourth order general circulation model. A 15 day integration starting on 1 August 1979 is used as the control case. This run produced peak values of 82 m s⁻¹ for the zonal wind at 200 mb during August 8 centered over Australia at about 26°S in conjunction with increased divergent circulations at about 6°N about 50° longitude upstream from the jet maximum. A simulation was started on 4 August and run for ten days suppressing tropical Pacific heating from 90°E through 120°W. Normal mode contributions were also obtained for the control and “no tropical heating” experiment. The case study and GCM simulations suggest the following time scale of response of subtropical latitudes to tropical latent heat release: divergent circulations which project mostly into inertia-gravity waves react to changes in latent heat release in 1–2 days, its impact in subtropical latitudes is felt in 2–4 days; and becomes fully established after about 6 days.

Abstract

Persistent anomalies with recurrent spatial patterns play an important role in the atmosphere's low-frequency variability. We establish a connection between statistical and dynamical methods of description and prediction of persistent anomalies. This is done by computing and analyzing the empirical orthogonal functions (EOFs) in a simple deterministic model, on the one hand, and in Southern Hemisphere geopotential heights, on the other.

The dynamical model is governed by the fully nonlinear, equivalent-barotropic vorticity equation on the sphere, with simplified forcing, dissipation and topography. Model solutions exhibit persistent anomalies identifiable with blocked, zonal and wave-train anomalies in Northern Hemisphere atmospheric data. Flow structures similar to the patterns above occur as high-variance EOFs of this nonlinear model.

The Southern Hemisphere data we analyze consist in gridded daily maps of 500 mb heights from June 1972 to July 1983. Two types of persistent anomalies appear in this time series, both having a strong wavenumber-three component; they differ by the value of the constant phase of this wave and by the strength of the wavenumber-one component. The first two EOFs bear a striking resemblance to these two patterns.

We conclude that the dynamical interpretation of EOFs is their pointing from the time mean to the most populated regions of the system's phase space. Pursuing this interpretation, we introduce a Markov-chain formulation of transitions from one persistent anomaly regime to another, and discuss the implications for long-range forecasting.

Abstract

We have studied a barotropic flow over sinusoidal topography in a beta channel in the presence of forcing and dissipation. Our work can be divided into two main parts: In the first part, we have computed numerically the stationary solutions of the model, using Newton's method, together with a finite difference approximation, and we compare our solutions with the solutions of the linear theory; in the second part, we successfully develop a nonlinear theory that reproduces our numerical solutions very closely.

In the first part of the paper we show that multiple equilibria do exist in our problem; i.e., multiple equilibria are not artificially introduced by the mathematical technique of severe truncation previously used, but are a consequence of the resonance present in our physical system. Then we show that even if we are dealing with a small topography (we have considered Fourier components of the real topography that have amplitudes of a few hundred meters, small values compared with the depth of the atmosphere) the linear theory fails when the system comes close enough to resonance to produce forced waves with an amplitude of any relevance.

In the second part, we study the nonlinear interactions, using an appropriate spectral decomposition, and we have pointed out how the linear solution must be modified when the system approaches resonance. We show how to split the nonlinear interaction term into two pieces, one always negligible (for low topography) and another one crucial when the system is close to the resonance. Based on these considerations, we have developed a nonlinear theory that successfully explains the qualitative and quantitative features of our numerical calculations.

Abstract

General Circulation Model (GCM) experiments have been performed to determine mechanisms that maintained the blocking episode in the Australian-New Zealand region during the period 8–22 June 1982. A control forecast reproduces the persistent ridge. Several mechanistic experiments lead to the following conclusions. (i) The block was not due to orographic forcing, which has only a small local influence on the winter atmospheric circulation in the Southern Hemisphere. (ii) The block was not produced by the sea surface temperature anomalies (SST). By comparing the relative location of low-level atmospheric vorticity and SST anomalies, we are able to show that during June 1982 the atmospheric blocking was the cause of the SST anomalies in the Pacific. (iii) The block was not a response to tropical heating or the Asian Monsoon. There are only weak effects on the block when the tropical heating or heating in the Pacific region is suppressed. (iv) The most important boundary forcing maintaining this blocking ridge is heating associated with the land-sea contrast. The height fields are more zonally symmetric when the land-sea contrast is suppressed. The local land-sea contrast in the Australian region also contributed to maintain the stationary blocking ridge. The sensible heat release in the subantarctic region is an important mechanism that maintains the block. (v) Finally, the daily spectral energetics of the control experiment suggests that the baroclinic amplification of planetary-scale waves forced by synoptic-scale disturbances played an important role in the evolution of this blocking process.

Abstract

Studies by van Loon and Jenne, van Loon et al., Trenberth and others indicate that stationary waves in the Southern Hemisphere are dominated by planetary scales. Kalnay and Halem reported the presence of large amplitude, short-scale stationary waves during the month of January 1979 in the lee of South America and their disappearance in February 1979. In this paper we present further observational evidence of the January waves.

We also perform two 15-day forecast experiments with the GLAS Fourth-Order General Circulation Model, and initial conditions corresponding to 5 January and 4 February 1979. These factors reproduce reasonably well the presence of the January waves and their absence in February. Several mechanistic experiments to determine the origin of the waves are then performed.

The principal conclusions are

a) Large amplitude stationary Rossby waves with zonal wavenumber ≈7 were present between 20° and 40°S both in the South Pacific and east of South America during January 1979. They appear in satellite observations as enhanced bands of high clouds associated with the South Pacific Convergence Zone (SPCZ) and the Amazon. Examination of satellite observations during 1974–79 indicates a correlation between the intensity of stationary cloud bands in the two regions.

b) The stationary waves in the lee of South America are not of orographic origin since they are associated with a ridge rather than a trough east of the Andes. A “no Andes” forecast experiment confirms this argument.

c) The waves could not be produced by a CISK mechanism suggested by Kalnay and Halem, because of their rather barotropic vertical structure. Sea surface temperature (SST) anomalies in the South Atlantic were of the same scale as the waves, but stronger at the end of January. This, and strong correlation between low level atmospheric cyclonic vorticity and cold SST anomalies indicate that the atmospheric stationary waves were the cause of the ocean temperature anomalies, which in turn provided a negative feedback to the atmosphere.

d) Several experiments modifying the coefficient of latent heat lead to the conclusion that tropical heating is important in the maintenance of the waves. Furthermore, the convection in the subtropical waves themselves is important in sustaining their amplitude and phase, and the Walker type of circulation associated with the SPCZ is also a contributor to the maintenance of the South American waves. These results confirm the existence of a relationship between the occurrence of a strong South Pacific Convergence Zone, somewhat eastward from its climatological position, and the strong “South Atlantic Convergence Zone” observed in outgoing longwave radiation maps.

Abstract

Low-frequency variability of large-scale atmospheric dynamics can be represented schematically by a Markov chain of multiple flow regimes. This Markov chain contains useful information for the long-range forecaster, provided that the statistical significance of the associated transition matrix can be reliably tested. Monte Carlo simulation yields a very reliable significance test for the elements of this matrix. The results of this test agree with previously used empirical formulae when each cluster of maps identified as a distinct flow regime is sufficiently large and when they all contain a comparable number of maps. Monte Carlo simulation provides a more reliable way to test the statistical significance of transitions to and from small clusters. It can determine the most likely transitions, as well as the most unlikely ones, with a prescribed level of statistical significance.

Abstract

The physical mechanisms responsible for the onset and maintenance of the 1993 summer floods were examined using the localized Eliassen-Palm flux diagnostics and solutions of a single-level primitive equation model linearized about a meridionally varying basic state. The unusually long persistent summer pattern is linked with the marked transient eddy activity in late May and June. The feedback of eddies in the time mean flow caused a strengthening and eastward extension of the Pacific jet and a strengthening of the jet over North America. Results from the model suggest that the summer pattern may be interpreted as that of a lee trough forced by the Rocky Mountains in the presence of a strong westerly mean flow maintained by the eddies upstream.

Composites from cases similar to that of the 1993 summer exhibit strong low-level southerly flow cast of the Rockies and suggest that the low-level jet may be an important mechanism to sustain the anomalous rainfall.

It is concluded that the effect of the eddies in maintaining a strong upper-level zonal flow, the role of the Rockies in sustaining a lee trough, and an associated low-level jet that brings in tropical moisture are essential ingredients in developing and maintaining floodlike conditions over the central United States.

Abstract

We present a new empirical orthogonal function (EOF) analysis of winter 500 mb geopotential height anomalies in the Southern Hemisphere. An earlier EOF analysis by two of the present authors prefiltered the anomalies to exclude wavenumbers 5 and higher; we do not. The different preprocessing of data affects the results. All three distinct planetary flow regimes identified in the winter circulation of the Southern Hemisphere by a pattern correlation method are captured by the new set of EOFs; only two of those regimes were captured by the earlier set. The new results, therefore, lend further support to the idea that EOFs point to distinct planetary