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M. K. Davey

Abstract

A three-layer model is used to study the effect of curvature in the vertical profile of the horizontal velocity on the linear baroclinic instability of a simple quasi-geostrophic flow. Potential vorticity is conserved and the β-effect is included. The results indicate that the range of unstable wavelengths increases as the curvature is increased from zero. For a given physical state, there may be more than one critical wavelength for which the waves are marginally stable. Growth rates for β = 0 and the structure of the fastest growing waves are given. A comparison is made between two-layer, three-layer and continuous systems.

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M. K. Davey

Abstract

Some simple solutions (mostly analytic) are presented for the large-scale baroclinic response to thermal forcing on a mid-latitude beta-plane. Surface heat flux is parameterized as (TATT)/tau;, with atmospheric temperature TA prescribed as a function of latitude, varying ocean surface temperature TT, and equilibration time τ. For long times (decades) benthic forcing is included, using a similar representation. The model allows horizontal density variations at each level.

When there are no meridional boundaries there is only a local response to the forcing. A geostrophic baroclinic zonal flow is driven by the north–south temperature gradient, but it has no associated advection or divergence effects. This picture is greatly changed when east and/or west coasts are added. Kelvin waves pass information rapidly (about 200 km day−1) along coasts, and Rossby waves travel slowly offshore, most effectively from the cut with speed c ≈ 1 km day−1. For spin-up problems (e.g., the response to a change in forcing) the long Rossby waves decay away from the eastern boundary on a scale Tτ. With TA decreasing poleward this creates a broad, relatively warm eastern region with weak downwelling. A steady state requires weaker vertical motion to balance benthic forcing and a corresponding larger decay scale. The narrow western boundary layer is relatively cold on average, with upwelling. (This two-level model does not adequately describe western boundary dynamical however.)

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R. Swinbank, T. N. Palmer, and M. K. Davey

Abstract

An integration of a general circulation model, with an ocean covered globe (or “aqua planet”), exhibits disturbances that are similar to observed eastward propagating waves of period 30 to 60 days (which we refer to as the Madden and Julian oscillation). The structure of the disturbances resembles a Kelvin wave, although the speed of propagation is slower than anticipated from theory as applied to a dry atmosphere. However, a simple model of the tropical atmosphere demonstrates that the wave speed is sensitive to moisture effects. This notion is confirmed by two further general circulation model experiments in which the latent beat release is increased; in both cases the intrinsic speed of the wave is reduced in inverse proportion to the vertical gradient of equivalent potential temperature.

The time-mean circulation of the basic aqua-planet integration exhibits some unusual features; for example a double Hadley cell, with wending branches displaced some 15° either side of the equate. Dynamical reasons for the maintenance of the aqua-planet circulations are discussed since these shed some light on the general circulation of the earth's atmosphere.

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M. K. Davey, D. L. T. Anderson, and S. Lawrence

Abstract

In many prediction schemes, the skill of long-range forecasts of ENSO events depends on the time of year. Such variability could be directly due to seasonal changes in the basic ocean-atmosphere system or due to the state of ENSO itself.

A highly idealized delayed oscillator model with seasonally varying internal parameters is used here to simulate such behavior. The skill of the artificial forecasts shows dependence on both seasonal and ENSO phase. Experiments with ENSO phase-locked to the seasonal cycle. but with no seasonal variation of model parameters. show that the ENSO cycle alone can induce variability in skill. Inclusion of seasonal parameters enhances seasonal skill dependence. It is suggested that the seasonal skill variations found in practice am due to a combination of seasonal changes in the basic state and the phase-locking of the ENSO and annual cycles.

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G. R. McGregor, M. Cox, Y. Cui, Z. Cui, M. K. Davey, R. F. Graham, and A. Brookshaw

Abstract

The winter climate of the British Isles is characterized by considerable interannual variability, which, because of the general climate sensitivity of a number of health outcomes, places at times considerable pressure on the provision of health services. Seasonal climate forecasts potentially could improve management within the health sector and assist in hedging against the vagaries of climatic variability. For this reason, an exploratory analysis of the potential utility of seasonal climate forecasting for the health sector in the United Kingdom is presented here. Study results revealed that the general level of winter mortality at the monthly to seasonal time scale possesses a strong association with simple descriptors of winter climate such as maximum temperature and the number of days below a given temperature threshold. Because such climate indices can be derived from the output of coupled seasonal climate prediction models, predictions of general levels of mortality may be possible using simple transfer functions that describe winter climate and health associations. Despite the potential one-month-ahead and one-season-ahead predictability of winter mortality levels, the predictability of the key climate indices by coupled climate models is shown to be somewhat limited, which compromises the ability to predict general levels of winter mortality for all months except February.

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C.R. Mechoso, A.W. Robertson, N. Barth, M.K. Davey, P. Delecluse, P.R. Gent, S. Ineson, B. Kirtman, M. Latif, H. Le Treut, T. Nagai, J.D. Neelin, S.G.H. Philander, J. Polcher, P.S. Schopf, T. Stockdale, M.J. Suarez, L. Terray, O. Thual, and J.J. Tribbia

Abstract

The seasonal cycle over the tropical Pacific simulated by 11 coupled ocean–atmosphere general circulation models (GCMs) is examined. Each model consists of a high-resolution ocean GCM of either the tropical Pacific or near-global means coupled to a moderate- or high-resolution atmospheric GCM, without the use of flux correction. The seasonal behavior of sea surface temperature (SST) and eastern Pacific rainfall is presented for each model.

The results show that current state-of-the-art coupled GCMs share important successes and troublesome systematic errors. All 11 models are able to simulate the mean zonal gradient in SST at the equator over the central Pacific. The simulated equatorial cold tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10°S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others by an ITCZ that migrates across the equator with the seasons; neither behavior is realistic. There is considerable spread in the simulated seasonal cycles of equatorial SST in the eastern Pacific. Some simulations do capture the annual harmonic quite realistically, although the seasonal cold tongue tends to appear prematurely. Others overestimate the amplitude of the semiannual harmonic. Nonetheless, the results constitute a marked improvement over the simulations of only a few years ago when serious climate drift was still widespread and simulated zonal gradients of SST along the equator were often very weak.

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