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G. L. Manney, J. D. Farrara, and C. R. Mechoso

Abstract

The evolution of the stratospheric flow during the major stratospheric sudden warming of February 1979 is studied using two primitive equation models of the stratosphere and mesosphere. The United Kingdom Meteorological Office Stratosphere-Mesosphere Model (SMM) uses log pressure as a vertical coordinate. A spectral, entropy coordinate version of the SMM (entropy coordinate model, or ECM) that has recently been developed is also used. Both models produce similar successful simulations through the peak of the warming, capturing the splitting of the vortex and the development of small-scale structures, such as narrow baroclinic zones. The ECM produces a more realistic recombination and recovery of the polar vortex in the midstratosphere after the warming, due mainly to better conservation properties for Rossby-Ertel potential vorticity in this model. Another advantage of the ECM is the automatic increase in vertical resolution near baroclinic zones. Comparison of SMM simulations with forecasts performed using the University of California, Los Angeles general circulation model confirms the previously noted sensitivity of stratospheric forecasts to tropospheric forecast and emphasizes the importance of adequate vertical resolution in modeling the stratosphere.

The ECM simulators provide a schematic description of the three-dimensional evolution of the polar vortex and the motion of air through it. During the warming, the two cyclonic vortices till westward and equatorward with height. Strong upward velocities develop in the lower stratosphere on the west (cold) side of a baroclinic zone as it forms over Europe and Asia. Strong downward velocities appear in the upper stratosphere on the east (warm) side, strengthening the temperature gradients. After the peak of the warming, vertical velocities decrease, downward velocities move into the lower stratosphere, and upward velocities move into the upper stratosphere. Transport calculations show that air with high ozone mixing ratios is advected toward the pole from low latitudes during the warming, and air with low ozone mixing ratios is transported to the midstratosphere from both higher and lower altitudes along the baroclinic zone in the polar regions. Trajectories of parcels moving around the vortex oscillate up and down as they move through regions of ascending and descending motions, with an overall increase in pressure in the polar regions. Tracer transport and trajectory calculations show enhanced diabatic descent in the region between cyclone and anticyclone during the warming, consistent with the temperature structure shown.

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Jui-Lin F. Li, Martin Köhler, John D. Farrara, and C. R. Mechoso

Abstract

When sea surface temperatures are prescribed at its lower boundary, the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) produces a realistic simulation of planetary boundary layer (PBL) stratocumulus cloud incidence. Despite this success, net surface solar fluxes are generally overpredicted in comparison to Earth Radiation Budget Experiment (ERBE) derived data in regions characterized by persistent stratocumulus cloud decks. It is suggested that this deficiency is due to the highly simplified formulation of the PBL cloud optical properties. A new formulation of PBL cloud optical properties is developed based on an estimate of the stratocumulus cloud liquid water path. The January and July mean net surface solar fluxes simulated by the revised AGCM are closer to ERBE-derived values in regions where stratocumulus clouds are frequently observed. The area-averaged estimated error reductions range from 24 (Peru region) to 53 W m−2 (South Pacific storm track region). The results emphasize that surface heat fluxes are very sensitive to the radiative properties of stratocumulus clouds and that a realistic simulation of both the geographical distribution of stratocumulus clouds and their optical properties is crucial.

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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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