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N. E. Davidson
,
J. L. McBride
, and
B. J. McAvaney

Abstract

A case study is presented of the onset of the Southern Hemisphere summer monsoon at longitudes near Australia during December 1978. The numerical analyses comprising this case study are used in conjunction with station data and operational manually derived analyses for other years to investigate the following: 1) the case or definition of monsoon onset; 2) the three-dimensional structure of the troposphere during an active monsoon situation; and 3) the flow changes preceding and during the transition from a period of suppressed to a period of enhanced cumulonimbus activity over tropical Australia.

A well-defined onset occurs in six of the seven years considered. Onset, defined as a satellite-observed, large-scale increase in tropical convection, is consistent with that determined by the wind criterion of Troup (1961).

In 1978 onset occurs in two stages: an increase in convergence, followed by an increase in convection. The monsoon cloudiness exists entirely in the region of low-level westerly wind. The convergence extends through a deep layer from the surface to 400 mb and exists in the upward branch of two linked Hadley cells, one from each of the Northern and Southern Hemispheres.

Observations of the flow changes prior to onset lead to the hypothesis that the trigger mechanism lies in the Southern Hemisphere subtropics. It is conjectured that the seasonal buildup of planetary-scale land-sea temperature gradients has reached a critical stage such that the troposphere is in a state of readiness for the monsoon. Before the onset can take place, however, it must wait for the Southern Hemisphere midlatitude synoptic systems to be in such a configuration that low-level trade wind easterlies are prevalent across the Australian continent.

The evidence is discussed also in favor of various alternative triggering mechanisms such as the influence of a Northern Hemisphere cold surge in the South China Sea and the westward propagation of equatorward westerlies from the Pacific Ocean near the international date line.

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N. E. Davidson
,
J. L. Mcbride
, and
B. J. McAvaney

Abstract

Large scale numerical analyses of divergence and the divergent component of wind are examined at two levels in the lower and upper troposphere. The synoptic sequence studied includes the onset of the Southern Hemisphere summer monsoon. Comparison with satellite-observed cloudiness leads to the conclusion that the analyzed patterns of divergence contain synoptically realistic meteorological information. Them seems to be virtually no information, however, in the day-to-day changes in magnitude of analyzed divergence in the lower troposphere, and only a weak signal in the upper troposphere.

The divergent wind analyses reveal the Intertropical Convergence Zone (ITCZ) to be a readily identifiable feature on individual days, and its location to he both vertically consistent and coincident with the satellite-observed cloud. Two days prior to monsoon onset the analyzed ITCZ moves poleward by 8° latitude. Monsoon convection exists at the intersection of Northern and Southern Hemisphere Hadley cells; it is well removed from the upward branch of any east-west Walker circulations in this situation.

The concept of a divergent surge is introduced to denote vertically consistent divergent circulations extending over distances greater than 20° latitude. This concept is shown to be useful in the physical interpretation of the role of the Southern Hemisphere subtropics in the triggering of monsoon onset. Use of the concept is also helpful in relating the day-to-day changes in tropical convection to simultaneous changes in location and intensity of (mean sea level) subtropical high pressure cells in both hemispheres.

In addition, solutions for the divergent component of wind calculated over a limited domain are compared with solutions calculated over a sphere.

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W. Bourke
,
K. Puri
,
R. Seaman
,
B. McAvaney
, and
J. Le Marshall

Abstract

A data assimilation scheme for the Southern Hemisphere has been incorporated into the ANMRC hemi-insertion frequency of six hours and has been performed with the FGGE data base for the period 17–26 and the model has been designed to accept data at any or all time steps. After each analysis a nonlinear normal mode initialization is performed. The initial evaluation of the analysis scheme has used a data insertion frequency of six hours and has been performed with the FGGE data base for the period 17–26 May 1979. Comparison with Australian Bureau of Meteorology operational analyses is presented, together with diagnostic evaluation of detailed aspects of the assimilation scheme.

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J. R. Garratt
,
A. J. Prata
,
L. D. Rotstayn
,
B. J. McAvaney
, and
S. Cusack

Abstract

An updated evaluation of the surface radiation budget in climate models (1994–96 versions; seven datasets available, with and without aerosols) and in two new satellite-based global datasets (with aerosols) is presented. All nine datasets capture the broad mean monthly zonal variations in the flux components and in the net radiation, with maximum differences of some 100 W m−2 occurring in the downwelling fluxes at specific latitudes. Using long-term surface observations, both from land stations and the Pacific warm pool (with typical uncertainties in the annual values varying between ±5 and 20 W m−2), excess net radiation (R N) and downwelling shortwave flux density (S o↓) are found in all datasets, consistent with results from earlier studies [for global land, excesses of 15%–20% (12 W m−2) in R N and about 12% (20 W m−2) in S o↓]. For the nine datasets combined, the spread in annual fluxes is significant: for R N, it is 15 (50) W m−2 over global land (Pacific warm pool) in an observed annual mean of 65 (135) W m−2; for S o↓, it is 25 (60) W m−2 over land (warm pool) in an annual mean of 176 (197) W m−2.

The effects of aerosols are included in three of the authors’ datasets, based on simple aerosol climatologies and assumptions regarding aerosol optical properties. They offer guidance on the broad impact of aerosols on climate, suggesting that the inclusion of aerosols in models would reduce the annual S o↓ by 15–20 W m−2 over land and 5–10 W m−2 over the oceans. Model differences in cloud cover contribute to differences in S o↓ between datasets; for global land, this is most clearly demonstrated through the effects of cloud cover on the surface shortwave cloud forcing. The tendency for most datasets to underestimate cloudiness, particularly over global land, and possibly to underestimate atmospheric water vapor absorption, probably contributes to the excess downwelling shortwave flux at the surface.

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S. B. Power
,
R. Kleeman
,
R. A. Colman
, and
B. J. McAvaney

Abstract

An atmospheric general circulation model (AGCM), a simplified atmospheric model (SAM) of surface heat flux, and various idealized analytic models have been used to investigate the atmospheric response over the North Atlantic to SST anomalies including a general cooling associated with a weakened thermohaline circulation. Latent heating dominates the surface heat flux response, while sensible heating plays an important secondary role. The total heat flux response is weaker than presumed in recent studies using ocean models under highly idealized surface boundary conditions. This implies that stability of the thermohaline circulation to high-latitude freshening in more sophisticated coupled systems (that incorporate either AGCMs or models like SAM) will be increased.

All three kinds of atmospheric models exhibit nonrestorative behavior away from the anomaly peak that is primarily associated with the advection of cooled air eastward. This simple picture is complicated in the AGCM by the fact that the winds weaken over the SST anomaly, which helps to moderate the response.

Analytic models for atmospheric temperature forced using imposed surface temperature anomalies highlight conditions under which a nonrestorative response can arise. Previous work has shown that the length scale of spatially periodic anomalies partially determines the magnitude of the response in a diffusive atmosphere. Here the authors show that this scale dependence has much wider applicability by considering more localized anomalies and by the inclusion of advective transport processes.

The modification of the response by sea ice changes and the absence of any statistically significant change in the basin-averaged hydrological cycle are also discussed.

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Gerald A. Meehl
,
Curt Covey
,
Thomas Delworth
,
Mojib Latif
,
Bryant McAvaney
,
John F. B. Mitchell
,
Ronald J. Stouffer
, and
Karl E. Taylor

A coordinated set of global coupled climate model [atmosphere–ocean general circulation model (AOGCM)] experiments for twentieth- and twenty-first-century climate, as well as several climate change commitment and other experiments, was run by 16 modeling groups from 11 countries with 23 models for assessment in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Since the assessment was completed, output from another model has been added to the dataset, so the participation is now 17 groups from 12 countries with 24 models. This effort, as well as the subsequent analysis phase, was organized by the World Climate Research Programme (WCRP) Climate Variability and Predictability (CLIVAR) Working Group on Coupled Models (WGCM) Climate Simulation Panel, and constitutes the third phase of the Coupled Model Intercomparison Project (CMIP3). The dataset is called the WCRP CMIP3 multimodel dataset, and represents the largest and most comprehensive international global coupled climate model experiment and multimodel analysis effort ever attempted. As of March 2007, the Program for Climate Model Diagnostics and Intercomparison (PCMDI) has collected, archived, and served roughly 32 TB of model data. With oversight from the panel, the multimodel data were made openly available from PCMDI for analysis and academic applications. Over 171 TB of data had been downloaded among the more than 1000 registered users to date. Over 200 journal articles, based in part on the dataset, have been published AMERICAN METEOROLOGICAL SOCIETY so far. Though initially aimed at the IPCC AR4, this unique and valuable resource will continue to be maintained for at least the next several years. Never before has such an extensive set of climate model simulations been made available to the international climate science community for study. The ready access to the multimodel dataset opens up these types of model analyses to researchers, including students, who previously could not obtain state-of-the-art climate model output, and thus represents a new era in climate change research. As a direct consequence, these ongoing studies are increasing the body of knowledge regarding our understanding of how the climate system currently works, and how it may change in the future.

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