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Thomas J. Phillips

Abstract

From an analysis of the heating associated with equatorial, subtropical and midlatitude ocean temperature anomalies in the Held-Suarez climate model, it is found that the magnitude of increase in vertical motion per unit heating in the vicinity of anomalies is relatively insensitive to latitude of the forcing, while the magnitudes of local 750-250 mb thickness changes per unit heating increase sharply as the ocean temperature anomaly is shifted poleward. For equatorial and subtropical anomalies the seasonal variation of the anomalous local vertical motion and thickness fields is in-phase with that of the anomalous local heating, but in the case of the midlatitude anomaly the seasonal changes of local thickness and heating are not synchronous. The steady-state linear shallow water equations on the equatorial and midlatitude beta planes provide a useful framework for explaining the dynamics of these phenomena.

Linear theory is less successfully applied to the analysis of the remote cross-latitudinal response of the model atmosphere to ocean temperature anomalies. Several possible physical explanations for the apparent violations or the predictions of linear theory are explored, but are ultimately rejected as credible hypotheses. Instead, it is shown that the errors arising from limited sampling of the seasonal climatic states of the model are probably the principal “cause” of the large-amplitude departures of 750–250 mb thickness in regions far-removed from the ocean temperature anomalies in certain seasons. The implications of these results for the prediction of seasonal climate are discussed.

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Thomas J. Phillips

Summary documentation of the numerics, dynamics, and physics of models participating in the Atmospheric Model Intercomparison Project is now available on the Internet's World Wide Web. In this article the principal attributes of the electronic model documentation are described and instructions on how to access it are provided.

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Thomas J. Phillips

Abstract

In this study, the sensitivity of the continental seasonal climate to initial conditions is estimated from an ensemble of decadal simulations of an atmospheric general circulation model with the same specifications of radiative forcings and monthly ocean boundary conditions, but with different initial states of atmosphere and land. As measures of the “reproducibility” of continental climate for different initial conditions, spatiotemporal correlations are computed across paired realizations of 11 model land surface variables in which the seasonal cycle is either included or excluded—the former case being pertinent to climate simulation and the latter to seasonal prediction.

It is found that the land surface variables that include the seasonal cycle are impacted only marginally by changes in initial conditions; moreover, their seasonal climatologies exhibit high spatial reproducibility. In contrast, the reproducibility of a seasonal land surface anomaly is generally low, although it is substantially higher in the Tropics; its spatial reproducibility also markedly fluctuates in tandem with warm and cold phases of the El Niño–Southern Oscillation. However, the overall degree of reproducibility depends on the particular land surface anomaly considered. It is also shown that the predictability of a land surface anomaly implied by its reproducibility statistics is consistent with what is inferred from more conventional predictability metrics. Implications of these results for climate model intercomparison projects and for operational forecasts of seasonal continental climate also are elaborated.

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Thomas J. Phillips

Abstract

A 50-year integration of a simple two-layer atmospheric model coupled to a prognostic-depth ocean mixed layer is used for a preliminary exploration of the potential for global climate prediction on seasonal to interannual time scales. Despite a number of quantitative deficiencies, the model simulation permits the investigation of a wider range of climate predictors and predictands than is usually possible from observations: ocean mixed-layer temperature, depth, heat content, and surface heat fluxes are tested as statistical predictors of atmospheric thickness, static stability, and zonal/meridional winds. The field significance of correlations of the atmospheric predictands lagging ocean predictors in different latitude sectors is assessed, and the predictive power and consistency of the ocean variables are determined as a function of time lag, latitude, atmospheric predictand, and season.

It is found that the ocean variables demonstrate a modest potential for predicting atmospheric climate at lags up to three seasons, but no more than about 40% of the local variance of an atmospheric field is explained by any ocean predictor. The most powerful predictors are situated in the tropics, while Northern Hemisphere subtropical and midlatitude ocean variables are substantially better predictors than their Southern Hemisphere counterparts. Ocean mixed-layer temperature is the strongest predictor, while the thickness is the most predictable atmospheric field. Spring and autumn ocean variables are most effective, and winter variables least effective, in predicting subsequent atmospheric seasonal states, but the atmosphere is more predictable in summer and winter than in the transitional seasons.

The implications of these results for global climate prediction are discussed, and some possible future research priorities are proposed.

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Thomas J. Phillips

Abstract

The interaction of different zonally symmetric and asymmetric flows with heating arising from a prescribed anomaly in surface temperature is investigated in linear and nonlinear quasi-geostrophic models. In cases of zonally asymmetric flows the climatic effects of changes in longitudinal position of the surface temperature anomaly with respect to the planetary wave structure are also examined. The modelled atmospheric responses to the surface temperature anomaly divide into two categories, depending upon whether the flow is baroclinically stable or unstable.

For baroclinically stable flows the climatic mean atmospheric response is baroclinic in structure above the surface temperature anomaly; it is equivalent barotropic elsewhere. Maximum changes in atmospheric fields occur in the vicinity of the surface temperature anomaly in such cases. The steady-state linear model is able to predict the basic features of the nonlinear response of the baroclinically stable zonal flows, but with discrepancies in amplitude and phase. Changes in longitudinal position of the surface temperature anomaly with respect to the zonally asymmetric stable flows cause perceptible changes in atmospheric response, both above the anomaly and in other regions. Analysis of the natural variability of the model climates defined by different values of mean vertical shear and static stability of the flows confirms that most of these changes in response arc statistically significant.

For baroclinically unstable flows the changes in climatic mean atmospheric fields are equivalent barotropic in structure everywhere, with large, statistically significant changes in regions far from the surface temperature anomaly. The steady-state linear response does not at all resemble the nonlinear solution in such cases. In addition, for baroclinically unstable zonally asymmetric flows, the response is relatively insensitive to the longitudinal position of the surface temperature anomaly.

The climatic implications of these model results are discussed.

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Thomas J. Phillips and Albert J. Semtner Jr.

Abstract

A ten-year integration of the Held-Suarez climate model with simplified continents and prescribed, but seasonally varying, ocean temperatures produces mean climatic states that are qualitatively similar to observed seasonal climatology. However, the model's temperature gradients, zonal winds and interannual variability are of lower magnitude than observed.

Differences between these seasonal control climates and those obtained from other decadal integrations with fixed temperature anomalies superimposed on the ocean at different latitudes indicate that the seasonal response of the model atmosphere is a function of anomaly position. Thus, although there are increases in upward motion and precipitation in the vicinity of all ocean temperature anomalies, these changes are of considerably greater magnitude for the equatorial and subtropical anomalies than for the midlatitude anomaly. The vertical motion forced by the equatorial ocean temperature anomaly is that of a Walker-type circulation, with overturning to the east and west of the anomalous heating in all seasons.

Local changes in 750-250 mb thickness are much less sensitive to anomaly position: annual thickness increases in the vicinity of all the anomalies are roughly the same. However, there are large seasonal variations in the responses of the 750–250 mb thickness to each ocean temperature anomaly. In the case of the midlatitude anomaly, the maximum local and downstream increases in thickness occur in summer, while a much weaker response characterizes spring and winter. Local and remote changes in thickness forced by the subtropical anomaly exhibit the greatest seasonal variation, with large and extensive thickness departures in summer and autumn, but comparatively small differences in spring and winter. Significant changes in thickness associated with the equatorial anomaly are confined mainly to the tropics in spring and summer, but in autumn and winter there is substantial interaction with extratropical latitudes in the Southern and Northern Hemisphere, respectively. In winter an intensification of the Northern Hemisphere subtropical jet stream also results from the anomalous equatorial heating.

If the geographically extensive changes in thickness associated with the equatorial and subtropical anomalies in certain seasons are the result of the propagation of waves forced by the heat anomalies, these features appear to contradict the predictions of the linear theory of critical levels. Alternative explanations for these phenomena are proposed, whose merits are examined in Part II of this paper.

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Ka-Ming Lau and Thomas J. Phillips

Abstract

Coherent fluctuations between extratropical circulation and tropical conviction in the intraseasonal time scale are studied. Possible relationships between 500-mb height field and outgoing longwave radiation (OLR) are examined using correlation, complex EOF and composite techniques. Results show that in the 20-70 day bands data, there is a systematic evolution of extratropical wavetrains from Eurasia across the Pacific to North America and the North Atlantic in the time scale of 5 to 10 days. Over the tropics, the dominant mode of intraseasonal variation in convection is an east-west dipole-like feature which propagates from the western Indian Ocean eastward to the dateline with a quasi-period of 40-50 days.

The space-time evolution of these extratropical wavetrains is found to he coherent with the tropical dipolar convection and with the strongest convection over Indonesia/central Pacific in approximate quadrature with the peak phase of the Eurasia and Pacific-North America wavetrains. While the extratropical anomalies are found to occur over fixed geographical locations along the entire latitude circle, tropical convection is restricted to the Indian Ocean/western Pacific region. Localized convection near the dateline and to the east appears to be either forced or decoupled from the extratropical circulation anomalies.

The above results are discussed in light of present theories of low frequency variability of the extratropical and tropical atmosphere. It is suggested that they may be the manifestations of the presence of coupled normal modes between the tropics and midlatitudes.

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Thomas W. Betige, Phillip J. Smith, and David B. Baumhefner

Abstract

A diagnostic energetics analysis of short-range, real-data forecasts produced by the NCAR General Circulation Model is performed by computing the forecast kinetic energy budget for a single case study and comparing it with budget statistics derived from observational data. Comparisons are made for the Northern Hemisphere as well as for a limited region encompassing most of North America.

The hemispheric analysis reveals that significant losses of kinetic energy occur in the first 48 h of the forecast period. Also, computations of the kinetic energy generation and dissipation reveal that the model may be underestimating the intensity of the atmospheric energy cycle. Within the limited region the model captures the essential features of the large-scale flow, but fails to reproduce adequately the energy sources and sinks necessary for the development and subsequent propagation of a short synoptic-scale wave system.

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Hua Lu, Thomas J. Bracegirdle, Tony Phillips, and John Turner

Abstract

The Eliassen–Palm (E-P) flux divergences derived from ERA-40 and ERA-Interim show significant differences during northern winter. The discrepancies are marked by vertically alternating positive and negative anomalies at high latitudes and are manifested via a difference in the climatology. The magnitude of the discrepancies can be greater than the interannual variability in certain regions. These wave forcing discrepancies are only partially linked to differences in the residual circulation but they are evidently related to the static stability in the affected regions. Thus, the main cause of the discrepancies is most likely an imbalance of radiative heating.

Two significant sudden changes are detected in the differences between the eddy heat fluxes derived from the two reanalyses. One of the changes may be linked to the bias corrections applied to the infrared radiances from the NOAA-12 High-Resolution Infrared Radiation Sounder in ERA-40, which is known to be contaminated by volcanic aerosol from the 1991 eruption of Mt. Pinatubo. The other change may be due in part to the use of uncorrected radiances from the NOAA-15 Advanced Microwave Sounding Units by ERA-Interim since 1998. These sudden changes have the potential to alter the wave forcing trends in the affected reanalysis, suggesting that extreme care is needed when one comes to extract trends from the highly derived wave forcing quantities.

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John Turner, Thomas J. Bracegirdle, Tony Phillips, Gareth J. Marshall, and J. Scott Hosking

Abstract

This paper examines the annual cycle and trends in Antarctic sea ice extent (SIE) for 18 models used in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that were run with historical forcing for the 1850s to 2005. Many of the models have an annual SIE cycle that differs markedly from that observed over the last 30 years. The majority of models have too small of an SIE at the minimum in February, while several of the models have less than two-thirds of the observed SIE at the September maximum. In contrast to the satellite data, which exhibit a slight increase in SIE, the mean SIE of the models over 1979–2005 shows a decrease in each month, with the greatest multimodel mean percentage monthly decline of 13.6% decade−1 in February and the greatest absolute loss of ice of −0.40 × 106 km2 decade−1 in September. The models have very large differences in SIE over 1860–2005. Most of the control runs have statistically significant trends in SIE over their full time span, and all of the models have a negative trend in SIE since the mid-nineteenth century. The negative SIE trends in most of the model runs over 1979–2005 are a continuation of an earlier decline, suggesting that the processes responsible for the observed increase over the last 30 years are not being simulated correctly.

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