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Robert G. Gallimore

Abstract

A zonal mean water vapor-energy balance (WEB) model is formulated to assess feedback interactions of the hydrologic cycle and lapse rate with the radiative fluxes, snow-dependent albedo and transport mechanisms. The WEB model is designed for comparative study and integration of many sensitivity experiments in which changes in feedback processes are introduced through variation in parameters and/or parameterizations.

Specific processes in the model include 1) an explicit hydrologic cycle with the predicted atmospheric water vapor used in the determination of the solar and longwave fluxes; 2) a land albedo and surface energy budget dependency on explicit model calculations of snowfall, snow melt and snow accumulation; 3) a lapse rate dependency for the longwave emission, sensible heating and mean energy transport; and 4) separate specification for mean and eddy energy and water vapor transports.

The sensitivity of the model energy and water vapor budgets is similar to that obtained in a GCM by Wetherald and Manabe (1975), with several of the more important ones for reduced solar constant being 1) the nonlinear decrease of temperature resulting in part from the rapid intrusion of snowfall into the middle latitude precipitation belt; 2) the large reduction of atmospheric latent enemy content and decreased intensity of the hydrologic cycle; and 3) the nearly invariant total atmospheric energy transport. Based on the similarity of the WEB model and GCM results, a forthcoming paper will examine the parameterizations governing interaction of the model radiative and transport terms with the ice-albedo mechanism.

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Robert G. Gallimore

Abstract

The life cycle and structure of dominant wintertime SST anomalies and associated atmospheric response in the extratropical North Pacific are examined using results from a 100-yr seasonal simulation of a low-resolution atmospheric model with realistic geography coupled to a simple mixed layer ocean. The study focuses on composited SST anomalies produced solely by ocean–atmosphere energy exchange. One key pattern shows a negative (positive) SST anomaly in the central Pacific, denoted CCP (WCP), flanked by opposite signed anomalies in the western and eastern Pacific. For the WCP case, the SST anomaly reaches about 1.0°C in the central Pacific, whereas for the CCP case it is −1.5°C.

During the growth phase of the WCP (CCP) SST anomalies, anomalous highs (lows) occur over the western Pacific and over western North America, and an anomalous low (high) is over the east-central Pacific. To the rear of the anomalous low (high), a negative (positive) SST anomaly develops in response to anomalous cold, dry (warm, moist) air. The effect of anomalous wind also contributes but to a lesser extent. The composited SST anomalies primarily develop in 1–2 months.

During the decay stage of the WCP SST anomaly, the atmospheric anomalies are essentially of opposite sign than during the growth stage and help to destroy the SST anomaly. In contrast, the atmospheric anomalies during the decay stage of the CCP SST anomaly are of the same sign but weaker than during the growth stage. For this case, a positive SST-atmosphere feedback involving ocean–atmosphere energy exchange helps maintain a more persistent SST anomaly than in the WCP composite case.

Comparison of results with prescribed SST anomaly experiments indicate that the ocean is in part forcing the atmosphere during the decay stage of CCP SST anomalies. The magnitude and position of the lower-tropospheric anomalies forced by SST anomalies are similar to those obtained in linear theory, while the vertical structure is more akin to that found in higher-resolution GCMs with realistic geography. The weaker atmospheric anomalies generated by the low-resolution model are likely a consequence of its simulation of weak transients. Differences between the composite and prescribed SST anomaly results (e.g., in position of the atmospheric anomalies relative to the SST anomaly) suggest that the anomalous response of the atmosphere to SST anomalies in a coupled ocean–atmosphere environment may not be the same as in a one-way forced system.

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Robert G. Gallimore
and
Donald R. Johnson

Abstract

In the linear theory of the isentropic zonally averaged circulation (Gallimore and Johnson, 1981), stable meridional circulations within the circumpolar vortex are forced by larger scale diabatic beating and angular momentum torques. The linear theory, however, could not describe important nonlinear interactions between the forcing of meridional circulations and the maintenance of the circumpolar vortex nor could the quasi-steady balance be ascertained.

In this study, an isentropic numerical model is developed to investigate the forcing of the meridional circulation and its role and the role of angular momentum torques in maintaining the circumpolar vortex. The model is based on a set of isentropic zonally averaged equations applied to the Northern Hemisphere. The horizontal resolution is 5° of latitude while nine equally spaced isentropic levels are used for vertical resolution. The basic forcing of the model is diabatic heating and zonal pressure and friction torques. The distribution of diabatic heating is prescribed from observational results while the meridional distribution of the zonal pressure torque is determined from a specified separation of the zonally averaged meridional mass transport into geostrophic and ageostrophic modes. The friction torque is internally time dependent. Eddy transport terms are not included and a fixed surface potential temperature is assumed at the lower boundary. The zonally averaged circulation and its forcing were studied in three experiments in which the meridional distribution of the diabatic heating and the pressure torque were varied. After 30 days of numerical simulation a slowly varying state was attained.

The simulation determined a realistic isentropic Hadley circulation spanning the Northern Hemisphere and a zonal wind structure with an upper tropospheric subtropical jet. The structure of the model's zonally averaged meridional circulation is consistent with observations and with the results of the linear theory, i.e., the vertical branches of the direct, stable circulation are forced through tropical heating and polar cooling while the meridional branches are forced by upper tropospheric sources and lower tropospheric sinks of pressure and friction torques. The zonal vortex is maintained through the balance of angular momentum torques and the convergence of the absolute angular momentum transport within the meridional circulation. The results further indicate that non-steady, freely convecting meridional circulations do not occur for realistic forcing.

A comparison of the results in which the heating is varied reveals that the changes in the intensity of the meridional circulation depend on the variations of the meridional heating distribution. Since changes occur in the meridional momentum transport, structural changes in the zonal momentum and torques are also produced. With the same heating, changes in the zonal vortex occur in conjunction with compensating changes in the meridional distribution of the zonal pressure and time-dependent friction torques while the intensity of the total meridional circulation remains independent of this factor.

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Robert G. Gallimore
and
Donald R. Johnson

Abstract

For the axisymmetric general circulation, Eliassen (1951) showed that stable meridional circulations are controlled by large-scale diabatic heating and friction torque. Because the real atmosphere is longitudinally disturbed, a direct meridional response to large-scale diabatic heating is not readily found by conventional zonal averaging in isobaric coordinates. By introducing, the concept of a zonally averaged general circulation in isentropic coordinates, a direct meridional response to diabatic heating is isolated.

Using an approach analogous to Eliassen's, this study has extended the concept of diabatically forced isentropic meridional circulations to include the role of angular momentum torques within the circumpolar vortex. The absolute angular momentum torques comprise three terms: divergence of eddy relative momentum transport, friction and zonal pressure torques. The heating is composed of hemispheric-scale diabatic processes. For a hydrodynamically and statically stable atmosphere, a configuration of tropical heating and higher latitude cooling coupled with a combined upper tropospheric sink and low tropospheric source by the angular momentum torques support a Hadley-type circulation spanning the entire hemisphere.

In low latitudes, an upper tropospheric sink by friction torque and divergence of the eddy relative momentum transport combine with a lower tropospheric source by friction torque to force a dominant ageostrophic mode for the meridional branch of the Hadley cell. Within large-scale middle latitude waves, the zonal pressure torque is a principal mechanism for the transfer of angular momentum from upper to lower isentropic layers and therefore is the dominant upper tropospheric sink and lower tropospheric source of angular momentum in high latitudes. Due to the dominance of the pressure torque in extratropical latitudes, the poleward branch of the upper layers and equatorward branch of the lower layers are mainly geostrophic and mask indirect ageostrophic motion that is forced by friction torque and the divergence of eddy relative momentum transport.

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Robert G. Gallimore
and
David D. Houghton

Abstract

The approximation of ocean heat storage by the net surface energy flux and the implications for zonal mean SST simulation using mixed layer ocean formulation are examined. The analysis considers both constant and variable depth mixed layers. Simulated zonal-mean net surface energy fluxes, taken from a low-resolution atmospheric GCM with prescribed SST, are compared to observed flux and ocean heat storage data. The impact of limitations of the mixed layer ocean formulation on SST simulation are assessed to impacts of simulation errors of the atmospheric model.

The results indicate the important in determining errors in the atmospheric model simulation for the net surface energy flux when assessing anticipated improvement in SST simulation as neglected physical processes (e.g., ocean heat transport) are incorporated in the ocean component of an interactive model. Noting the current limited availability of observations, the approximation of ocean heat storage by the simulated net surface energy flux is cautiously assessed for middle latitudes of the Northern Hemisphere. Due to the uncertainty in observational estimates for both seasonal net surface energy flux and seasonal ocean heat transport, the quality of the flux simulation and the question as to whether or not the model heat storage approximation would improve with the addition of seasonal ocean heat transport are assessed with less certainty.

The inferred annual cycle of zonal mean SST is calculated by applying both model and observed net surface energy fluxes (approximated heat storage) and observed heat storage data to the mixed-layer ocean formulations. The results show that the change from a constant 50 m depth to a variable depth mixed layer ocean formulation (after Meehl), yields significant improvement in zonal mean SST simulation with the sensitivity to the approximation for heat storage being a lesser factor. The large uncertainty in seasonal heat transport data, however, warrants sensitivity examination of their impact on climate in a coupled ocean–atmosphere model. The results demonstrate that examination of biases in atmospheric model simulation and calculation of inferred SST using the atmospheric model results can be useful in diagnosing SST simulation in coupled ocean–atmosphere models.

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Robert G. Gallimore
and
David D. Houghton

Abstract

The simulation of zonal mean ocean surface temperature and heat storage (time rate of change of heat content) obtained from a low resolution, atmospheric general circulation model (GCM) coupled to a variable depth upper-ocean formulation (A/VO model) is compared with observations and with a parallel simulation using a constant (50 m) depth upper-ocean component (A/CO model). Variable depths are used to specify the heat content in the seasonal mixed layer and thermocline; the depths are prescribed to vary latitudinally and seasonally but not longitudinally. The Northern Hemisphere depths are taken from Meehl; for the Southern Hemisphere, Meehl's depths are modified to account for differences in ocean thermal structure between the hemispheres.

The annual variation of upper ocean depth produces important effects on seasonal simulation of ocean surface temperature in the extratropics. In this study, the extremes of ocean temperature in both hemispheres occur earlier by about 30 days in the A/VO model, compared to the A/CO model. This leads to a warmer ocean in spring/summer and a colder ocean in fall/winter for the A/VO model. In contrast to the A/CO model, an important asymmetry in the structure of the monthly departures of ocean temperature from the annual mean is produced by the A/VO model. In the southern extratropics, the A/VO model simulates a reduced annual cycle of ocean temperature (by about 25–30%) from that produced by the A/CO) model.

Both models underestimate the observed seasonal amplitude of zonal mean heat storage; the underestimation is greatest for the A/VO model. The differences in simulations of heat storage are linked to differences in ocean surface temperature computation between the two models.

Errors in zonal mean ocean surface temperature for the A/VO model are less than for the A/CO model in the extratropics. particularly the Southern Hemisphere. However, the errors in the coupled A/VO model simulation are larger in the northern extratropics than in the previously published uncoupled calculations using prescribed heat storage estimates. It is argued that significant improvement in ocean temperature simulation by the A/VO model can be achieved with better approximation of heat storage in conjunction with a small adjustment to the prescribed variable depths.

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Robert G. Gallimore
,
Bette L. Otto-Bliesner
, and
John E. Kutzbach

Abstract

The sensitivity of a low resolution, spectral general circulation model (GCM) to specification of physical processes is examined using a new version of the model with refined parameterizations. Specific refinements in parameterization include: 1) smoothing the original orography to greatly diminish undesirable topographic “ripples” occurring near high mountain ranges; 2) adding snowcover on the Tibetan plateau and representing winter snowcover in middle latitudes more realistically; 3) decreasing the land ground wetness and adjusting the drag coefficient and parameters governing condensation-moist convective adjustment.

Results of comparative 5-year integrations show that better parameterization in the low resolution model produces significant improvement in simulation without resorting to the use of higher horizontal or vertical resolution. The combined changes in ground wetness, drag coefficient and condensation-moist convective parameters produce more realistic zonal banding of precipitation belts and a better representation of continental precipitation relative to the ocean. In addition, mass is more nearly conserved and mean sea level pressure and temperature patterns are in better agreement with observations than in the previous model. Major deficiencies in simulation that are not improved include zonal jet and stratospheric temperature structures. Overall, the improvements in simulation suggest a wider applicability of the low resolution model for use in climate sensitivity studies.

Analysis of sensitivity experiments assessing specific effects of parameterization indicate that decreased ground

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David D. Houghton
,
Robert G. Gallimore
, and
Linda M. Keller

Abstract

Two 100-year seasonal simulators, one performed with a low resolution atmospheric general circulation model (GCM) coupled to a mixed-layer ocean formulation and the other made with the GCM forced by prescribed ocean conditions, are compared to assess the effects of an interactive ocean and sea-ice component on the stability and interannual variability of a climate system. Characteristics of the time variation of surface temperature, 700 mb temperature and sea-ice coverage are analyzed for selected land and ocean areas. Both simulations showed stable seasonal cycles of basic variables, although small trends were found. These trends were roughly linear in nature and quite distinct from all other components of variability. Detrended time series were used to describe the other aspects of variability.

There was pronounced interannual variability in the simulations from both models as seen in the time series for temperature and sea ice over the entire 100-year time period. Consistent with observations, variations tended to be larger in polar areas and over land. The inclusion of the interactive ocean and sea-ice component produced a red spectrum for surface temperature but not for 700-mb temperature. Using a linearized air-sea model patterned after the coupled models, this result is shown to be linked to the combined effects of the model longwave cooling and ocean-atmosphere energy exchange. The shift towards lower frequency in surface temperature was most evident in polar regions and occurred in conjunction with very low frequency (even decadal-scale) variability in the computed sea-ice coverage. The simulated mean and variability characteristics of sea ice corresponded fairly well with observations. This suggests that the low resolution model is able to represent some relevant aspects of the physics of climate fluctuations and thus provide useful simulations for studies of interannual variability.

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Robert M. Chervin
,
Jouhn E. Kutzbach
,
David D. Houghton
, and
Robert G. Gallimore

Abstract

The sensitivity of a six-layer NCAR atmospheric general circulation model (GCM) to a variety of idealized, very large amplitude, midlatitude and subtropical North Pacific Ocean surface temperature (OST) anomalies is analyzed. In the Pacific sector, the model exhibits a differential sensitivity depending on the latitudinal position of the imposed anomaly. Typically, the model response is a combination of a relative direct thermal circulation, an alteration in the pattern of cyclonic activity, and a selective wave response dependent on the planetary waves present in the unmodified control case. The extent to which the background planetary waves affect the model response is dependent on several factors including latitude-dependent features of the control case and the relative position of the anomaly. Analogous experiments with a simple quasi-geostrophic model are useful in isolating important physical and dynamical processes, and thereby assist in the interpretation of the GCM results. An analysis of the correlation between observed tropospheric thickness and North Pacific OST provides some confirmation of the consistent model result of an anomalously warm troposphere over a warm OST anomaly.

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