Annual oceanic heat transports computed from an atmospheric model

Mean annual estimates of the oceanic poleward energy transport are obtained using a global atmospheric general circulation model. The computations are carried out by using the atmospheric model to determine the net annual heat flux into the ocean on an 8° × 10° grid. Assuming no net annual heat storage, the annual surface heat fluxes into any zonal band must be accompanied by a corresponding meridional heat transport in the ocean. Heat is transported northward at all latitudes in the Atlantic Ocean and is transported poleward in both hemispheres in the Pacific Ocean. To account for the net northward transport throughout the Atlantic, heat is transported into the Atlantic from the Indian and Pacific basins. The results are compared with several recent direct and indirect calculations of oceanic meridional heat transports.     

Seasonal oceanic heat transports computed from an atmospheric model

Seasonal estimates of the oceanic poleward heat transport are obtained using a climate model that is a global atmospheric general circulation model on an 8° × 10° grid. The climate model is used to calculate the surface heat flux into each ocean grid point for each day of the year. The rate of ocean heat storage is calculated using climatological surface temperatures, mixed layer depths, and ice amounts. By assuming that the rate of change of heat storage in the deep ocean is spatially constant, the horizontal transports are calculated from the vertical fluxes and the upper ocean storage rates. The oceanic meridional transport for each latitude and for each ocean basin are derived, and results are compared with other calculations of the seasonal transports. In the Northern Hemisphere, comparisons between the simulated seasonal transports indicate that the annual variation is much greater in the Pacific than in the Atlantic.     

Energy transports by ocean and atmosphere based on an entropy extremum principle. Part II: Two-dimensional transports

Summary The maximum entropy production (MEP) principle used in Part J has been extended to separate the two-dimensional required energy transports determined from Nimbus 7 satellite net radiation measurements into atmospheric and oceanic components. In terms of the meridional component of the ocean transport vectors, results show northward ocean transports throughout the entire Atlantic ocean from southern hemisphere high latitudes to northern hemisphere polar regions, southward transports throughout the entire Indian Ocean, and poleward transports separated at approximately 10°S in the Pacific Ocean. The ocean transport patterns are consistent with well-known features concerning heat transport within the three ocean basins. However, uncertainty remains in the magnitudes of the transports. Because of the large remaining discrepancies between published estimates based on direct measurements and indirect estimates derived from energy budget methods, assessing the accuracy of the magnitudes is difficult, although there is evidence that the limited model resolution leads to synergistic biases in the North Atlantic and North Pacific. In terms of the crossmeridional energy transport component, results suggest that most of the net energy transfer in the tropics takes place within the ocean. In the southern hemisphere high latitudes, the Pacific and Indian Oceans export heat cross-meridionally to the Atlantic Ocean through the passages below Cape Horn and the Cape of Good Hope, although the magnitudes of these inter-ocean heat exchanges are small. Another important aspect of the southern hemisphere results is that poleward transports are dominated by the atmospheric component with strong zonal asymmetry. By contrast, in the northern hemisphere, atmospheric transports over the ocean are generally weaker than the corresponding southern hemisphere terms, indicating that the northern hemisphere oceans are relatively more effective in transferring heat poleward. Finally, poleward atmospheric transports over the continental areas exceed those over the ocean at equivalent latitudes as a result of the generally greater energy deficits over the land areas.With 7 Figures     

The atmospheric energy budget and implications for surface fluxes and ocean heat transports

Comprehensive diagnostic comparisons and evaluations have been carried out with the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and European Centre for Medium Range Weather Forecasts (ECMWF) reanalyses of the vertically integrated atmospheric energy budgets. For 1979 to 1993 the focus is on the monthly means of the divergence of the atmospheric energy transports. For February 1985 to April 1989, when there are reliable top-of-the-atmosphere (TOA) radiation data from the Earth Radiation Budget Experiment (ERBE), the implied monthly mean surface fluxes are derived and compared with those from the assimilating models and from the Comprehensive Ocean Atmosphere Data Set (COADS), both locally and zonally integrated, to deduce the implied ocean meridional heat transports. While broadscale aspects and some details of both the divergence of atmospheric energy and the surface flux climatological means are reproducible, especially in the zonal means, differences are also readily apparent. Systematic differences are typically ∼20 W m⁻². The evaluation highlights the poor results over land. Land imbalances indicate local errors in the divergence of the atmospheric energy transports for monthly means on scales of 500 km (T31) of 30 W m⁻² in both reanalyses and ∼50 W m⁻² in areas of high topography and over Antarctica for NCEP/NCAR. Over the oceans in the extratropics, the monthly mean anomaly time series of the vertically integrated total energy divergence from the two reanalyses correspond reasonably well, with correlations exceeding 0.7. A common monthly mean climate signal of about 40 W m⁻² is inferred along with local errors of 25 to 30 W m⁻² in most extratropical regions. Except for large scales, there is no useful common signal in the tropics, and reproducibility is especially poor in regions of active convection and where stratocumulus prevails. Although time series of monthly anomalies of surface bulk fluxes from the two models and COADS agree very well over the northern extratropical oceans, the total fields all contain large systematic biases which make them unsuitable for determining ocean heat transports. TOA biases in absorbed shortwave, outgoing longwave and net radiation from both reanalysis models are substantial (>20 W m⁻² in the tropics) and indicate that clouds are a primary source of problems in the model fluxes, both at the surface and the TOA. Time series of monthly COADS surface fluxes are shown to be unreliable south of about 20^∘N where there are fewer than 25 observations per 5^∘ square per month. Only the derived surface fluxes give reasonable implied meridional ocean heat transports. Received: 21 March 2000 / Accepted: 21 June 2000     

Estimation of the Pacific Ocean meridional heat flux at 35°N

Abstract

The meridional heat flux in the North Pacific Ocean at 35°N is estimated primarily using hydrographic section data, following the method of Bryan (1962) and Bennett (1978). The meridional heat flux in the Kuroshio, computed using the Worthington and Kawai section across the current, was 1.76 PW (positive northward), with over 80% of the flux occurring in the upper 400 m. The large‐scale baroclinic heat flux across the rest of the section (145°? to North America) was —1.0 PW for the indopac (1976) section and —0.5 PW for the IOS‐72 section. The fluxes across the sections were also concentrated in the upper ocean with the upper 300 m accounting for over 75% of the flux. The mean horizontal barotropic gyre circulation results in little (0.1 PW) net heat flux because the northward‐moving water is only about 0.5°C warmer than the southward‐moving water. The contributions due to Ekman heat flux (—0.16 PW) and flow through the Japan Sea (0.13 PW) are also relatively small. The eddy heat flux is quite uncertain, but estimated to be about 0.3 PW. The total meridional heat flux, for the 1976 section, is calculated to be about 1.0 PW. The total is very dependent on the baroclinic heat flux in the highly variable Kuroshio region. The northward heat flux found in this study is more consistent with large‐scale atmospheric estimates and with Bryden et al. ‘s (1990) estimate for 24°? in the Pacific.     

Fluxes of latent heat over the oceans: climatological studies and application of satellite observations

A short review is given of the different methods by which latent heat fluxes (or evaporation) over oceans are determined. In more detail, the applicability of the bulk aerodynamical formula is discussed. This formula is mainly used for climatological studies of heat fluxes and for the application of satellite data. As an example, for climatological studies we selected the work of Isemer and Hasse, who did a re-processing of the so-called Bunker data set to determine heat fluxes over the North Atlantic Ocean. In order to check their results, Isemer and Hasse calculated the annual mean heat budget for each latitudinal delt and derived from it the required ocean heat transport. With the aid of inverse modelling, the derived ocean transport was compared with the observed ocean transport and some of the used coefficients (e.g. bulk coefficients for latent and sensible heat flux) were altered. Though the ocean heat transport is changed by a large amount (at the equator 0.3 PW, original Bunker data; 0 PW, Isemer and Hasse; 0.76 PW, after inverse modelling (all northwards)) the overall patterns of the fields of the energy fluxes remain almost unchanged. The bulk coefficient for latent heat flux for example is altered by 5.6%.The geophysical parameters necessary for the bulk aerodynamic method can be determined from satellite observations: SST, q₀, u₀. Studies are described which used data from a microwave radiometer on SEASAT and NIMBUS7 to determine latent heat flux. An error calculation shows that the obtained accuracy is between 26 and 35 W m⁻². This accuracy is adequate enough to allow reasonable estimates to be made of these fluxes. More satellites are planned for launch with microwave radiometers and scatterometers which will increase the possibility of determining geophysical parameters more accurately for use in the bulk aerodynamic formula. They will provide the database from which large-scale fieldsof latent heat flux (for time scales shorter than a month or even for actual situations) can be derived.     

Interocean circulation and heat and freshwater budgets of the South China Sea based on a numerical model

The South China Sea (SCS) interocean circulation and its associated heat and freshwater budgets are examined using the results of a variable-grid global ocean model. The ocean model has a 1/6° resolution in the SCS and its adjacent oceans. The model results from 1982 to 2003 show that the western Pacific waters enter the SCS through the Luzon Strait with an annual mean volume transport of 4.80 Sv, of which 1.71 Sv returns to the western Pacific through the Taiwan Strait and East China Sea and 3.09 Sv flows toward the Indian Ocean. The heat in the western Pacific is transported to the SCS with a rate of 0.373 PW (relative to a reference temperature 3.72 °C), while the total heat transport through the outflow straits is 0.432 PW. The net heat transport out of the SCS is thus 0.059 PW, which is balanced by a mean net downward heat flux of 17 W/m² across the SCS air–sea interface. Therefore, the interocean circulation acts as an “air conditioner”, cooling the SCS and its overlaying atmosphere. The SCS contributes a heat transport of 0.279 PW to the Indian Ocean, of which 0.240 PW is from the Pacific Ocean through the Luzon Strait and 0.039 PW is from the SCS interior gained from the air–sea exchange. The Luzon Strait salt transport is greater than the total salt transport leaving the SCS by 3.97 Gg/s, implying a mean freshwater flux of 0.112 Sv (or 3.54 × 10¹² m³/year) from the land discharge and P − E (precipitation minus evaporation). The total annual land discharge to the SCS is estimated to be 1.60 × 10¹² m³/year, the total annual P − E over the SCS is thus 1.94 × 10¹² m³/year, equivalent to a mean P − E of 0.55 m/year. The SCS freshwater contribution to the Indian Ocean is 0.096 Sv. The pattern of the SCS interocean circulation in winter differs greatly from that in summer. The SCS branch of the Pacific-to-Indian Ocean throughflow exists in winter, but not in summer. In winter this branching flow starts at the Luzon Strait and extends to the Karimata Strait. In summer the interocean circulation is featured by a north-northeastward current starting at the Karimata Strait and extending to the Taiwan and Luzon Straits, and a subsurface inflow from the Luzon Strait that upwells into the surface layer in the SCS interior to supply the outward transports.     

Aircraft Observations of Sea-Surface Turbulent Fluxes Near the California Coast

Aircraft turbulence data from the Autonomous Ocean Sampling Network project were analyzed and compared to the Coupled Ocean–Atmosphere Response Experiment (COARE) bulk parametrization of turbulent fluxes in an ocean area near the coast of California characterized by complex atmospheric flow. Turbulent fluxes measured at about 35 m above the sea surface using the eddy-correlation method were lower than bulk estimates under unstable and stable atmospheric stratification for all but light winds. Neutral turbulent transfer coefficients were used in this comparison because they remove the effects of mean atmospheric conditions and atmospheric stability. Spectral analysis suggested that kilometre-scale longitudinal rolls affect significantly turbulence measurements even near the sea surface, depending on sampling direction. Cross-wind sampling tended to capture all the available turbulent energy. Vertical soundings showed low boundary-layer depths and high flux divergence near the sea surface in the case of sensible heat flux but minimal flux divergence for the momentum flux. Cross-wind sampling and flux divergence were found to explain most of the observed discrepancies between the measured and bulk flux estimates. At low wind speeds the drag coefficient determined with eddy correlation and an inertial dissipation method after corrections were applied still showed high values compared to bulk estimates. This discrepancy correlated with the dominance of sea swell, which was a usually observed condition under low wind speeds. Under stable atmospheric conditions measured sensible heat fluxes, which usually have low values over the ocean, were possibly affected by measurement errors and deviated significantly from bulk estimates.     

Sensitivity of the thermohaline circulation in coupled oceanic GCM — atmospheric EBM experiments

We analyze the sensitivity of the oceanic thermohaline circulation (THC) regarding perturbations in fresh water flux for a range of coupled oceanic general circulation — atmospheric energy balance models. The energy balance model (EBM) predicts surface air temperature and fresh water flux and contains the feedbacks due to meridional transports of sensible and latent heat. In the coupled system we examine a negative perturbation in run-off into the southern ocean and analyze the role of changed atmospheric heat transports and fresh water flux. With mixed boundary conditions (fixed air temperature and fixed surface fresh water fluxes) the response is characterized by a completely different oceanic heat transport than in the reference case. On the other hand, the surface heat flux remains roughly constant when the air temperature can adjust in a model where no anomalous atmospheric transports are allowed. This gives an artificially stable system with nearly unchanged oceanic heat transport. However, if meridional heat transports in the atmosphere are included, the sensitivity of the system lies between the two extreme cases. We find that changes in fresh water flux are unimportant for the THC in the coupled system.     

Discrepancies in Simulated Ocean Net Surface Heat Fluxes over the North Atlantic

The change in ocean net surface heat flux plays an important role in the climate system. It is closely related to the ocean heat content change and ocean heat transport, particularly over the North Atlantic, where the ocean loses heat to the atmosphere, affecting the AMOC(Atlantic Meridional Overturning Circulation) variability and hence the global climate.However, the difference between simulated surface heat fluxes is still large due to poorly represented dynamical processes involving multisca...     

Subgrid-scale vertical heat fluxes for the African region

Summary A diagnostic model for complete heat budgets in the free atmosphere is presented and is applied to the African-Atlantic sector between 35°S–30°N for May 1979. The model is based on the conservation equations for latent and sensible heat. These are evaluated in a form integrated over 24 hours in time and over atmospheric boxes of 2.5°×2.5° in horizontal and 100 hPa in vertical direction. Grid-scale input data are the 3D-ECMWF-diagnoses of the FGGE period plus parameterized fields of surface rain, evaporation and sensible Heat flux. This leads to an overspecification of latent and sensible heat budgets for any atmospheric column between surface and top of the atmosphere and thus yields an objective column imbalance. In order to separate the vertical subscale fluxes of rain, moisture and heat in the free atmosphere the model uses a closure assumption for the coupling between moisture and sensible heat flux as well as one for the vertical imbalance profiles; it is demonstrated that the budgets are not too sensitive with respect to these parameterizations.Results are presented in terms of vertical profiles of the subscale vertical fluxes of rain, moisture and heat. These are interpeted as measures of convective activity, with particular emphasis on the ITCZ. May 1979 averages as well as results for a respresentative single day are discussed. The imbalance (=the error) can be sufficiently well separated from the signal. It is shown that the low-level mass flux divergence does not coincide with the position of the ITCZ while the maximum of the subscale fluxes does coincide. Over the continent, it is not the horizontal mass flux convergence which feeds the ITCZ and the rainbelt but rather the subscale moisture flux and its convergence in the vertical. Over the Saharan latitudes, there is considerable convective flux of sensible heat, but not of latent heat. Over the ocean, deep convection in the ITCZ is weaker than over Africa, and it is consistently correlated with upward converging subscale moisture flux. The fields of the subscale vertical fluxes are coherent in space and time. It is argued from these results that the presented diagnostic model is potentially useful for testing parameterizations of convection in general circulation and climate models.With 19 Figures     

Transports and budgets of volume,heat, and salt from a global eddy-resolving ocean model

The results from an integration of a global ocean circulation model have been condensed into an analysis of the volume, heat, and salt transports among the major ocean basins. Transports are also broken down between the model's Ekman, thermocline, and deep layers. Overall, the model does well. Horizontal exchanges of mass, heat, and salt between ocean basins have reasonable values; and the volume of North Atlantic Deep Water (NADW) transport is in general agreement with what limited observations exist. On a global basis the zonally integrated meridional heat transport is poleward at all latitudes except for the latitude band 30°S to 45°S. This anomalous transport is most likely a signature of the model's inability to form Antarctic Intermediate (AAIW) and Antarctic bottom water (AABW) properly. Eddy heat transport is strong at the equator where its convergence heats the equatorial Pacific about twice as much as it heats the equatorial Atlantic. The greater heating in the Pacific suggests that mesoscale eddies may be a vital mechanism for warming and maintaining an upwelling portion of the global conveyor-belt circulation. The model's fresh water transport compares well with observations. However, in the Atlantic there is an excessive southward transport of fresh water due to the absence of the Mediterranean outflow and weak northward flow of AAIW. Eddies in the mid-latitudes act to redistribute heat and salt down the mean gradients. Residual fluxes calculated from a sum of the computed advective (including eddies), forced, and stored fluxes of heat and salt represent transport mostly due to vertical sub-grid scale mixing processes. Perhaps the model's greatest weakness is the lack of strong AAIW and AABW circulation cells. Accurate thermohaline forcing in the North Atlantic (based on numerous hydrographic observations) helps the model adequately produce NADW. In contrast, the southern ocean is an area of sparse observation. Better thermohaline observations in this area may be needed if models such as this are to produce the deep convection that will achieve more accurate simulations of the global 3-dimensional circulation.     

Validation of an annual cycle simulation with a T42-L20 GCM

An annual cycle of an atmospheric general circulation model (AGCM) is presented. The winter and summer zonal averages of the atmospheric fields are compared with an observed climatology. The main features of the observed seasonal means are well reproduced by the model. One of the main discrepancies is that the simulated atmosphere is too cold, particularly in its upper part. Some other discrepancies might be explained by the interannual variability. The AGCM surface fluxes are directly compared to climatological estimates. On the other hand, the calculation of meridional heat transport by the ocean, inferred from the simulated energy budget, can be compared to transport induced from climatologies. The main result of this double comparison is that AGCM fluxes generally are within the range of climatological estimates. The main deficiency of the model is poor partitioning between solar and non-solar heat fluxes in the tropical belt. The meridional heat transport also reveals a significant energy-loss by the Northern Hemisphere ocean north of 45° N. The possible implications of model surface flux deficiencies on coupling with an oceanic model are discussed.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

Ocean heat transport into the Arctic in the twentieth and twenty-first century in EC-Earth

The ocean heat transport into the Arctic and the heat budget of the Barents Sea are analyzed in an ensemble of historical and future climate simulations performed with the global coupled climate model EC-Earth. The zonally integrated northward heat flux in the ocean at 70°N is strongly enhanced and compensates for a reduction of its atmospheric counterpart in the twenty first century. Although an increase in the northward heat transport occurs through all of Fram Strait, Canadian Archipelago, Bering Strait and Barents Sea Opening, it is the latter which dominates the increase in ocean heat transport into the Arctic. Increased temperature of the northward transported Atlantic water masses are the main reason for the enhancement of the ocean heat transport. The natural variability in the heat transport into the Barents Sea is caused to the same extent by variations in temperature and volume transport. Large ocean heat transports lead to reduced ice and higher atmospheric temperature in the Barents Sea area and are related to the positive phase of the North Atlantic Oscillation. The net ocean heat transport into the Barents Sea grows until about year 2050. Thereafter, both heat and volume fluxes out of the Barents Sea through the section between Franz Josef Land and Novaya Zemlya are strongly enhanced and compensate for all further increase in the inflow through the Barents Sea Opening. Most of the heat transported by the ocean into the Barents Sea is passed to the atmosphere and contributes to warming of the atmosphere and Arctic temperature amplification. Latent and sensible heat fluxes are enhanced. Net surface long-wave and solar radiation are enhanced upward and downward, respectively and are almost compensating each other. We find that the changes in the surface heat fluxes are mainly caused by the vanishing sea ice in the twenty first century. The increasing ocean heat transport leads to enhanced bottom ice melt and to an extension of the area with bottom ice melt further northward. However, no indication for a substantial impact of the increased heat transport on ice melt in the Central Arctic is found. Most of the heat that is not passed to the atmosphere in the Barents Sea is stored in the Arctic intermediate layer of Atlantic water, which is increasingly pronounced in the twenty first century.     

On the incompatibility of ocean and atmosphere models and the need for flux adjustments

The surface heat and freshwater fluxes from equilibrium ocean (OGCM) and atmospheric (AGCM) general circulation model climates are examined in order to determine the minimum flux adjustment required to prevent climate drift upon coupling. This is accomplished by integrating an OGCM with specified surface fluxes. It is shown that a dramatic climate drift of the coupled system is inevitable unless ocean meridional heat and freshwater (salt) transports are used as constraints for tuning the AGCM present-day climatology. It is further shown that the magnitude of the mismatch between OGCM and AGCM fluxes is not as important for climate drift as the difference in OGCM and implied AGCM meridional heat and freshwater (salt) transports. Hence a minimum flux adjustment is proposed, which is zonally-uniform in each basin and of small magnitude compared to present flux adjustments. This minimum flux adjustment acts only to correct the AGCM implied oceanic meridional transports of heat and freshwater (salt). A slight extension is also proposed to overcome the drift in the surface waters when the minimum flux adjustment is used. Finally, it is suggested that the flux adjustments which arise from current methods used to determine them are all very similar, leading to adjustment fields which are significantly larger than both AGCM and climatological fields over large regions.     

Surface heat fluxes from a numerical weather prediction system

Twenty-two months (July 1983-April 1985) of surface heat fluxes predicted at day 1 from a numerical weather prediction system have been processed. Monthly means and monthly standard deviations of available surface short-wave, long-wave, latent and sensible heat fluxes as well as annual means have been computed. The global mean of the annual net sea-surface heat flux is about 40 Wm^–2 and is therefore far from equilibrium. When used to force an oceanic model, these fluxes would tend to warm the ocean and would produce an unrealistic transport of heat by the oceanic general circulation. They therefore need to be corrected. This correction appears feasible because the main difference between these fluxes and long-term climatologies appears largely independent of the month and the latitude. This suggests that the errors have a systematic origin. The corrected fluxes allow both the reproduction of a realistic seasonal migration of the zero net heat-flux line and the reproduction of the annual meridional heat transport in the different oceans, within the range of previous estimates.     

Low-frequency variability of the arctic climate: the role of oceanic and atmospheric heat transport variations

Changes in meridional heat transports, carried either by the atmosphere (HTRA) or by the ocean (HTRO), have been proposed to explain the decadal to multidecadal climate variations in the Arctic. On the other hand, model simulations indicate that, at high northern latitudes, variations in HTRA and HTRO are strongly coupled and may even compensate each other. A multi-century control integration with the Max Planck Institute global atmosphere-ocean model is analyzed to investigate the relative role of the HTRO and HTRA variations in shaping the Arctic climate and the consequences of their possible compensation. In the simulation, ocean heat transport anomalies modulate sea ice cover and surface heat fluxes mainly in the Barents Sea/Kara Sea region and the atmosphere responds with a modified pressure field. In response to positive HTRO anomalies there are negative HTRA anomalies associated with an export of relatively warm air southward to Western Siberia and a reduced inflow of heat over Alaska and northern Canada. While the compensation mechanism is prominent in this model, its dominating role is not constant over long time scales. The presence or absence of the compensation is determined mainly by the atmospheric circulation in the Pacific sector of the Arctic where the two leading large-scale atmospheric circulation patterns determine the lateral fluxes with varying contributions. The degree of compensation also determines the heat available to modulate the large-scale Arctic climate. The combined effect of atmospheric and oceanic contributions has to be considered to explain decadal-scale warming or cooling trends.     

Surface flux response to interannual tropical Pacific sea surface temperature variability in AMIP models

 A systematic comparison of observed and modeled atmospheric surface heat and momentum fluxes related to sea surface temperature (SST) variability on interannual time scales in the tropical Pacific is conducted. This is done to examine the ability of atmospheric general circulation models (AGCMs) in the Atmospheric Model Intercomparison Project (AMIP) to simulate the surface fluxes important for driving the ocean on interannual time scales. In order to estimate the model and observed response to such SST variability, various regression calculations are made between a time series representing observed ENSO SST variability in the tropical Pacific and the resulting surface flux anomalies. The models exhibit a range of differences from the observations. Overall the zonal wind stress anomalies are most accurately simulated while the solar radiation anomalies are the least accurately depicted. The deficiencies in the solar radiation are closely related to errors in cloudiness. The total heat flux shows some cancellation of the errors in its components particularly in the central Pacific. The performance of the GCMs in simulating the surface flux anomalies seems to be resolution dependent and low-resolution models tend to exhibit weaker flux responses. The simulated responses in the western Pacific are more variable than those of the central and eastern Pacific but in the west the observed estimates are less robust as well. Further improvements in atmospheric GCM flux simulation through better physical parametrization is clearly required if such models are to be used to their full potential in coupled modeling and climate forecasting. Received: 24 August 1999 / Accepted: 11 September 2000     

The simulation of SST, sea ice extents and ocean heat transports in a version of the Hadley Centre coupled model without flux adjustments

Results are presented from a new version of the Hadley Centre coupled model (HadCM3) that does not require flux adjustments to prevent large climate drifts in the simulation. The model has both an improved atmosphere and ocean component. In particular, the ocean has a 1.25° × 1.25° degree horizontal resolution and leads to a considerably improved simulation of ocean heat transports compared to earlier versions with a coarser resolution ocean component. The model does not have any spin up procedure prior to coupling and the simulation has been run for over 400 years starting from observed initial conditions. The sea surface temperature (SST) and sea ice simulation are shown to be stable and realistic. The trend in global mean SST is less than 0.009 °C per century. In part, the improved simulation is a consequence of a greater compatibility of the atmosphere and ocean model heat budgets. The atmospheric model surface heat and momentum budget are evaluated by comparing with climatological ship-based estimates. Similarly the ocean model simulation of poleward heat transports is compared with direct ship-based observations for a number of sections across the globe. Despite the limitations of the observed datasets, it is shown that the coupled model is able to reproduce many aspects of the observed heat budget. Received: 1 October 1998 / Accepted: 20 July 1999     

Measured And Simulated Latent And Sensible Heat Fluxes At Two Marine Sites In The Baltic Sea

In this study, turbulent heat flux data from two sites within the Baltic Sea are compared with estimates from two models. The main focus is on the latent heat flux. The measuring sites are located on small islands close to the islands of Bornholm and Gotland. Both sites have a wide wind direction sector with undisturbed over-water fetch. Mean parameters and direct fluxes were measured on masts during May to December 1998.The two models used in this study are the regional-scale atmospheric model HIRLAM and the ocean model PROBE-Baltic. It is shown that both models overestimate the sensible and latent heat fluxes. The overestimation can, to a large extent, be explained by errors in the air-water temperature and humidity differences. From comparing observed and modelled data, the estimated 8-month mean errors in temperature and humidity are up to 1 °C and 1 g kg^-1, respectively. The mean errors in the sensible and latent heat fluxes for the same period are approximately 15 and 30 W m^-2, respectively.Bulk transfer coefficients used for calculating heat and humidity fluxes at the surface were shown to agree rather well with the measurements, at least for the unstable data. For stable stratification, the scatter in data is generally large, and it appears that the bulk formulation chosen overestimates turbulent heat fluxes.