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Abstract
This paper describes field observations in the Bay of Biscay, and presents convincing evidence for the existence of a broad beam of internal tidal energy propagating downward from a source region on the upper continental slopes, which, after penetrating the deep ocean interior along a theoretical ray path, reaches the abyssal plain, at a depth of about 4.2 km, 58 km from the source. The results concentrate on the region between 50 and 60 km from the generating area, where the beam of energy is below a depth of 2 km. A marked change in phase of the beam (4 hours) was observed over a distance of 12 km at a depth of about 3 km. In addition, semidiurnal currents were noticeably increased near the sea floor. These results are interpreted as demonstrating the occurrence of partial bottom reflection of the internal tidal energy, in agreement with theoretical predictions, at a distance of about 58 km from the generating region on the upper slope, and extend our previous results farther into the ocean.
Abstract
This paper describes field observations in the Bay of Biscay, and presents convincing evidence for the existence of a broad beam of internal tidal energy propagating downward from a source region on the upper continental slopes, which, after penetrating the deep ocean interior along a theoretical ray path, reaches the abyssal plain, at a depth of about 4.2 km, 58 km from the source. The results concentrate on the region between 50 and 60 km from the generating area, where the beam of energy is below a depth of 2 km. A marked change in phase of the beam (4 hours) was observed over a distance of 12 km at a depth of about 3 km. In addition, semidiurnal currents were noticeably increased near the sea floor. These results are interpreted as demonstrating the occurrence of partial bottom reflection of the internal tidal energy, in agreement with theoretical predictions, at a distance of about 58 km from the generating region on the upper slope, and extend our previous results farther into the ocean.
Abstract
In a companion paper, a spinup integration of the North Atlantic Ocean with the Miami isopycnic-coordinate model was presented. The wintertime mixed layer in the central North Atlantic was subject to relatively little change in salinity or depth but cooled markedly, most probably because of heat loss associated with a partial surface relaxation to climatological sea surface temperatures in a region in which the Gulf Stream was too far to the north. This mixed layer cooling caused the isopycnic layers in the model ventilated subtropical gyre to rise and, surprisingly, to warm. While the experiment was not an attempt to simulate changes in the real Atlantic Ocean, it nevertheless appears from observations that, in recent decades, the mixed layer in nature has undergone a change similar to that exhibited by the model mixed layer. Since it is expected that changes in the ventilated subtropical gyre will be governed largely by changes in the mixed layer in the central North Atlantic, from where the ventilating water masses are subducted, one might therefore anticipate similarities between dine changes in the ventilated regions of the gyres in the model and the real world, even though the cause of the mixed layer changes in the real world may have been different from that in the model. The present paper shows that this is indeed so. In particular, the model behavior closely parallels observed changes in the ventilated subtropical gyre reported by Levitus, in a study of differences between two pentads. The degree of similarity between the model and the observations, including in particular warming of the isopycnic surfaces, leads to a proposal that the changes Levitus observed were caused largely by the subduction of water masses from a cooler mixed layer. Historical changes in the characteristics of the warm North Atlantic Central Water may also be explained by this mechanism. Changes in the wind stress or Ekman pumping fields do not necessarily need to be invoked. Overall, the model provides a framework in which observations from a number of different sources can be understood in a coherent fashion and allows new insights to be gained into the interdecadal variability of the Atlantic Ocean.
Abstract
In a companion paper, a spinup integration of the North Atlantic Ocean with the Miami isopycnic-coordinate model was presented. The wintertime mixed layer in the central North Atlantic was subject to relatively little change in salinity or depth but cooled markedly, most probably because of heat loss associated with a partial surface relaxation to climatological sea surface temperatures in a region in which the Gulf Stream was too far to the north. This mixed layer cooling caused the isopycnic layers in the model ventilated subtropical gyre to rise and, surprisingly, to warm. While the experiment was not an attempt to simulate changes in the real Atlantic Ocean, it nevertheless appears from observations that, in recent decades, the mixed layer in nature has undergone a change similar to that exhibited by the model mixed layer. Since it is expected that changes in the ventilated subtropical gyre will be governed largely by changes in the mixed layer in the central North Atlantic, from where the ventilating water masses are subducted, one might therefore anticipate similarities between dine changes in the ventilated regions of the gyres in the model and the real world, even though the cause of the mixed layer changes in the real world may have been different from that in the model. The present paper shows that this is indeed so. In particular, the model behavior closely parallels observed changes in the ventilated subtropical gyre reported by Levitus, in a study of differences between two pentads. The degree of similarity between the model and the observations, including in particular warming of the isopycnic surfaces, leads to a proposal that the changes Levitus observed were caused largely by the subduction of water masses from a cooler mixed layer. Historical changes in the characteristics of the warm North Atlantic Central Water may also be explained by this mechanism. Changes in the wind stress or Ekman pumping fields do not necessarily need to be invoked. Overall, the model provides a framework in which observations from a number of different sources can be understood in a coherent fashion and allows new insights to be gained into the interdecadal variability of the Atlantic Ocean.
Abstract
The control climates of two coupled climate models are intercompared. The first is the third climate configuration of the Met Office Unified Model (HadCM3), while the second, the Coupled Hadley–Isopycnic Model Experiment (CHIME), is identical to the first except for the replacement of its ocean component by the Hybrid-Coordinate Ocean Model (HYCOM). Both models possess realistic and similar ocean heat transports and overturning circulation. However, substantial differences in the vertical structure of the two ocean components are observed, some of which are directly attributed to their different vertical coordinate systems. In particular, the sea surface temperature (SST) in CHIME is biased warm almost everywhere, particularly in the North Atlantic subpolar gyre, in contrast to HadCM3, which is biased cold except in the Southern Ocean. Whereas the HadCM3 ocean warms from just below the surface down to 1000-m depth, a similar warming in CHIME is more pronounced but shallower and confined to the upper 400 m, with cooling below this. This is particularly apparent in the subtropical thermoclines, which become more diffuse in HadCM3, but sharper in CHIME. This is interpreted as resulting from a more rigorously controlled diapycnal mixing in the interior isopycnic ocean in CHIME. Lower interior mixing is also apparent in the better representation and maintenance of key water masses in CHIME, such as Subantarctic Mode Water, Antarctic Intermediate Water, and North Atlantic Deep Water. Finally, the North Pacific SST cold error in HadCM3 is absent in CHIME, and may be related to a difference in the separation position of the Kuroshio. Disadvantages of CHIME include a nonconservation of heat equivalent to 0.5 W m−2 globally, and a warming and salinification of the northwestern Atlantic.
Abstract
The control climates of two coupled climate models are intercompared. The first is the third climate configuration of the Met Office Unified Model (HadCM3), while the second, the Coupled Hadley–Isopycnic Model Experiment (CHIME), is identical to the first except for the replacement of its ocean component by the Hybrid-Coordinate Ocean Model (HYCOM). Both models possess realistic and similar ocean heat transports and overturning circulation. However, substantial differences in the vertical structure of the two ocean components are observed, some of which are directly attributed to their different vertical coordinate systems. In particular, the sea surface temperature (SST) in CHIME is biased warm almost everywhere, particularly in the North Atlantic subpolar gyre, in contrast to HadCM3, which is biased cold except in the Southern Ocean. Whereas the HadCM3 ocean warms from just below the surface down to 1000-m depth, a similar warming in CHIME is more pronounced but shallower and confined to the upper 400 m, with cooling below this. This is particularly apparent in the subtropical thermoclines, which become more diffuse in HadCM3, but sharper in CHIME. This is interpreted as resulting from a more rigorously controlled diapycnal mixing in the interior isopycnic ocean in CHIME. Lower interior mixing is also apparent in the better representation and maintenance of key water masses in CHIME, such as Subantarctic Mode Water, Antarctic Intermediate Water, and North Atlantic Deep Water. Finally, the North Pacific SST cold error in HadCM3 is absent in CHIME, and may be related to a difference in the separation position of the Kuroshio. Disadvantages of CHIME include a nonconservation of heat equivalent to 0.5 W m−2 globally, and a warming and salinification of the northwestern Atlantic.
Abstract
This paper describes a model intercomparison between a Bryan-Cox-type ocean model and an isopycnic-coordinate ocean model. The two models are integrated for 30 years on a domain of the North Atlantic stretching from 20°S to 82°N. The main purpose of this work is to illuminate aspects of the respective models that give a realistic representation of the North Atlantic circulation and the physical processes that occur therein, and to identify those which need to be improved. To this end, the same forcing fields and as many of the same parameter settings as possible were chosen so that differences between the models would be due to distinct model features rather than choice of parameters. Where we felt that using the same setup between the models was inappropriate or impossible, we examined the possible difference this could make to the simulators.
In the isopycnic model, the path of the North Atlantic Current after separation is simulated quite realistically, whereas in the Bryan-Cox model it becomes much too zonal in the central North Atlantic. This difference has an impact on the simulation in the subpolar gyre and in the Greenland-Iceland-Norway basin.
The representation of dense overflows across the Greenland-Iceland-Scotland ridge is found to be the main underlying difference in the way the simulators develop in the two models. The isopycnic model has specified isopycnic and diapycnic mixing, and its deep, dense flows over the ridge system retain their water properties. This is not the case in the Bryan-Cox model, in which the quasi-isopycnal mixing tensor includes both explicit background horizontal mixing (which could have a diapycnic component) and implicit diapycnic mixing arising from a limitation on the allowable slope of the mixing tensor. It is found that in this model the dense overflows mix vigorously with the surrounding warmer, saltier water as they flow over the ridge so that their water properties change relatively quickly as they travel downstream.
The formation of subpolar mode waters occurs primarily in the Irminger Basin in both models, and in the isopycnic model this mode water has reasonable characteristics. In the Bryan-Cox model the processes down-stream of the dense overflows are degraded due to the development of a homogeneous water mass below the surface, originating from the mixed overflow water. This water mass prevents the mixed layer from deepening substantially in the subpolar gyre, and so prevents the formation of realistic amounts of mode water. The growth of this homogeneous water mass may also be at least partly responsible for the zonality of the North Atlantic Current in the Bryan-Cox model.
The results provide guidance on the future development of both types of model.
Abstract
This paper describes a model intercomparison between a Bryan-Cox-type ocean model and an isopycnic-coordinate ocean model. The two models are integrated for 30 years on a domain of the North Atlantic stretching from 20°S to 82°N. The main purpose of this work is to illuminate aspects of the respective models that give a realistic representation of the North Atlantic circulation and the physical processes that occur therein, and to identify those which need to be improved. To this end, the same forcing fields and as many of the same parameter settings as possible were chosen so that differences between the models would be due to distinct model features rather than choice of parameters. Where we felt that using the same setup between the models was inappropriate or impossible, we examined the possible difference this could make to the simulators.
In the isopycnic model, the path of the North Atlantic Current after separation is simulated quite realistically, whereas in the Bryan-Cox model it becomes much too zonal in the central North Atlantic. This difference has an impact on the simulation in the subpolar gyre and in the Greenland-Iceland-Norway basin.
The representation of dense overflows across the Greenland-Iceland-Scotland ridge is found to be the main underlying difference in the way the simulators develop in the two models. The isopycnic model has specified isopycnic and diapycnic mixing, and its deep, dense flows over the ridge system retain their water properties. This is not the case in the Bryan-Cox model, in which the quasi-isopycnal mixing tensor includes both explicit background horizontal mixing (which could have a diapycnic component) and implicit diapycnic mixing arising from a limitation on the allowable slope of the mixing tensor. It is found that in this model the dense overflows mix vigorously with the surrounding warmer, saltier water as they flow over the ridge so that their water properties change relatively quickly as they travel downstream.
The formation of subpolar mode waters occurs primarily in the Irminger Basin in both models, and in the isopycnic model this mode water has reasonable characteristics. In the Bryan-Cox model the processes down-stream of the dense overflows are degraded due to the development of a homogeneous water mass below the surface, originating from the mixed overflow water. This water mass prevents the mixed layer from deepening substantially in the subpolar gyre, and so prevents the formation of realistic amounts of mode water. The growth of this homogeneous water mass may also be at least partly responsible for the zonality of the North Atlantic Current in the Bryan-Cox model.
The results provide guidance on the future development of both types of model.
Abstract
In a companion paper, two ocean general circulation models were implemented in order to simulate and intercompare the main features of the North Atlantic circulation: the Atlantic Isopycnic Model (AIM) and the Hadley Centre Bryan-Cox-type ocean model (HC). Starting from the same initial state and using the same mechanical and thermohaline forcing datasets, both models were spun up from rest for 30 years. This paper examines the western boundary currents, meridional heat transport and subtropical gyre ventilation.
AIM transports more heat poleward in the subtropics (with peak annual-mean meridional heat transport of 0.63 PW) than HC (which transports up to 0.48 PW), a difference that arises primarily due to surface-poleward and deep-equatorward flows, which are stronger, and at warmer and colder extremes, than in HC. However, HC displays stronger heat transport across the subpolar gyre (with a secondary maximum of 0.36 PW compared to 0.24 PW in AIM), consistent with stronger subpolar gyre heat gain (due to a more zonal North Atlantic Current path, leading to larger relaxation surface heat fluxes).
To quantify the effect of diapycnic mixing and bathymetry two separate 30-year integrations of the isopycnic model, without diapycnal mixing and with the same bathymetry as HC, were undertaken. The isopycnic model is relatively insensitive to these two aspects of model setup on the 30-year timescale.
Both models develop subtropical gyres of annual mean strength ∼45 Sv (Sv ≡ 106 m3 s−1 (due to essentially identical Sverdrup responses), although AIM displays stronger seasonal cycles of Gulf Stream transport than HC (probably due to differences in topographic responses). At subtropical latitudes deep western boundary currents are weaker in AIM (∼5 Sv) than in HC (∼10 Sv). although in HC them is an approximate halving in strength of the DWBC as it progresses south of Florida, due to abyssal recirculation and upwelling.
In the subtropical gyro AIM displays a clear pattern of ventilation, and potential vorticity is, to a large degree, conserved along particle trajectories inside the thermocline. Ventilation pathways are less sharply defined in HC and, compared to AIM, horizontal mixing of temperature and salinity more strongly limits the degree to which water properties (including potential vorticity) are conserved along isopycnals. Both models annually renew realistic quantities of subtropical mode water, AIM forming 15 Sv compared to 20 Sv in HC. Subsurface isopycnal warming in AIM is related to 30-year trends of surface cooling with little corresponding change in salinity. Subsurface isopycnal cooling in HC is due to surface cooling and freshening.
Abstract
In a companion paper, two ocean general circulation models were implemented in order to simulate and intercompare the main features of the North Atlantic circulation: the Atlantic Isopycnic Model (AIM) and the Hadley Centre Bryan-Cox-type ocean model (HC). Starting from the same initial state and using the same mechanical and thermohaline forcing datasets, both models were spun up from rest for 30 years. This paper examines the western boundary currents, meridional heat transport and subtropical gyre ventilation.
AIM transports more heat poleward in the subtropics (with peak annual-mean meridional heat transport of 0.63 PW) than HC (which transports up to 0.48 PW), a difference that arises primarily due to surface-poleward and deep-equatorward flows, which are stronger, and at warmer and colder extremes, than in HC. However, HC displays stronger heat transport across the subpolar gyre (with a secondary maximum of 0.36 PW compared to 0.24 PW in AIM), consistent with stronger subpolar gyre heat gain (due to a more zonal North Atlantic Current path, leading to larger relaxation surface heat fluxes).
To quantify the effect of diapycnic mixing and bathymetry two separate 30-year integrations of the isopycnic model, without diapycnal mixing and with the same bathymetry as HC, were undertaken. The isopycnic model is relatively insensitive to these two aspects of model setup on the 30-year timescale.
Both models develop subtropical gyres of annual mean strength ∼45 Sv (Sv ≡ 106 m3 s−1 (due to essentially identical Sverdrup responses), although AIM displays stronger seasonal cycles of Gulf Stream transport than HC (probably due to differences in topographic responses). At subtropical latitudes deep western boundary currents are weaker in AIM (∼5 Sv) than in HC (∼10 Sv). although in HC them is an approximate halving in strength of the DWBC as it progresses south of Florida, due to abyssal recirculation and upwelling.
In the subtropical gyro AIM displays a clear pattern of ventilation, and potential vorticity is, to a large degree, conserved along particle trajectories inside the thermocline. Ventilation pathways are less sharply defined in HC and, compared to AIM, horizontal mixing of temperature and salinity more strongly limits the degree to which water properties (including potential vorticity) are conserved along isopycnals. Both models annually renew realistic quantities of subtropical mode water, AIM forming 15 Sv compared to 20 Sv in HC. Subsurface isopycnal warming in AIM is related to 30-year trends of surface cooling with little corresponding change in salinity. Subsurface isopycnal cooling in HC is due to surface cooling and freshening.
Abstract
This paper describes a 30-yr spinup experiment of the North Atlantic Ocean with the Miami isopycnic-coordinate ocean model, which, when compared with previous experiments, possesses improved horizontal resolution, surface forcing functions, and bathymetry, and is extended to higher latitudes. Overall, there is a conversion of lighter to heavier water masses, and waters of densities 1027.95 and 1028.05 kg m−3 are produced in the Greenland-lceland Norwegian basin, and of density 1027.75 kg m−3 in the Labrador and Irminger basins. These water masses flow primarily southward. The main purpose of this present study, however, is to investigate the ventilation of the subtropical gyre. The role of Ekman pumping and lateral induction in driving the subduction process is examined and the relative importance of the latter is confirmed. The paper also illustrates how the mixed layer waters are drawn southward and westward into the ocean interior in a continuous spectrum of mode waters with densities ranging between 1026.40 and 1027.30 kg m−3. These are organized into a regular fashion by the model from a relatively disorganized initial state. The evolution of the model gyre during spinup is governed by mixed layer cooling in the central North Atlantic, which causes the ventilation patterns to move southwestward, the layers to rise, and surprisingly, to become warmer. This warming is explained by thermodynamic considerations. Finally, it is shown that the rate of change of potential vorticity following a fluid pathway in the subtropical gyre is governed by the diffusion of layer thickness, which represents subgrid-scale mixing processes in the model. This leads to increasing potential vorticity along pathways that ventilate from the thickest outcrop regions as fluid is diffused laterally and to decreasing potential vorticity along neighboring trajectories.
Abstract
This paper describes a 30-yr spinup experiment of the North Atlantic Ocean with the Miami isopycnic-coordinate ocean model, which, when compared with previous experiments, possesses improved horizontal resolution, surface forcing functions, and bathymetry, and is extended to higher latitudes. Overall, there is a conversion of lighter to heavier water masses, and waters of densities 1027.95 and 1028.05 kg m−3 are produced in the Greenland-lceland Norwegian basin, and of density 1027.75 kg m−3 in the Labrador and Irminger basins. These water masses flow primarily southward. The main purpose of this present study, however, is to investigate the ventilation of the subtropical gyre. The role of Ekman pumping and lateral induction in driving the subduction process is examined and the relative importance of the latter is confirmed. The paper also illustrates how the mixed layer waters are drawn southward and westward into the ocean interior in a continuous spectrum of mode waters with densities ranging between 1026.40 and 1027.30 kg m−3. These are organized into a regular fashion by the model from a relatively disorganized initial state. The evolution of the model gyre during spinup is governed by mixed layer cooling in the central North Atlantic, which causes the ventilation patterns to move southwestward, the layers to rise, and surprisingly, to become warmer. This warming is explained by thermodynamic considerations. Finally, it is shown that the rate of change of potential vorticity following a fluid pathway in the subtropical gyre is governed by the diffusion of layer thickness, which represents subgrid-scale mixing processes in the model. This leads to increasing potential vorticity along pathways that ventilate from the thickest outcrop regions as fluid is diffused laterally and to decreasing potential vorticity along neighboring trajectories.
Abstract
A global isopycnal coordinate GCM is used to investigate the processes that drive the meridional circulation, transformation, and interocean exchange of water masses in the Southern Ocean. The noneddy-resolving model (mesh size 1.25°) includes an active mixed layer, parameterized bolus transport, and seasonally varying surface fluxes. The model gives a plausible picture of the formation and circulation of subantarctic mode water (SAMW) and Antarctic Intermediate Water (AAIW). Progressively denser versions of SAMW and AAIW form in the Indian and Pacific Oceans as the Antarctic Circumpolar Current drifts south and loses buoyancy.
SAMW forms predominantly in the Indian Ocean, at a rate of 20 Sv (Sv ≡ 106 m3 s−1), while AAIW forms mainly in the Pacific sector, at a rate of 8.5 Sv. Throughout the circumpolar zone 25°–42.5°S, there is a net formation of 11 Sv of SAMW, largely by surface cooling. This SAMW is exported northward across 25°S into the subtropical gyres. The properties, distribution, and recirculation of SAMW and AAIW compare well with observations. The authors differentiate the effects of surface fluxes and mixing in transforming water masses in two distinct circumpolar zones. South of 42.5°S, surface buoyancy gain (due to a slight dominance of freshening over cooling) and diapycnal mixing are shown to play a roughly equal role in lightening water (at a peak diapycnal flux of 9 Sv across σ = 27.3), and in forming AAIW.
The meridional overturning is computed as a function of density and decomposed. The parameterized bolus transport opposes the northward surface Ekman drift and southward deep geostrophic flow. Denser waters are not in steady state and the meridional overturning streamfunction gives a misleading impression of dense water transformation in the Southern Ocean. A “transformation streamfunction” is introduced that gives the correct (model) transformation rates; this is believed to be a powerful tool in diagnosing models that drift.
The implications for model North Atlantic Deep Water (NADW) are considerable. In the model, NADW is transported southward across 25°S in the Atlantic sector at a rate of 15.7 Sv. South of 25°S, NADW and Circumpolar Deep Water (CDW) are consumed by interior diapycnal mixing at a rate of 5.7 Sv. NADW and CDW are exported northward across 25°S in the Indo-Pacific sector at a rate of 19.5 Sv. The 9.5 Sv imbalance amounts to a steady loss of NADW and CDW from the Southern Ocean, highlighting the unsteadiness of dense water masses in the model.
Abstract
A global isopycnal coordinate GCM is used to investigate the processes that drive the meridional circulation, transformation, and interocean exchange of water masses in the Southern Ocean. The noneddy-resolving model (mesh size 1.25°) includes an active mixed layer, parameterized bolus transport, and seasonally varying surface fluxes. The model gives a plausible picture of the formation and circulation of subantarctic mode water (SAMW) and Antarctic Intermediate Water (AAIW). Progressively denser versions of SAMW and AAIW form in the Indian and Pacific Oceans as the Antarctic Circumpolar Current drifts south and loses buoyancy.
SAMW forms predominantly in the Indian Ocean, at a rate of 20 Sv (Sv ≡ 106 m3 s−1), while AAIW forms mainly in the Pacific sector, at a rate of 8.5 Sv. Throughout the circumpolar zone 25°–42.5°S, there is a net formation of 11 Sv of SAMW, largely by surface cooling. This SAMW is exported northward across 25°S into the subtropical gyres. The properties, distribution, and recirculation of SAMW and AAIW compare well with observations. The authors differentiate the effects of surface fluxes and mixing in transforming water masses in two distinct circumpolar zones. South of 42.5°S, surface buoyancy gain (due to a slight dominance of freshening over cooling) and diapycnal mixing are shown to play a roughly equal role in lightening water (at a peak diapycnal flux of 9 Sv across σ = 27.3), and in forming AAIW.
The meridional overturning is computed as a function of density and decomposed. The parameterized bolus transport opposes the northward surface Ekman drift and southward deep geostrophic flow. Denser waters are not in steady state and the meridional overturning streamfunction gives a misleading impression of dense water transformation in the Southern Ocean. A “transformation streamfunction” is introduced that gives the correct (model) transformation rates; this is believed to be a powerful tool in diagnosing models that drift.
The implications for model North Atlantic Deep Water (NADW) are considerable. In the model, NADW is transported southward across 25°S in the Atlantic sector at a rate of 15.7 Sv. South of 25°S, NADW and Circumpolar Deep Water (CDW) are consumed by interior diapycnal mixing at a rate of 5.7 Sv. NADW and CDW are exported northward across 25°S in the Indo-Pacific sector at a rate of 19.5 Sv. The 9.5 Sv imbalance amounts to a steady loss of NADW and CDW from the Southern Ocean, highlighting the unsteadiness of dense water masses in the model.
Abstract
An ocean mixed layer heat budget methodology is used to investigate the physical processes determining subpolar North Atlantic (SPNA) sea surface temperature (SST) and ocean heat content (OHC) variability on decadal to multidecadal time scales using the state-of-the-art climate model HadGEM3-GC2. New elements include development of an equation for evolution of anomalous SST for interannual and longer time scales in a form analogous to that for OHC, parameterization of the diffusive heat flux at the base of the mixed layer, and analysis of a composite Atlantic meridional overturning circulation (AMOC) event. Contributions to OHC and SST variability from two sources are evaluated: 1) net ocean–atmosphere heat flux and 2) all other processes, including advection, diffusion, and entrainment for SST. Anomalies in OHC tendency propagate anticlockwise around the SPNA on multidecadal time scales with a clear relationship to the phase of the AMOC. AMOC anomalies lead SST tendencies, which in turn lead OHC tendencies in both the eastern and western SPNA. OHC and SST variations in the SPNA on decadal time scales are dominated by AMOC variability because it controls variability of advection, which is shown to be the dominant term in the OHC budget. Lags between OHC and SST are traced to differences between the advection term for OHC and the advection–entrainment term for SST. The new results have implications for interpretation of variations in Atlantic heat uptake in the CMIP6 climate model assessment.
Abstract
An ocean mixed layer heat budget methodology is used to investigate the physical processes determining subpolar North Atlantic (SPNA) sea surface temperature (SST) and ocean heat content (OHC) variability on decadal to multidecadal time scales using the state-of-the-art climate model HadGEM3-GC2. New elements include development of an equation for evolution of anomalous SST for interannual and longer time scales in a form analogous to that for OHC, parameterization of the diffusive heat flux at the base of the mixed layer, and analysis of a composite Atlantic meridional overturning circulation (AMOC) event. Contributions to OHC and SST variability from two sources are evaluated: 1) net ocean–atmosphere heat flux and 2) all other processes, including advection, diffusion, and entrainment for SST. Anomalies in OHC tendency propagate anticlockwise around the SPNA on multidecadal time scales with a clear relationship to the phase of the AMOC. AMOC anomalies lead SST tendencies, which in turn lead OHC tendencies in both the eastern and western SPNA. OHC and SST variations in the SPNA on decadal time scales are dominated by AMOC variability because it controls variability of advection, which is shown to be the dominant term in the OHC budget. Lags between OHC and SST are traced to differences between the advection term for OHC and the advection–entrainment term for SST. The new results have implications for interpretation of variations in Atlantic heat uptake in the CMIP6 climate model assessment.
Abstract
This article describes the development and evaluation of the U.K.’s new High-Resolution Global Environmental Model (HiGEM), which is based on the latest climate configuration of the Met Office Unified Model, known as the Hadley Centre Global Environmental Model, version 1 (HadGEM1). In HiGEM, the horizontal resolution has been increased to 0.83° latitude × 1.25° longitude for the atmosphere, and 1/3° × 1/3° globally for the ocean. Multidecadal integrations of HiGEM, and the lower-resolution HadGEM, are used to explore the impact of resolution on the fidelity of climate simulations.
Generally, SST errors are reduced in HiGEM. Cold SST errors associated with the path of the North Atlantic drift improve, and warm SST errors are reduced in upwelling stratocumulus regions where the simulation of low-level cloud is better at higher resolution. The ocean model in HiGEM allows ocean eddies to be partially resolved, which dramatically improves the representation of sea surface height variability. In the Southern Ocean, most of the heat transports in HiGEM is achieved by resolved eddy motions, which replaces the parameterized eddy heat transport in the lower-resolution model. HiGEM is also able to more realistically simulate small-scale features in the wind stress curl around islands and oceanic SST fronts, which may have implications for oceanic upwelling and ocean biology.
Higher resolution in both the atmosphere and the ocean allows coupling to occur on small spatial scales. In particular, the small-scale interaction recently seen in satellite imagery between the atmosphere and tropical instability waves in the tropical Pacific Ocean is realistically captured in HiGEM. Tropical instability waves play a role in improving the simulation of the mean state of the tropical Pacific, which has important implications for climate variability. In particular, all aspects of the simulation of ENSO (spatial patterns, the time scales at which ENSO occurs, and global teleconnections) are much improved in HiGEM.
Abstract
This article describes the development and evaluation of the U.K.’s new High-Resolution Global Environmental Model (HiGEM), which is based on the latest climate configuration of the Met Office Unified Model, known as the Hadley Centre Global Environmental Model, version 1 (HadGEM1). In HiGEM, the horizontal resolution has been increased to 0.83° latitude × 1.25° longitude for the atmosphere, and 1/3° × 1/3° globally for the ocean. Multidecadal integrations of HiGEM, and the lower-resolution HadGEM, are used to explore the impact of resolution on the fidelity of climate simulations.
Generally, SST errors are reduced in HiGEM. Cold SST errors associated with the path of the North Atlantic drift improve, and warm SST errors are reduced in upwelling stratocumulus regions where the simulation of low-level cloud is better at higher resolution. The ocean model in HiGEM allows ocean eddies to be partially resolved, which dramatically improves the representation of sea surface height variability. In the Southern Ocean, most of the heat transports in HiGEM is achieved by resolved eddy motions, which replaces the parameterized eddy heat transport in the lower-resolution model. HiGEM is also able to more realistically simulate small-scale features in the wind stress curl around islands and oceanic SST fronts, which may have implications for oceanic upwelling and ocean biology.
Higher resolution in both the atmosphere and the ocean allows coupling to occur on small spatial scales. In particular, the small-scale interaction recently seen in satellite imagery between the atmosphere and tropical instability waves in the tropical Pacific Ocean is realistically captured in HiGEM. Tropical instability waves play a role in improving the simulation of the mean state of the tropical Pacific, which has important implications for climate variability. In particular, all aspects of the simulation of ENSO (spatial patterns, the time scales at which ENSO occurs, and global teleconnections) are much improved in HiGEM.
