Poleward Heat Transport Research Articles

Isotopic evidence for Early to mid-Cretaceous ocean temperature variability

Stable isotopic measurements have been made on planktonic and benthic foraminifera of Early to mid-Cretaceous (Early Albian–Cenomanian) age from DSDP site 463 located in the Pacific Ocean. The degree of alteration of the foraminifera has been assessed through the application of scanning electron microscopy. The oxygen isotope data support the view that during the Cenomanian ‘globigerine’ foraminifera, such as the genus Hedbergella, inhabited near-surface waters, whilst the flattened, keeled, morphotypes lived deeper within the water column. The carbon isotope data reveal a more complex pattern as the δ 13C values from planktonic species essentially show a large overlap, which is interpreted in terms of a weak δ 13C gradient existing in the upper part of the ocean. In contrast, the oxygen isotope data from planktonic foraminifera from the Late Albian display no discernible depth stratification pattern and the δ 13C data also show a large overlap of values indicative of a complete disruption of the upper part of the thermocline. A relatively low surface to bottom water temperature gradient within the Early–Middle Albian part of the succession may reflect a well-mixed ocean. Inferred cool ocean temperatures (and/or high salinities) for the Early Albian may reflect increased oceanic poleward heat transport. An increase in temperature (and/or decrease in salinity) in the Cenomanian suggests a reduction of the poleward heat flux, promoting the build-up of limited polar ice during this period of tropical warmth.

Poleward heat transport by the atmospheric heat engine.

The atmospheric heat transport on Earth from the Equator to the poles is largely carried out by the mid-latitude storms. However, there is no satisfactory theory to describe this fundamental feature of the Earth's climate. Previous studies have characterized the poleward heat transport as a diffusion by eddies of specified horizontal length and velocity scales, but there is little agreement as to what those scales should be. Here we propose instead to regard the baroclinic zone--the zone of strong temperature gradients and active eddies--as a heat engine which generates eddy kinetic energy by transporting heat from a warmer to a colder region. This view leads to a new velocity scale, which we have tested along with previously proposed length and velocity scales, using numerical climate simulations in which the eddy properties have been varied by changing forcing and boundary conditions. The experiments show that the eddy velocity varies in accordance with the new scale, while the size of the eddies varies with the well-known Rhines beta-scale. Our results not only give new insight into atmospheric eddy heat transport, but also allow simple estimates of the intensities of mid-latitude storms, which have hitherto only been possible with expensive general circulation models.

Meltwater and the global ocean conveyor: northern versus southern connections

The sensitivity of the ocean circulation to changes in North Atlantic surface fluxes has become a major factor in explaining climate variability. The role of the Antarctic Bottom Water in modulating this variability has received much less attention, limiting the development of a complete understanding of decadal to millennial time-scale climate change. New analyses indicate that the southern deepwater source may change dramatically (e.g., experience a decrease of as much as two thirds during last 800 years). Such change can substantially alter the ocean circulation patterns of the last millennium. Additional analyses indicate that the Southern Hemisphere led the Northern Hemisphere changes in some of the glacial cycles of Pleistocene, implying a seesaw-type oscillation of the global ocean conveyor. The potential for melting of sea ice and ice sheets in the Antarctica associated with global warming can cause a further slowdown of the southern deepwater source. These results demand an assessment of the role of the Southern Ocean in driving changes of the global ocean circulation and climate. Systematic model simulation targeting the ocean circulation response to changes in surface salinity in the high latitudes of both Northern and Southern Hemispheres demonstrate that meltwater impacts in one hemisphere may lead to a strengthening of the thermohaline conveyor driven by the source in the opposite hemisphere. This, in turn, leads to significant changes in poleward heat transport. Further, meltwater events can lead to deep-sea warming and thermal expansion of abyssal water, that in turn cause a substantial sea-level change even without a major ice sheet melting.

Revisiting the South Pacific subtropical circulation: A synthesis of World Ocean Circulation Experiment observations along 32°S

Hydrographic data about latitudes 30°–32°S in the Pacific Ocean are presented and used together with direct velocity observations from floats and moorings to investigate the spatial distribution of meridional flow and its associated transports. An initial velocity scheme is developed with reference to the water property distributions that is subsequently adjusted to meet constraints on net mass and dissolved silica transports. The resulting circulation is found to be sensitive to a priori scaling assumptions and differs in detail from past estimates of the flow in this region. Additionally constraining the flow with directly measured time averaged estimates of the velocity from floats and a current meter array significantly reduces the uncertainty in the derived flow field and improves consistency between the velocity and the property distributions. The derived circulation includes a nearly closed bottom‐to‐deep‐water overturning circulation of 16–20 Sv, below a subtropical gyre flow of thermocline and surface waters consisting of nearly distinct Tasman Sea (∼30 Sv) and basin interior (∼10 Sv) circulations and (a prescribed) ∼10±5 Sv of net northward flow to supply the Indonesian and Bering Strait throughflows. On the basis of this circulation scheme, the heat flux divergence for the Pacific north of the P6 line is estimated to be 0.75±0.56 pW (ocean gaining heat from the atmosphere) and the net freshwater divergence is estimated to be 0.1±0.1 Sv (ocean gains fresh water via air‐sea exchange and runoff). The former is highly dependent on the strength of the Indonesian throughflow; the latter also depends on the intensity of the lateral subtropical gyre circulation, which is constrained by the float data. Combined with estimates from the Indian and Atlantic Oceans, we find the subtropical Southern Hemisphere oceans transport 0.46±0.38 PW poleward, less than half the poleward heat transport achieved by the subtropical oceans in the Northern Hemisphere.

Increased Pliocene North Atlantic Deep Water: Cause or consequence of Pliocene warming?

Raymo et al. [1996] suggested that the mid‐Pliocene (∼3 Ma) warm period was associated with increased North Atlantic Deep Water (NADW) production. Is this circulation change a cause or consequence of Pliocene warming? We test the hypothesis that increased strength of NADW was a consequence of the warming around Antarctica affecting deep Antarctic outflow. A sensitivity experiment with an ocean general circulation model with Pliocene surface conditions changed only over the Southern Ocean (SO) indicates that warmer temperatures around Antarctica result in lower rates of sea ice formation and SO deep water outflow. The decreased abyssal density gradient in the SO directly leads to about a 20% increase in NADW outflow at 30°S, a 10% increase in NADW overturning in the subpolar North Atlantic, and a 20% increase in poleward heat transport in the North Atlantic. We postulate that the largest initial Pliocene climate change was in the SO because the greater sea ice area in this region is more sensitive to inferred slightly higher CO2 levels in the mid‐Pliocene.

Ocean Feedback in Response to 6 kyr BP Insolation

Abstract The role of the ocean in the 6 kyr BP climate change is investigated by comparing a coupled ocean–atmosphere simulation and a simulation with the same atmospheric component for which SSTs are kept to the modern ones. For these simulations, Earth’s orbital parameters have been changed from their current values to those valid 6 kyr ago. The resulting change in insolation strengthens the seasonal cycle in the Northern Hemisphere, and the summer African and Asian monsoons are more vigorous. The results show that the summer monsoon flow penetrates farther north into the Sahara when ocean feedbacks are included. The SST response lags by 2–3 months the insolation forcing. This delay has an impact on the timing of the monsoon change. In most coastal regions, the climate response is very different, and even of opposite sign, between the coupled and atmosphere-alone experiments, stressing the climatic impact of the ocean in these regions. The impact of the interactive ocean on the poleward heat transport i...

Comparison of Thermally Driven Circulations from a Depth-Coordinate Model and an Isopycnal-Layer Model. Part I: Scaling-Law Sensitivity to Vertical Diffusivity

Abstract Two different types of numerical ocean circulation models are used in a classical idealized problem, the thermally induced circulation in an ocean basin bounded by two meridians to the east and west and by the equator and a line of constant latitude. A simple scaling theory exists for predicting poleward heat transport and the strength of meridional overturning as a function of vertical diffusivity and other external factors. However, previous studies have indicated conflicting results, and other scaling laws have been proposed. Experiments with two widely used types of numerical models, one based on depth coordinates and the other based on isopycnal layers, provide insight into the discrepancies of previous studies. In the numerical experiments vertical diffusivity is varied over a range of 200. The source of the difficulty in previous studies is in part traced to applying a fixed restoring coefficient at the upper boundary and considering the buoyancy forcing at the surface fixed irrespective o...

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.

Climate and the Tropical Oceans*

An attempt is made to determine the role of the ocean in establishing the mean tropical climate and its sensitivity to radiative perturbations. A simple two-box energy balance model is developed that includes ocean heat transports as an interactive component of the tropical climate system. It is found that changes in the zonal mean ocean heat transport can have a considerable affect on the mean tropical sea surface temperature (SST) through their effect on the properties of subtropical marine stratus clouds or on the water vapor greenhouse effect of the tropical atmosphere. The way that the tropical climate adjusts to changes in the ocean heat transport is primarily through the atmospheric heat transport, without changing the net top of the atmosphere radiative balance. Thus, the total amount of low-latitude poleward heat transport is invariant with respect to changes in ocean circulation in this model. These results are compared with analogous experiments with general circulation models. Doubled CO2 experiments are performed with different values of ocean heat transport. It is found that the sensitivity of the mean tropical SST to doubled CO2 depends on the strength of the ocean heat transport due to feedbacks between the ocean and subtropical marine stratus clouds and the water vapor greenhouse effect. In this model, the results are the same whether the ocean heat transports are determined interactively or are fixed. Some recent studies have suggested that an increased meridional overturning in the ocean due to changes in the zonally asymmetric circulation can reduce the sensitivity of the tropical climate to increased CO2. It is found that, in equilibrium, this is not that case, but rather an increase in ocean heat transport, which involves increased equatorial upwelling, actually warms the tropical climate.

An assessment of the Geophysical Fluid Dynamics Laboratory ocean model with coarse resolution: Annual‐mean climatology

The Geophysical Fluid Dynamics Laboratory Modular Ocean Model 2.2 code with coarse resolution (4° × 3°) is assessed by performing three experiments and comparing their equilibrated solutions with recent observationally based analyses (OBAs). The first experiment (E1) uses subgrid‐scale horizontal diffusion and surface boundary conditions which relax surface temperature and salinity toward observations. The second (E2) replaces the physically incorrect heat and moisture flux boundary conditions of E1 by flux conditions taken from OBAs, plus a term relaxing surface temperatures toward observations. The third (E3) uses the same surface boundary conditions as E2 but replaces the horizontal diffusion by the Gent‐McWilliams (GM) parameterization of isopycnal diffusion. Under the restoring surface boundary conditions (E1), the North Atlantic overturning rate is about 17 Sv, smaller than in OBAs, the maximum poleward heat transport in the Northern Hemisphere is 1.2 Petawatts (PW), also smaller than in OBAs, and in the Antarctic Circumpolar Current (ACC) region the poleward heat transport is 1.3 PW, much larger than in OBAs. Under the more realistic flux boundary condition (E2) the overturning rate increases to an unrealistically large level of 40 Sv, and the poleward heat transports are only slightly improved. When the GM parameterization is employed (E3), the overturning is reduced to 28 Sv, and the poleward heat transport in the ACC region is reduced to 0.3 PW; both results are consistent with OBAs. However, there is only a slight further improvement in the poleward heat transport in the Northern Hemisphere, which now has a peak value of 1.6 PW, still about 0.5 PW less than in OBAs. The sea surface temperature errors in E3 are consistent with the conclusion that the heat transport in the Northern Hemisphere is still being underestimated. All the experiments show strong systematic biases in the salinity field.

Late Ordovician glaciation under high atmospheric CO2: A coupled model analysis

The analysis of the geologic record has revealed a question concerning how the Late Ordovician glaciation could have occurred simultaneously with high CO2 levels (10–18x). Sensitivity studies using a coupled atmosphere‐ocean‐sea ice model show that it is possible to maintain a permanent snow cover (which corresponds to 60% of all the glacial deposits found on Gondwana) under 10x CO2 levels, warm fall/cool spring orbital parameters, a 4.5% reduction in solar luminosity, a length of day of 21.5 hours, and an enhanced snow/sea ice albedo of 0.3. A cold summer orbit experiment with 10x CO2 and a reduced snow/sea ice albedo of 0.1 also sustains a permanent (albeit less extensive) snow cover. The geographic configuration of the Late Ordovician results in an up to ∼42% increase in the global ocean poleward heat transport in the Southern Hemisphere relative to present‐day and a significant asymmetry relative to the equator.

The Relative Influence of Diapycnal Mixing and Hydrologic Forcing on the Stability of the Thermohaline Circulation*

Abstract Scaling analysis of the oceanic thermohaline circulation has been done under two types of surface boundary conditions: (i) Under “relaxation” conditions (sea surface temperature and salinity are relaxed to prescribed values), there is a two-thirds power law dependence of the meridional overturning (and the poleward heat transport) on the diapycnal diffusivity. For any given external forcing, there is only one equilibrium state for the thermohaline circulation. (ii) Under “mixed” boundary conditions (temperature is relaxed to prescribed values and a virtual salt flux condition is used for salinity), multiple equilibria become possible. For a given thermal forcing, the existence of multiple equilibria depends on the relative contributions of diapycnal diffusivity and the hydrologic forcing: for each diapycnal diffusivity K, there is a threshold freshwater flux Ec = CK2/3 (C is a constant) below which three modes are possible with one stable thermal mode, one unstable thermal mode, and a stable hali...

On the influence of the parameterization of lateral boundary layers on the thermohaline circulation in coarse-resolution ocean models

Because of the first order geostrophic balance in the ocean interior, the parameterization of lateral boundary layers has more influence than the parameterization of viscosity on the thermohaline overturning and the deep water properties in coarse-resolution ocean circulation models. Different formulations of momentum dissipation and associated boundary conditions are implemented within a planetary-geostrophic ocean circulation model for a Cartesian coordinate, flat-bottomed, β-plane, with restoring boundary conditions for the surface density and zero wind stress. Traditional Laplacian friction with a no-slip boundary condition produces an interior circulation in good agreement with geostrophy and the Sverdrup balance, but generates very large vertical (diapycnal) transports at lateral boundaries, especially upwelling in the western boundary current and downwelling in the northeast corner. The meridional and zonal overturning are thus enhanced, but drive to depth surface waters that are not as cold as the ones in the deep convection regions. Rayleigh friction with various frictional closures for the alongshore velocities within a no-normal-flow boundary condition framework efficiently reduces the diapycnal vertical transports along the boundaries, by allowing horizontal recirculation of geostrophic currents impinging into coasts. Hence, these parameterizations induce weaker overturnings, with colder deep water and a sharper thermocline resulting in higher poleward heat transports. We suggest that the upwelling along the boundaries is a consequence of the coarse-resolution dynamics and not only horizontal diffusion (termed the Veronis effect, horizontal diffusion produces large diapycnal fluxes once the isopycnals are tilted by coastal upwellings). Alternative parameterizations for the lateral boundary layers reduce this effect without the need for rotating the mixing tensor along isopycnals. This model comparison proves the need to clearly assess the extent of the diapycnal upwelling in the western boundary currents and to develop physically-based parameterizations of lateral boundary layers in order to improve coarse-resolution OGCMs.

The response of atmospheric heat transport to zonally-averaged SST trends

We compute the atmospheric heat transport in a realistic atmospheric general circulation modelunder five different configurations of implied heat transport in the ocean. The implied oceanicheat transport is varied by changing the meridional gradient of sea surface temperature (SST).Climatological SSTs are employed for the control run. The other runs differ in that a zonallysymmetric component is added to or subtracted from the climatological SST field. The meridionalstructure of the variation in SST gradient is based on the observed change in zonallyaveraged SST over the last century. The SST trend has maxima of about 1 K at high latitudesof both hemispheres. Elsewhere, the change in SST over the last century is fairly uniform atabout 0.5 K. We find that in the annual mean, the atmosphere adjusts so that the total meridionalheat transport (by atmosphere and ocean) is rather insensitive to the change in zonally averaged SST. Interannual variability in the annual mean heat transport is minor in each of these cases. There is a large degree of compensation even between the different components of atmosphericheat transport such that changes in latent heat transport usually go hand in hand with changesin dry static energy transport of an opposite sign. The radiative flux at the top of the atmosphereis affected the most by the change in SST in the tropics, where the shortwave component showsa strong negative feedback and the longwave component shows a weak positive feedback. Concentrating on the winter season in the Northern Hemisphere, we find that when we decreasethe meridional SST gradient (i.e., warm the sea surface at high latitudes the most), the stationarywaves accomplish more of the poleward heat transport than before. When we increase themeridional SST gradient, the heat flux due to both transient and stationary waves increases, although not by nearly as much as most eddy parameterization schemes would predict. Thewinter season in the Southern Hemisphere shows a substantial increase in heat transport bytransient waves when the meridional SST gradient is increased. Their maximum heat transportis greater and extends over a wider band of latitudes than in the control case. Because theSouthern Hemisphere is mostly covered by ocean, the stationary waves are weak and play aminor roôle in atmospheric heat transport.

Do We Require Adiabatic Dissipation Schemes in Eddy-Resolving Ocean Models?

Use of horizontal diffusion of temperature and salinity in numerical ocean models causes spurious diapycnal transfers—the ‘‘Veronis effect’’—leading to erosion of the thermocline and reduced poleward heat transports. The authors derive a relation between these diapycnal transfers and the dissipation of vorticity gradients. An increase in model resolution does not significantly reduce the diapycnal transfers since vorticity gradients cascade to smaller scales and must ultimately be dissipated to maintain numerical stability. This is confirmed in an idealized primitive equation ocean model at a variety of resolutions between 18 and 1 /88. Thus, the authors conclude that adiabatic dissipation schemes are required, even in eddy-resolving ocean models. The authors propose and implement a new biharmonic form of the Gent and McWilliams scheme, which adiabatically dissipates at the grid scale while preserving larger-scale features.

Effects of the seasonal variation on forming the steady state of an atmosphere-ocean coupled system

A three-dimensional, coupled atmosphere-ocean general circulation model is developed and effects of the seasonal variation on forming a steady-state coupled atmosphere-ocean system are studied. Case studies are carried out for a basin of idealized geometry by changing the period of the seasonal variation in the solar forcing. The coupled system shows significant differences depending on the existence of the seasonal variation, where the heat transport by the oceanic circulation plays a central role. For the regular seasonal variation, a stronger oceanic meridional circulation, which accompanies larger poleward heat transport, is realized compared with that for the annual-mean solar forcing. The stronger meridional circulation is associated with a larger wintertime meridional gradient of the sea surface temperature. The meridional gradient of the annual-mean sea surface temperature is smaller for the regular seasonal variation, which results in a smaller atmospheric poleward heat transport. In consequence, the total of the atmospheric and the oceanic poleward heat transport is almost identical for the two cases. Cases with the seasonal variation of the doubled period and with the time-filtered flux exchange between the atmosphere and the ocean are also studied, showing that the system is not sensitive to those factors.

Electromagnetic forces on the earth's core due to the poleward transport of heat in the oceans

Poleward heat transport in the oceans generate global-scale electric currents and associated Lorentz forces in the Earth's core. We make a first estimate of this effect using a simple zonally averaged model from which we obtain analytical/numerical solutions. The results indicate that the poleward heat transport in the presence of Main field reversals having periods of 10,000 to 10 million years lead to large-scale electric currents in the core of order 10-11 to 10-13 A/m2 which although extremely small, imply poleward forces in the core which are likely to have acted systematically throughout the history of the planet.

A dynamically consistent analysis of circulation and transports in the southwestern Weddell Sea

Abstract. An inverse model is applied for the analysis of hydrographic and current meter data collected on the repeat WOCE section SR4 in the Weddell Sea in 1989–1992. The section crosses the Weddell Sea cyclonic gyre from Kapp Norvegia to the northern end of the Antarctic Peninsula. The concepts of geostrophy, conservation of planetary vorticity and hydrostatics are combined with advective balances of active and passive properties to provide a dynamically consistent circulation pattern. Our variational assimilation scheme allows the calculation of three-dimensional velocities in the section plane. Current speeds are small except along the coasts where they reach up to 12 cm/s. We diagnose a gyre transport of 34 Sverdrup which is associated with a poleward heat transport of 28×1012 W corresponding to an average heat flux of 15 Wm–2 in the Weddell Sea south of the transect. This exceeds the estimated local flux on the transect of 2 Wm–2. As the transect is located mostly in the open ocean, we conclude that the shelf areas contribute significantly to the ocean-atmosphere exchange and are consequently key areas for the contribution of the Weddell Sea to global ocean ventilation. Conversion of water masses occuring south of the section transform 6.6±1.1 Sv of the inflowing warm deep water into approximately equal amounts of Weddell Sea deep water and Weddell Sea bottom water. The volume transport of surface water equals in the in- and outflow. This means that almost all newly formed surface water is involved in the deep and bottom water formation. Comparison with the results obtained by pure velocity interpolation combined with a hydrographic data subset indicates major differences in the derived salt transports and the water mass conversion of the surface water. The differences can be explained by deviations in the structure of the upper ocean currents to which shelf areas contribute significantly. Additionally a rigorous variance analysis is performed. When only hydrographic data are used for the inversion both the gyre transport and the poleward heat transport are substantially lower. They amount to less than 40% of our best estimate while the standard deviations of both quantities are 6.5 Sv and 37×1012 W, respectively. With the help of long-term current meter measurements these errors can be reduced to 2 Sv and 8×1012 W. Our result underlines the importance of velocity data or equivalent information that helps to estimate the absolute velocities.Key words. Oceanography: General (Arctic and antarctic oceanography) · Oceanography: Physical (General circulation; Hydrography)

Northern Hemispheric Interdecadal Variability: A Coupled Air-Sea Mode

A coupled air–sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean–atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific. The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal. The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air–sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration.

Northern Hemispheric Interdecadal Variability: A Coupled Air–Sea Mode

AbstractA coupled air–sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean–atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific.The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal.The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air–sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration.