Statistical downscaling of sea-surface wind over the Peru�CChile upwelling region: diagnosing the impact of climate change from the IPSL-CM4 model

The key aspect of the ocean circulation off Peru?CChile is the wind-driven upwelling of deep, cold, nutrient-rich waters that promote a rich marine ecosystem. It has been suggested that global warming may be associated with an intensification of upwelling-favorable winds. However, the lack of high-resolution long-term observations has been a limitation for a quantitative analysis of this process. In this study, we use a statistical downscaling method to assess the regional impact of climate change on the sea-surface wind over the Peru?CChile upwelling region as simulated by the global coupled general circulation model IPSL-CM4. Taking advantage of the high-resolution QuikSCAT wind product and of the NCEP reanalysis data, a statistical model based on multiple linear regressions is built for the daily mean meridional and zonal wind at 10?m for the period 2000?C2008. The large-scale 10?m wind components and sea level pressure are used as regional circulation predictors. The skill of the downscaling method is assessed by comparing with the surface wind derived from the ERS satellite measurements, with in situ wind observations collected by ICOADS and through cross-validation. It is then applied to the outputs of the IPSL-CM4 model over stabilized periods of the pre-industrial, 2?×?CO₂ and 4?×?CO₂ IPCC climate scenarios. The results indicate that surface along-shore winds off central Chile (off central Peru) experience a significant intensification (weakening) during Austral winter (summer) in warmer climates. This is associated with a general decrease in intra-seasonal variability.     

On the response of the oceanic wind-driven circulation to atmospheric CO2 increase

A global, flux-corrected climate model is employed to predict the surface wind stress and associated wind-driven oceanic circulation for climate states corresponding to a doubling and quadrupling of the atmospheric CO₂ concentration in a simple 1% per year CO₂ increase scenario. The model indicates that in response to CO₂ increase, the position of zero wind stress curl in the mid-latitudes of the Southern Hemisphere shifts poleward. In addition, the wind stress intensifies significantly in the mid-latitudes of the Southern Hemisphere. As a result, the rate of water circulation in the subpolar meridional overturning cell in the Southern Ocean increases by about 6 Sv (1 Sv=10⁶ m³ s⁻¹) for doubled CO₂ and by 12 Sv for quadrupled CO₂, implying an increase of deep water upwelling south of the circumpolar flow and an increase of Ekman pumping north of it. In addition, the changes in the wind stress and wind stress curl translate into changes in the horizontal mass transport, leading to a poleward expansion of the subtropical gyres in both hemispheres, and to strengthening of the Antarctic Circumpolar Current. Finally, the intensified near-surface winds over the Southern Ocean result in a substantial increase of mechanical energy supply to the ocean general circulation.     

What dynamics drive future wind scenarios for coastal upwelling off Peru and Chile?

The dynamics of the Peru–Chile upwelling system (PCUS) are primarily driven by alongshore wind stress and curl, like in other eastern boundary upwelling systems. Previous studies have suggested that upwelling-favorable winds would increase under climate change, due to an enhancement of the thermally-driven cross-shore pressure gradient. Using an atmospheric model on a stretched grid with increased horizontal resolution in the PCUS, a dynamical downscaling of climate scenarios from a global coupled general circulation model (CGCM) is performed to investigate the processes leading to sea-surface wind changes. Downscaled winds associated with present climate show reasonably good agreement with climatological observations. Downscaled winds under climate change show a strengthening off central Chile south of 35°S (at 30°S–35°S) in austral summer (winter) and a weakening elsewhere. An alongshore momentum balance shows that the wind slowdown (strengthening) off Peru and northern Chile (off central Chile) is associated with a decrease (an increase) in the alongshore pressure gradient. Whereas the strengthening off Chile is likely due to the poleward displacement and intensification of the South Pacific Anticyclone, the slowdown off Peru may be associated with increased precipitation over the tropics and associated convective anomalies, as suggested by a vorticity budget analysis. On the other hand, an increase in the land–sea temperature difference is not found to drive similar changes in the cross-shore pressure gradient. Results from another atmospheric model with distinct CGCM forcing and climate scenarios suggest that projected wind changes off Peru are sensitive to concurrent changes in sea surface temperature and rainfall.     

Parallel climate model (PCM) control and transient simulations

The Department of Energy (DOE) supported Parallel Climate Model (PCM) makes use of the NCAR Community Climate Model (CCM3) and Land Surface Model (LSM) for the atmospheric and land surface components, respectively, the DOE Los Alamos National Laboratory Parallel Ocean Program (POP) for the ocean component, and the Naval Postgraduate School sea-ice model. The PCM executes on several distributed and shared memory computer systems. The coupling method is similar to that used in the NCAR Climate System Model (CSM) in that a flux coupler ties the components together, with interpolations between the different grids of the component models. Flux adjustments are not used in the PCM. The ocean component has 2/3° average horizontal grid spacing with 32 vertical levels and a free surface that allows calculation of sea level changes. Near the equator, the grid spacing is approximately 1/2° in latitude to better capture the ocean equatorial dynamics. The North Pole is rotated over northern North America thus producing resolution smaller than 2/3° in the North Atlantic where the sinking part of the world conveyor circulation largely takes place. Because this ocean model component does not have a computational point at the North Pole, the Arctic Ocean circulation systems are more realistic and similar to the observed. The elastic viscous plastic sea ice model has a grid spacing of 27?km to represent small-scale features such as ice transport through the Canadian Archipelago and the East Greenland current region. Results from a 300?year present-day coupled climate control simulation are presented, as well as for a transient 1% per year compound CO₂ increase experiment which shows a global warming of 1.27?°C for a 10?year average at the doubling point of CO₂ and 2.89?°C at the quadrupling point. There is a gradual warming beyond the doubling and quadrupling points with CO₂ held constant. Globally averaged sea level rise at the time of CO₂ doubling is approximately 7?cm and at the time of quadrupling it is 23?cm. Some of the regional sea level changes are larger and reflect the adjustments in the temperature, salinity, internal ocean dynamics, surface heat flux, and wind stress on the ocean. A 0.5% per year CO₂ increase experiment also was performed showing a global warming of 1.5?°C around the time of CO₂ doubling and a similar warming pattern to the 1% CO₂ per year increase experiment. El Niño and La Niña events in the tropical Pacific show approximately the observed frequency distribution and amplitude, which leads to near observed levels of variability on interannual time scales.     

Heat balance and eddies in the Peru-Chile current system

The Peru-Chile current System (PCS) is a region of persistent biases in global climate models. It has strong coastal upwelling, alongshore boundary currents, and mesoscale eddies. These oceanic phenomena provide essential heat transport to maintain a cool oceanic surface underneath the prevalent atmospheric stratus cloud deck, through a combination of mean circulation and eddy flux. We demonstrate these behaviors in a regional, quasi-equilibrium oceanic model that adequately resolves the mesoscale eddies with climatological forcing. The key result is that the atmospheric heating is large (>50 W m^?2) over a substantial strip >500 km wide off the coast of Peru, and the balancing lateral oceanic flux is much larger than provided by the offshore Ekman flux alone. The atmospheric heating is weaker and the coastally influenced strip is narrower off Chile, but again the Ekman flux is not sufficient for heat balance. The eddy contribution to the oceanic flux is substantial. Analysis of eddy properties shows strong surface temperature fronts and associated large vorticity, especially off Peru. Cyclonic eddies moderately dominate the surface layer, and anticyclonic eddies, originating from the nearshore poleward Peru-Chile Undercurrent (PCUC), dominate the subsurface, especially off Chile. The sensitivity of the PCS heat balance to equatorial intra-seasonal oscillations is found to be small. We demonstrate that forcing the regional model with a representative, coarse-resolution global reanalysis wind product has dramatic and deleterious consequences for the oceanic circulation and climate heat balance, the eddy heat flux in particular.     

Climate sensitivity due to increased CO2: experiments with a coupled atmosphere and ocean general circulation model

A version of the National Center for Atmospheric Research community climate model — a global, spectral (R15) general circulation model — is coupled to a coarse-grid (5° latitude-] longitude, four-layer) ocean general circulation model to study the response of the climate system to increases of atmospheric carbon dioxide (CO₂). Three simulations are run: one with an instantaneous doubling of atmospheric CO₂ (from 330 to 660 ppm), another with the CO₂ concentration starting at 330 ppm and increasing linearly at a rate of 1% per year, and a third with CO₂ held constant at 330 pm. Results at the end of 30 years of simulation indicate a globally averaged surface air temperature increase of 1.6° C for the instantaneous doubling case and 0.7°C for the transient forcing case. Inherent characteristics of the coarse-grid ocean model flow sea-surface temperatures (SSTs) in the tropics and higher-than-observed SSTs and reduced sea-ice extent at higher latitudes] produce lower sensitivity in this model after 30 years than in earlier simulations with the same atmosphere coupled to a 50-m, slab-ocean mixed layer. Within the limitations of the simulated meridional overturning, the thermohaline circulation weakens in the coupled model with doubled CO₂ as the high-latitude ocean-surface layer warms and freshens and westerly wind stress is decreased. In the transient forcing case with slowly increasing CO₂ (30% increase after 30 years), the zonal mean warming of the ocean is most evident in the surface layer near 30°–50° S. Geographical plots of surface air temperature change in the transient case show patterns of regional climate anomalies that differ from those in the instantaneous CO₂ doubling case, particularly in the North Atlantic and northern European regions. This suggests that differences in CO₂ forcing in the climate system are important in CO₂ response in regard to time-dependent climate anomaly regimes. This confirms earlier studies with simple climate models that instantaneous CO₂ doubling simulations may not be analogous in all respects to simulations with slowly increasing CO₂.A portion of this study is supported by the US Department of Energy as part of its Carbon Dioxide Research Program     

Tropospheric adjustment to increasing CO2: its timescale and the role of land–sea contrast

Physical processes responsible for tropospheric adjustment to increasing carbon dioxide concentration are investigated using abrupt CO₂ quadrupling experiments of a general circulation model (GCM) called the model for interdisciplinary research on climate version 5 with several configurations including a coupled atmosphere–ocean GCM, atmospheric GCM, and aqua-planet model. A similar experiment was performed in weather forecast mode to explore timescales of the tropospheric adjustment. We found that the shortwave component of the cloud radiative effect (SWcld) reaches its equilibrium within 2 days of the abrupt CO₂ increase. The change in SWcld is positive, associated with reduced clouds in the lower troposphere due to warming and drying by instantaneous radiative forcing. A reduction in surface turbulent heat fluxes and increase of the near-surface stability result in shoaling of the marine boundary layer, which shifts the cloud layer downward. These changes are common to all experiments regardless of model configuration, indicating that the cloud adjustment is primarily independent of air–sea coupling and land–sea thermal contrast. The role of land in cloud adjustment is further examined by a series of idealized aqua-planet experiments, with a rectangular continent of varying width. Land surface warming from quadrupled CO₂ induces anomalous upward motion, which increases high cloud and associated negative SWcld over land. The geographic distribution of continents regulates the spatial pattern of the cloud adjustment. A larger continent produces more negative SWcld, which partly compensates for a positive SWcld over the ocean. The land-induced negative adjustment is a factor but not necessary requirement for the tropospheric adjustment.     

Impact of extreme CO2 levels on tropical climate: a CGCM study

A coupled general circulation model has been used to perform a set of experiments with high CO₂ concentration (2, 4, 16 times the present day mean value). The experiments have been analyzed to study the response of the climate system to strong radiative forcing in terms of the processes involved in the adjustment at the ocean–atmosphere interface. The analysis of the experiments revealed a non-linear response of the mean state of the atmosphere and ocean to the increase in the carbon dioxide concentration. In the 16 × CO₂ experiment the equilibrium at the ocean–atmosphere interface is characterized by an atmosphere with a shut off of the convective precipitation in the tropical Pacific sector, associated with air warmer than the ocean below. A cloud feedback mechanism is found to be involved in the increased stability of the troposphere. In this more stable condition the mean total precipitation is mainly due to large-scale moisture flux even in the tropics. In the equatorial Pacific Ocean the zonal temperature gradient of both surface and sub-surface waters is significantly smaller in the 16 × CO₂ experiment than in the control experiment. The thermocline slope and the zonal wind stress decrease as well. When the CO₂ concentration increases by about two and four times with respect to the control experiment there is an intensification of El Niño. On the other hand, in the experiment with 16 times the present-day value of CO₂, the Tropical Pacific variability weakens, suggesting the possibility of the establishment of permanent warm conditions that look like the peak of El Niño.     

On the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing

An ocean general circulation model coupled to an energy-moisture balance atmosphere model is used to investigate the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing. The climate sensitivity of the coupled model using three different parametrisations of sub-grid scale mixing is 3°C for a doubling of CO₂ (6°C for a quadrupling of CO₂). This suggests that the ocean has only a weak feedback on global mean surface air temperature although significant regional differences, notably at high latitudes, exist with different sub-grid scale parametrisations. In the experiment using the Gent and McWilliams parametrisation for mixing associated with mesoscale eddies, an enhancement of the surface response in the Southern Ocean is found. This enhancement is largely due to the existence of more realistic sea-ice in the climatological control integration and the subsequent enhanced ice-albedo feedback upon warming. In accordance with earlier analyses, the Gent and McWilliams scheme decreases the global efficiency of ocean heat uptake. During the transient phase of all experiments, the North Atlantic overturning initially weakened but ultimately recovered, surpassing its former strength. This suggests that in the region around the North Atlantic the ocean acts as a negative feedback on local warming during the transient phase but a positive feedback at equilibrium. During the transient phase of the experiments with a more sophisticated and realistic parametrisation of sub-grid scale mixing, warmed Atlantic water was found to penetrate at depth into the Arctic, consistent with recent observations in the region. Received: 14 October 1998 / Accepted: 27 April 1999     

Hydrological changes in the climate system from leaf responses to increasing CO2

Vegetation is a major component of the climate system because of its controls on the energy and water balance over land. This functioning changes because of the physiological response of leaves to increased CO₂. A climate model is used to compare these changes with the climate changes from radiative forcing by greenhouse gases. For this purpose, we use the Community Earth System Model coupled to a slab ocean. Ensemble integrations are done for current and doubled CO₂. The consequent reduction of transpiration and net increase of surface radiative heating from reduction in cloudiness increases the temperature over land by a significant fraction of that directly from the radiative warming by CO₂. Large-scale atmospheric circulation adjustments result. In particular, over the tropics, a low-level westerly wind anomaly develops associated with reduced geopotential height over land, enhancing moisture transport and convergence, and precipitation increases over the western Amazon, the Congo basin, South Africa, and Indonesia, while over mid-latitudes, land precipitation decreases from reduced evapotranspiration. On average, land precipitation is enhanced by 0.03 mm day^?1 (about 19 % of the CO₂ radiative forcing induced increase). This increase of land precipitation with decreased ET is an apparent negative feedback, i.e., less ET makes more precipitation. Global precipitation is slightly reduced. Runoff increases associated with both the increased land precipitation and reduced evapotranspiration. Examining the consistency of the variations among ensemble members shows that vegetation feedbacks on precipitation are more robust over the tropics and in mid to high latitudes than over the subtropics where vegetation is sparse and the internal climate variability has a larger influence.     

Response of the Subtropical Gyre Circulation in the North Pacific Ocean to CO2 Quadrupling

ABSTRACT

This study investigates the response of the subtropical gyre circulation in the North Pacific Ocean to quadrupled CO₂ using the Community Earth System Model, version 1 (CESM1). In particular, an overriding technique is applied to isolate and quantify the effects of wind stress and thermal warming caused by CO₂ emissions. Results show that, in response to the increase in CO₂, the total mass transport in the subtropical gyre is reduced by approximately 11%. This reduction results mainly from negative anomalies of the wind stress curl over the subtropical region, with a smaller contribution from the thermal warming effect. Furthermore, a detailed analysis finds that the change in the subtropical gyre is baroclinic in nature [i.e., the gyre appears to be spin-up in the upper ocean (above 300?m) but spin-down in the lower thermocline (from 300 to 1500?m)]. This reversal between the upper ocean and lower thermocline is a result of the thermal warming effect, which intensifies ocean stratification, hindering the transfer of momentum from the upper layers to the lower layers and leading to an acceleration of the gyre in the upper ocean but a deceleration in the lower thermocline. Another feature of the response of the subtropical gyre to quadrupled CO₂ is the respective poleward and equatorward movements of its northern and southern boundaries, which is a result of the change in the zero wind stress curl lines.     

Summertime land–sea thermal contrast and atmospheric circulation over East Asia in a warming climate—Part I: Past changes and future projections

Land–sea surface air temperature (SAT) contrast, an index of tropospheric thermodynamic structure and dynamical circulation, has shown a significant increase in recent decades over East Asia during the boreal summer. In Part I of this two-part paper, observational data and the results of transient warming experiments conducted using coupled atmosphere–ocean general circulation models (GCMs) are analyzed to examine changes in land–sea thermal contrast and the associated atmospheric circulation over East Asia from the past to the future. The interannual variability of the land–sea SAT contrast over the Far East for 1950–2012 was found to be tightly coupled with a characteristic tripolar pattern of tropospheric circulation over East Asia, which manifests as anticyclonic anomalies over the Okhotsk Sea and around the Philippines, and a cyclonic anomaly over Japan during a positive phase, and vice versa. In response to CO₂ increase, the cold northeasterly winds off the east coast of northern Japan and the East Asian rainband were strengthened with the circulation pattern well projected on the observed interannual variability. These results are commonly found in GCMs regardless of future forcing scenarios, indicating the robustness of the East Asian climate response to global warming. The physical mechanisms responsible for the increase of the land–sea contrast are examined in Part II.     

Equilibrium and transient climatic warming induced by increased atmospheric CO2

The change in the Earth's equilibrium global mean surface temperature induced by a doubling of the CO₂ concentration has been estimated as 0.2 to 10 K by surface energy balance models, 0.5 to 4.2 K by radiative-convective models, and 1.3 to 4.2 K by general circulation models. These wide ranges are interpreted and quantified here in terms of the direct radiative, forcing of the increased CO₂, the response of the climate system in the absence of feedback processes, and the feedbacks of the climate system. It is the range in the values of these feedbacks that leads to the ranges in the projections of the global mean surface warming. The time required for a CO₂-induced climate change to reach equilibrium has been characterized by an e-folding time _e with values estimated by a variety of climate/ocean models as 10 to 100 years. Analytical and numerical studies show that this wide range is due to the strong dependence of _e on the equilibrium sensitivity of the climate model and on the effective vertical thermal diffusivity of the ocean model. A coupled atmosphere-ocean general circulation model simulation for doubled CO₂ suggestes that, as a result of the transport of the CO₂-induced surface heating into the interior of the ocean, _e 50 to 100 years. Theoretical studies for a realistic CO₂ increase between 1850 and 1980 indicate that this sequestering of heat into the ocean's interior is responsible for the concomittant warming being only about half that which would have occurred in the absence of the ocean. These studies also indicate that the climate sytem will continue to warm towards its as yet unrealized equilibrium temperature change, even if there is no further increase in the CO₂ concentration.     

Deep-ocean heat uptake and equilibrium climate response

We integrate the coupled climate model ECHAM5/MPIOM to equilibrium under atmospheric CO₂ quadrupling. The equilibrium global-mean surface-temperature change is 10.8 K. The surface equilibrates within about 1,200 years, the deep ocean within 5,000 years. The impact of the deep ocean on the equilibrium surface-temperature response is illustrated by the difference between ECHAM5/MPIOM and ECHAM5 coupled with slab ocean model (ECHAM5/SOM). The equilibrium global-mean surface temperature response is 11.1 K in ECHAM5/SOM and is thus 0.3 K higher than in ECHAM5/MPIOM. ECHAM5/MPIOM shows less warming over the northern-hemisphere mid and high latitudes, but larger warming over the tropical ocean and especially over the southern-hemisphere high latitudes. ECHAM5/MPIOM shows similar polar amplification in both the Arctic and the Antarctic, in contrast to ECHAM5/SOM, which shows stronger polar amplification in the northern hemisphere. The southern polar warming in ECHAM5/MPIOM is greatly delayed by Antarctic deep-ocean warming due to convective and isopycnal mixing. The equilibrium ocean temperature warming under CO₂ quadrupling is around 8.0 K and is near-uniform with depth. The global-mean steric sea-level rise is 5.8 m in equilibrium; of this, 2.3 m are due to the deep-ocean warming after the surface temperature has almost equilibrated. This result suggests that the surface temperature change is a poor predictor for steric sea-level change in the long term. The effective climate response method described in Gregory et al. (2004) is evaluated with our simulation, which shows that their method to estimate the equilibrium climate response is accurate to within 10 %.     

Relative roles of climate sensitivity and forcing in defining the ocean circulation response to climate change

The response of the ocean’s meridional overturning circulation (MOC) to increased greenhouse gas forcing is examined using a coupled model of intermediate complexity, including a dynamic 3-D ocean subcomponent. Parameters are the increase in CO₂ forcing (with stabilization after a specified time interval) and the model’s climate sensitivity. In this model, the cessation of deep sinking in the north “Atlantic” (hereinafter, a “collapse”), as indicated by changes in the MOC, behaves like a simple bifurcation. The final surface air temperature (SAT) change, which is closely predicted by the product of the radiative forcing and the climate sensitivity, determines whether a collapse occurs. The initial transient response in SAT is largely a function of the forcing increase, with higher sensitivity runs exhibiting delayed behavior; accordingly, high CO₂-low sensitivity scenarios can be assessed as a recovering or collapsing circulation shortly after stabilization, whereas low CO₂-high sensitivity scenarios require several hundred additional years to make such a determination. We also systemically examine how the rate of forcing, for a given CO₂ stabilization, affects the ocean response. In contrast with previous studies based on results using simpler ocean models, we find that except for a narrow range of marginally stable to marginally unstable scenarios, the forcing rate has little impact on whether the run collapses or recovers. In this narrow range, however, forcing increases on a time scale of slow ocean advective processes results in weaker declines in overturning strength and can permit a run to recover that would otherwise collapse.     

On the predictability of decadal changes in the North Pacific

 The predictability of decadal changes in the North Pacific is investigated with an ocean general circulation model forced by simplified and realistic atmospheric conditions. First, the model is forced by a spatially fixed wind stress anomaly pattern characteristic for decadal North Pacific climate variations. The time evolution of the wind stress anomaly is chosen to be sinusoidal, with a period of 20 years. In this experiment different physical processes are found to be important for the decadal variations: baroclinic Rossby waves dominate the response. They move westward and lead to an adjustment of the subtropical and subpolar gyre circulations in such a way that anomalous temperatures in the central North Pacific develop as a delayed response to the preceding wind stress anomalies. This delayed response provides not only a negative feedback but also bears the potential for long-term predictions of upper ocean temperature changes in the central North Pacific. It is shown by additional experiments that once these Rossby waves have been excited, decadal changes of the upper ocean temperatures in the central North Pacific evolve without any further anomalous atmospheric forcing. In the second part, the model is forced by surface heat flux and wind stress observations for the period 1949–1993. It is shown that the same physical processes which were found to be important in the simplified experiments also govern the evolution of the upper ocean in this more realistic simulation. The 1976/77 cooling can be mainly attributed to anomalously strong horizontal advection due to the delayed response to persistent wind stress curl anomalies in the early 1970s rather than local anomalous atmospheric forcing. This decadal change could have been predicted some years in advance. The subsequent warming in the late 1980s, however, cannot be mainly explained by advection. In this case, local anomalous atmospheric forcing needs to be considered. Received: 6 July 1998 / Accepted: 16 October 1999     

Effects of tropical instability wave (TIW)-induced surface wind feedback in the tropical Pacific Ocean

Tropical instability waves (TIWs) arise from oceanic instability in the eastern tropical Pacific and Atlantic Oceans, having a clear atmospheric signature that results in coupled atmosphere–ocean interactions at TIW scales. In this study, the extent to which TIW-induced surface wind feedback influences the ocean is examined using an ocean general circulation model (OGCM). The TIW-induced wind stress (τ_TIW) part is diagnostically determined using an empirical τ_TIW model from sea surface temperature (SST) fields simulated in the OGCM. The interactively represented TIW wind tends to reduce TIW activity in the ocean and influence the mean state, with largest impacts during TIW active periods in fall and winter. In December, the interactive τ_TIW forcing induces a surface cooling (an order of ?0.1 to ?0.3 °C), an increased heat flux into the ocean, a shallower mixed layer and a weakening of the South Equatorial Current in the eastern equatorial Pacific. Additionally, the TIW wind effect yields a pronounced latitudinal asymmetry of sea level field across the equator, and a change to upper thermal structure, characterized by a surface cooling and a warming below in the thermocline, leading to a decreased temperature gradient between the mixed layer and the thermocline. Processes responsible for the τ_TIW–induced cooling effects are analyzed. Vertical mixing and meridional advection are the two terms in the SST budget that are dominantly affected by the TIW wind feedback: the cooling effect from the vertical mixing on SST is enhanced, with the maximum induced cooling in winter; the warming effect from the meridional advection is reduced in July–October, but enhanced in November–December. Additional experiments are performed to separate the relative roles the affected surface momentum and heat fluxes play in the cooling effect on SST. This ocean-only modeling work indicates that the effect of TIW-induced wind feedback is small but not negligible, and may need to be adequately taken into account in large-scale climate modeling.     

An explanation for the difference between twentieth and twenty-first century land–sea warming ratio in climate models

A land–sea surface warming ratio (or φ) that exceeds unity is a robust feature of both observed and modelled climate change. Interestingly, though climate models have differing values for φ, it remains almost time-invariant for a wide range of twenty-first century climate transient warming scenarios, while varying in simulations of the twentieth century. Here, we present an explanation for time-invariant land–sea warming ratio that applies if three conditions on radiative forcing are met: first, spatial variations in the climate forcing must be sufficiently small that the lower free troposphere warms evenly over land and ocean; second, the temperature response must not be large enough to change the global circulation to zeroth order; third, the temperature response must not be large enough to modify the boundary layer amplification mechanisms that contribute to making φ exceed unity. Projected temperature changes over this century are too small to breach the latter two conditions. Hence, the mechanism appears to show why both twenty-first century and time-invariant CO₂ forcing lead to similar values of φ in climate models despite the presence of transient ocean heat uptake, whereas twentieth century forcing—which has a significant spatially confined anthropogenic tropospheric aerosol component that breaches the first condition—leads to modelled values of φ that vary widely amongst models and in time. Our results suggest an explanation for the behaviour of φ when climate is forced by other regionally confined forcing scenarios such as geo-engineered changes to oceanic clouds. Our results show how land–sea contrasts in surface and boundary layer characteristics act in tandem to produce the land–sea surface warming contrast.     

Factors influencing anthropogenic carbon dioxide uptake in the North Atlantic in models of the ocean carbon cycle

The uptake and storage of anthropogenic carbon in the North Atlantic is investigated using different configurations of ocean general circulation/carbon cycle models. We investigate how different representations of the ocean physics in the models, which represent the range of models currently in use, affect the evolution of CO₂ uptake in the North Atlantic. The buffer effect of the ocean carbon system would be expected to reduce ocean CO₂ uptake as the ocean absorbs increasing amounts of CO₂. We find that the strength of the buffer effect is very dependent on the model ocean state, as it affects both the magnitude and timing of the changes in uptake. The timescale over which uptake of CO₂ in the North Atlantic drops to below preindustrial levels is particularly sensitive to the ocean state which sets the degree of buffering; it is less sensitive to the choice of atmospheric CO₂ forcing scenario. Neglecting physical climate change effects, North Atlantic CO₂ uptake drops below preindustrial levels between 50 and 300 years after stabilisation of atmospheric CO₂ in different model configurations. Storage of anthropogenic carbon in the North Atlantic varies much less among the different model configurations, as differences in ocean transport of dissolved inorganic carbon and uptake of CO₂ compensate each other. This supports the idea that measured inventories of anthropogenic carbon in the real ocean cannot be used to constrain the surface uptake. Including physical climate change effects reduces anthropogenic CO₂ uptake and storage in the North Atlantic further, due to the combined effects of surface warming, increased freshwater input, and a slowdown of the meridional overturning circulation. The timescale over which North Atlantic CO₂ uptake drops to below preindustrial levels is reduced by about one-third, leading to an estimate of this timescale for the real world of about 50 years after the stabilisation of atmospheric CO₂. In the climate change experiment, a shallowing of the mixed layer depths in the North Atlantic results in a significant reduction in primary production, reducing the potential role for biology in drawing down anthropogenic CO₂.     

Climate Change to the End of the Millennium

Anthropogenic climate change will continue long after anthropogenic CO₂ emissions cease. Atmospheric CO₂, global warming and ocean circulation will approach equilibrium on the millennial timescale, whereas thermal expansion of the ocean, ice sheet melt and their contributions to sea level rise are unlikely to be complete. Atmospheric CO₂ in year 3000 depends non-linearly on the total amount of CO₂ emitted and is very likely to exceed the present level of ∼380 ppmv. CO₂ is doubled for ∼2500 GtC emitted, quadrupled if all ∼5000 GtC of conventional fossil fuel resources are emitted, and increases by a factor of ∼32 if a further 20,000 GtC of exotic fossil fuel resources are emitted. Global warming in year 3000 will also depend on climate sensitivity to doubling CO₂, which is most probably ∼3 ^∘C but highly uncertain. Thermal expansion will contribute 0.5–2 m to millennial sea level rise for each doubling of CO₂. The Greenland ice sheet could melt completely within the millennium under > 8×CO₂, adding a further ∼7 m to sea level. The rate of melt depends on the magnitude of forcing above a regional warming threshold of 1–3 ^∘C. The West Antarctic ice sheet could be threatened by 4–10 ^∘C local warming, and its potential contribution to millennial sea level rise exceeds current maximum estimates of ∼1 m. The fate of the ocean thermohaline circulation may depend on the rate as well as the magnitude of forcing.