Oleg A. Saenko

The UVic earth system climate model: Model description, climatology, and applications to past, present and future climates

Abstract A new earth system climate model of intermediate complexity has been developed and its climatology compared to observations. The UVic Earth System Climate Model consists of a three‐dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea‐ice model, an energy‐moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land‐ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrized through Fickian diffusion, and precipitation is assumed to occur when the relative humidity is greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to return instantaneously to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrization of water vapour/planetary longwave feedbacks, although the radiative forcing associated with changes in atmospheric CO2 is prescribed as a modification of the planetary longwave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present‐day winds in its climatology, although a dynamical wind feedback is included which exploits a latitudinally‐varying empirical relationship between atmospheric surface temperature and density. The ocean component of the coupled model is based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model 2.2, with a global resolution of 3.6° (zonal) by 1.8° (meridional) and 19 vertical levels, and includes an option for brine‐rejection parametrization. The sea‐ice component incorporates an elastic‐viscous‐plastic rheology to represent sea‐ice dynamics and various options for the representation of sea‐ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection. Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0°C for a doubling of CO2, in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback, leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but is eventually re‐established to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere General Circulation Models (GCMs). This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning is reestablished to a strength that is greater than its initial condition. When applied to the climate of the Last Glacial Maximum (LGM), the model obtains tropical cooling (30°N‐30°S), relative to the present, of about 2.1°C over the ocean and 3.6°C over the land. These are generally cooler than CLIMAP estimates, but not as cool as some other reconstructions. This moderate cooling is consistent with alkenone reconstructions and a low to medium climate sensitivity to perturbations in radiative forcing. An amplification of the cooling occurs in the North Atlantic due to the weakening of North Atlantic Deep Water formation. Concurrent with this weakening is a shallowing of, and a more northward penetration of, Antarctic Bottom Water. Climate models are usually evaluated by spinning them up under perpetual present‐day forcing and comparing the model results with present‐day observations. Implicit in this approach is the assumption that the present‐day observations are in equilibrium with the present‐day radiative forcing. The comparison of a long transient integration (starting at 6 KBP), forced by changing radiative forcing (solar, CO2, orbital), with an equilibrium integration reveals substantial differences. Relative to the climatology from the present‐day equilibrium integration, the global mean surface air and sea surface temperatures (SSTs) are 0.74°C and 0.55°C colder, respectively. Deep ocean temperatures are substantially cooler and southern hemisphere sea‐ice cover is 22% greater, although the North Atlantic conveyor remains remarkably stable in all cases. The differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene. It is also demonstrated that a global warming simulation that starts from an equilibrium present‐day climate (cold start) underestimates the global temperature increase at 2100 by 13% when compared to a transient simulation, under historical solar, CO2 and orbital forcing, that is also extended out to 2100. This is larger (13% compared to 9.8%) than the difference from an analogous transient experiment which does not include historical changes in solar forcing. These results suggest that those groups that do not account for solar forcing changes over the twentieth century may slightly underestimate (∼3% in our model) the projected warming by the year 2100.

Coupling of the hemispheres in observations and simulations of glacial climate change.

Abstract We combine reconstructions, climate model simulations and a conceptual model of glacial climate change on millennial time scales to examine the relation between the high latitudes of both hemispheres. A lead-lag analysis of synchronised proxy records indicates that temperature changes in Greenland preceded changes of the opposite sign in Antarctica by 400– 500 yr . A composite record of the Dansgaard–Oeschger events shows that rapid warming (cooling) in Greenland was followed by a slow cooling (warming) phase in Antarctica. The amplitudes, rates of change and time lag of the interhemispheric temperature changes found in the reconstructions are in excellent agreement with climate model simulations in which the formation of North Atlantic Deep Water is perturbed. The simulated time lag between high northern and southern latitudes is mainly determined by the slow meridional propagation of the signal in the Southern Ocean. Our climate model simulations also show that increased deep water formation in the North Atlantic leads to a reduction of the Antarctic Circumpolar Current through diminishing meridional density gradients in the Southern Ocean. We construct a simple conceptual model of interhemispheric Dansgaard–Oeschger oscillations. This model explains major features of the recorded temperature changes in Antarctica as well as the general shape of the north–south phase relation found in the observations including a broad peak of positive correlations for a lead of Antarctica over Greenland by 1000– 2000 yr . The existence of this peak is due to the regularity of the oscillations and does not imply a southern hemisphere trigger mechanism, contrary to previous suggestions. Our findings thus further emphasise the role of the thermohaline circulation in millennial scale climate variability.

The Atlantic–Pacific Seesaw

Abstract A global, oceanic teleconnection of salinity, meridional overturning circulation (MOC), and climate of the North Atlantic and North Pacific is proposed. Simulations with a global climate model show that an extraction of freshwater from the Pacific results not only in an increase of salinity there, but also in a decrease of salinity in the Atlantic. As a result, a Pacific MOC develops while the Atlantic MOC collapses without freshwater perturbation in the Atlantic. Similarly, an input of freshwater to the Atlantic leads not only to a decrease of salinity there, but also to an increase of salinity in the Pacific. The Atlantic MOC collapses, whereas the Pacific MOC develops without freshwater perturbation in the Pacific. The mechanism behind this antiphase Atlantic– Pacific relationship is the positive feedback between ocean circulation and salinity contrasts, originally proposed by Stommel to operate between low and high latitudes. Here the authors show that the same mechanism operates on the Atlan...

The Role of Poleward-Intensifying Winds on Southern Ocean Warming

Recent analyses of the latest series of climate model simulations suggest that increasing CO2 emissions in the atmosphere are partly responsible for (i) the observed poleward shifting and strengthening of the Southern Hemisphere subpolar westerlies (in association with shifting of the southern annular mode toward a higher index state), and (ii) the observed warming of the subsurface Southern Ocean. Here the role that poleward-intensifying westerlies play in subsurface Southern Ocean warming is explored. To this end a climate model of intermediate complexity was driven separately, and in combination with, time-varying CO2 emissions and time-varying surface winds (derived from the fully coupled climate model simulations mentioned above). Experiments suggest that the combination of the direct radiative effect of CO2 emissions and poleward-intensified winds sets the overall magnitude of Southern Ocean warming, and that the polewardintensified winds are key in terms of determining its latitudinal structure. In particular, changes in wind stress curl associated with poleward-intensified winds significantly enhance pure CO2-induced subsurface warming around 45°S (through increased downwelling of warm surface water), reduces it at higher latitudes (through increased upwelling of cold deep water), and reduces it at lower latitudes (through decreased downwelling of warm surface water). Experiments also support recent high-resolution ocean model experiments suggesting that enhanced mesoscale eddy activity associated with poleward-intensified winds influences subsurface (and surface) warming. In particular, it is found that increased poleward heat transport associated with increased mesoscale eddy activity enhances the warming south of the Antarctic Circumpolar Current. Finally, a mechanism involving offshore Ekman sea ice transport (modulated by enhanced mesoscale activity) that acts to significantly limit the human-induced high-latitude Southern Hemisphere surface temperature response is reported on.

On the Link between the Two Modes of the Ocean Thermohaline Circulation and the Formation of Global-Scale Water Masses

A close link between the formation of global-scale water masses, such as North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW), and two stable modes of the thermohaline circulation (THC) is investigated in a coupled model. In the upper 2‐3 km of the Atlantic, the THC modes are characterized by meridional overturning circulations of opposite sign, with either a dominance of the AAIW cell over the NADW cell (‘‘off’’ THC mode) or vice versa (‘‘on’’ THC mode). A transition between these THC modes is controlled by the relationship between the densities in the source regions of formation of AAIW and NADW water masses. This is shown by applying a freshwater perturbation in the region of enhanced AAIW formation in the Southern Ocean to obtain a hysteresis loop of the NADW circulation. Transitions between the two modes of the THC occur when the densities in the source regions of AAIW and NADW become comparable to each other.

Rapid subsurface warming and circulation changes of Antarctic coastal waters by poleward shifting winds

The southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s. This wind trend is projected to persist under continued anthropogenic forcing, but the impact of the changing winds on Antarctic coastal heat distribution remains poorly understood. Here we show that a poleward wind shift at the latitudes of the Antarctic Peninsula can produce an intense warming of subsurface coastal waters that exceeds 2°C at 200–700 m depth. The model simulated warming results from a rapid advective heat flux induced by weakened near-shore Ekman pumping and is associated with weakened coastal currents. This analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around Antarctica, with potentially significant ramifications for global sea level rise.

Human-Induced Change in the Antarctic Circumpolar Current

Abstract Global climate models indicate that the poleward shift of the Antarctic Circumpolar Current observed over recent decades may have been significantly human induced. The poleward shift, along with a significant increase in the transport of water around Antarctica, is predicted to continue into the future. To appreciate the magnitude of the poleward shift it is noted that by century’s end the concomitant shrinking of the Southern Ocean is predicted to displace a volume of water close to that in the entire Arctic Ocean. A simple theory, balancing surface Ekman drift and ocean eddy mixing, explains these changes as the oceanic response to changing wind stress.

The Effect of Localized Mixing on the Ocean Circulation and Time-Dependent Climate Change

Observations indicate that intense mixing in the ocean is localized above complex topography and near the boundaries. Model experiments presented here illustrate that accounting for this fact can be important. In particular, it is found that in the case of localized mixing, the rate of overturning circulation is proportional to the net rate of generation of potential energy by the vertical mixing, linked to the net downward heat diffusion, rather than to the value of the mean vertical diffusivity coefficient. Furthermore, it is shown that two climate models, having the same vertical profile of diffusivity but differing in their distribution (horizontally uniform versus topography/boundary intensified) can simulate significantly different meridional oceanic circulations, vertical heat transfers, and responses of simulated climate to atmospheric CO2 increase. This is found for relatively large [O(1.0 cm 2 s 1 )] horizontal-mean values of vertical diffusivity in the pycnocline. However, in cases of relatively small [O(0.1 cm 2 s 1 )] mean diffusivity in the pycnocline, the simulated integral quantities such as meridional mass and heat transports do not depend much on the details of the mixing distribution. Even so, it is found that the deep western boundary currents are more localized near the boundaries in the case of topography/boundary-intensified mixing; also, the stratification in the deep ocean is set through the localized regions of intense vertical mixing. In addition, it is shown that reconciling the observed basin-mean values of diffusivity in the abyssal ocean of O(10 cm 2 s 1 ) with realistic stratification can be problematic, unless the regions of enhanced vertical mixing are localized.

Atlantic deep circulation controlled by freshening in the Southern Ocean

Received 5 May 2003; accepted 18 June 2003; published 24 July 2003. [1] Numerical simulations with a climate model of intermediate complexity are used to illustrate the effect of meridional moisture transport in the Southern Hemisphere mid-latitudes on the meridional overturning circulation (MOC) and heat transport in the Atlantic. A novel feature of the model is a diapycnal mixing scheme in the ocean, which ensures low values of diffusivity (about 10 � 5 m 2 s � 1 ) in the pycnocline. It is shown that the Atlantic MOC, northward oceanic heat transport and the associated air-sea heat flux anomalies are all proportional to the southward moisturetransport from subtropical tosubpolarregions inthe Southern Hemisphere. The effect of the intensified ocean circulation on sea surface temperature and salinity is also illustrated. INDEX TERMS: 4532 Oceanography: Physical: General circulation; 4516 Oceanography: Physical: Eastern boundary currents; 4283 Oceanography: General: Water masses. Citation: Saenko, O. A., A. J. Weaver, and A. Schmittner, Atlantic deep circulation controlled by freshening in the Southern Ocean, Geophys. Res. Lett., 30(14), 1754, doi:10.1029/ 2003GL017681, 2003.

Importance of wind‐driven sea ice motion for the formation of Antarctic Intermediate Water in a global climate model

An ocean-atmosphere-sea ice model is used to show the importance of wind-driven sea ice motion in the formation of Antarctic Intermediate Water (AAIW). The model is able to reasonably simulate a tongue of low salinity AAIW even when the direct momentum transfer from wind to the ocean is neglected, provided that the wind stress is applied to sea ice. In contrast, when the wind stress exclusively drives the ocean, the model fails to capture the properties of AAIW. The growth and subsequent offshore transport of sea ice acts as a freshwater conduit from near-shore regions, where AABW is formed, to subpolar regions, where AAIW is formed. Sea ice dynamics are also shown to be important in the simulation of a local salinity minimum at intermediate depths in the southern Indian Ocean and a local salinity maximum in the western Weddell Sea.