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Siobhan P. O'Farrell

Abstract

This study focuses on the uptake of a passive idealized tracer in the Northern Hemisphere oceans from two coupled ocean–atmosphere simulations: a standard horizontal diffusion case and the second case including the Gent and McWilliams (GM) eddy mixing parameterization. The results are compared with tracer uptake in stand-alone synchronous and asynchronous ocean simulations for the same cases. The GM set of integrations shows tracer penetration reduced from the standard set in all water mass formation regions. There is a strong similarity in the tracer distributions in the stand-alone ocean simulations in both the standard and GM cases. Changes in the velocity fields between the stand-alone ocean and coupled simulations explain many of the differences in the modeled tracer concentrations.

There is a particular focus in the study on the dynamics of modeled water mass formation for North Pacific Intermediate Water, Labrador Sea Water, northeast Atlantic mode water, and North Atlantic Deep Water. The model representation of these water masses is compared with observational data of passive tracers, and gives mixed results as the model water masses are on lighter density surfaces than the real ocean though the timing of the advance of the tracer plume within the water masses appears to be realistically modeled. For the coupled simulations, the North Atlantic and North Pacific Oceans have interdecadal signals that alter the circulation and hence tracer patterns. Some issues arising from the interdecadal signal are discussed in relation to tracer distribution on density surfaces within the ocean.

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Hal B. Gordon and Siobhan P. O’Farrell

Abstract

The CSIRO coupled model has been used in a “transient” greenhouse experiment. This model contains atmospheric, oceanic, comprehensive sea-ice (dynamic/thermodynamic plus leads), and biospheric submodels. The model control run (over 100 years long) employed flux corrections and displayed only a small amount of cooling, mainly at high latitudes. The transient experiment (1% increase in CO2 compounding per annum) gave a 2°C warming at time of CO2 doubling. The model displayed a “cold start” effect with a (maximum) value estimated at 0.3°C. The warming in the transient run had an asymmetrical response as seen in other coupled models, with the Northern Hemisphere (NH) warming more than the Southern Hemisphere (SH). However, the land surface response in this model is different from some other transient experiments in that there is not a pronounced drying of the midlatitudes in the NH in summer.

In the control run the ice model gave realistic ice distributions at both poles, with the NH ice in particular displaying considerable interdecadal variability. In the transient run the ice amount decreased more in the NH than the SH (corresponding with a greater NH warming). The NH ice extent and volume in summer was considerably reduced in depth and extent compared to the control. However, the model ice dynamics and thermodynamics allowed for a successful regrowth of the ice during the winter season to give a coverage comparable to that of the control run, although thinner.

During the transient run there is a freshening of the surface salinity in the oceans at high latitudes. In the SH this is caused mainly by increases in precipitation over evaporation. The same is true for the NH, but it is found that there is a similar magnitude contribution to the polar freshening from ice melt and land runoff changes. The freshening in the North Atlantic reduces the strength of the meridional overturning by 35% at time of doubling of CO2. Other changes in the global climate at the end of the transient run relative to the control run are also investigated.

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Wenju Cai, Jozef Syktus, Hal B. Gordon, and Siobhan O’Farrell

Abstract

The response of a coupled oceanic–atmospheric–sea ice climate model to an imposed North Atlantic high-latitude freshening is examined. The imposed freshening lasts for 5 yr with a total salt deficit equivalent to about eight times the observed Great Salinity Anomaly during the late 1960s and early 1970s.

The thermohaline circulation associated with North Atlantic Deep Water Formation (NADWF) initially weakens, but it recovers within 20 yr of the imposed freshening being removed. The effect of the weakened NADWF is gradually transmitted from high latitudes to the entire Atlantic Ocean. The response at the equator lags that at 62°N by about 10 yr. In the midlatitude (from 30° to 58°N) region, the lag causes a warming during the initial weakening and a cooling during the recovery. Changes in the thermohaline circulation significantly modify the large-scale North Atlantic circulation. In particular, the barotropic Gulf Stream weakens by about 18%.

An interesting feature is the dipole structure of the initial response in sea surface temperature, with cooling in the sinking region and warming south of it. This dipole structure plays an important role for the recovery of the NADWF once the imposed freshening is removed. It increases the surface density in the sinking region and increases the north–south pressure gradient. Thus, the conditions set up during the initial weakening contribute to the recovery process.

Modifications of the thermal structure of the ocean surface lead to changes in the atmospheric circulation, in particular, a weakening of the westerlies over the midlatitude North Atlantic and a southward shift over Western Europe. The North Atlantic oscillation (NAO) index under the imposed freshening is negative, consistent with findings from observational studies. The associated climate changes are similar to those observed with negative NAO values.

Effects of various oceanic and atmospheric feedbacks are discussed. The results are also compared with those from ocean-only models, where the atmosphere–ocean interactions and some of the oceanic feedbacks are excluded.

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Anthony C. Hirst, Siobhan P. O’Farrell, and Hal B. Gordon

Abstract

The Gent and McWilliams (GM) parameterization for large-scale water transport caused by mesoscale oceanic eddies is introduced into the oceanic component of the Commonwealth Scientific and Industrial Research Organisation global coupled ocean–atmosphere model. Parallel simulations with and without the GM scheme are performed to examine the effect of this parameterization on the model behavior for integrations lasting several centuries under conditions of constant atmospheric CO2. The solution of the version with GM shows several significant improvements over that of the earlier version. First, the generally beneficial effects of the GM scheme found previously in studies of stand-alone ocean models, including more realistic deep water properties, increased stratification, reduced high-latitude convection, elimination of fictitious horizontal diffusive heat transport, and more realistic surface fluxes in some regions, are all maintained during the coupled integration. These improvements are especially pronounced in the high-latitude Southern Ocean. Second, the magnitude of flux adjustment is reduced in the GM version, mainly because of smaller surface fluxes at high southern latitudes in the GM ocean spinup. Third, the GM version displays markedly reduced climate drift in comparison to the earlier version. Analysis in the present study verifies previous indications that changes in the pattern of convective heat flux are central to the drift in the earlier version, supporting the view that reduced convective behavior in the GM version contributes to the reduction in drift. Based on the satisfactory behavior of the GM model version, the GM coupled integration is continued for a full 1000 yr. Key aspects of the model behavior during this longer period are also presented. Interannual variability of surface air temperature in the two model versions is briefly examined using some simple measures of magnitude. The variability differs between the two versions regionally, but is little changed on the global scale. In particular, the magnitude of variability in the tropical Pacific is little changed between the versions.

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