Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: Hal B. Gordon x
  • Refine by Access: All Content x
Clear All Modify Search
T. Yonetani and Hal B. Gordon

Abstract

Following a transient increase in the atmospheric carbon dioxide to double the current level, and a subsequent maintenance at the doubled level, there is a climate shift toward a new equilibrium state. Changes in the mean temperature and precipitation, and changes in the occurrence frequencies of their extremes for the doubled carbon dioxide conditions have been assessed at the continental scale. There is a characteristic spatial pattern that involves a maximum annual mean warming in high northern latitudes and a minimum annual mean warming around Antarctica and in the northern North Atlantic. Under maintained doubled carbon dioxide, this interhemispheric asymmetry disappears except for an ocean–land asymmetry. A possible mechanism for this is considered in terms of changes in effective thermal capacity due to a reduction of overturning in the oceans that continues to decline after the atmospheric carbon dioxide stops increasing. It is also found that global warming becomes most noticeable in the occurrence frequency of high extremes in the annual mean temperature in the low latitudes, even though the temperature rise is largest in the high northern latitudes in autumn and winter. In addition, extremes of large (small) annual and seasonal total precipitation are recorded much more frequently in regions where the mean precipitation increases (decreases).

Full access
Wenju Cai and Hal B. Gordon

Abstract

Climate drift in coupled models affects the response of the coupled system to an external forcing. In most existing coupled models that employ flux adjustments, the southern high latitudes, in particular, are still affected by some climate drift. In the CSIRO coupled model, within 100 years following coupling, the Antarctic Circumpolar Current (ACC) intensifies by about 30 Sv (Sv ≡ 106 m3 s−1). This happens despite the use of flux adjustments. Many other model fields such as sea ice, surface albedo, and heat fluxes of the coupled system also experience drift from the precoupled spinup states. It is therefore important to study the processes that give rise to these drifts.

The primary cause of drift in the CSIRO model is due to changes in the pattern of convection in the Southern Ocean relative to the spinup steady state. Upon coupling, the pattern of convection alters systematically regardless of surface boundary conditions. Consequently, overturning at shallow to intermediate depths (from the surface to about 2000 m) weakens, while that below these depths intensifies. The decline of overturning at shallow to intermediate depths leads to reduced surface temperatures because a lesser amount of warm subsurface water is mixed up into the colder surface mixed layer. The cooler surface temperature leads to an initial increase in sea ice, which is exacerbated by a significant albedo–temperature–sea ice feedback. The resulting increase in sea ice formation at the higher southern latitudes leads to increased brine rejection and a general increase in salinity throughout much of the high-latitude water column. This increase in salinity intensifies deep convection and bottom water formation, driving a stronger ACC.

Several additional experiments are performed to trace various oceanic and ocean–atmosphere feedbacks that give the drift its character. It is demonstrated that the feedbacks significant to the drift in the present model are the positive albedo–temperature–sea ice feedback and a negative feedback between sea ice and overturning. The role of these two feedbacks in the interconnection between the drifts in various model fields is discussed.

Full access
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.

Full access
Wenju Cai, Peter G. Baines, and Hal B. Gordon
Full access
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.

Full access
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.

Full access
Wenju Cai, Peter G. Baines, and Hal B. Gordon

Abstract

Variability in the southern atmospheric circulation at mid- to high latitudes with a dominant quasi-stationary wavenumber-3 pattern has been reported in many observational studies. The variability is barotropic in nature with signals in the middle troposphere as well as at the atmosphere–ocean interface. Moreover, there are preferred fixed centers for the strongest anomalies. These features are well reproduced by the Commonwealth Scientific and Industrial Research Organisation coupled model on various timescales. On the interannual timescale, an index of the modeled wavenumber-3 pattern shows little correlation with the modeled Southern Oscillation index, suggesting that the variability associated with wavenumber-3 anomalies is separate to modeled ENSO-like events. However, the variation of the pattern index is strikingly similar to, and highly correlated with, the modeled oceanic variability. The associated oceanic anomalies move eastward and are similar to those of the observed Antarctic circumpolar wave (ACW). The modeled ACW-like anomalies exist not only at the surface but also through middle ocean depths, with a similar barotropic nature to those of the atmospheric anomalies. The oceanic anomalies also display a wavenumber-3 pattern.

The essential elements of the dynamics of the modeled ACW are the advection of SST anomalies by the surface Antarctic Circumpolar Current (ACC), and the interactions between anomalies of SST and mean sea level pressure (MSLP). Associated with the standing wavenumber-3 pattern, there are fixed centers for the strongest MSLP anomalies. As a positive SST anomaly advected by surface ACC approaches a center of a positive MSLP anomaly, the MSLP decreases. The positive (negative) SST anomalies are generated by anomalous latent and heat fluxes, which are in turn induced by southward (northward) meridional wind stress anomalies resulting from geostrophic balance. These MSLP anomalies change sign when the positive (negative) SST anomalies move to a location near the centers. Once MSLP anomalies change sign, positive (negative) SST anomalies are generated again reinforcing the anomalies entering from the west. The time for the surface ACC to advect one-sixth of the circuit around the pole corresponds to the time of a half-cycle of the standing MSLP oscillations. Thus the surface ACC determines the frequency of the standing oscillation. In the present model, the speed of the surface ACC is such that the period of the standing oscillation is 4–5 yr, and it would take 12–16 yr for an anomaly to encircle the pole. These and other features of the modeled ACW, together with associated dynamic processes, are analyzed and discussed.

Full access
Wenju Cai, Mark A. Collier, Hal B. Gordon, and Linda J. Waterman

Abstract

Simulations of El Niño–Southern Oscillation (ENSO) variability with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 3 coupled climate model, which is not flux adjusted and has an ocean north–south resolution of approximately 0.9°, are described. Major indices, periodicity, and spatial patterns of the modeled ENSO compare well to those observed over the last 100 yr. This good simulation is achieved despite some deficiencies in the model climatology, in particular the climatological tropical Pacific sea surface temperature (SST). The model SST climatology has a “cold tongue” that is too strong and extends too far into the western equatorial Pacific, a common problem experienced by many climate models. Although this cold tongue problem also affects the model rainfall climatology over the tropical ocean, the ENSO–rainfall teleconnection pattern is realistically simulated, particularly over the Indonesian and northeast Australian regions, where in reality rainfall is significantly affected by ENSO cycles.

Comparisons between modeled and observed equatorial thermocline structure reveal that the model thermocline depth (depth of the 20°C isotherm) is shallower, whereas the spread or thickness (depth difference between 16° and 22°C isotherms) of the modeled thermocline is greater, than the observed. The former is favorable, whereas the latter is unfavorable, for generating strong ENSO variability, because a shallower thermocline with smaller spread of isotherms and steeper slope makes it easier for the equatorial upwelling to draw the colder subthermocline water to the surface. On balance, the model is capable of producing ENSO cycles with realistic amplitude. This model capability is further highlighted by what is called here a “super-ENSO” pair: a super-El Niño event followed by a super-La Niña event, both with a Niño-3.4 index (SST average over 5°S–5°N, 120°–170°W) exceeding 3°C in amplitude.

The pairing of the two superevents is unique, and the dynamics are explored. It is found that during the super-El Niño event, the surface zonal wind stress, SST, and the equatorial upwelling anomalies are proportionately large. In contrast, during the super-La Niña event, the response of SST anomalies to easterly and upwelling anomalies is disproportionately large. It is demonstrated that this exceptionally large cooling of SST is linked to an exceptionally strong shallowing of the equatorial thermocline depth, and that the shallowing is induced by the exceptionally strong westerly wind anomalies associated with the super-ENSO. In the context of the recently proposed recharge–oscillator paradigm, which is shown to operate in the present model, the strong shallowing can be seen as a result of the discharge of the equatorial Pacific warm water volume in response to the exceptionally strong westerly anomalies associated with the super-El Niño event.

Full access