Hal B. Gordon

Transient Climate Change in the CSIRO Coupled Model with Dynamic Sea Ice

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 particula...

Simulated changes in daily rainfall intensity due to the enhanced greenhouse effect: Implications for extreme rainfall events

In this study we present rainfall results from equilibrium 1 ×− and 2 × CO2 experiments with the CSIRO 4-level general circulation model. The 1 × CO2 results are discussed in relation to observed climate. Discussion of the 2 × CO2 results focuses upon changes in convective and non-convective rainfall as simulated in the model, and the consequences these changes have for simulated daily rainfall intensity and the frequency of heavy rainfall events. In doing this analysis, we recognize the significant shortcomings of GCM simulations of precipitation processes. However, because of the potential significance of any changes in heavy rainfall events as a result of the enhanced greenhouse effect, we believe a first examination of relevant GCM rainfall results is warranted. Generally, the model results show a marked increase in rainfall originating from penetrative convection and, in the mid-latitudes, a decline in largescale (non-convective) rainfall. It is argued that these changes in rainfall type are a consequence of the increased moisture holding capacity of the warmer atmosphere simulated for 2 × CO2 conditions. Related to changes in rainfall type, rainfall intensity (rain per rain day) increases in the model for most regions of the globe. Increases extend even to regions where total rainfall decreases. Indeed, the greater intensity of daily rainfall is a much clearer response of the model to increased greenhouse gases than the changes in total rainfall. We also find a decrease in the number of rainy days in the middle latitudes of both the Northern and Southern Hemispheres. To further elucidate these results daily rainfall frequency distributions are examined globally and for four selected regions of interest. In all regions the frequency of high rainfall events increases, and the return period of such events decreases markedly. If realistic, the findings have potentially serious practical implications in terms of an increased frequency and severity of floods in most regions. However, we discuss various important sources of uncertainty in the results presented, and indicate the need for rainfall intensity results to be examined in enhanced greenhouse experiments with other GCMs.

Comparison of a coupled ocean-atmosphere model with and without oceanic eddy-induced advection. Part I: Ocean spinup and control integrations

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 especia...

Global warming in a coupled climate model including oceanic eddy-induced advection

The Gent and McWilliams (GM) parameterization for large-scale water transport caused by mesoscale oceanic eddies is introduced into the oceanic component of a global coupled ocean-atmosphere model. Parallel simulations with and without the GM scheme are performed to examine the effect of this parameterization on model behavior under constant atmospheric CO2 and on the model response to increasing CO2. The control (constant CO2) runs show substantial differences in the oceanic stratification and extent of convection, similar to differences found previously using uncoupled ocean models. The transient (increasing CO2) runs show moderate differences in the rate of oceanic heat sequestration (less in the GM case), as expected based on passive tracer uptake studies. However, the surface warming is weaker in the GM case, especially over the Southern Ocean, which is contrary to some recent supposition. Reasons for the reduced warming in the GM case are discussed.

Comments on ''Southern Mid- to High-Latitude Variability, a Zonal Wavenumber-3 Pattern, and the Antarctic Circumpolar Wave in the CSIRO Coupled Model''

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 circum...

Simulated changes in the frequency of extremes and regional features of seasonal/annual temperature and precipitation when atmospheric CO2 is doubled

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...

Strong ENSO Variability and a Super-ENSO Pair in the CSIRO Mark 3 Coupled Climate Model

Abstract Simulations of El Nino–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 sign...

The problem of “naturally”-occurring drought

Naturally-occurring drought is defined here to be drought arising from the nonlinear interactions which are an inherent part of the dynamics of the climatic system. As such it has no specific excitation mechanism, in contrast to forced drought where sea surface temperature anomalies are frequently cited as an important precursor. The essential difference between these two types of drought is that the former is very local and isolated spatially, whereas the latter is widespread and coherent. Observations for Australia are used to illustrate these points. Results are given for a 10-year general circulation model integration which clearly simulated naturally occurring drought and highlighted its unique characteristics. Multi-annual time series for specific geographical regions in the model show that no differences in monthly mean values of relative humidity or zonal and meridional fluxes of moisture were apparent for years with or without drought. More detailed analysis indicated that rather small differences exist in atmospheric temperatures and absolute humidities between drought and nondrought years which are important factors in determining the onset of precipitation in the model.Overall the analysis emphasises the subtlety of the processes involved. These processes, however, were able to produce completely different precipitation histories from one year to the next at a given point. The smallness of the changes involved in the atmospheric processes indicates that the nonlinearities were able to modulate conditions at a given point within an existing synoptic system only slightly, rather than initiate a new climatic regime in drought years. The problem of naturally-occurring drought, of course, is that it is intrinsically unpredictable.

Southern High-Latitude Ocean Climate Drift in a Coupled Model

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 su...

Sensitivity of a coupled atmosphere-dynamic upper ocean GCM to variations of CO2, solar constant, and orbital forcing

Sensitivity of a coupled atmosphere-dynamic upper ocean GCM to varying CO2, solar constant, and orbital forcing was examined. Response to atmospheric CO2 concentrations ranging from 100–3500 ppm is logarithmic at all latitudes and seasons, with highest sensitivity at high latitudes, during the winter season. Solar constant response is approximately linear over the range of values +2%, but the sensitivity at high latitudes is less than for equivalent CO2 forcing. Sensitivity to “cold northern summer” orbital forcing, which occurred at the start of the last glacial cycle, is strongly affected by CO2. For CO2 at or below the present level, perennial snow cover in the northern hemisphere expands dramatically with “cold summer” orbital forcing, but this effect becomes very small for CO2 levels in the range 410–460 ppm. This result suggests that the Quaternary “ice age” mode of climatic behaviour may have been initiated by an atmospheric CO2 decrease below a critical value, probably around 350–450 ppm.