Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: Ian G. Watterson x
  • All content x
Clear All Modify Search
Kevin Walsh and Ian G. Watterson

Abstract

A climate simulation of a limited area model implemented over the Australian region is analyzed for the presence of low pressure systems that have some of the physical characteristics of tropical cyclones. The model is run at a horizontal resolution of 125 km and is nested within a GCM simulation of 10 Januarys. The model simulates those variables that are believed to be important for tropical cyclone formation reasonably well, as evaluated using Gray’s Seasonal Genesis Parameter. Objective criteria are used to detect tropical cyclone-like vortices (TCLVs) in the model. The composite structure of the simulated storms and the life cycle of a typical TCLV are described. Like tropical cyclones, the simulated TCLVs have warm cores, low-level wind maxima, and their tracks and regions of occurrence are similar to those observed for tropical cyclones. In general, the TCLVs simulated by the limited area model are weaker than observed, as determined by a measure of the area-averaged low-level tangential wind speed, but they are much more realistic than those vortices similarly generated by the GCM. Maximum wind speeds also occur farther from the center of the storm on average than observed. A multiply nested limited area model simulation at a horizontal resolution of 30 km shows further improvement in the TCLV simulation. While the 125-km resolution model may have some potential for predicting genesis regions, numbers, and tracks of TCLVs, it does not yet show such potential for predicting intensities.

Full access
Wenju Cai and Ian G. Watterson

Abstract

This study examines the capability of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) climate model in simulating the observed modes of interannual variability of the Southern Hemisphere circulation. Modes of variability in the 500-hPa geopotential height (Z500) field of the following three experiments are examined: 1) a coupled experiment, in which the atmosphere and the ocean are fully coupled, producing El Niño–Southern Oscillation (ENSO) cycles and allowing full air–sea interactions; 2) a mixed layer experiment, in which the atmosphere is coupled to an ocean mixed layer heat equation allowing limited air–sea interactions; and 3) a climatology experiment, in which the atmosphere is forced by an observed SST climatology with a fixed annual cycle, allowing no air–sea interactions. It is found that the observed modes are reasonably simulated in all three experiments, although the amplitude of the model modes is generally smaller than that of the observed. These modes include the high-latitude mode (i.e., the Antarctic Oscillation), the Pacific–South American (PSA) mode, and the wavenumber-3 mode. It is also found that the response of the mid- to high-latitude atmosphere circulation to the model ENSO forcing projects mainly onto the PSA mode. Many features of the PSA mode are similar to those associated with the Pacific–North American mode in the Northern Hemisphere. In response to these Z500 modes, the ocean produces coherent modes of variability, but the oceanic feedback effect appears to be weak. The amplitude of anomalies associated with each mode of the Z500 field in the three experiments shows little difference, suggesting that these Z500 modes can be generated by atmospheric internal dynamics alone, and that the ocean dynamics, air–sea interactions, and ENSO forcing are not essential.

Full access
Mark R. Sinclair and Ian G. Watterson

Abstract

An automated weather system identification and tracking scheme is used to appraise the skill of the CSIRO9 GCM in replicating contemporary extratropical cyclone and anticyclone behavior, and to assess possible changes as a result of doubled CO2. Cyclones are identified as centers of cyclonic vorticity rather than pressure minima, which can vanish if the background pressure gradient increases. Comparison with an observational dataset from ECMWF revealed that the GCM control simulation realistically reproduced the present-day storm track locations, but with slightly fewer and generally weaker systems overall. These errors are consistent with the coarser resolution of the GCM and its underestimation of the strength and baroclinicity of the polar vortex in both hemispheres.

Comparison between 1 and 2 × CO2 GCM simulations revealed increases in both 500-hPa geopotential height and 1000–500-hPa thickness for doubled CO2. As in other studies, these changes are largest near the poles, resulting in weaker westerlies and reduced tropospheric baroclinicity. Decreases of 10%–15% in both cyclone and anticyclone activity consistent with these circulation changes are found. However, there is some evidence of increased winter cyclone activity near the downstream end of the principal storm tracks. There is also a general reduction in the number and strength of intense storms, despite generally lower central pressures, which arise from global-scale decreases in sea level pressure in the doubled CO2 atmosphere rather than from greater storm vigor. This underscores the need for GCM projections of midlatitude “storminess” to employ more realistic measures of storm activity and intensity.

Full access
John W. Kidson and Ian G. Watterson

Abstract

The CSIRO9 general circulation model shows a zonally symmetric mode of variability, which closely resembles the high-latitude mode (HLM) in middle and high latitudes of the Southern Hemisphere. The leading EOF of the zonal mean zonal wind between 30° and 68°S, whose amplitude has been taken as an index of the HLM, shows opposing variations centered near 40° and 60°S accounting for 43% of the daily variance. Analysis has concentrated on composites for periods when the index changed quickly between significant peaks of the opposite sign or persisted with a large amplitude for an extended period. The momentum flux variations are small at the northern and southern boundaries and the principal variations are centered near 49°S between the maxima in the zonal wind. The changes in angular momentum content are around 30% smaller in the southern band. Eddy heat fluxes are less coherent but help in maintaining the zonal wind anomalies against friction.

A simple model of the zonal wind index with stochastic forcing and linear damping reproduces its short period variations well but is less successful in simulating the observed continuity over 10- to 20-day lags.

Full access
Edwin K. Schneider and Ian G. Watterson

Abstract

The linearized shallow water equations on a sphere are solved numerically to examine the sensitivity of the steady response to midlatitude mountain forcing to the zonal mean basic state. The zonal mean basic state consists of meridionally varying zonal winds ū(y) and meridional winds v̄(y). Cases are considered where ū is westerly everywhere, outside a tropical region where it is easterly. A zonal wavenumber three mountain confined to the Northern Hemisphere midlatitudes, where ū>0, provides the forcing.

When v̄≡0 the usual result of negligible Southern Hemisphere response to the mountain forcing is found. However, a modest mean meridional velocity [0(3 m s−1)] that is directed from north to south through the easterly layer leads to significant Southern Hemisphere response. An argument based on the local dispersion relation is offered to explain this effect. It is concluded that critical latitude effects on wave propagation are sensitive to the structure of the mean meridional circulation in the critical latitude region of the model. The result of the simplified model suggests that a more relevant model with a zonally symmetric basic state consisting of zonal winds and meridional circulation varying with height as well as latitude should be investigated.

Full access
Ian G. Watterson, Jenni L. Evans, and Brian F. Ryan

Abstract

Gray's seasonal genesis parameter (SGP) is reassessed as a diagnostic quantity for both climatological and single-season tropical cyclogenesis. The SGP applied to global analyses from recent years is able to locate the regions of genesis activity during 1967–86. The SGP based on the climatology of a simulation by the CSIR09 atmospheric model using prescribed ocean temperatures for 1979–88 has similar skill. The SGP applied to single-season means is then assessed as a diagnostic for interannual variation of cyclogenesis. Increased cyclogenesis in the central Pacific during the 1982/83 El Niño coincides with increased SGP. CSIRO9 simulated similar variations in the SGP. Moderate correlations are found between the time series of the observed and inferred simulated cyclogenesis numbers in the central Pacific, eastern North Pacific, and North Atlantic regions during 1979–88. However, elsewhere the correlations were poor.

Full access
Terence J. O’Kane, Dougal T. Squire, Paul A. Sandery, Vassili Kitsios, Richard J. Matear, Thomas S. Moore, James S. Risbey, and Ian G. Watterson

Abstract

Recent studies have shown that regardless of model configuration, skill in predicting El Niño–Southern Oscillation (ENSO), in terms of target month and forecast lead time, remains largely dependent on the temporal characteristics of the boreal spring predictability barrier. Continuing the 2019 study by O’Kane et al., we compare multiyear ensemble ENSO forecasts from the Climate Analysis Forecast Ensemble (CAFE) to ensemble forecasts from state-of-the-art dynamical coupled models in the North American Multimodel Ensemble (NMME) project. The CAFE initial perturbations are targeted such that they are specific to tropical Pacific thermocline variability. With respect to individual NMME forecasts and multimodel ensemble averages, the CAFE forecasts reveal improvements in skill when predicting ENSO at lead times greater than 6 months, in particular when predictability is most strongly limited by the boreal spring barrier. Initial forecast perturbations generated exclusively as disturbances in the equatorial Pacific thermocline are shown to improve the forecast skill at longer lead times in terms of anomaly correlation and the random walk sign test. Our results indicate that augmenting current initialization methods with initial perturbations targeting instabilities specific to the tropical Pacific thermocline may improve long-range ENSO prediction.

Free access
Terence J. O’Kane, Paul A. Sandery, Vassili Kitsios, Pavel Sakov, Matthew A. Chamberlain, Dougal T. Squire, Mark A. Collier, Christopher C. Chapman, Russell Fiedler, Dylan Harries, Thomas S. Moore, Doug Richardson, James S. Risbey, Benjamin J. E. Schroeter, Serena Schroeter, Bernadette M. Sloyan, Carly Tozer, Ian G. Watterson, Amanda Black, Courtney Quinn, and Richard J. Matear

Abstract

The CSIRO Climate retrospective Analysis and Forecast Ensemble system, version 1 (CAFE60v1) provides a large (96 member) ensemble retrospective analysis of the global climate system from 1960 to present with sufficiently many realizations and at spatiotemporal resolutions suitable to enable probabilistic climate studies. Using a variant of the ensemble Kalman filter, 96 climate state estimates are generated over the most recent six decades. These state estimates are constrained by monthly mean ocean, atmosphere, and sea ice observations such that their trajectories track the observed state while enabling estimation of the uncertainties in the approximations to the retrospective mean climate over recent decades. For the atmosphere, we evaluate CAFE60v1 in comparison to empirical indices of the major climate teleconnections and blocking with various reanalysis products. Estimates of the large-scale ocean structure, transports, and biogeochemistry are compared to those derived from gridded observational products and climate model projections (CMIP). Sea ice (extent, concentration, and variability) and land surface (precipitation and surface air temperatures) are also compared to a variety of model and observational products. Our results show that CAFE60v1 is a useful, comprehensive, and unique data resource for studying internal climate variability and predictability, including the recent climate response to anthropogenic forcing on multiyear to decadal time scales.

Open access