Search Results
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.
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.
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.
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.
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.
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.
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.
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.
Abstract
We present four 140-yr wind-wave climate simulations (1961–2100) forced with surface wind speed and sea ice concentration from two CMIP6 GCMs under two different climate scenarios: SSP1–2.6 and SSP5–8.5. A global three-grid system is implemented in WAVEWATCH III to simulate the wave–ice interactions in the Arctic and Antarctic regions. The models perform well in comparison with global satellite altimeter and in situ buoys climatology. The comparison with traditional trend analyses demonstrates the present GCM-forced wave models’ ability to reproduce the main historical climate signals. The long-term datasets allow a comprehensive description of the twentieth- and twenty-first-century wave climate and yield statistically robust trends. Analysis of the latest IPCC ocean climatic regions highlights four regions where changes in wave climate are projected to be most significant: the Arctic, the North Pacific, the North Atlantic, and the Southern Ocean. The main driver of offshore wave climate change is the wind, except for the Arctic where the significant sea ice retreat causes a sharp increase in the projected wave heights. Distinct changes in the wave period and the wave direction are found in the Southern Hemisphere, where the poleward shift of the Southern Ocean westerlies causes an increase in the wave period of up to 5% and a counterclockwise change in wave direction of up to 5°. The new CMIP6 forced wave models improve in performance compared to previous CMIP5 forced wave models, and will ultimately contribute to a new CMIP6 wind-wave climate model ensemble, crucial for coastal adaptation strategies and the design of future marine offshore structures and operations.
Significance Statement
The purpose of this study is to advance the understanding of ocean wind-wave climate evolution over the twentieth and twenty-first centuries and to effectively communicate the long-term impacts of climate change in diverse wind-wave climatic regions across the globe. The 140-yr continuous model results produced in this work are crucial to studying changes in extreme sea states and investigating the relationship between interdecadal periodic oscillations and long-term climate trends. The dataset produced can be used to gain further insight into the substantial long-term changes of the polar wind-wave climate caused by the rapid decrease of sea ice coverage, and the evolution of the directional changes in the sea states triggered by climate change.
Abstract
We present four 140-yr wind-wave climate simulations (1961–2100) forced with surface wind speed and sea ice concentration from two CMIP6 GCMs under two different climate scenarios: SSP1–2.6 and SSP5–8.5. A global three-grid system is implemented in WAVEWATCH III to simulate the wave–ice interactions in the Arctic and Antarctic regions. The models perform well in comparison with global satellite altimeter and in situ buoys climatology. The comparison with traditional trend analyses demonstrates the present GCM-forced wave models’ ability to reproduce the main historical climate signals. The long-term datasets allow a comprehensive description of the twentieth- and twenty-first-century wave climate and yield statistically robust trends. Analysis of the latest IPCC ocean climatic regions highlights four regions where changes in wave climate are projected to be most significant: the Arctic, the North Pacific, the North Atlantic, and the Southern Ocean. The main driver of offshore wave climate change is the wind, except for the Arctic where the significant sea ice retreat causes a sharp increase in the projected wave heights. Distinct changes in the wave period and the wave direction are found in the Southern Hemisphere, where the poleward shift of the Southern Ocean westerlies causes an increase in the wave period of up to 5% and a counterclockwise change in wave direction of up to 5°. The new CMIP6 forced wave models improve in performance compared to previous CMIP5 forced wave models, and will ultimately contribute to a new CMIP6 wind-wave climate model ensemble, crucial for coastal adaptation strategies and the design of future marine offshore structures and operations.
Significance Statement
The purpose of this study is to advance the understanding of ocean wind-wave climate evolution over the twentieth and twenty-first centuries and to effectively communicate the long-term impacts of climate change in diverse wind-wave climatic regions across the globe. The 140-yr continuous model results produced in this work are crucial to studying changes in extreme sea states and investigating the relationship between interdecadal periodic oscillations and long-term climate trends. The dataset produced can be used to gain further insight into the substantial long-term changes of the polar wind-wave climate caused by the rapid decrease of sea ice coverage, and the evolution of the directional changes in the sea states triggered by climate change.
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.
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.
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.
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.
