Carsten S. Frederiksen

Recent apparent changes in relationships between the El Niño‐Southern Oscillation and Australian rainfall and temperature

High quality historical temperature and rainfall data sets have been used to produce time series of annual rainfall and temperature, averaged over Australia. The relationships between these series, and with the Southern Oscillation Index, have been examined. A change in the relationships between the variables appeared in the early 1970s. Since then, for any value of the SOI or rainfall, maximum temperature has tended to be higher than previously. Likewise, rainfall, for any value of the SOI, has tended to be greater than would have been expected for such an SOI value in previous years. Artificial changes, such as changes in instrumentation, seem unlikely to account for these observed changes in relationships. Model experiments have duplicated some of the changes in the relationships. Increased Indian Ocean temperatures may be the causal factor underlying these changes.

Interdecadal changes in southern hemisphere winter storm track modes

The interdecadal changes in southern hemisphere (SH) winter cyclogenesis have been studied using a global two-level primitive equation instability-model with reanalysed observed July three-dimensional basic states for the periods 1949–1968 and 1975–1994. The early to mid-1970s were a time of quite dramatic reduction in the winter rainfall in the southwest of western Australia (SWWA). We find that the rainfall reduction is associated with a decrease in the vertical mean meridional temperature gradient and in the peak upper tropospheric jet-stream zonal winds near 30° south throughout most of the SH. These changes are reflected in the properties of the leading SH cyclogenesis modes: for 1975–1994 both the fastest growing mode, and on average the 10 leading SH cyclogenesis modes that cross Australia, have growth rates which are around 30% smaller than for the corresponding modes for 1949–1968. The sensitivity of our results, to the strengths of physical parametrizations and to the choice of basic states based on different data sets, is examined. Our results suggest that a primary cause of the rainfall reduction over SWWA in the period after 1975 is the reduction of the intensity of cyclogenesis and the southward deflection of some storms.

Southern Hemisphere Storm Tracks, Blocking, and Low-Frequency Anomalies in a Primitive Equation Model

Abstract The results of a study are presented that indicate three-dimensional instability theory is able to generate analogs of a wide variety of Southern Hemisphere observed fluctuations including those associated with cyclogenesis, blocking, and low-frequency teleconnection patterns. This study has been conducted with a two-level primitive equation eigenvalue model and the growing modes for both January and July averaged basic states are examined. In both seasons, the fastest-growing cyclogenesis modes have largest amplitudes in the eastern part of the hemisphere just downstream of the polar jet stream maxima. In July, there is a tendency to form elongated eddies in the region downstream of Australia where growth on both polar and subtropical jet streams occurs. These results are in general agreement with the observational studies of Southern Hemisphere storm tracks. For both January and July, larger-scale slower-propagating dipole or multipole modes are found that are consistent with blocking in some o...

Monsoon disturbances, intraseasonal oscillations, teleconnection patterns, blocking, and storm tracks of the global atmosphere during January 1979: Linear theory

Abstract The results of a study are presented that indicate that a wide variety of atmospheric disturbances, including those associated with storm tracks and blocking in both hemispheres, quasi-stationary global teleconnection patterns, and localized monsoon disturbances, as well as intraseasonal oscillations, may be generated through the instability of the three-dimensional global basic state for January 1979 including a wave-CISK cumulus heating parameterization. The analysis has been conducted with a two-level primitive equation eigenvalue model, and the growing disturbances for various specifications of the strengths of the cumulus heating have been analyzed. Within the parameter range studied, inclusion of explicit moisture in the basic state has little effect on the structures of the storm track and onset-of-blocking modes in both hemispheres, but it increases growth rates as expected. It is also responsible for generating a significant amplitude of the tropical shear streamfunction of low-frequency...

Validating Interannual Variability in an Ensemble of AGCM Simulations

This paper is an effort to explore general statistical procedures for validating the interannual variability in ensembles of atmospheric general circulation model simulations. The authors propose an estimation method for the correlation coefficient between the SST-forced components of simulated and observed seasonal means. The proposed methodology has been applied to study the interannual variability of New Zealand surface temperature as simulated by the Australian Bureau of Meteorology Research Centre nine-level atmospheric general circulation model. Compared with observations, it is found that the model simulates this variability well in most cases, except for summer maximum temperatures. It is suggested that deficiencies in the simulated storm tracks during summer may account for some apparent deficiency in the model. The failure in simulating the summer maximum temperatures also indicates the need for model validation.

Multidecadal Simulations of Australian Rainfall Variability: The Role of SSTs

Australian rainfall variability and its relationship with the Southern Oscillation index (SOI) and global sea surface temperature (SST) variability is considered in both observational datasets and ensembles of multidecadal simulations using two different atmospheric general circulation models forced by observed SSTs and sea ice extent. Monthly and seasonal time series have been constructed to examine the observed and modeled relationships. The models show some success in the Australian region, largely reproducing the observed relationships between rainfall, the SOI, and global SSTs, albeit better in some seasons and geographical regions than others. A partition of the rainfall variance into components due to SST forcing and internal variability, suggests that both models have too much internal variability over the central eastern half of the continent, especially during austral winter and spring. Consequently, the strength of the SOI and SST relationships tend to be underestimated in this region. The largest impact of SST forcing is seen over the tropical and western parts of the continent. A principal component analysis reveals two dominant rotated modes of rainfall variability that are very similar in both the observed and modeled cases. One of these modes is significantly correlated with SST anomalies to the north-northwest of Australia (in the case of the models) and the SST gradient between the Indonesian archepelago and the central Indian Ocean (in the observed case). The other mode is significantly correlated with

A Theoretical Model of Australian Northwest Cloudband Disturbances and Southern Hemisphere Storm Tracks: The Role of SST Anomalies

Abstract The author examine the effects of an enhanced sea surface temperature gradient, between the central Indian Ocean and the Indonesian archipelago, on the structure of the monthly averaged three-dimensional July global circulation and particularly on its consequent instability properties. This study has been conducted using a two-level primitive-equation model including a wave-CISK cumulus heating parameterization and using basic states taken from two AGCM simulations of the July circulation, with and without an enhanced SST gradient. The growing disturbances have been analyzed for various strengths of the cumulus heating. The study has focused on circulation changes associated with the development of Australian northwest cloudband disturbances and changes in Southern Hemisphere storm tracks. With an enhanced SST gradient, the authors have found a new group of modes that can be associated with circulation changes surrounding the onset of the simulated northwest cloudband events. Such modes are shown...

A Study of Predictable Patterns for Seasonal Forecasting of New Zealand Rainfall

Abstract A recently developed variance decomposition approach is applied to study the causes of the predictability of New Zealand seasonal mean rainfall. In terms of predictability, the Southern Oscillation is identified as being the most important cause of variability for both the winter and summer New Zealand rainfall, especially for the North Island. Indian Ocean sea surface temperature variability and the Southern Hemisphere annular mode are the second most important causes of variability for winter and summer rainfall, respectively. Based on this study, a statistical prediction scheme has been developed. May Nino-3 (5°N–5°S, 150°–90°W) SSTs and March–May (MAM) central Indian Ocean SSTs are identified as being the most important predictors for the winter rainfall, while September–November (SON) Nino-3 SSTs, November local New Zealand SSTs, and the SON Southern Hemisphere annular mode index are the most important predictors for the summer rainfall. The predictive skill, in term of the percentage explai...

Northern Hemisphere Storm Tracks and Teleconnection Patterns in Primitive Equation and Quasigeostrophic Models

Abstract A study is made of the importance of horizontally varying static stability and nongeostrophic effects upon the location of Northern Hemisphere storm track instability modes and mature anomaly teleconnection pattern modes during January 1979. The analysis has been conducted with a two-level primitive equation model, and the results compared with corresponding results from one-, two-, and five-level quasigeostrophic models, although the main emphasis is a comparison between the two-level results.January 1979 was a period of frequent and severe storm activity in the Northern Hemisphere and a time of transition from high-latitude blocking over northwestern Canada and the Beaufort Sea area to persistent and large-scale blocking in the North Atlantic region. The three-dimensional instability modes from all models are discussed in the context of these synoptic developments. In particular, it is found that the inclusion of a horizontally varying static stability and nongeostrophic effects influence the s...

Dynamical Seasonal Forecasts during the 1997/98 ENSO Using Persisted SST Anomalies

Abstract An evaluation of trial seasonal forecasts during the 1997/98 El Nino, using an atmospheric GCM forced by persisted sea surface temperature and sea-ice anomalies, is presented. Generally, forecasts of seasonal anomalies of precipitation, surface air temperature, 200-hPa geopotential height, and mean sea level pressure (MSLP) are shown to have statistically significant skill in the Tropics and subtropics, but predominantly over the oceans. Surface air temperature and 200-hPa height anomalies are also skillfully forecast over land in the 30°S–30°N latitudinal band, and, in contrast to precipitation and MSLP, also show significant skill in the extratropics. The global pattern of significant skill seems not to be oversensitive to the use of a Kuo or a mass-flux convection scheme (Tiedtke), although the global root-mean-square errors are consistently larger, in the latter case. Results from multidecadal simulations of the model, when forced by observed sea surface temperature and sea-ice, show that the...