Southern Hemisphere Sea Level Pressure Research Articles

South African droughts and decadal variability

We explore the historical occurrence of extensive droughts over South Africa within the context of decadal climate variability. A Standardized Precipitation Index dataset is developed and used to assess the spatial extent of droughts in South Africa for the period 1920–2014. The most extensive droughts over the period at various time scales are identified and discussed. Results of a wavelet analysis are also presented towards identifying statistically significant regional climate variation with which the occurrence of droughts is associated. The occurrence of drought with respect to the El Nino Southern Oscillation and decadal climate variability is also considered. Significant associations between short-term droughts and decadal variability are pointed out. An overview of global sea surface temperature and Southern Hemisphere sea-level pressure associations with three prominent scales of multi-year climate variability is given. Dry epochs at the most prominent time scales are shown to be significantly negatively related to the Southern Annular Mode and associated sea surface temperature anomalies in the mid-to-high southern latitudes. Relations with the Pacific Decadal Oscillation, Inter-Decadal Pacific Oscillation and ENSO, which are all associated with a Southern Annular Mode of opposite sign, are also highlighted.

Intercomparison of four different Southern Hemisphere sea level pressure datasets

Four different monthly sea level pressure (MSLP) datasets for the Southern Hemisphere (SH, south of 15°S) are intercompared for the 1961–2000 period, contrasting the differences in decadal‐average pressure and differences between correlations for the 1961–80 (pre‐satellite measurement era) and the 1991–2000 (post‐satellite) period. Agreement is very dependent on the locations in the observing network, and this becomes even clearer for the two decades (1961–80) before the satellite era. Away from the station locations it is impossible to say which provides the ‘best’ decadal average MSLP values, due to assumptions made in the dataset derivation. Improved reanalyses will be undertaken in the next few years, but they are only likely to provide the true decadal average values of MSLP for the 1961–80 period if a much higher percentage of observations made for this period are digitized, and provided they are quality controlled, assessed for homogeneity and given higher priority within the reanalysis system.

Detection of external influence on sea level pressure with a multi‐model ensemble

Over the past fifty years, December–February mean sea level pressure has decreased markedly over both poles, corresponding to a trend toward strengthened westerlies in both hemispheres. In this study we compare observed sea level pressure trends with those simulated in response to natural and anthropogenic influence in a suite of eight up‐to‐date coupled general circulation models. A global analysis indicates that sea level pressure trends may be attributed to external influence. However, while simulated Southern Hemisphere sea level pressure trends are consistent with those observed, simulated Northern Hemisphere sea level pressure trends are not: Observations show a large negative trend in the Arctic and a positive trend over the subtropical North Atlantic and Mediterranean which is not reproduced in the simulations.

Sea Ice and Polar Climate in the NCAR CSM*

The Climate System Model (CSM) consists of atmosphere, ocean, land, and sea-ice components linked by a flux coupler, which computes fluxes of energy and momentum between components. The sea-ice component consists of a thermodynamic formulation for ice, snow, and leads within the ice pack, and ice dynamics using the cavitating-fluid ice rheology, which allows for the compressive strength of ice but ignores shear viscosity. The results of a 300-yr climate simulation are presented, with the focus on sea ice and the atmospheric forcing over sea ice in the polar regions. The atmospheric model results are compared to analyses from the European Centre for Medium-Range Weather Forecasts and other observational sources. The sea-ice concentrations and velocities are compared to satellite observational data. The atmospheric sea level pressure (SLP) in CSM exhibits a high in the central Arctic displaced poleward from the observed Beaufort high. The Southern Hemisphere SLP over sea ice is generally 5 mb lower than observed. Air temperatures over sea ice in both hemispheres exhibit cold biases of 2‐4 K. The precipitationminus-evaporation fields in both hemispheres are greatly improved over those from earlier versions of the atmospheric GCM. The simulated ice-covered area is close to observations in the Southern Hemisphere but too large in the Northern Hemisphere. The ice concentration fields show that the ice cover is too extensive in the North Pacific and subarctic North Atlantic Oceans. The interannual variability of the ice area is similar to observations in both hemispheres. The ice thickness pattern in the Antarctic is realistic but generally too thin away from the continent. The maximum thickness in the Arctic occurs against the Bering Strait, not against the Canadian Archipelago as observed. The ice velocities are stronger than observed in both hemispheres, with a consistently greater turning angle (to the left) in the Southern Hemisphere. They produce a northward ice transport in the Southern Hemisphere that is 3‐4 times the satellite-derived value. Sensitivity tests with the sea-ice component show that both the pattern of wind forcing in CSM and the air-ice drag parameter used contribute to the biases in thickness, drift speeds, and transport. Plans for further development of the ice model to incorporate a viscousplastic ice rheology are presented. In spite of the biases of the sea-ice simulation, the 300-yr climate simulation exhibits only a small degree of drift in the surface climate without the use of flux adjustment. This suggests a robust stability in the simulated climate in the presence of significant variability.

Low-Frequency Variability of Southern Hemisphere Sea Level Pressure and Weather System Activity

Abstract This study examines the month-to-month variations in the tracks of Southern Hemisphere weather systems and their relation to low-frequency circulation variability. Cyclones and anticyclones are identified and tracked from ECMWF analyses during 1980–94 via an automated method and the principal patterns of variation identified by EOF analysis of monthly track density anomaly fields. Only the first three EOFs of cyclone track density involving about one-third of the total track variance were distinguishable from noise. Spatial patterns derived from both unrotated and rotated EOF analysis were not reproducible on subsets of the data, pointing to secular changes in the variance structure of the cyclone dataset. An increase in cyclone numbers over the Southern Ocean during the 1980s suggested that detection of small-scale cyclones is sensitive to changes in data coverage and analysis procedure, as associated changes in the mean circulation were small during this period. EOFs of anticyclone track data w...

The Impact of Manually Derived Southern Hemisphere Sea Level Pressure Data upon Forecasts from a Global Model

Abstract Experiments have been conducted within the framework of the Australian Bureau of Meteorology's global data assimilation and prediction system to assess the forecast impact of Southern Hemisphere sea level pressure estimates [paid observations (PAOBs)] from manual analyses. Parallel assimilation cycles with and without PAOBs, for two periods each of about 4 weeks in different seasons, while inconclusive, suggest a small positive impact of PAOBs in Southern Hemisphere midlatitudes.

Intercomparison of Two Different Southern Hemisphere Sea Level Pressure Datasets

Two different sea-level pressure datasets for the strip between 20 o and 40 o S are intercompared. At large space-time scales the sets are virtually identical. However, for studies of short-term, regional changes caution must be exercised because the datasets may differ substantially

Intra‐ and inter‐hemispheric teleconnections using seasonal southern hemisphere sea level pressure

AbstractIn this study spatial and temporal relationships involving Southern Hemisphere (SH) sea level pressure (SLP) were examined using a 25‐year data set assembled for this purpose, with special emphasis on discovering inter‐hemispheric teleconnections in the seasonal SLP field. Principal component and correlation analysis was employed. The most important findings include: Season‐to‐season autocorrelation of SLP is at least as large in the SH as in the Northern Hemisphere (NH), though the amount of area of significant autocorrelation is not great in either hemisphere (i.e. a total global area of 17‐24 per cent). Principal component (PC) analysis performed on SH SLP in conjunction with Monte Carlo tests of significance showed that three or four PCs, explaining 40 to 58 per cent of the variance, represent the climatic signal in the data set. Field significance tests applied to each teleconnection map, produced for each grid point in the SH, implied that there is a strong likelihood of real inter‐hemispheric relationships in NH winter. There was much less certainty in the other seasons. Examination of significant teleconnection maps suggested SH links to the Southern Oscillation, the North Atlantic Oscillation, Siberian High strength, and the ‘West Pacific pattern’ described by Wallace and Gutzler (1981).

Intra- and inter-hemispheric teleconnections using seasonal Southern Hemisphere sea level pressure : Harnack, R.P. and Jeremi Harnack, 1985. J. Climatol., 5(3):283–296

Intra- and inter-hemispheric teleconnections using seasonal Southern Hemisphere sea level pressure : Harnack, R.P. and Jeremi Harnack, 1985. J. Climatol., 5(3):283–296

A Southern Hemisphere sea level pressure data set for use in climatic studies

AbstractA 25‐year (1956–1980) Southern Hemisphere sea level pressure (SLP) data set (G‐SLP) for the region 10°S–50°S was created by manually extracting interpolated grid point data from monthly SLP maps contained in the German publication Die Witterung in Übersee. Comparison of long term means with published means indicated that the major large‐scale circulation features on the original maps were retained through the extraction process. Comparison between a short term subset (1972–1979) of the data set and A‐SLP, derived from a SLP data set produced by the Australian National Meteorological Analysis Centre of the Australian Bureau of Meteorology, substantiated the reasonableness of the extraction process and suggested areas of possible data problems. The 50°S data was determined to be insufficient for statistical studies owing to missing data. Most areas of significant difference between G‐SLP and A‐SLP, as determined by a Student's t test, can probably be accounted for by the fact that A‐SLP includes satellite data not included in G‐SLP. Analysis of the informational content of the G‐SLP data set is presented, the results of which strongly suggest that the derived data set will be useful in large‐scale climatic studies involving interannual variability of Southern Hemisphere SLP.