John W. Kidson

An analysis of New Zealand synoptic types and their use in defining weather regimes

A new set of 12 daily weather types for the New Zealand region has been derived from the 40-year NCEP/NCAR reanalysis dataset. Cluster analysis of the monthly frequencies of these patterns has led to the definition of three ‘regimes’, characterized by (i) frequent troughs crossing the country, (ii) highs to the north with strong zonal flow to the south of the New Zealand, and (iii) blocking patterns with highs more prominent in the south. Blocking regimes are more frequent in summer and autumn and are associated with above-normal temperatures, less precipitation in the southwest of the country and more precipitation in the northeast. The Zonal regime, which brings below-normal precipitation to the northeast and milder conditions in the south, is less common in summer. The Trough regime is less frequent in autumn and is linked to cooler temperatures in the west and above-normal precipitation over the entire country. The monthly frequencies of individual synoptic types are only weakly related to the Southern Oscillation Index (SOI) and other indices of the hemispheric-scale flow, with variance reductions from regression equations ranging from between 3 and 33%. Similar predictions of weighted combinations of the synoptic types corresponding to each regime gave mean variance reductions 70–80% higher. The correct regime could be specified from the hemispheric indices by the use of discriminant analysis for 56% of the 474 months in the dependent dataset. The monthly frequency of synoptic types was also tested as an alternative to monthly mean 1000 hPa patterns in the selection of analogues over the New Zealand region. The success of the selection process was measured by the variance of temperature and rainfall patterns over homogeneous regions for sets of two, four and eight analogues. The mean 1000 hPa patterns scored better in nearly all cases, but gave variance reductions of only 13–19% over randomly chosen analogues. Copyright

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 Structure and Predictability of the “High-Latitude Mode” in the CSIRO9 General Circulation Model

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...

The Southern Hemisphere Evolution of ENSO during 1981–99

Abstract Improvements in observing systems over the last two decades now permit more detailed examinations of variability in the mid- and high-latitude Southern Hemisphere circulation. In this study the leading modes of variation in the extratropical sea surface temperature are identified from the Reynolds optimal interpolation (OI) dataset between November 1981 and December 1999. Although attempts were made to exclude the strong contribution due to El Nino–Southern Oscillation (ENSO) by confining the analysis between 60° and 15°S, the three leading modes were all ENSO-related. The first EOF with 13.9% of the monthly variance lags the Southern Oscillation index (SOI) by 2 months and depicts sea temperature variations characteristic of mature ENSO events. The second EOF, accounting for 8.8% of the variance, shows a “precursor” pattern that is strongly correlated with the SOI and with EOF1, which it leads by 9–10 months. This pattern is characterized by anomalies west and east of Australia (extending north ...

THE UTILITY OF SURFACE AND UPPER AIR DATA IN SYNOPTIC CLIMATOLOGICAL SPECIFICATION OF SURFACE CLIMATIC VARIABLES

The relative merits of applying 1000 and 500 hPa map classifications to specifying departures in daily and monthly climatic elements have been examined for a network of 82 stations over New Zealand. The map classification technique was based on eigenanalysis of twice daily geopotential height fields at 2.5° resolution, followed by cluster analysis. Five significant EOFs were obtained at each level and subsequent cluster analysis resulted in stable patterns of seven and ten synoptic classes at the 1000 and 500 hPa levels respectively. The application of EOF analysis and cluster analysis to the combined data from both levels led in each case to equivalent-barotropic patterns, so that it was not possible to distinguish between varying vertical structures associated with the same low-level circulation. On a daily basis, the ten 500 hPa classes were better able to distinguish between maximum and minimum temperatures and daily precipitation, but comparable results with the seven 1000 hPa map types were obtained for the duration of bright sunshine and daily wind run. The difference in skill was at least partly related to the number of synoptic classes at each level. Monthly mean departures for all variables over 1980-1993 were estimated by regression from the relative frequencies of each synoptic class, from the weighted daily mean departure patterns for each class, and from the mean EOF values for the month. In general the first two methods showed similar skill, with estimates based on the 500 hPa data slightly superior to those based on 1000 hPa analyses, but combinations of the predictors at both levels gave the best results. Direct estimates from the monthly mean EOFs proved better overall than those based on daily map classifications, limiting the value of the stochastic weather generator approach to the down-scaling of New Zealands regional climate.

Convection Patterns in the Tropical Pacific and Their Influence on theAtmospheric Circulation at Higher Latitudes

Abstract Patterns of outgoing longwave radiation (OLR) have been analyzed over the tropical Indian and Pacific Oceans in order to identify the varying influence of their associated convective anomalies on the circulation at higher latitudes. Particular attention has been given to the changes related to El Nino–Southern Oscillation (ENSO) events. The two leading EOFs (emperical orthogonal functions) of monthly OLR anomaly patterns for the region between 20°N–20°S and 70°E–120°W, express complementary variations between centers located 1) near 170°W just south of the equator and over the Philippines, and 2) slightly south of the equator near 145°W and slightly north of the equator near 165°E. Cluster analysis over a smaller area between 10°S–10°N and 140°E–140°W has highlighted ENSO-related changes with two of the six clusters associated with “moderate” (EN) and “strong” (EN+) El Nino events, and a third including most La Nina (LN) events. The OLR anomaly patterns associated with the 1986/87 and 1991/92 war...

The potential long-range predictability of precipitation over New Zealand

It is assumed that the interannual variance of seasonal precipitation totals is made up of a component reflecting daily weather variations which, as a result, is unpredictable beyond deterministic predictability limits of about 2 weeks. The second component is any additional variance that is, at least, potentially predictable. The first component is considered noise and is estimated using a statistical model whose parameters are determined from daily, within season, precipitation. Estimates are compared with the total variance and where the total variance exceeds the estimated noise it is concluded that there is potential for long-range prediction. Results indicate that only 30% or less of the total variance at stations is potentially predictable. Countrywide totals do not improve the situation. Persistence of the ENSO signal may be able to help realize a small fraction of the potential predictability or about 5% of the total variance. Copyright

Interpreting Low-Frequency Modes of Southern Hemisphere Atmospheric Variability as the Rotational Response to Divergent Forcing

Abstract The principal modes of Southern Hemisphere low-frequency variability have recently been calculated using a 39-yr record of 300-hPa streamfunction fields from the NCEP–NCAR reanalysis dataset. The authors attempt to interpret these modes as the rotational response to some divergent forcing. For a range of mean states the linearized barotropic vorticity equation (BVE) is used to solve for the divergent wind that would generate (or at least be consistent with) the observed vorticity modes. Several of these low-frequency modes can be generated by forcing the BVE with fairly simple divergent wind fields that could easily be interpreted as resulting from anomalous tropical convection. In particular this is found to be true for streamfunction anomalies with El Nino–Southern Oscillation (ENSO), high-latitude mode, South Pacific wave, and Madden–Julian oscillation structure. The authors speculate that it may be possible to relate these calculated divergent wind fields to recently observed OLR fields and h...

Climatic influences on the survival of southern gemfish (Rexea solandri, Gempylidae) in New Zealand waters

Catches of the southern stock of New Zealand gemfish, Rexea solandri, reached a peak of nearly 7000 tonnes in 1985‐1986 but have since declined dramatically to only 130 tonnes in 1995‐1996. Analysis of commercial and research data over the last 15 years suggests that the fish stock has undergone a major decline in abundance in New Zealand waters and that there have been large interannual fluctuations in the numbers of young fish recruiting into the commercial fishery. Such fluctuations appear to be strongly related to the frequency of occurrence of southwesterly wind flow over New Zealand and to sea surface temperature variations during the winter spawning period. Spawning success is reduced when local sea surface temperatures are lower than normal, and there is a higher than normal frequency of southwesterly flow patterns. A causal relationship has not been established, but a number of possible mechanisms capable of accounting for the statistical results, are discussed.

The potential long‐range predictability of temperature over New Zealand

Estimates of climate noise are made from temperature records of 20 New Zealand stations. The climate noise is variability of finite time averages, which is due to day-to-day fluctuations in weather, and it is unpredictable at long range (exceeding deterministic predictability limits of about 2 weeks). This unpredictable part of seasonal or monthly averages is compared with the actual variance of these averages. The amount by which the actual variance exceeds the noise variance is taken to be a measure of the potential long-range predictability. We find that about 50 per cent of the variance of seasonally averaged temperatures is potentially predictable except for winter (June-July-August), when it is less.