Showing papers in "International Journal of Climatology in 1989"

The spatial and temporal patterns of the East African seasonal rainfall derived from principal component analysis

Abstract: In this study, rotated principal component analysis (RPCA) was used to study the spatial and temporal characteristics of seasonal rainfall over East Africa during the period 1922–1983. The RPCA solutions were derived from both spatial and temporal correlation matrices. The spatial correlation matrices described intercorrelation between pairs of stations, while the temporal matrices gave correlations between pairs of map patterns. Results obtained with the spatial correlation matrices indicated seasonal shifts in the patterns of the dominant RPCA modes which closely resembled the seasonal migration patterns of the rainfall belts induced by the Inter Tropical Convergence Zone (ITCZ). The influence of the large water bodies, especially Lake Victoria and the Indian Ocean, were however, outstanding throughout the year. Twenty-six homogeneous regional groups were delineated from the spatial characteristics of the dominant eigenvectors. Solutions based on the temporal correlation matrices clustered together some of the wet and dry episodes. Some of the map patterns clustered together could be associated significantly with the El-Nino/Southern Oscillation events.

Homogenized long‐period Southern Oscillation indices

Abstract: Many research workers require indices of the Southern Oscillation that are continuous for a long period, homogeneous and easily updated. This paper aims to provide such indices. The method used is to compare indices based on sea-surface temperature (SST), rainfall and pressure, and thereby to make some corrections to improve homogeneity. To fill data gaps, substitutions are proposed based on regressions between indices over 1950–1979. Tabulations are presented of the recommended long-period indices: SST monthly 1872–1985; rainfall monthly 1893–1983; and pressure seasonal 1851–1984. The most extreme years according to the two monthly indices are listed.

A study of climatic variability in Nigeria based on the onset, retreat, and length of the rainy season

Abstract: The long-term variability of rainfall conditions in Nigeria in terms of the onset, retreat, and length of the rainy season has been analysed, using pentad data for the period 1919–1985. Data were grouped into four areas, arranged in a south-north transect; the Coastal, Guinea-Savanna, Midland and Sahelian Zones. The series for retreat of rainfall showed evidence for quasi-triennial and quasi-6-year oscillations, while that for rainy season length displayed quasi-biennial and quasi-triennial oscillations. No consistent spectral peaks emerged for changes in the date of onset of the rainy season. There is spatial coherence in variation in the date of the retreat of rainfall over the whole country, whilst for the date of onset of the season spatial coherence is limited to southern Nigeria (Coastal and Guinea-Savanna Zones). Northern Nigeria (Midland and Sahelian Zones) and southern Nigeria (Coastal and Guinea-Savanna) emerge as distinct areas in terms of spatial coherence in the variation of the length of the rainy season. There is also evidence for a secular change in the date of the retreat of rainfall for the whole country during the period 1939–1985, and in the date of onset of rainfall for southern Nigeria for 1968–1985.

Statistical distribution of daily rainfall and its association with the coefficient of variation of rainfall series

Abstract: The paper deals with the statistical analysis of the daily rainfall series (monthly, seasonal, and annual) of 15 Indian stations representing a wide variety of rainfall regimes, utilizing the data for the period 1901–1980. The study focuses attention on the normalized rainfall curve (NRC) depicting the association between cumulated percentage rain amount (x) and cumulated percentage number of rain days (y) of the rainfall series. It is shown that the NRC is uniquely determined by the coefficient of variation (CV) of the rainfall series. There is no universal NRC that can represent all rainfall regimes. The equation x = y exp[ — b(100 — y)c], where b and c are two empirical constants, gives a good analytical representation of the NRCs over a wide range of CV values of the rainfall series. This analytical equation is able to account for the occurrence of high rainfall intensities towards the upper extremity of the NRC for rainfall series with high values of CV. The rain intensity corresponding to any point on the NRC is inversely proportional to the slope of the tangent at that point. The point where the slope is 45° corresponds to the mean rain amount per rain day (r) of the rainfall series. It is shown that days with rain amount greater than r (considered as days of significant rainfall) constitute about 30 per cent of the rain days and contribute 75–80 per cent of the total rain amount, with some local and seasonal variations.

Tropical-extratropical cloudband climatology based on satellite data

Abstract: A global tropical-extratropical cloudband (TECB) climatology, using a clear definition of TECBs, and based on NOAA IR satellite imagery for the period 1979–1983, is presented. It is shown that on a global scale we can distinguish between 14 different TECBs, seven of them in each hemisphere. The cross-correlations of the TECB frequency anomalies with tropical pressure and sea-surface temperature anomalies indicate that the activity of these phenomena can respond strongly to tropical forcing from different parts of the world. Whereas only 10 TECBs show a significant response in their activity to the Southern Oscillation, the frequency anomalies of all 14 TECBs correlate significantly with other pressure anomaly indices.

Macro-regional definition and characteristics of Indian summer monsoon rainfall, 1871–1985

Abstract: A case is made for regional-scale Indian summer monsoon rainfall data and analysis, intermediate between the widely used all-India and meteorological subdivision data sets. Macro-regional units are constructed from the data provided by Parthasarathy et al., 10 being defined using principal components analysis and a classification algorithm. The temporal changes of summer monsoon rainfall over the period 1871–1985 are analysed and described for each of these regions, and the marked diversity of fluctuations between the regions is emphasized by a variety of methods. The degree of relationship is considered for each region between drier and wetter conditions and (i) El Nino and non-El Nino years, and (ii) SST anomalies in the eastern tropical Pacific. Clear regional differences are apparent, but even statistically highly significant relationships are not large in any absolute sense. The need for explanatory analyses at the regional scale, in addition to those at the more common all-India scale, is stressed.

Weather types at Brisbane, Queensland: An example of the use of principal components and cluster analysis

Abstract: Principal components analysis was conducted on 15 years data of surface weather observations at Brisbane, Queensland. Component scores from each year's analysis were grouped using an hierarchical clustering method. Each of the 10,920 observations used in the study was able to be placed into a distinctive class so that the weather in Brisbane from 1967 to 1981 may now be described in terms of nine broad weather complexes, or weather yatypes’, and 63 hybrid types at a finer level of refinement. The nine broad weather types uniquely describe variation in Brisbane's climate during the period studied and hint at an ability to identify important variations in controlling mechanisms—in this case the subtropical ridge. Copyright

Heavy rainfalls over East Asia

Abstract: The mean annual and monthly frequency distribution of heavy rainfalls for the period 1971–1980 are investigated over the whole of East Asia, including Japan, China, Korea, USSR, and Hong Kong. The highest frequency region (< 10 times per year) is shown to be located along the southern coast of Japan. The second highest region (< 8 times per year) is located along the southern coast of China. The high-frequency area advances northward over China during the summer monsoon season, in accordance with the northward advance of the ‘Mei-yu frontal zone’. Maps of the axes of high-frequency heavy rainfall areas (frequency per 10-day period) and of maximum-frequency 10-day periods at each station, clearly reveal the intra-seasonal shift of the heavy rainfall area during the summer monsoon season. According to the dominant causes of heavy rainfalls, four regions are identified. The investigated area is also divided into three regions (I-III) with respect to the annual frequency distribution of heavy rainfalls. Based on these regional divisions, the northern limit of the influence of the ‘Mei-yu front’ is determined in terms of heavy rainfall occurrence.

The dust storms of Mexico City

Abstract: Dust storms represent an important environmental problem affecting the comfort and health of one of the largest urban agglomerations in the tropical world. Data for dust events of varied intensities have been used in order to describe the seasonal and annual variation of dust storms in Mexico City. The maximum frequency of dust events occurs in March at the end of a three-month period characterized by monthly rainfall amounts of about 13 mm. The frequency of intense (visibility less than 1 km) dust storms has markedly declined (to 0.6 year−1 for the period 1981–1987), apparently as a result of repasturing of Lake Texcoco's dry bed. However, dust activity of low magnitude as observed from Tacubaya has considerably increased in the Mexico Basin, apparently as a result of both accelerated deforestation and urbanization as well as a decrease in precipitation in recent years at the dust source. In spite of the low frequency of intense/moderate dust events, dustfall deposition rates observed near the dust sources are comparable to those measured elsewhere in arid environments.

Hawaiian drought and the Southern Oscillation

Abstract: Long-term rainfall records at key locations on the Hawaiian Islands and the Southern Oscillation Index (SOI) are examined to reveal the impact of large-scale atmospheric circulation on Hawaiian rainfall. By compositing six major drought winters, it is found that they were preceded by a persistently low phase of the Southern Oscillation starting from March of the preceding year. Correlation analysis indicates that the spring SOI is not significantly correlated to rainfall in the subsequent seasons. In contrast, changes of the SOI in summer lead corresponding changes of rainfall by two to three seasons. Similarly, changes of the SOI in autumn lead changes of rainfall by one to two seasons. These results are also confirmed by scatter diagrams. Although a large negative SOI anomaly in summer or autumn can be regarded as a precursor to a drought for the following winter or spring, the inverse relation, namely, a positive SOI anomaly leading abundant rainfall does not necessarily follow. As an empirical rule, a value for the summer SOI of −2 or below, or a value for the autumn SOI of −1 or below, or both, indicate the occurrence of a dry winter. During drought episodes, Hawaii was located under an area of strong subsidence, and synoptic systems that normally produce winter and spring rainfall in the Hawaiian Islands were retarded. An anomalously cold sea-surface temperature in the north-central Pacific may also further enhance and prolong the dryness by reducing the rate of evaporation from the sea surface and by stabilizing the overlying air. Individual forecasts carried out for recent years reveal that severe drought during an El Nino/Southern Oscillation (ENSO) winter could be predicted two seasons in advance using simple regression models. For the non-ENSO drought winter, forecasting ability degrades somewhat. Overall, by means of a cross-validation technique, antecedent conditions of the large-scale pressure see-saw are useful predictors for Hawaiian winter rainfall anomalies, particularly when an ENSO-like phenomenon has started to develop.

Reconstruction of New Zealand summer temperatures to 1730 ad using dendroclimatic techniques

Abstract: The first statistically calibrated and verified reconstructions of past New Zealand temperatures are presented. The reconstructions of summer (December-March) temperature are derived from a grid of ten subalpine Nothofagus menziesii (Hook. f.) Oerst. and N. solandri (Hook. f.) Oerst. tree-ring chronologies using dendroclimatological techniques. A regression model where climate in year i is estimated as a function of tree growth in years i−1, i and i + 1 for each chronology is used. Two reconstruction models are developed (for 1853–1915 AD and 1916–1979 AD) and account for 55 per cent and 66 per cent of the temperature variance respectively. Verification of these models accounts for 34 per cent and 42 per cent of the temperature variance over an independent time period. Analysis of the spectral properties of the reconstructions indicates that relatively short-period variability ( 60 years). The reconstructions, extending back to 1730 AD, provide statistically reliable estimates of annual to decadal temperature variations. The relative coolness of the 1780s and late 1840s and warmth of the 1750s appear to be notable features of the reconstruction. The reconstructions indicate a far more complicated sequence of palaeotemperature variations than have previously been inferred for New Zealand.

Onset dates of the south-west monsoon over Kerala for the period 1870-1900

Abstract: This communication presents the dates on onset of the south-west monsoon over south Kerala for the period 1890-1900 and over north Kerala for the period 1870-1900.

The influence of general circulation patterns on humid and dry years in the Cuyo andean region of Argentina

Abstract: Due to the population growth in the agricultural areas under irrigation in Cuyo (West Central Argentina), the consequences of fluctuations in cordillerean river regimes, whose snowfalls provide water for irrigation, have begun to be felt acutely in recent years. Hemispheric and regional circulation patterns associated with two extreme hydrological (drought and excess) situations in Cuyo were investigated, searching for links between both scales. Important anomalies of hemispheric and regional occurrences have been observed when droughts and floods occur at the regional scale (upper level of the mountain range central Andes and Chilean Mediterranean zone). Precipitation fluctuations show a direct or inverse association with other South American Regions (Uruguay and Islas Orcadas del Sur) and also an important teleconnection between phenomena of extratropical latitudes. The anomalous monthly average pressure fields (regional scale) are generated by an increase (20 to 30 per cent over the normal frequency) of a few persistent synoptic models, and by a westerly wave with ridge or trough over Islas Orcadas del Sur (South Orkneys Is.) or towards the west of Juan Fernandez.

Regionalization of thunderstorm rainfall in the contiguous United States

Abstract: The amount of precipitation recorded during individual thunderstorms was determined at 220 stations in the contiguous United States of America for a 30-year period. The probability distribution of thunderstorm rainfall amounts at each station during each season was summarized by use of the incomplete gamma distribution. The incomplete gamma distribution is completely described by two parameters, which were used in a clustering routine to determine whether regionalizing tendencies exist in the probability distributions. Large-scale regions were found when the results of the clustering were mapped. In general, these regions corresponded well with the areas of dominance of many of the proposed mechanisms for thunderstorm development.

Principal components of the surface wind field in the United States: a comparison of analyses based upon wind velocity, direction, and speed

Abstract: Three statistical summaries of the 1975 surface wind field in the United States are computed using principal components analysis (PCA). Separate analyses of wind velocity and direction are made with vector-based PCA, and the scalar (zonal and meridional) components of wind are described using a scalar-based PCA. Comparisons indicate that results of the three PCAs are not equivalent. Vector PCA yields the most physically comprehensive components because the velocity observations are not decomposed into direction and speed, or into zonal and meridional coordinates a priori. Directional and scalar-based PCAs appear similar but since they only summarize one coordinate of velocity at a time, they represent incomplete descriptions of the wind field. Vector-based PCA then is the most appropriate of the three for investigating wind fields or other vector-valued data sets.

Global‐scale sea‐surface temperature anomalies and rainfall characteristics in northern Nigeria

Abstract: Composite rainfall data over the 35-year period 1951–1985 from seven Nigerian synoptic stations, all around latitude 12°N, have been used to examine the correlation, or otherwise, between global-scale sea-surface temperature (SST) anomalies and rainfall characteristics on the fringe of the Sahel. Results show that SST anomalies of the Indian, North Atlantic and South Atlantic Oceans are correlated with total rainfall and number of rain days while the anomalies of the tropical East Pacific may have influence on rainfall intensity at the beginning of the rainy season. It is indicated that Pacific El Nino events may affect the frequency of occurrence of the main rain-producing mechanism in the Sahel; namely, squall lines. Extraordinary warmings in the South Atlantic are also shown to cause considerable shortfalls in expected mean monthly distribution of rainfall, especially in August, the main rainy month in the Sahel.

Recent decadal climate variations in the tropical Pacific

Abstract: The existence of two major variations in the climate of the tropical Pacific during the period 1920–1983 is demonstrated. During 1937–1946 an anomaly of much longer duration than, but of somewhat similar character to, an El Nino event occurred. It contained within it several swings of the Southern Oscillation Index previously thought to be separate events, but which we hypothesize to be part of a longer coherent disruption to the tropical Pacific ocean-atmosphere system. This anomaly is described using ship observations of winds and sea-surface temperature, island sea-level pressure records and length-of-day measurements. In all records examined the anomaly was persistent, and as strong as the recent 1982/1983 El Nino, though of far different character. Evidence for a long-term change in the atmospheric circulation over the tropical Pacific in the 30 years since 1950 is also presented. A combination of the trade winds gradually strengthening by 1 ms−1 over this time and changes in subtropical pressure of up to 1 mb leads to a hypothesized change in the circulation.

Topographical and coastal influence on spatial precipitation patterns in Tunisia

Abstract: The spatial properties of the correlation structure based on daily rainfall data from 67 stations throughout Tunisia over a 5-year period are examined. The correlation structure is generally markedly anisotropic as influenced by topography and the coast. These two factors seem to influence the correlation pattern differently during different months. The mean areas within the 0.7 correlation isoline for all subgroups vary from about 1000 km2 in the early autumn to about 7000 km2 in the winter months.

The influence of upwelling extent upon fog incidence at Lüderitz, southern Africa

Abstract: This paper examines the influence of the horizontal extent of the cold-water upwelling on the incidence of fog at Luderitz, Namibia, during 1983–1984. The spatial extent of the upwelling was measured on days with synoptic conditions conducive to fog formation, i.e. when a coastal low was centred to the south of the study area. A significant relationship was identified, indicating increased fog occurrence with an upwelling extent of less than 200 km and vice versa. The moisture necessary for fog formation was postulated as originating from the warm water to the seaward side of the upwelling zone. With a narrow upwelling, the circulation around the coastal low extends beyond the cold-water mass, whereas when the extent of the upwelling exceeds that of the coastal low circulation, evaporation is limited, resulting in a relatively dry onshore air flow.

Sahel drought: Meteorological or agricultural?

Abstract: Meteorological definitions of drought are deemed to reveal little of the effects of water deficiencies upon the inhabitants of arid regions An agricultural definition of drought is obtained using a water-balance model of the moisture available to millet (Pennisetum typhoides) The model accepts inputs of rainfall, potential evapotranspiration, and crop development for 5-day periods Values for field capacity, wilting point, bulk density, crop coefficients, and rooting depth are obtained, and changes in soil moisture, actual evapotranspiration, and drainage are computed The resulting seasonal millet moisture deficits are mapped for ‘millet drought’ in Niger from 1967 to 1978 These maps indicate that for traditional areas of millet cultivation, ‘millet drought’ was a rare event during the 1970s, but was common and widespread in the more northerly, marginal lands

Classification of the climate of England and Wales based on agroclimatic data

Abstract: Classifications of the climate of England and Wales, based on agroclimatic data from L. P. Smith, are described. A principal component analysis was carried out on these data in order to simplify them by removing inessentials. This showed that most variation in the variables was associated with maximum soil moisture deficit in summer and effective transpiration, both important for plant response. Six methods of cluster analysis were then used, two of which, further neighbour and minimum variance, were selected for comparison. The groups into which areas were classified are discussed in terms of the two climatological variables. The resulting classification was compared with others in the literature.

Spatial and temporal variations in Australo‐Indonesian region cloudiness

Abstract: Spatial and temporal variations in cloudiness of the Australo-Indonesian region are examined with the help of a newly created cloudiness data set, based on GMS-IR imagery. As represented by the first Varimax rotated principal component pattern of the Australo-Indonesian region cloudiness, the band of tropical cloudiness is dominated by a clearly defined eastward cluster propagation, as well as a more complicated 40–50-day oscillation pattern. Details of these phenomena are discussed.

Morphology of bistable 180° phase switches in 18.6‐year induced rainfall over the north‐eastern United States of America

Abstract: Using a novel data presentation technique, bistable 180° phase switches in 18.6-year induced precipitation over the northeastern United States of America are re-examined. It is found that bistable switching in 19-year wavetrains occurred principally at epochs 1898–9, 1917.5, and 1954.7, although some occurred at mid-epoch 1908–2, and at epochs 1936.1 and 1973.3 (epochs are dates of maximum in the luni-solar 18.6-year tide). The 180° phase switch at 1898.9 occurred along the northern Atlantic seaboard, and by epoch 1954.7 this invading ‘cell’ from the east blanketed virtually the entire region. O'Brien and Currie (1988) have provided a construct in mathematical physics which can explain how such sudden readjustments in the standing wave pattern for air pressure occur on subcontinental scales, and such adjustments are found in other climatic parameters such as air temperature and pressure. Results for seven tree-ring chronologies in the region are also given, and are found to have serious inconsistencies both among themselves and also with respect to instrumental rain-gauge data.

Diurnally varying regional climatic simulations

Abstract: A detailed analysis has been made of the atmospheric variability at a number of specific geographical points as simulated with a global general circulation model. The objective of this study was to assess the model's performance as regards diurnal and interdiurnal variability, but more importantly to identify critical features which should be monitored in climatic-change experiments. Since the model contains a number of physical processes, including cloud forecasting, three independent rain-producing mechanisms, a very responsive surface-hydrology formulation, sea-ice determination, etc., complex interactions among the various climatic components can occur. Results are presented for a number of longitudinal points at three selected latitudes, representative of summer, winter, and tropical conditions. One of the clearest features of the model output was the very marked longitudinal asymmetry in atmospheric forcing arising from land-sea contrast. The vital role of the soil moisture, which depends critically on its model formulation, in determining whether this forcing is due to latent or sensible heating over the land is also demonstrated. The unique diurnal variations which were simulated at individual geographical points suggest that systematic distortions must occur in most near-surface processes in non-diurnal models. The extreme sensitivity of the convective mechanisms and the consequent impact on rainfall is noted, together with its implications for experiments involving climatic change. Finally, the interdiurnal variability of the model is illustrated by a number of examples, one of which highlights the potential misrepresentation of air-sea exchanges in models employing fixed sea-surface temperatures.

A satellite-derived climatology of polar-low evolution in the North Pacific

Abstract: A climatology of polar-low evolution for seven low-sun seasons (November to March) in the North Pacific is presented. The data were collected by the subjective interpretation of Defense Meteorological Satellite Program (DMSP) infra-red imagery, using a cloud vortex classification scheme designed especially for use with medium to high resolution imagery. For the study period, about two-thirds of all polar lows that formed over the North Pacific evolved to a higher stage. Approximately one-half of those (one-third of the total) passed through all stages of development from cyclogenesis to decay. The more common form of cyclogenetic polar low in the North Pacific, the comma cloud, continued to develop two-thirds of the time, while the less common spiraliform only evolved to a higher stage of development in two-fifths of the cases. Those polar lows that formed in certain key areas were likely to evolve to a higher stage, while other specific locations discouraged continued development. No readily discernible difference between gross size and cloud characteristics of evolving and non-evolving polar lows was present in the satellite data, suggesting that additional data are needed to forecast correctly which will develop into mature cyclones.

Local and remote response to zonally uniform sea-surface temperature in a July general circulation model

Abstract: To investigate the effect of longitudinal sea‐surface temperature gradients this paper reports on three experiments in which they were removed over the entire globe, and separately in the two hemispheres. The first of these is compared to an analogous experiment for January and the response is found to be generally greater. An almost‐hemispheric region of sea‐level pressure increase is simulated centred on Papua New Guinea with decreases centred on the Atlantic. In the anomalous upper level streamfunction there is an anticyclonic pair straddling the Equator in the Western Hemisphere with a corresponding cyclonic pair in the Eastern Hemisphere. The velocity potential is dominated by a wave 1 response. Precipitation anomalies tend to be confined to and follow the sea‐surface temperature anomalies and their magnitudes are much larger than evaporation changes. This highlights the role of moisture advection in maintaining these anomalies. The Hadley cell intensifies while the Walker circulation is weakened. The root‐mean‐square deviation of geopotential height waves generally decreases, particularly in the Southern Hemisphere, mainly due to reductions in the strength of wave 1. The experiments with hemispheric anomalies allow us to investigate the relative importance of local and remote forcing. It is found that most of the global response can be explained by the Northern Hemisphere forcing alone. This is true even in the Southern Hemisphere. The implications of this for cross‐equatorial propagation of information are discussed. The results highlight the importance of the meridional component of the wind in this connection. Copyright © 1989 John Wiley & Sons, Ltd

Comments on Power spectra and coherence of drought in the interior plains by E. O. Oladipo

Abstract: Oladipo failed to detect any long-period signals in precipitation records from North America, in particular those from the states of Arkansas, Illinois and Iowa. In this paper evidence is presented for signals near 19 years and 10 to 11 years in 64 records from these three states. A series of errors led to Oladipo's failure to detect the terms, and these are discussed.

An approach to the variability of urban surface energy budgets under stratified synoptic weather types

Abstract: A simulation-based method for examining the response of urban canyon energy budgets to stratified synoptic weather types is outlined and briefly illustrated. The approach contains three steps: (1) an objective grouping procedure consisting of a principal components analysis of sixteen climatic variables followed by a cluster analysis of the resulting component scores is applied to the Boston, Massachusetts Typical Meteorological Year data set to group each day into a limited number of synoptic weather types; (2) a characteristic day is identified for each weather type based upon a minimized sum-of-squares criterion; (3) the URBAN3 numerical model is used to simulate the surface energy budget of a standard urban canyon for each of the synoptic weather types. Results illustrate the response of urban canyon surface energetic patterns to stratified synoptic weather types. The objective grouping procedure utilized should be applicable to a wide variety of computer modelling applications involving complete daily climatic data sets.