Sea Level Pressure Fields Research Articles

Surface and 700 hPa Atmospheric Circulation Patterns for the Great Lakes Basin and Eastern North America and Relationship to Atmospheric Teleconnections

Many studies have identified continental-scale atmospheric circulation regimes, and some have been employed for various regions, but none have involved a regional categorization of circulation around the Great Lakes basin. Such an analysis is important not only because of the economic and recreational importance of the lakes, but in an effort to relate the regional circulation types to the broaderscale modes of atmospheric circulation, such as that forced by El Niño (ENSO). In this study, rotated principal components analysis (RPCA) is performed on the monthly mean sea-level pressure field around the Great Lakes basin, and in a separate analysis, on the mean 700 hPa field in eastern North America. An average-linkage clustering algorithm is applied to the RPCA scores to classify the monthly surface circulation in the Great Lakes region and the 700 hPa circulation over eastern North America. The classification is used to determine whether the various categories of regional circulation patterns are coincident with distinct hemispheric-scale flow regimes. To do this, indices of the modes of variability in some of the most well-known atmospheric teleconnections during months that fall within each circulation mode are subjected to ANOVA tests by cluster. Results suggest that the regional atmosphere over the Great Lakes basin undergoes long-term shifts in preferred modes of circulation. Furthermore, flow variability associated with the 700 hPa North Atlantic Oscillation (NAO) and Pacific/North American (PNA) teleconnections are more strongly tied to variability in both the Great Lakes regional surface circulation and the 700 mb eastern North American flow regimes than is the ENSO-forced Southern Oscillation.

A case study of statistical downscaling in Australia using weather classification by recursive partitioning

Because of their coarse resolution General Circulation Models (GCMs) lack the ability to predict reliably surface variables that may be needed for climate effects studies on a regional scale. Stochastic downscaling methods, based on the relationship between large-scale circulation types and regional surface variables, offer one approach to bridge this scale mismatch. The method explored in this paper uses recursive partitioning (also known as Classification Tree Analysis) to identify circulation patterns (weather states) in observed sea level pressure (SLP) fields that are most closely related to certain rainfall occurrence patterns (dry/wet) at multiple stations. The joint distribution functions of rainfall amounts at a set of stations are then used to define a statistical model for rainfall conditional on these weather states and the previous day's rainfall occurrence/absence. This model is applied to SLP fields from control, 2×CO 2 equilibrium and transient 100 year GCM integrations performed at the Max-Planck-Institut für Meteorologie, Hamburg to simulate local rainfall corresponding to the large-scale GCM climates. The weather generator is applied for several case studies in Australia. Four regions, one each in the West, the North, the Southeast and East of Australia are considered, for the summer and winter seasons, respectively. These regions correspond to different climate zones: the North has mainly summer monsoon rain, the West has dominant frontal winter rainfall, and moderate Southeast has a relatively uniform rainfall distribution throughout the year. Thus, the performance of the weather generator in different climatic regimes can be evaluated. Since rainfall amounts are simulated by sampling from historical data a statistical uncertainty is attached to the generated rainfall sequences which is due to the procedure used. This is reflected in uncertainties of the downscaled mean rainfall of generally less than ±10%, with larger numerical intervals for dry stations and seasons. The effects of a 2×CO 2 signal in the equilibrium run on local rainfall are not dramatic. For the transient experiment, there is an indication of more rainy days and a decrease in the length of dry periods in West Australia in winter. The strongest change is identified in the North in the monsoon season which would have decreasing rainfall. No unequivocal changes were found for the Southeast. However, consistent with earlier studies, the bias of GCM control simulations strongly affects the results, as the difference between precipitation statistics based on observed and control run pressure fields often exceeds the difference between GCM control and altered climate. Considering this deficiency in current GCM simulations, the disagreement between different climate models and the uncertainty attached to the statistical downscaling method, not too much reliance can be put on the relevance of these results for regional hydrologic impact modeling.

The association between the north Indian Ocean and summer monsoon rainfall over India

Utilizing the marine meteorological data of the period 1961-81, the sea level pressure (SLP) and sea surface temperature (SST) distributions have been obtained on a 5° grid-mesh over the north Indian Ocean area bounded by 0°- 25°N, 50°- l00°E for each individual year. It has been found that the SLP and SST fields for the month of May provide predictive indications of subsequent summer monsoon rainfall over India. Significant negative correlations have been found between the mean SLPs of May over the latitudinal belts 5°-10°, 10°- 15°, 15°-20° and 20°-25°N of Arabian Sea and Bay of Bengal and all India rainfall departures of succeeding summer monsoon season. The mean SST gradient over the Arabian Sea between 7.5°- 17 .5°N during May has been found to have significant positive correlation with all India rainfall of subsequent monsoon. The study suggests that certain functions of SLP and SST of May over the north Indian Ocean can prove to be useful predictors for subsequent summer monsoon rainfall over India.

Regional climate change in Portugal: precipitation variability associated with large-scale atmospheric circulation

Four major circulation patterns, associated with daily precipitation in Portugal, are classified from daily sea level pressure fields over the northeastern Atlantic and western Europe, based on the K-means clustering algorithm coupled with principal component analysis. A rainy pattern is clearly identified with a probability of rain of 74.6%, as well as two distinct dry patterns, one prevailing in summer and the other occurring frequently in winter; a blocking-like pattern with a probability of rain of 36.8% has also been identified. These patterns are quasi-stationary, normally persisting for 1 week and sometimes even for 1 month, especially the dry ones; they represent the principal weather regimes associated with precipitation in Portugal. Interannual variations in monthly precipitation associated with the circulation patterns are also investigated; results show that these variations match fluctuations in the frequencies of occurrence of both the rainy and the dry patterns. The decreasing trend of March monthly rainfall in southern Portugal is closely related to corresponding trends in the frequencies of both the rainy pattern and the summer dry pattern. Long term trends are not significant either in other monthly rainfall sequences or in the frequencies of different circulation patterns. Interannual variations seem, in most months, to be quasi-periodic. Singular spectrum analysis (SSA) is performed on these sequences to detect quasi-periodic oscillations. Relationships between oscillations in rainfall and in frequencies of occurrence of circulation patterns are studied. Results show that four weather circulation patterns or weather regimes are important for investigating regional climate change in Portugal and its relationship with variability of large-scale atmospheric circulation. © 1998 Royal Meteorological Society

The Arctic oscillation signature in the wintertime geopotential height and temperature fields

The leading empirical orthogonal function of the wintertime sea‐level pressure field is more strongly coupled to surface air temperature fluctuations over the Eurasian continent than the North Atlantic Oscillation (NAO). It resembles the NAO in many respects; but its primary center of action covers more of the Arctic, giving it a more zonally symmetric appearance. Coupled to strong fluctuations at the 50‐hPa level on the intraseasonal, interannual, and interdecadal time scales, this “Arctic Oscillation” (AO) can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft. It is proposed that the zonally asymmetric surface air temperature and mid‐tropospheric circulation anomalies observed in association with the AO may be secondary baroclinic features induced by the land‐sea contrasts. The same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature, sea‐level pressure, and 50‐hPa height over the past 30 years: parts of Eurasia have warmed by as much as several K, sea‐level pressure over parts of the Arctic has fallen by 4 hPa, and the core of the lower stratospheric polar vortex has cooled by several K. These trends can be interpreted as the development of a systematic bias in one of the atmosphere's dominant, naturally occurring modes of variability.

Numerical Modeling of a Coastal Trapped Disturbance. Part I: Comparison with Observations

Abstract The coastal trapped disturbance (CTD) of 15–17 May 1985 represents an example of a strong mesoscale trapped event along the west coast of North America with abrupt transitions in many basic meteorological parameters. In this study, a comparison between observations and a numerical simulation of this event using the Regional Atmospheric Modeling System (RAMS) is presented. The model is shown to realistically reproduce CTD characteristics such as the coastal transition from northerly to southerly flow, as a mesoscale coastal ridge of higher pressure with associated drops in marine-layer temperature propagates northward along the west coast of North America. Simulated sea level pressure and temperature fields near the surface match well with observations, especially at the synoptic scale. The model realistically simulates mesoscale sea level pressure and 6-h pressure changes during the event. The modeled hourly time evolution of sea level pressure and the southerly transitions at a series of coastal...

南～東アジアにおける夏季降水量の広域変動とそれに関連した熱帯域の海面気圧偏差

The relationships between the summer (June to September) rainfall variability in South and East Asia and the sea level pressure (SLP) field in the tropics are clarified both by principal component analysis (PCA) and by singular value decomposition analysis (SVDA). First, we applied PCA to summer rainfall for detecting temporal and spatial variations in this area. Then, components of common variations between summer rainfall and the SLP field in the tropics from the Indian Ocean to the Pacific were revealed by SVDA. We found that the first 2 components of summer rainfall variation derived from PCA and SVDA have the same spatial patterns and temporal variations. The first SVD mode (SVD 1) has large signals of rainfall variations in India and north China, and its time coefficients show the relation to the warm event of ENSO. The corresponding spatial pattern of the SLP field indicates variations in the east-west circulation pattern between the areas from the Indian Ocean to the western Pacific and the eastern Pacific in the tropics. Time series of the first principal component (PC I) for summer rainfall and SVD 1 have decadal scale variations with a remarkable change in the latter half of the 1970 s. The second SVD mode (SVD 2) shows a variation in summer rainfall in central China associated with variation in the SLP field and convective activities in the western tropical Pacific. We discuss the relationships between the variation in summer rainfall and the SLP field for the first two SVD modes in terms of sea surface wind anomalies accompanied by variation in the SLP field. The rainfall anomalies over India in SVD I are explained by wind anomalies of meridional components in the southwestern monsoon over the Arabian Sea. These wind anomalies are accompanied by large SLP anomalies over the Arabian Sea near the Eurasian continent. On the other hand, the variation in summer rainfall in north China appears to be closely connected to the formation of a relative pressure anomaly field over the western tropical Pacific in the Northern Hemisphere. Positive (negative) SLP anomalies both in south China and in the western equatorial Pacific develop relative cyclonic (anticyclonic) circulations. This cyclonic (anticyclonic) circulation weakens (strengthens) the southerly wind component in the middle latitudes over East Asia. As a result, the water vapor transport to north China seems to be inhibited (promoted). Concerning SVD 2, SLP variations in the northwestern tropical Pacific are accompanied by the convergence or divergence of surface wind and variation in convective activities in the corresponding region. The variation in summer rainfall in central China is considered to be related to the impact of convective activities in- the northwestern tropical Pacific. Variation in the sea surface temperature (SST) accompanied by these variations in the SLP field were investigated using correlation analysis. The variation in SLP for SVD 1 is correlated significantly with those in the SST in the eastern tropical Pacific and the tropical Indian Ocean. The variation in east-west circulation from the Indian Ocean to the Pacific is considered to be caused by the movement of tropical convective regions with the variation in SST over the eastern tropical Pacific.

Seasonality of Large-Scale Atmosphere–Ocean Interaction over the North Pacific

This paper attempts to resolve a long-standing paradox concerning the season-to-season memory of sea surface temperature anomalies (SSTAs) over the North Pacific. Summertime SSTAs are confined to a shallow mixed layer that is obliterated by wind-driven mixing in late autumn or early winter storms. The mixing exposes waters that were last in contact with the surface during the previous spring. Hence, SSTAs at fixed locations exhibit little memory from summer to the next winter. Yet despite this apparent lack of memory, a well-defined pattern of summertime and autumn SSTAs exhibits significant correlations with the sea level pressure field over the North Pacific a season later. It is shown that the dominant mode of SSTA variability over the North Pacific, as inferred from empirical orthogonal function (EOF) analysis, exhibits a rather similar spatial structure year-round, with highest amplitude during summer. By means of singular value decomposition analysis, it is shown that this pattern is much more persistent from one season to the next (and particularly from summer to the next winter) than SSTAs at fixed grid points. It is substantially more persistent from one summer to the next than from one winter to the next, reflecting the relatively greater prominence of the interdecadal variability in the summertime SSTAs. The minor differences in the structure of the winter and summer EOFs can be attributed to the coupling with the atmospheric Pacific North American pattern during winter, which induces SSTAs off the west coast of North America opposite in polarity to those in the central and western Pacific.

Role of Low Clouds in Summertime Atmosphere–Ocean Interactions over the North Pacific

The summer-to-summer variability of the areal extent of marine stratiform cloudiness (MSC; stratus, stratocumulus, and fog) over the North Pacific is examined for the period of record 1952‐92 using a dataset based on surface observations. Variability is largest in two regions: the central and western Pacific along 358N coincident with a strong meridional gradient in climatological MSC amount, and the eastern Pacific near 158N downstream of the persistent stratocumulus deck off Baja California. The MSC amount in both regions tends to be negatively correlated with local sea surface temperature (SST), suggestive of a positive cloud feedback on SST. The MSC amounts in the two regions also tend to be negatively correlated by virtue of their relationship to the basinwide sea level pressure (SLP) field: a strengthening of the seasonal mean subtropical anticyclone is accompanied by increased cloudiness in the trade wind regime and decreased cloudiness in the southerly flow farther toward the west. These relationships are reflected in the leading modes derived from empirical orthogonal function analysis and singular value decomposition analysis of the MSC, SST, and SLP fields. From the 1950s to the 1980s, summertime MSC amounts increased in the central and western Pacific and decreased in the trade wind region, while SST exhibited the opposite tendencies. Although these trends contributed to the relationships described above, similar patterns are obtained when the analysis is performed on 1-yr difference fields (e.g., 1953 minus 1952, 1954 minus 1953, etc.). Hence, it appears that MSC plays an important role in atmosphere‐ocean coupling over the North Pacific during the summer season when latent and sensible heat fluxes are not as dominant and the coupling between atmospheric circulation and SST is not as strong as in winter.

Forecasting ENSO Events: A Neural Network–Extended EOF Approach

The authors constructed neural network models to forecast the sea surface temperature anomalies (SSTA) for three regions: Niño 4, Niño 3.5, and Niño 3, representing the western-central, the central, and the eastern-central parts of the equatorial Pacific Ocean, respectively. The inputs were the extended empirical orthogonal functions (EEOF) of the sea level pressure (SLP) field that covered the tropical Indian and Pacific Oceans and evolved for a duration of 1 yr. The EEOFs greatly reduced the size of the neural networks from those of the authors’ earlier papers using EOFs. The Niño 4 region appeared to be the best forecasted region, with useful skills up to a year lead time for the 1982–93 forecast period. By network pruning analysis and spectral analysis, four important inputs were identified: modes 1, 2, and 6 of the SLP EEOFs and the SSTA persistence. Mode 1 characterized the low-frequency oscillation (LFO, with 4–5-yr period), and was seen as the typical ENSO signal, while mode 2, with a period of 2–5 yr, characterized the quasi-biennial oscillation (QBO) plus the LFO. Mode 6 was dominated by decadal and interdecadal variations. Thus, forecasting ENSO required information from the QBO, and the decadal–interdecadal oscillations. The nonlinearity of the networks tended to increase with lead time and to become stronger for the eastern regions of the equatorial Pacific Ocean.

Analysis of the sea-level variability along the Chinese coast and estimation of the impact of a CO2-perturbed atmospheric circulation

The relationship between monthly sea-level data measured at stations located along the Chinesecoast and concurrent large-scale atmospheric forcing in the period 1960–1990 is examined. Itis found that sea-level varies quite coherently along the whole coast, despite the geographicalextension of the station set. A canonical correlation analysis between sea-level and sea-levelpressure (SLP) indicates that a great part of the sea-level variability can be explained by theaction of the wind stress on the ocean surface. The relationship between sea-level and sea-levelpressure is analyzed separately for the summer and winter half-years. In winter, one factoraffecting sea-level variability at all stations is the SLP contrast between the continent and thePacific Ocean, hence the intensity of the winter Monsoon circulation. Another factor that affectscoherently all stations is the intensity of the zonal circulation at mid-latitudes. In the summerhalf year, on the other hand, the influence of SLP on sea-level is spatially less coherent: thestations in the Yellow Sea are affected by a more localized circulation anomaly pattern, whereasthe rest of the stations is more directly connected to the intensity of the zonal circulation. Basedon this analysis, statistical models (different for summer and winter) to hindcast coastal sealevelanomalies from the large-scale SLP field are formulated. These models have been testedby fitting their internal parameters in a test period and reproducing reasonably the sea-levelevolution in an independent period. These statistical models are also used to estimate thecontribution of the changes of the atmospheric circulation on sea-level along the Chinese coastin an altered climate. For this purpose the ouput of 150 year-long experiment with the coupled ocean-atmosphere model ECHAM1-LSG has been analyzed, in which the atmospheric concentrationof greenhouse gases was continuously increased from 1940 until 2090, according to theScenario A projection of the Intergovermental Panel on Climate Change. In this experimentthe meridional (zonal) circulation relevant for sea-level tends to become weaker (stronger) inthe winter half year and stronger (weaker) in summer. The estimated contribution of thisatmospheric circulation changes to coastal sea-level is of the order of a few centimeters at theend of the integration, being in winter negative in the Yellow Sea and positive in the China Seawith opposite signs in the summer half-year.

Surface Flux Variability over the North Pacific and North Atlantic Oceans

Daily fields obtained from a 17-yr atmospheric GCM simulation are used to study the surface sensible and latent heat flux variability and its relationship to the sea level pressure (SLP) field. The fluxes are analyzed over the North Pacific and Atlantic Oceans during winter. The leading mode of interannual SLP variability consists of a single center associated with the Aleutian low in the Pacific, and a dipole pattern associated with the Icelandic low and Azores high in the Atlantic. The surface flux anomalies are organized by the low-level atmospheric circulation associated with these modes in agreement with previous observational studies. The surface flux variability on all of the timescales examined, including intraseasonal, interannual, 3‐10 day, and 10‐30 day, is maximized along the north and west edges of both oceans and between Japan and the date line at ;358N in the Pacific. The intraseasonal variability is approximately 3‐5 times larger than the interannual variability, with more than half of the total surface flux variability occuring on timescales of less than 1 month. Surface flux variability in the 3‐10-day band is clearly associated with midlatitude synoptic storms. Composites indicate upward (downward) flux anomalies that exceed |30 W m 22| occur to the west (east) of storms, which move eastward across the oceans at 108‐158 per day. The SLP and surface flux anomalies are also strong and coherent in the 10‐30-day band but are located farther north, are broader in scale, and propagate ;3‐4 times more slowly eastward than the synoptic disturbances. The sensible and latent heat flux are proportional to the wind speed multiplied by the air‐sea temperature and humidity difference, respectively. The anomalous wind speed has the greatest influence on surface flux anomalies in the subtropics and western Pacific, while the air temperature and moisture anomalies have the greatest impact in the northeast Pacific and north of 408N in the Atlantic. The covariance between the wind speed and the air temperature or humidity anomalies, while generally small, is nonnegligible on synoptic timescales.

Summertime U.S. Precipitation Variability and Its Relation toPacific Sea Surface Temperature

The year-to-year fluctuations in summertime precipitation over the U.S. Great Plains are examined in this study using data from 1950 to 1990. There are large interannual variabilities in precipitation amounts over the Great Plains during the period considered. A long-term trend in Great Plains precipitation from relatively wet conditions in the 1950s to relatively dry conditions in the 1980s is also identified. The spatial scale of the anomalous precipitation covers a large portion of the United States on seasonal mean timescales. It is shown that the Great Plains precipitation fluctuations are significantly correlated with the tropical, as well as North Pacific, sea surface temperature (SST) variations. Two leading modes of covariation between Pacific SST and the U.S. precipitation are identified, with the first mode having spatial and temporal characteristics of the El Niño–La Niña SST variation, while the second mode is confined to the North Pacific and contains the decadal trend. The relationship of both the SST and the precipitation variation with the atmospheric circulation is established through 500-mb height, as well as the sea level pressure fields. A well-defined wave train over the Pacific and North American region is found to be associated with the two leading modes. A southward-shifted jet stream over the central United States brings more synoptic storms into the region and causes excessive precipitation during wet events. The tropical SST and the U.S. precipitation may be connected through the anomalous tropical convection and its effects on the circulation. The relation between North Pacific SST and the U.S. precipitation is consistent with a strong atmospheric forcing on the North Pacific SST at a 1-month lead. It is also hypothesized that North Pacific SST feeds back onto the circulation through an enhanced (reduced) Pacific jet due to the increase (decrease) of the meridional SST gradient during dry (wet) summers. This appears to be consistent with the enhanced convection along the Pacific storm track and the intensified Pacific jet stream in the two recent dry summers (1983 and 1988).

Seasonal Sea-level Variations in San Francisco Bay in Response to Atmospheric Forcing, 1980

The seasonal response of sea level in San Francisco Bay (SFB) to atmospheric forcing during 1980 is investigated. The relations between sea-level data from the Northern Reach, Central Bay and South Bay, and forcing by local wind stresses, sea-level pressure (SLP), runoff and the large-scale sea-level pressure field are examined in detail. The analyses show that the sea-level elevations and slopes respond to the along-shore wind stressTyat most times of the year, and to the cross-shore wind stressTxduring two transition periods in spring and autumn. River runoff raises the sea-level elevation during winter. It is shown that winter precipitation in the SFB area is mainly attributed to the atmospheric circulation associated with the Aleutian Low, which transports the warm, moist air into the Bay area. A multiple linear regression model is employed to estimate the independent contributions of barometric pressure and wind stress to adjusted sea level. These calculations have a simple dynamical interpretation which confirms the importance of along-shore wind to both sea level and north-south slope within the Bay.

An analysis of the relationship between precipitation and discharge fields over a karstic river basin

The main goal of the study is to establish a relation- ship between monthly precipitation and discharge fields over the karstic Cetina river basin situated on the eastern Adriatic coast (Croatia). For that purpose a principal component analysis (PCA) of Cetina's time series (1961–1985) of monthly precipitation and discharge data was used. In addition, European sea-level pressure data were examined. The results indicate the existence of a high correlation between precipitation and discharge fields in the area considered, except for the hydrological stations closely below Peruca lake where human influence is significant. Also, these fields are connected with the European sea-level pressure field. Thus, a forecast of this field can be used for estimation of the other two. Also, this relationship can be used for reduction of global climate model (GCM) scenarios to the regional scale. © 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 891–901 (1997) (No. of Figures: 7. No. of Tables: 3. No. of

ENSO-like Interdecadal Variability: 1900–93

A number of recent studies have reported an ENSO-like EOF mode in the global sea surface temperature (SST) field, whose time variability is marked by an abrupt change toward a warmer tropical eastern Pacific and a colder extratropical central North Pacific in 1976‐77. The present study compares this pattern with the structure of the interannual variability associated with the ENSO cycle and documents its time history back to 1900. The analysis is primarily based on the leading EOFs of the SST anomaly and ‘‘anomaly deviation’’ fields in various domains and the associated expansion coefficient (or principal component) time series, which are used to construct global regression maps of SST, sea level pressure (SLP), and a number of related variables. The use of ‘‘anomaly deviations’’ (i.e., departures of local SST anomalies from the concurrent global-mean SST anomaly) reduces the influence of global-mean SST trends upon the structure of the EOFs and their expansion coefficient time series. An important auxiliary time series used in this study is a ‘‘Southern Oscillation index’’ based on marine surface observations. By means of several different analysis techniques, the time variability of the leading EOF of the global SST field is separated into two components: one identified with the ‘‘ENSO cycle-related’’ variability on the interannual timescale, and the other a linearly independent ‘‘residual’’ comprising all the interdecadal variability in the record. The two components exhibit rather similar spatial signatures in the global SST, SLP, and wind stress fields. The SST signature in the residual variability is less equatorially confined in the eastern Pacific and it is relatively more prominent over the extratropical North Pacific. The corresponding SLP signature is also stronger over the extratropical North Pacific, and its counterpart in the cold season 500-mb height field more closely resembles the PNA pattern. The amplitude time series of the ENSO-like pattern in the residual variability reflects the above-mentioned shift in 1976‐77, as well as a number of other prominent features, including a shift of opposite polarity during the 1940s.

MID-LATITUDE CYCLONES SOUTH OF AFRICA IN THE GENESIS GCM

This paper examines the density, distribution and characteristics of mid-latitude cyclones in the oceans south of Africa in the GENESIS general circulation model (GCM). The latest version of the GENESIS GCM (version 2.0.a), as well as its predecessor (version 1.02), are evaluated to assess whether version 2.0.a is an improvement over version 1.02 in terms of mid-latitude cyclones. An automated cyclone finding program was used to identify cyclone centres. This program was applied to 5 years of twice daily GENESIS (versions 1.02 and 2.0.a) sea-level pressure data, as well as to 10 years of a gridded assimilation of observed data for the winter season (June, July, August). The results show that version 1.02 does not simulate the full meridional sea-level pressure range over the analysis window, whereas version 2.0.a is closer to that of the observed data. The circumpolar trough in version 1.02 is between 10 and 15 hPa too weak and it extends too far equatorward. The trough is captured better by version 2.0.a, probably due to its finer grid resolution (T31) compared with that of version 1.02 (R15 resolution). In both versions, cyclone densities north of 55°S are higher than observed and the high cyclone density band around the pole in version 1.02 is poorly defined. Discrepancies between the two versions, and the model and observed data, have been related to factors such as grid resolutions, topography, heat transport and sea-ice extent. Version 1.02 does not simulate the full meridional gradient of cyclone central pressures, whereas version 2.0.a is a better representation of the observed data. Both versions exhibit greater variability in the sea-level pressure field compared with the observed data. GENESIS version 2.0.a is a considerable improvement over its predecessor in terms of mid-latitude cyclones in the African region of the southern oceans. ©1997 by the Royal Meteorological Society. Int. J. Climatol. 17: 459–473, 1997.

THE CLIMATE CHANGE IN SOUTH AMERICA DUE TO A DOUBLING IN THE CO2 CONCENTRATION: INTERCOMPARISON OF GENERAL CIRCULATION MODEL EQUILIBRIUM EXPERIMENTS

Climatic mean sea-level (MSL) pressure, temperature, and precipitation fields simulated by five general circulation models are compared with observations within the area of the South American subcontinent in order to assess their accuracy. Equilibrium climate experiments with doubled atmospheric CO2 concentration are also intercompared to find consistent climate trend patterns in the region. Results are analysed through statistical methods to appreciate the relative model performance at a regional scale. Descriptive analysis is applied to evaluate each model's ability to simulate outstanding climate features. Statistical analysis of control runs indicates comparable and acceptable model performance in seasonal MSL pressure field and surface temperature field simulations, but less satisfactory results in precipitation. Descriptive analysis reveals that some of the local climate characteristics, such as the location of the summer continental warm centre, and the seasonal variation in the mean latitudinal pressure gradient at the southern end of the continent, are not simulated adequately. Regarding precipitation, even those models that perform better in the statistical comparison fail to simulate some of the major precipitation regimes, either in mean magnitude or seasonality of the rainfall rate. Comparison of the 2×CO2 equilibrium experiments allow us to establish some related and physically consistent trend patterns. Among them: (i) a southward shift in two main pressure systems, the summer continental low and the Pacific anticyclone, consistent with a simulated displacement in the arid zone of central Chile and western Argentina, and a noticeable warm-up in the region; (ii) maximum warming in Paraguay and southern Brazil up to the Atlantic coast and concurrent weakening of two pressure systems, the Atlantic anticyclone and the continent al high, during winter; (iii) precipitation increase in the tropical central and eastern part of the continent south of about 15°S, and an opposite trend to the north of this latitude, coincident with a maximum warming in the lower Amazon basin and northern coast of South America, and an intensification of the longitudinal pressure difference between the equatorial Pacific and Atlantic Oceans during summer; (iv) enhanced convective activity in the Inter Tropical Convergence Zone (ITCZ) along the Pacific coast of South America north of the Equator; (v) precipitation increase in the southern end of the continent, consistent with increasing warming at higher latitudes. © 1997 by the Royal Meteorological Society. Int. J. Climatol. 17, 377–398 (1997).

A statistical model to downscale local daily temperature extremes from synoptic-scale atmospheric circulation patterns in the Australian region

The study seeks to describe one method of deriving information about local daily temperature extremes from larger scale atmospheric flow patterns using statistical tools. This is considered to be one step towards downscaling coarsely gridded climate data from global climate models (GCMs) to finer spatial scales. Downscaling is necessary in order to bridge the spatial mismatch between GCMs and climate impact models which need information on spatial scales that the GCMs cannot provide. The method of statistical downscaling is based on physical interaction between atmospheric processes with different spatial scales, in this case between synoptic scale mean sea level pressure (MSLP) fields and local temperature extremes at several stations in southeast Australia. In this study it was found that most of the day-to-day spatial variability of the synoptic circulation over the Australian region can be captured by six principal components. Using the scores of these PCs as multivariate indicators of the circulation a substantial part of the local daily temperature variability could be explained. The inclusion of temperature persistence noticeably improved the performance of the statistical model. The model established and tested with observations is thought to be finally applied to GCM-simulated pressure fields in order to estimate pressure-related changes in local temperature extremes under altered CO2 conditions.

Icelandic Low Cyclone Activity: Climatological Features, Linkages with the NAO, and Relationships with Recent Changes in the Northern Hemisphere Circulation

Abstract Output from a cyclone detection and tracking algorithm, applied to twice-daily sea level pressure (SLP) fields for the period 1966–93, is used to examine the characteristics of cyclone activity associated with the locus of the mean Icelandic low (IL), variability during extremes of the North Atlantic oscillation (NAO), and recent changes in relation to circulation over the Northern Hemisphere. Cyclone events within the climatological IL display a modest seasonal cycle with a winter maximum. However, winter systems are considerably deeper than their summer counterparts with much larger maximum deepening rates. During the cold season (October–March), IL cyclone intensities are typical of oceanic systems but exhibit lower maximum deepening rates. During the warm season (April–September), intensities are typical of Northern Hemisphere values with deepening characteristics similar to those for all extratropical oceans. Depending on the month, 10%–15% (13%–18%) of cyclone events in the IL region repres...