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Level Pressure (level + pressure)
Kinds of Level Pressure Selected AbstractsAnalysis of snow cover variability and change in Québec, 1948,2005HYDROLOGICAL PROCESSES, Issue 14 2010Ross D. Brown Abstract The spatial and temporal characteristics of annual maximum snow water equivalent (SWEmax) and fall and spring snow cover duration (SCD) were analysed over Québec and adjacent area for snow seasons 1948/1949,2004/2005 using reconstructed daily snow depth and SWE. Snow cover variability in Québec was found to be significantly correlated with most of the major atmospheric circulation patterns affecting the climate of eastern North America but the influence was characterized by strong multidecadal-scale variability. The strongest and most consistent relationship was observed between the Pacific Decadal Oscillation (PDO) and fall SCD variability over western Québec. El Niño-Southern Oscillation (ENSO) was found to have a limited impact on Québec snow cover. Evidence was found for a shift in circulation over the study region around 1980 associated with an abrupt increase in sea level pressure (SLP) and decreases in winter precipitation, snow depth and SWE over much of southern Québec, as well as changes in the atmospheric patterns with significant links to snow cover variability. Trend analysis of the reconstructed snow cover over 1948,2005 provided evidence of a clear north,south gradient in SWEmax and spring SCD with significant local decreases over southern Québec and significant local increases over north-central Québec. The increase in SWEmax over northern Québec is consistent with proxy data (lake levels, tree growth forms, permafrost temperatures), with hemispheric-wide trends of increasing precipitation over higher latitudes, and with projections of global climate models (GCMs). Copyright © 2010 Her Majesty the Queen in right of Canada. Published by John Wiley & Sons. Ltd [source] Long-term variability in precipitation and streamflow in Iceland and relations to atmospheric circulationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2009Jóna Finndís Jónsdóttir Abstract How the variability of the atmospheric circulation affects precipitation in Iceland is not completely understood. Also, the sea surface temperature (SST) has a strong influence on the temperature over the country, and thereby, snow and glacial melt. This study, therefore, aims at explaining how atmospheric circulation and sea surface temperature influence seasonal and annual precipitation, and, consequently, runoff in Iceland. Empirical orthogonal function (EOF) analysis is performed on annual and seasonal time series of precipitation and discharge to identify their key modes of variability during the period 1966,2004. The correlation between the time series of each EOF mode with individual time series of sea level pressure (SLP), air temperature and SST was then evaluated. The analyses evidenced how large-scale climate variables are connected to the regional precipitation and runoff in Iceland. They showed that the strength of the polar vortex may be, at least, as important for the precipitation in some areas of Iceland as the strength of the Icelandic Low (IL). Moreover, the location of the semi-permanent IL often defines the predominant wind direction over the country and, as such, the regions of preferred precipitation. Since the watersheds act as large precipitation gauges with response patterns depending on the geology and glaciers, the variability of the annual discharge closely resembles the variability of precipitation, except for the glacial rivers. Glacial melt is highly correlated to air temperature and SST, and the spring discharge is affected by winter and spring temperatures. The results also revealed that Icelandic hydrological conditions in the spring can be forecasted by precipitation and temperature of the autumn and winter seasons, as well as by the general prevalent circulation patterns. Additionally, a potential for seasonal forecast of precipitation, and river discharge in other seasons was identified, particularly if seasonal forecast of SLP is available. Copyright © 2008 Royal Meteorological Society [source] Recent accumulation variability and change on the Antarctic Peninsula from the ERA40 reanalysisINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2008Georgina M. Miles Abstract The Antarctic Peninsula has displayed significant climate change over recent decades. Understanding contemporaneous changes in accumulation is made difficult because the region's complex orography means that ice-core data are not necessarily representative of a wider area. In this paper, the patterns of regional spatial accumulation variability across the Antarctic Peninsula region are presented, based on an Empirical Orthogonal Function (EOF) analysis of European Centre for Medium Range Forecasts Reanalysis (ERA40) data over the 23-year period from 1979 through 2001. Annual and seasonal trends in the sign and strength of these patterns are identified, as is their relationship with mean sea level pressure, temperature and indices of large-scale circulation variability. The results reveal that the first pattern of accumulation variability on the Peninsula is primarily related to pressure in the circumpolar trough and the second pattern to temperature: together the two EOFs explain ,45,65% of the annual/seasonal accumulation. The strongest positive trend in an EOF occurs with EOF2 in the austral autumn March-April-May (MAM). This is highly correlated with the Southern Annular Mode (SAM) in this season, suggesting stronger westerly winds have caused an increase in orographic precipitation along the west Antarctic Peninsula. A significant correlation with ENSO occurs only in the winter EOF1, associated with blocking in the Bellingshausen Sea. Inter-annual ERA40 accumulation is shown to compare favourably with an ice core in the south of the Peninsula, but, for a variety of reasons, correlates poorly with accumulation as measured in an ice core from the northern tip. Opposite trends in accumulation at these two sites can be explained by the spatial pattern and trend of EOF2 in MAM and thus by recent changes in the SAM. The results of this study will aid in the understanding of temporal accumulation changes observed in the regional ice-core record. Copyright © 2007 Royal Meteorological Society [source] In search of simple structures in climate: simplifying EOFsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2006A. Hannachi Abstract Empirical orthogonal functions (EOFs) are widely used in climate research to identify dominant patterns of variability and to reduce the dimensionality of climate data. EOFs, however, can be difficult to interpret. Rotated empirical orthogonal functions (REOFs) have been proposed as more physical entities with simpler patterns than EOFs. This study presents a new approach for finding climate patterns with simple structures that overcomes the problems encountered with rotation. The method achieves simplicity of the patterns by using the main properties of EOFs and REOFs simultaneously. Orthogonal patterns that maximise variance subject to a constraint that induces a form of simplicity are found. The simplified empirical orthogonal function (SEOF) patterns, being more ,local', are constrained to have zero loadings outside the main centre of action. The method is applied to winter Northern Hemisphere (NH) monthly mean sea level pressure (SLP) reanalyses over the period 1948,2000. The ,simplified' leading patterns of variability are identified and compared to the leading patterns obtained from EOFs and REOFs. Copyright © 2005 Royal Meteorological Society. [source] Decadal variability of the Danube river flow in the lower basin and its relation with the North Atlantic OscillationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2002Norel Rîmbu Abstract The decadal variability (>5 years) of the Danube river flow in the lower basin and its connection with the North Atlantic Oscillation (NAO) is analysed for the period 1931,95. Associated linkages with precipitation (PP) in the European sector, global sea surface temperature (SST) and atmospheric circulation for the period 1931,81, and the 500 hPa geopotential heights (G500) over the Northern Hemisphere for the period 1948,95 are also investigated. The results show that there is an out-of-phase relationship between the time series of the Danube river flow anomalies and the NAO index. The time series of a PP index, defined as the average of normalized precipitation anomalies over a large area including the Danube basin, presents a time evolution similar to that of the river flow anomalies. The correlation maps between the river flow anomalies and global SST show coherent large-scale patterns. High values of the Danube river flow are associated with a tripole-like SST structure in the North Atlantic similar to that appearing during the negative phase of the NAO, and with negative SST anomalies in the central North Pacific and positive SST anomalies in the eastern and central tropical Pacific. Physically consistent sea level pressure and 500 hPa geopotential height are obtained. Copyright © 2002 Royal Meteorological Society. [source] Meteorological conditions associated with sea surges in Venice: a 40 year climatologyINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2002Isabel F. Trigo Abstract The frequency of sea surges in Venice has increased during the 20th century, and the trend has been particularly pronounced in the last four decades. However, the time series of independent surge events (i.e. events separated by at least 1 week) has remained nearly stationary during that period. This suggests that, although the sea level rise (due to global warming and human activity in the region) is leading to more multiple events, the frequency of meteorological conditions that trigger independent events seems to be nearly balancing the effects of sea level change. Such meteorological conditions are identified by compositing sea level pressure (SLP) and 995 hPa wind during and before independent sea surge events in Venice. The composite analysis shows that these correspond to storms orographically induced over the western Mediterranean basin, when an Atlantic synoptic system is perturbed by the Pyrenees and/or the Alps. It is, however, the persistence, intensity and relative position of such storms to the Adriatic Sea that contribute to the optimum conditions for the occurrence of floodings in its northern embayment. It is shown that the synoptic picture is translated into persistent low SLP over the Venice region, negative north,south SLP gradient over the Adriatic, and south-southeasterly to southeasterly wind over the central and northern parts of the sea. During the 40 year period under analysis, the persistence and intensity of the most adverse scenarios for the occurrence of sea surges in Venice have been generally decreasing; significant trends have been found in the tails of the distributions of Venice SLP, SLP north,south gradient, and of surface wind over the northern Adriatic. It is the balance between these trends and the continuing sea level rise that may account for the near-stationarity of independent sea surge events during the last four decades. Copyright © 2002 Royal Meteorological Society [source] Interdecadal Pacific Oscillation and South Pacific climateINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 14 2001M.J. Salinger Abstract The Interdecadal Pacific Oscillation (IPO) has been shown to be associated with decadal climate variability over parts of the Pacific Basin, and to modulate interannual El Niño,Southern Oscillation (ENSO)-related climate variability over Australia. Three phases of the IPO have been identified during the 20th century: a positive phase (1922,1944), a negative phase (1946,1977) and another positive phase (1978,1998). Climate data are analysed for the two most recent periods to describe the influence of the IPO on decadal climate trends and interannual modulation of ENSO teleconnections throughout the South West Pacific region (from the equator to 55°S, and 150°E to 140°W). Data coverage was insufficient to include the earliest period in the analysis. Mean sea level pressure (SLP) in the region west of 170°W increased for the most recent positive IPO period, compared with the previous negative phase. SLP decreased to the east of 170°W, with generally more southerly quarter geostrophic flow over the region. Annual surface temperature increased significantly southwest of the South Pacific Convergence Zone (SPCZ) at a rate similar to the average Southern Hemisphere warming. Northwest of the SPCZ temperature increases were less, and northeast of the SPCZ more than the hemispheric warming in surface temperature. Increases of annual precipitation of 30% or more occurred northeast of the SPCZ, with smaller decreases to the southwest, associated with a movement in the mean location of the SPCZ northeastwards. The IPO modulates teleconnections with ENSO in a complex way, strengthening relationships in some areas and weakening them in others. For New Zealand, there is a consistent bias towards stronger teleconnections for the positive IPO period. These results demonstrate that the IPO is a significant source of climate variation on decadal time scales throughout the South West Pacific region, on a background which includes global mean surface temperature increases. The IPO also modulates interannual ENSO climate variability over the region. Copyright © 2001 Royal Meteorological Society [source] Low frequency and quasi-biennial oscillations in the Malaysian precipitation anomalyINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2001Fredolin T. Tangang Abstract The spatial and temporal variability of the precipitation anomaly in Malaysia in the quasi-biennial (1.5,2.5 years) and the low-frequency (3,7 years) bands were investigated. The oscillations in both bands contributed a significant proportion of the variance in the precipitation anomaly. The most dominant modes for both bands had large impacts on the stations in Sabah, Sarawak and on the east coast of the Malaysian peninsula. These dominant modes had a similar manifestation, i.e. the stations experienced enhanced rainfall during periods of large negative anomaly of sea surface temperature (SST) in the central-eastern Pacific Ocean and suppressed rainfall during periods of large positive SST anomaly. Hence, these areas may face severe drought during an El Niño event and flood during La Niña. Both bands appeared to have large-scale coherence with SST, sea level pressure and zonal wind over the tropical Pacific and Indian Oceans and South China Sea. However, the two signals differed with respect to timing and propagation. Copyright © 2001 Royal Meteorological Society [source] Performance of statistical downscaling models in GCM validation and regional climate change estimates: application for Swedish precipitationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2001Aristita Busuioc Abstract This study deals with an analysis of the performance of a general circulation model (GCM) (HadCM2) in reproducing the large-scale circulation mechanisms controlling Swedish precipitation variability, and in estimating regional climate changes owing to increased CO2 concentration by using canonical correlation analysis (CCA). Seasonal precipitation amounts at 33 stations in Sweden over the period 1899,1990 are used. The large-scale circulation is represented by sea level pressure (SLP) over the Atlantic,European region. The link between seasonal Swedish precipitation and large-scale SLP variability is strong in all seasons, but especially in winter and autumn. For these two seasons, the link is a consequence of the North Atlantic Oscillation (NAO) pattern. In winter, another important mechanism is related to a cyclonic/anticyclonic structure centred over southern Scandinavia. In the past century, this connection has remained almost unchanged in time for all seasons except spring. The downscaling model that is built on the basis of this link is skilful in all seasons, but especially so in winter and autumn. This observed link is only partially reproduced by the HadCM2 model, while large-scale SLP variability is fairly well reproduced in all seasons. A concept about optimum statistical downscaling models for climate change purposes is proposed. The idea is related to the capability of the statistical downscaling model to reproduce low frequency variability, rather than having the highest skill in terms of explained variance. By using these downscaling models, it was found that grid point and downscaled climate signals are similar (increasing precipitation) in summer and autumn, while in winter, the amplitudes of the two signals are different. In spring, both signals show a slight increase in the northern and southern parts of Sweden. Copyright © 2001 Royal Meteorological Society [source] Climate of the seasonal cycle in the North Pacific and the North Atlantic oceansINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 4 2001Igor M. Yashayaev Abstract Time series of monthly sea-surface temperature (SST), air temperature (AT) and sea level pressure (SLP) were constructed from merged releases of the Comprehensive Ocean-Atmosphere Data Set (COADS). The time series were decomposed into seasonal and non-seasonal (short and long-term) components. The contribution of the seasonal cycle to the total variance of SST and AT exceeds 80% in the mid and in some high latitude locations and reaches its peak (>95%) in the centres of subtropical gyres. In most cases, a combination of annual and semiannual harmonics accounts for more than 95% of the seasonal variability. Amplitudes of SST and AT annual cycles are highest near the western boundaries of the oceans; annual phases of SST and AT increase toward the eastern tropical oceans, revealing a southeastern propagation of the annual cycle over the Northern Hemisphere oceans. The annual cycle of AT leads that of SST by 1,3 weeks. The largest phase differences are observed in the regions of western boundary currents in the North Pacific and the North Atlantic oceans. This is consistent with spatial patterns of integral air,sea heat fluxes. Annual phases of SST increase along the Gulf Stream and the Kuroshio Current. This points to the importance of signal transport by the major ocean currents. The lowest annual amplitudes of SLP are observed along the equator (0°,10°N) in both oceans. There are three distinct areas of high annual amplitudes of SLP in the North Pacific Ocean: Asian, Aleutian and Californian. Unlike the North Pacific, only one such area exists in the North Atlantic centred to the west of Iceland. A remarkable feature in the climate of the North Pacific is a maximum of semiannual SLP amplitudes, centred near 40°N and 170°W. It is also an absolute maximum in the entire Northern Hemisphere. Analysis of phases of harmonics of SLP seasonal cycle has revealed the trajectories of propagation of the annual and semiannual cycles. Analysis of semiannual to annual amplitudes ratio has revealed the regions of semiannual cycle dominance. Copyright © 2001 Royal Meteorological Society [source] Objective classification of atmospheric circulation over southern ScandinaviaINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2001Maj-Lena Linderson Abstract A method for calculating circulation indices and weather types following the Lamb classification is applied to southern Scandinavia. The main objective is to test the ability of the method to describe the atmospheric circulation over the area, and to evaluate the extent to which the pressure patterns determine local precipitation and temperature in Scania, southernmost Sweden. The weather type classification method works well and produces distinct groups. However, the variability within the group is large with regard to the location of the low pressure centres, which may have implications for the precipitation over the area. The anticyclonic weather type dominates, together with the cyclonic and westerly types. This deviates partly from the general picture for Sweden and may be explained by the southerly location of the study area. The cyclonic type is most frequent in spring, although cloudiness and amount of rain are lowest during this season. This could be explained by the occurrence of weaker cyclones or low air humidity during this time of year. Local temperature and precipitation were modelled by stepwise regression for each season, designating weather types as independent variables. Only the winter season-modelled temperature and precipitation show a high and robust correspondence to the observed temperature and precipitation, even though <60% of the precipitation variance is explained. In the other seasons, the connection between atmospheric circulation and the local temperature and precipitation is low. Other meteorological parameters may need to be taken into account. The time and space resolution of the mean sea level pressure (MSLP) grid may affect the results, as many important features might not be covered by the classification. Local physiography may also influence the local climate in a way that cannot be described by the atmospheric circulation pattern alone, stressing the importance of using more than one observation series. Copyright © 2001 Royal Meteorological Society [source] Flow dependence of background errors and their vertical correlations for radiance-data assimilationTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 647 2010Reinhold Hess Abstract This article examines the dependence of background-error statistics on synoptic conditions and flow patterns. Error variances and vertical correlations of background temperatures as used for variational radiance-data assimilation are estimated for two different weather regimes over Europe using the NMC method. The results are validated with real observations, i.e. radiosonde data and microwave satellite radiances and generalised with half a year of global data from the ECMWF forecasting system, where weather conditions are distinguished using model fields of wind speed, mean sea level pressure, and relative vorticity. Strong winds, low pressure, and cyclonic flow generally induce larger background errors of 500 hPa temperature than calm winds, high pressure, and anticyclonic flow, and also broader temperature correlations in the vertical with other tropospheric levels. Copyright © 2010 Royal Meteorological Society [source] Low-frequency climate variability in the Atlantic basin during the 20th centuryATMOSPHERIC SCIENCE LETTERS, Issue 3 2010Y. M. Tourre Abstract From joint sea surface temperature/sea level pressure (SST/SLP) EOF analyses, low-frequency variability modes are compared. The multi-decadal oscillation (MDO) changed phases twice during the 20th century, with its north Atlantic SST patterns resembling the Atlantic multi-decadal oscillation (AMO). The quasi-decadal oscillation (QDO) SST patterns displayed a double tripole configuration over the entire Atlantic basin, leading to tropical inter-hemispheric out-of-phase relationship. From the mid-1960s onward, while SST anomalies were negative to the north (negative phases of MDO/AMO), the Sahelian drought persisted with a weaker hurricane power dissipation index (PDI). During that period, the QDO modulated the intensity of the Sahelian drought. Copyright © 2010 Royal Meteorological Society [source] Bottom-up, top-down, and within-trophic level pressures on a cactus-feeding insectECOLOGICAL ENTOMOLOGY, Issue 2 2008TOM E. X. MILLER Abstract 1.,The relative importance of host-plants and predators in the population dynamics of herbivorous insects, and the frequency and intensity of inter-specific competition among herbivores, have both been intensively studied and debated. The joint effects of bottom-up, top-down, and within-trophic level interactions, however, have rarely been integrated in a single system. 2.,I studied the dynamics of the cactus bug (Narnia pallidicornis), a specialist feeder on tree cholla cactus (Opuntia imbricata), in response to variable host-plant quality, spider predation, and interactions with cactus-feeding beetles (Moneilema appressum). Previous work suggests that cactus reproductive effort (the proportion of meristems allocated to reproduction) is an important component of host-plant quality for Narnia. I conducted a 2-year field experiment to test the hypotheses that Narnia abundance is positively related to host-plant reproductive effort, and that interactions with predators and putative competitors alter the shape of this relationship. 3.,I found strong support for the first prediction (positive Narnia,plant quality relationship) in both years, but neither predator removal nor beetle exclusion had detectable effects on this relationship in either year. I conclude that the dynamics of this insect herbivore are driven predominantly from the bottom-up, and that available data from this work and from previous studies are too variable to permit broad generalisations for the combined effects of host-plants, predation, and competition on herbivore dynamics. [source] Predicting summer rainfall in the Yangtze River basin with neural networksINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2008Heike Hartmann Abstract Summer rainfall in the Yangtze River basin is predicted using neural network techniques. Input variables (predictors) for the neural network are the Southern Oscillation Index (SOI), the East Atlantic/Western Russia (EA/WR) pattern, the Scandinavia (SCA) pattern, the Polar/Eurasia (POL) pattern and several indices calculated from sea surface temperatures (SST), sea level pressures (SLP) and snow data from December to April for the period from 1993 to 2002. The output variable of the neural network is rainfall from May to September for the period from 1994 to 2002, which was previously classified into six different regions by means of a principal component analysis (PCA). Rainfall is predicted from May to September 2002. The winter SST and SLP indices are identified to be the most important predictors of summer rainfall in the Yangtze River basin. The Tibetan Plateau snow depth, the SOI and the other teleconnection indices seem to be of minor importance for an accurate prediction. This may be the result of the length of the available time series, which does not allow a deeper analysis of the impact of multi-annual oscillations. The neural network algorithms proved to be capable of explaining most of the rainfall variability in the Yangtze River basin. For five out of six regions, our predictions explain at least 77% of the total variance of the measured rainfall. Copyright © 2007 Royal Meteorological Society [source] The parametrization of drag induced by stratified flow over anisotropic orographyTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 568 2000J. F. Scinocca Abstract A new parametrization of drag arising from the flow over unresolved topography (UT) in a general-circulation model (GCM) is presented. It is comprised of three principle components: a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation, a parametrization of the drag associated with low-level wave breaking, and a parametrization of low-level drag associated with upstream blocking and lee-vortex dynamics. Novel features of the scheme include: a new procedure for defining the UT in each GCM grid cell which takes account of the GCM resolution and includes only the scales represented by the parametrization scheme, a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the UT in each GCM grid cell, and a new application of form drag in the lowest levels which can change the direction of the low-level flow so that it is more parallel to unresolved two-dimensional topographic ridges. The new scheme is tested in the Canadian Centre for Climate Modelling and Analysis third generation atmospheric GCM at horizontal resolutions of T47 and T63. Five-year seasonal means of present-day climate show that the new scheme improves mean sea level pressures (or mass distribution) and improves the tropospheric circulation when compared with the gravity-wave drag scheme used currently in the GCM. The benefits are most pronounced during northern hemisphere winter. It is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity-wave field with two waves rather than just one allows 30-50% more gravity-wave momentum flux up into the middle atmosphere, depending on the season. The additional momentum flux into the middle atmosphere is expected to have a beneficial impact on GCMs that employ a more realistic representation of the stratosphere. [source] | ||||||||||