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Sea-level Pressure (sea-level + pressure)
Selected AbstractsLong-term trends and cycles in the hydrometeorology of the Amazon basin since the late 1920sHYDROLOGICAL PROCESSES, Issue 22 2009José A. Marengo Abstract Rainfall and river indices for both the northern and southern Amazon were used to identify and explore long-term climate variability on the region. From a statistical analysis of the hydrometeorological series, it is concluded that no systematic unidirectional long-term trends towards drier or wetter conditions have been identified since the 1920s. The rainfall and river series showing variability at inter-annual scales linked to El Niño Southern Oscillation was detected in rainfall in the northern Amazon. It has a low-frequency variability with a peak at , 30 years identified in both rainfall and river series in the Amazon. The presence of cycles rather than a trend is characteristic of rainfall in the Amazon. These cycles are real indicators of decadal and multi-decadal variations in hydrology for both sides of the basin. Sea-level pressure (SLP) gradients between tropics and sub topics were explored in order to explain variability in the hydrometeorology of the basin. Sea surface temperature (SST) gradients inside the tropical Atlantic and between the tropical Atlantic and the sub-tropical Atlantic have been assessed in the context of changes in rainfall in the Amazon, as compared to northern Argentina. Trends in SSTs in the subtropical Atlantic are linked to changes in rainfall and circulation in northern Argentina, and they seem to be related to multi-decadal variations of rainfall in the Amazon. Copyright © 2009 John Wiley & Sons, Ltd. [source] European snow cover extent variability and associations with atmospheric forcingsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2010Gina R. Henderson Abstract Snow cover in Europe represents an important component of the region's climatic system. Variability in snow cover extent can have major implications on factors such as low-level atmospheric temperatures, soil temperatures, soil moisture, stream discharge, and energy allocation involved in the warming and melting of the snowpack. The majority of studies investigating Northern Hemisphere snow cover identify European snow cover extent as a portion of the Eurasian record, possibly masking complexities of this subset. This study explores the variability of European snow cover extent from 1967,2007, with the region in question including the area of Europe extending eastward to the Ural Mountains (60°E). Using the 89 × 89 gridded National Oceanic and Atmospheric Administration (NOAA) Northern Hemisphere weekly satellite snow cover product, area estimates of seasonal snow cover were calculated, and their relationship to gridded temperature, precipitation, and sea-level pressure data analysed. The spatial variability of snow cover extent was also explored using geographical information systems (GIS). The combined results from both surface temperature and precipitation analyses point towards snow cover extent in Europe being primarily temperature dependent. Atmospheric variables associated with extremes in snow cover extent were investigated. Large (small) European snow extent is associated with negative (positive) 850 hPa zonal wind anomalies, negative (positive) European 1000,500 hPa thickness anomalies, and generally positive (negative) Northern European precipitation anomalies. Sea-level pressure and 500 hPa results indicate strong associations between large (small) snow cover seasons and the negative (positive) phase of the North Atlantic Oscillation. Copyright © 2009 Royal Meteorological Society [source] Variability of the impact of El Niño,southern oscillation on sea-level pressure anomalies over the North Atlantic in January to March (1874,1996)INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 13 2003I. Gouirand Abstract Sea-level pressure (SLP) anomalies over the North Atlantic and European (NAE) sector (25,70°N, 100°W,50°E) and over a larger domain encompassing the entire North Pacific domain are studied to demonstrate that SLP anomalies (SLPAs) during boreal winter (January,March) vary widely between years characterized by the same El Niño,southern oscillation (ENSO) phase. The typical cold ENSO signal tends to be more stable than the warm one during the 1874,1996 period. The typical cold ENSO pattern (e.g. positive SLPA south of 55°N across the North Atlantic and negative SLPA in the northern North Atlantic) is similar to the positive phase of the North Atlantic oscillation (NAO) and occurs throughout the 20th century, except during the 1950s and 1960s when the basinwide westerlies are particularly slow. On the contrary, the typical warm ENSO pattern (e.g. positive SLPA from central Canada to Scandinavia and negative SLPA from the southeastern USA to central Europe, corresponding to the negative phase of the NAO) occurs mainly from 1930 to 1970. Another robust warm ENSO pattern is associated with a large positive (negative) SLPA between Newfoundland and western Europe (between Greenland and Scandinavia), and occurs mainly at the beginning and the end of the 20th century when the basinwide North Atlantic westerlies are strengthened. All these patterns stay statistically significant when the multi-decadal variability is removed from the North Atlantic SLPA. It is shown that the low-frequency variability of the north tropical Atlantic sea-surface temperature anomalies could exert a modulating effect on the ENSO teleconnection. NAE SLPAs tend to be strong during warm (cold) ENSO winters and consistent with a negative (positive) phase of the NAO when the north tropical Atlantic is anomalously warm (cold). Lastly, the magnitude of the SLPA patterns over the NAE sector appears poorly related to the intensity of sea-surface temperature anomalies in the central and eastern equatorial Pacific. Copyright © 2003 Royal Meteorological Society [source] Is there a connection between weather at departure sites, onset of migration and timing of soaring-bird autumn migration in Israel?GLOBAL ECOLOGY, Issue 6 2006Judy Shamoun-Baranes ABSTRACT Aims, Different aspects of soaring-bird migration are influenced by weather. However, the relationship between weather and the onset of soaring-bird migration, particularly in autumn, is not clear. Although long-term migration counts are often unavailable near the breeding areas of many soaring birds in the western Palaearctic, soaring-bird migration has been systematically monitored in Israel, a region where populations from large geographical areas converge. This study tests several fundamental hypotheses regarding the onset of migration and explores the connection between weather, migration onset and arrival at a distant site. Location, Globally gridded meteorological data from the breeding areas in north-eastern Europe were used as predictive variables in relation to the arrival of soaring migrants in Israel. Methods, Inverse modelling was used to study the temporal and spatial influence of weather on initiation of migration based on autumn soaring-bird migration counts in Israel. Numerous combinations of migration duration and temporal influence of meteorological variables (temperature, sea-level pressure and precipitable water) were tested with different models for meteorological sensitivity. Results, The day of arrival in Israel of white storks, honey buzzards, Levant sparrowhawks and lesser spotted eagles was significantly and strongly related to meteorological conditions in the breeding area days or even weeks before arrival in Israel. The cumulative number of days or cumulative value above or below a meteorological threshold performed significantly better than other models tested. Models provided reliable estimates of migration duration for each species. Main conclusions, The meteorological triggers of migration at the breeding grounds differed between species and were related to deteriorating living conditions and deteriorating migratory flight conditions. Soaring birds are sensitive to meteorological triggers at the same period every year and their temporal response to weather appears to be constrained by their annual routine. [source] Simulation of New Zealand's climate using a high-resolution nested regional climate modelINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 9 2007Frank Drost Abstract A regional climate model (RCM) for New Zealand has been developed. The RCM is embedded within a GCM and both models are run under pre-industrial conditions. Seasonal mean output of the RCM is compared against NCEP data and the New Zealand national climate database. Regional and seasonal aspects of modelled surface temperature and precipitation are to a large extent simulated correctly. The main anomalies are related to the difficulty of incorporating New Zealand's orography appropriately and to the initial and lateral boundary conditions, which were supplied by the GCM. The largest anomalies occur over the Southern Alps, where the modelled temperatures are too low and where the amount of precipitation is too high. Many parts of the east coasts in both the North and South Island are too warm and too dry. Correlation patterns of temperature and precipitation with mean sea-level pressure differ considerably from the equivalent patterns constructed from NCEP data, but do show in general the dominant relationships between wind direction and temperature and precipitation. Copyright © 2006 Royal Meteorological Society [source] Remote weather associated with North Pacific subtropical sea level high propertiesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2007Richard Grotjahn Abstract Remote events influencing North Pacific (NP) subtropical high properties in monthly and daily data are identified. Variability in the NP during summer is far more strongly dominated by midlatitude events than in South Pacific (SP); low-pass filtering is required to see tropical associations. The dominant pattern in composites, correlations, and regressions is a midlatitude wave train. A stronger NP high was led by higher sea-level pressure (SLP) just east of Japan and lower SLP over central Canada and to a lesser extent over western tropical Pacific. Various mechanisms have been proposed to force the NP high: (1)Heating over southwestern North America (with cooling off the west coast). However, higher temperatures over North America follow stronger SLP over the NP high and occur much further east than postulated. Higher SLP occurs where temperatures are lower over western North America and adjacent ocean. Thermal pattern is consistent with temperature advection between NP high and Canadian low. (2)Precipitation over and near Central America. However, SLP increase on the SE side of the high is led by higher SLP (and higher outgoing longwave radiation (OLR)) along the west coast of Mexico and Central America. Normalized regressions find a very weak lower OLR in North American monsoon preceding stronger NP high, but the region is much smaller in size and magnitude than other significant areas. (3)Precipitation over Indonesia and southeast Asia. Statistics provide some support for lower SLP and OLR over Indonesia preceding higher SLP in the center, west, and northwest sides of NP high. The lower SLP and OLR appear to migrate into southeast Asia, perhaps independently, perhaps from stronger NP high. (4)The NP high has a strong connection to El Niño during winter, but no significant link during summer. Only the south side of NP high appears (weakly) linked to the Madden Julian oscillation (MJO). Copyright © 2006 Royal Meteorological Society [source] Impact of global warming on ENSO variability using the coupled giss GCM/ZC modelINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2006Dr. Timothy Eichler Research Scientist Abstract This study uses a hybrid coupled model (referred to as the general-circulation model (GCM)/Zebiak/Cane (ZC) model), which consists of the Goddard Institute for Space Studies' (GISS) Atmospheric general-circulation model (AGCM) coupled to the oceanic component of the ZC intermediate model to assess the impact of global warming on El Niño behavior, with and without the influence of heat introduced from the subtropical Pacific (via subtropical cell (STC) pathway). The baseline GCM/ZC model produces El Niño variability with a two year periodicity and an amplitude of approximately half the magnitude of observed El Niño. The GCM/ZC model also produces an appropriate atmospheric global response to El Niño/southern oscillation (ENSO) as shown by composites of 500 hPa heights, sea-level pressure (SLP), 200 hPa wind, and precipitation during El Niño and La Niña periods. To evaluate the importance of global warming on ENSO variability, 2× CO2 and 4× CO2 transient simulations were done increasing the atmospheric CO2 one percent per year, then extending the runs for an additional 70 years to obtain equilibrium climates for each run. An additional set of global-warming simulations was run after including a STC parameterization generated by computing 5-year running means of the sea-surface temperature (SST) difference between a transient run and the 1× CO2 GCM/ZC run at the anticipated subduction zones (160,130°W, 20,40°N and 20,44°S, 160,130°W) and adding it to the base of the equatorial mixed-layer of the ZC model with a time lag of 15 years. This effectively alters the vertical temperature gradient of the ZC model, which affects SST via upwelling. Two features of the GCM/ZC response to global warming are emphasized. Firstly, the inclusion of the STC results in a major redistribution of heat across the equatorial Pacific, leading to an El Niño-like response in the final equilibrium solution with less variability about the mean. The global warming aspect due to the El Niño-like response results in a positive feedback on global warming, which causes a higher global surface-air temperature (SAT) than identical transient simulations without inclusion of the STC. Secondly, including the STC effect produces a far greater magnitude of global ENSO-like impact because of the reduction of, or even the reversal of, the equatorial Pacific longitudinal SST gradient. The implications of such an extreme climate scenario are discussed. Copyright © 2006 Royal Meteorological Society [source] The behavior of extreme cold air outbreaks under greenhouse warmingINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 9 2006S. Vavrus Abstract Climate model output is used to analyze the behavior of extreme cold-air outbreaks (CAOs) under recent and future climatic conditions. The study uses daily output from seven GCMs run under late-twentieth century and projected twenty-first century radiative conditions (SRES A1B greenhouse gas emission scenario). We define a CAO as an occurrence of two or more consecutive days during which the local mean daily surface air temperature is at least two standard deviations below the local wintertime mean temperature. In agreement with observations, the models generally simulate modern CAOs most frequently over western North America and Europe and least commonly over the Arctic. These favored regions for CAOs are located downstream from preferred locations of atmospheric blocking. Future projections indicate that CAOs,defined with respect to late-twentieth century climatic conditions,will decline in frequency by 50 to 100% in most of the Northern Hemisphere during the twenty-first century. Certain regions, however, show relatively small changes and others actually experience more CAOs in the future, due to atmospheric circulation changes and internal variability that counter the thermodynamic tendency from greenhouse forcing. These areas generally experience greater near-surface wind flow from the north or the continent during the twenty-first century and/or are especially prone to atmospheric blocking events. Simulated reductions in CAOs are smallest in western North America, the North Atlantic, and in southern regions of Europe and Asia. The Eurasian pattern is driven by a strong tendency for the models to produce sea-level pressure (SLP) increases in the vicinity of the Mediterranean Sea (intermodel mean of 3 hPa), causing greater advection of continental air from northern and central Asia, while the muted change over western North America is due to enhanced ridging along the west coast and the increased frequency of blocking events. The North Atlantic response is consistent with a slowdown of the thermohaline circulation, which either damps the warming regionally or results in a cooler mean climate in the vicinity of Greenland. Copyright © 2006 Royal Meteorological Society. [source] A mid-shelf, mean wave direction climatology for southeastern Australia, and its relationship to the El Niño,Southern Oscillation since 1878 A.D.INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 13 2005Ian D. Goodwin Abstract Coastal systems behave on timescales from days to centuries. Shelf and coastal wave climatological data from the Tasman Sea are only available for the past few decades. Hence, the records are too short to investigate inter- and multidecadal variability and their impact on coastal systems. A method is presented to hindcast monthly mid-shelf mean wave direction (MWD) for southeastern Australia, based on the monthly, trans-Tasman mean sea-level pressure (MSLP) difference between northern NSW (Yamba) and the north island of New Zealand (Auckland). The MSLP index is calibrated to instrumental (Waverider buoy) MWD data for the Sydney shelf and coast. Positive/negative trans-Tasman MSLP difference is significantly correlated to southerly/easterly Sydney MWD, and to long/short mean wave periods. The 124-year Sydney annual (MWD) time series displays multidecadal variability, and identifies a significant period of more southerly annual MWD during 1884 to 1914 than in the period since 1915. The Sydney MWD is significantly correlated to the Southern Oscillation Index (SOI). The correlation with the SOI is enhanced during periods when the Interdecadal Pacific Oscillation (IPO) is in its negative state and warm SST anomalies occur in the southwest Pacific region. The Sydney MWD was found to be associated with Pacific basin-wide climate fluctuations associated with the El Niño-Southern Oscillation (ENSO). Southerly/easterly Sydney MWD is correlated with low/high MSLP anomalies over New Zealand and the central Pacific Ocean. Southerly/easterly Sydney MWD is also correlated with cool/warm SST anomalies in the southwest Pacific, particularly in the eastern Coral Sea and Tasman Sea. Copyright © 2005 Royal Meteorological Society. [source] The latitude of the subtropical ridge over Eastern Australia: The L index revisitedINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2005Wasyl Drosdowsky Abstract Recent research has suggested that the position of the subtropical ridge in mean sea-level pressure over eastern Australia, referred to as the L index, has undergone a substantial poleward shift over the past century. These findings are not supported by calculations of the L index based on daily National Centers for Environmental Prediction reanalysis from 1948 to 2002, and on monthly mean Australian pressure data from 1890 to 2003. Reasons for these differences are discussed, together with the relationship between the L index and Australian rainfall. Copyright © 2005 Royal Meteorological Society [source] Solar-induced and internal climate variability at decadal time scalesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2005Mihai Dima Abstract Statistical analyses of long-term instrumental and proxy data emphasize a distinction between two quasi-decadal modes of climate variability. One mode is linked to atmosphere,ocean interactions (,the internal mode') and the other one is associated with the solar sunspots cycle (,the solar mode'). The distinct signatures of these two modes are also detected in a high-resolution sediment core located in the Cariaco basin. In the oceanic surface temperature the internal mode explains about three times more variance than the solar mode. In contrast, the solar mode dominates over the internal mode in the sea-level pressure and upper atmospheric fields. The heterogeneous methods and data sets used in this study underline the distinction between these decadal modes and enable estimation of their relative importance. The distinction between these modes is important for the understanding of climate variability, the recent global warming trend and the interpretation of high-resolution proxy data. Copyright © 2005 Royal Meteorological Society [source] Antarctic climate change during the last 50 yearsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2005John Turner Abstract An erratum has been published for this article in International Journal of Climatology 25 (8) 2005, 1147,1148. The Reference Antarctic Data for Environmental Research (READER) project data set of monthly mean Antarctic near-surface temperature, mean sea-level pressure (MSLP) and wind speed has been used to investigate trends in these quantities over the last 50 years for 19 stations with long records. Eleven of these had warming trends and seven had cooling trends in their annual data (one station had too little data to allow an annual trend to be computed), indicating the spatial complexity of change that has occurred across the Antarctic in recent decades. The Antarctic Peninsula has experienced a major warming over the last 50 years, with temperatures at Faraday/Vernadsky station having increased at a rate of 0.56 °C decade,1 over the year and 1.09 °C decade,1 during the winter; both figures are statistically significant at less than the 5% level. Overlapping 30 year trends of annual mean temperatures indicate that, at all but two of the 10 coastal stations for which trends could be computed back to 1961, the warming trend was greater (or the cooling trend less) during the 1961,90 period compared with 1971,2000. All the continental stations for which MSLP data were available show negative trends in the annual mean pressures over the full length of their records, which we attribute to the trend in recent decades towards the Southern Hemisphere annular mode (SAM) being in its high-index state. Except for Halley, where the trends are constant, the MSLP trends for all stations on the Antarctic continent for 1971,2000 were more negative than for 1961,90. All but two of the coastal stations have recorded increasing mean wind speeds over recent decades, which is also consistent with the change in the nature of the SAM. Copyright © 2005 Royal Meteorological Society [source] Atmospheric circulation patterns related to heavy snowfall days in Andorra, PyreneesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2005Pere Esteban Abstract Heavy snowfalls over mountain regions are often a direct cause of avalanches. Specific synoptic-scale atmospheric situations are responsible for these kinds of extreme snowfall event, and this is indeed the case for Andorra, a small country located in the Pyrenees, between France and Spain. Based on days with an intensity of at least 30 cm of snow in a 24 h period, the present study uses principal component analysis (PCA) and clustering techniques to characterize the synoptic circulation patterns for these days during the winter season. The area of analysis encompasses the region 30,60°N by 30°W,15°E and the period covers the winter seasons from 1986,87 to 2000,01. The methodology proposed involves a preprocessing approach consisting of a spatial standardization of the data used for the PCA, an alternative approximation to decide the centroids and the number of groups for the K -means clustering, and the rejection of the iterations for this algorithm. This approach enables the synoptic classification of every heavy snowfall day, and composite maps were constructed for sea-level pressure, 500 hPa geopotential height, and 1000,500 m thickness (the 5270 m, 5400 m and 5520 m contour lines). The results show seven circulation patterns, most of them with an Atlantic component of the wind, and others with a clear Mediterranean advection that could be combined with cold continental air. The results, as weather charts, could be a useful tool to assist meteorological models in heavy snowfall forecasting, and the day's classification obtained opens up future possibilities for detailed meteorological and climatological analysis of the established types. Copyright © 2005 Royal Meteorological Society [source] Weather regimes and their connection to the winter rainfall in PortugalINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2005J.A. Santos Abstract Wintertime rainfall over Portugal is strongly coupled with the large-scale atmospheric flow in the Euro-Atlantic sector. A K -means cluster analysis, on the space spanned by a subset of the empirical orthogonal functions of the daily mean sea-level pressure fields, is performed aiming to isolate the weather regimes responsible for the interannual variability of the winter precipitation. Each daily circulation pattern is keyed to a set of five weather regimes (C, W, NAO,, NAO+ and E). The dynamical structure of each regime substantiates the statistical properties of the respective rainfall distribution and validates the clustering technique. The C regime is related to low-pressure systems over the North Atlantic that induce southwesterly and westerly moist winds over the country. The W regime is characterized by westerly disturbed weather associated with low-pressure systems mainly located over northern Europe. The NAO, regime is manifested by weak low-pressure systems near Portugal. The NAO+ regime corresponds to a well-developed Azores high with generally settled and dry weather conditions. Finally, the E regime is related to anomalous strong easterly winds and rather dry conditions. Although the variability in the frequencies of occurrence of the C and NAO, regimes is largely dominant in the interannual variability of the winter rainfall throughout Portugal, the C regime is particularly meaningful over northern Portugal and the NAO, regime acquires higher relevance over southern Portugal. The inclusion of the W regime improves the description of the variability over northern and central Portugal. Dry weather conditions prevail in both the NAO+ and E regimes, with hardly any exceptions. The occurrence of the NAO+ and the NAO, regimes is also strongly coupled with the North Atlantic oscillation. Copyright © 2005 Royal Meteorological Society [source] Relationship between atmospheric circulation types over Greece and western,central Europe during the period 1958,97INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 14 2004Christina Anagnostopoulou Abstract An attempt is made to examine the relationship of the surface circulation prevailing over Europe with the corresponding surface and 500 hPa over Greece by correlating Lamb weather types for western Europe and Hess and Brezowsky (HB) types for central Europe with those derived from a new classification scheme for the Greek area. It was found that it was difficult to formulate rules controlling the frequency distributions of the circulation types over the Greek area in relation to the circulation over western and central Europe. However, statistically significant correlation was found between certain types with high frequency, which is greater between Lamb and HB types with the surface circulation types over the Greek area, compared with 500 hPa circulation types. For the most correlated pairs, seasonal composites of mean sea-level pressure and 500 hPa geopotential height anomalies demonstrated that the formation of the circulation types over the Greek area depends on the extent, intensity of the anticyclonic or cyclonic centres, air mass characteristics, and stability profile in the lower troposphere over the regions examined, but especially over the central and eastern Mediterranean. Copyright © 2004 Royal Meteorological Society [source] Remote weather associated with South Pacific subtropical sea-level high propertiesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2004Richard Grotjahn Abstract The subtropical highs in sea-level pressure (SLP) are little studied and incompletely understood. In recent years, three groups of theories, i.e. tropical divergent circulations, subtropical Rossby wavetrains, and midlatitude frontal cyclone interactions, have been proposed for remote maintenance of these highs. The latter is presented here as a remote forcing of these highs for the first time in the reviewed literature. The focus of the study is upon illuminating associations between these mechanisms and the South Pacific subtropical high in SLP (SP high). Precipitation, outgoing longwave radiation, velocity potential, and divergent winds are used as proxy markers for the remote forcing mechanisms. The tools used include composites, one-point correlations, autocorrelations, cross-correlations, and cross-spectra. Observational evidence, in monthly and daily data, is examined that appears to support each mechanism. Associations seen in monthly data are better understood in daily data at various lags. Convection over Amazonia, coordinated with suppressed convection in the western tropical Pacific, leads to enhanced SLP on the tropical side of the high. Midlatitude weather systems are the strongest influence upon the maximum SLP and the SLP on the higher latitude side of the high. The western side is associated with both middle-and lower-latitude phenomena, such as the South Pacific convergence zone. Various properties of the high have a strong period around 45 days. Associations to the Madden,Julian oscillation and El Niño,southern oscillation are explored and are strong only for the tropical side of the SP high. Copyright © 2004 Royal Meteorological Society [source] Sea-level pressure variability in the Po Plain (1765,2000) from homogenized daily secular recordsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 4 2004Maurizio Maugeri Abstract A homogenized 236 year daily regional sea-level pressure (SLP) record is constructed for the Po Plain (north Italy) by means of six station records: Bologna, Genoa, Lugano, Milan, Padua and Turin. Station records are subjected to a first homogenization in order to reduce all observations to sea level and to 0°C and to correct the bias introduced by calculating daily means using different sets of observation hours. A second homogenization is performed by means of comparison with other Italian and European series. After homogenization, the root-mean square error of the yearly station records is evaluated within 0.2 hPa after 1880, whereas for the periods 1834,1880 and 1765,1833 it is estimated as between 0.2 and 0.3 hPa and as around 0.4 hPa respectively. Trend analysis is applied to the annual and seasonal regional records and concerns both SLP and its day-to-day variability. The results show that neither the SLP nor its day-to-day variability have an evident trend when considering the entire 1765,2000 period. However, if the series is divided roughly in two parts, then significant trends can be highlighted. The annual and seasonal regional SLP records are also compared with corresponding regional temperature series. The results show that, especially in spring and in summer, temperature and SLP are in good agreement. Copyright © 2004 Royal Meteorological Society [source] Changes of air temperature and precipitation in Poland in the period 1951,2000 and their relationship to atmospheric circulationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2004Jan Degirmend Abstract An analysis of trends of mean monthly temperature and precipitation totals in Poland in the period 1951,2000 was carried out. Areal means of temperature and precipitation were used, averaged for 51 meteorological stations evenly distributed within Poland's borders. Sensitivity of air temperature and precipitation variations towards circulation was assessed. Circulation variations were expressed by sea-level pressure in the 52.5°N, 20°E grid point and geostrophic wind calculated from meridional (45,60°N) and latitudinal (10,30°E) pressure gradients. It was shown that the circulation factor explains up to 77% and 44% of temperature and precipitation variance respectively. Significant upward trends of temperature in March and May were detected. Also, the precipitation total in March was on the increase. The last decade of the 20th century was slightly ,too warm' in comparison with the circulation-induced temperature level, as well as with the temperature change scenario according to HadCM2 GS model. An attempt was made to evaluate the contribution of the frequency of snow cover occurrence to the temperature increase in winter, i.e. the temperature,albedo feedback. Copyright © 2004 Royal Meteorological Society [source] Decadal changes in the link between El Niño and springtime North Atlantic oscillation and European,North African rainfallINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2003Peter Knippertz Abstract The link between El Niño,southern oscillation (ENSO) variability in boreal winter (represented by the NIÑO3 index, i.e. East Pacific sea-surface temperature anomalies) and the large-scale circulation and weather conditions over Europe,northwest Africa in spring is explored, considering station reports of precipitation, sea-level pressure (SLP) anomalies and two North Atlantic oscillation (NAO) indices. It is found that these relations have undergone consistent and simultaneous changes in the 20th century. Three characteristic periods can be identified. During 1900,25 and 1962,87, positive NIÑO3 index values are associated with enhanced precipitation over central Europe and reduced rainfall in southern Europe and northern Africa. The ENSO influence on precipitation over Scotland and Norway is small. The rainfall anomalies can be explained from the advective and dynamical implications of a north,south dipole in SLP correlations (warm ENSO events followed by low pressure in northern Europe and high pressure over the Mediterranean Sea,North Africa). This dipole hardly projects on the commonly used NAO centres (Iceland and Azores/Gibraltar) and thus ENSO,NAO correlations are insignificant. During 1931,56 the NIÑO3 index reveals little influence on precipitation over the Iberian Peninsula and Morocco, but there are large negative correlations with precipitation over Scotland and Norway. This is related to an alteration of the NIÑO3,SLP correlation pattern, which implies high pressure over northern Europe and low pressure over central Europe after warm events, and thus a virtually inverted dipole with respect to the other two periods. The large westward extension of the dipole leads to a significant NAO,NIÑO3 correlation of r = ,0.5. These alterations were accompanied by substantial large-scale circulation changes during the period 1931,56, as revealed by anomalously high pressure and dry conditions over central,western Europe, a change in precipitation-producing SLP patterns for Morocco and an anomalously low number of positive NAO and NIÑO3 index values. It is left for discussion as to whether the decadal variations described are due to a change in the physics of the teleconnection or to stochastic fluctuations. Copyright © 2003 Royal Meteorological Society [source] The annual cycle and interannual variability of atmospheric pressure in the vicinity of the North PoleINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2003Richard I. Cullather Abstract A comparison of National Centers for Environmental Prediction,National Center for Atmospheric Research reanalysis six-hourly sea-level pressure data with former Soviet drifting station observations over the central Arctic Basin reveals high monthly correlations throughout the period 1950,91, but also a preferred winter season negative bias of about 1.4 hPa. Using the reanalysis, supplemented by Arctic Ocean Buoy Program fields and in situ observations, a generalized depiction of the annual cycle of pressure fields over the Arctic may be constructed. Above the Canada Basin,Laptev Sea side of the Arctic, the annual cycle of surface pressure is dominated by the first harmonic, which has an amplitude of about 5 hPa and maximum pressure occurring in March. Along the periphery of northern Greenland and extending to the North Pole, a weak semiannual cycle is found in surface pressure with maxima in May and November. The presence of the semiannual variation over time is highly variable. Dynamically, this progression of the annual cycle may be attributed to the transfer of atmospheric mass from Eurasia and into the Canadian Archipelago in spring and the reverse condition in autumn. Over the central Arctic Basin, springtime pressure increases result from an enhanced poleward mass transport from Eurasia. An increase of equatorward transport over the Canadian Archipelago in May and June results in central Arctic pressure decreases into summer. A less distinct temporal separation between the poleward Canadian transport and the equatorward Eurasian transport results in the weaker second pressure maximum in autumn. On interannual time-scales, atmospheric mass over the central Arctic is exchanged with the storm track centres of action in the North Atlantic and North Pacific. In particular, the large decrease in central Arctic Basin sea-level pressure during the late 1980s is due to a large transfer of atmospheric mass into the North Pacific. Copyright © 2003 Royal Meteorological Society [source] Australian drought: the interference of multi-spectral global standing modes and travelling wavesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2003Warren B. White Abstract Extreme drought has devastated the flora, fauna, and regional economy in rangeland grazing districts over Australia for 3,5 years duration every 20 to 30 years throughout the 20th century. We investigate the source of drought occurring in five example grazing districts in eastern and central Australia. We find year-to-year variability in grazing district rainfall (GDR) during the summer rainy season (November to March) composed of quasi-biennial, interannual, quasi-decadal, and interdecadal signals from 1900 to 1999. However, the longer period signals dominate, accounting for the interdecadal quasi-periodicity of the drought/flood cycle. We find these GDR signals associated with corresponding global standing modes and travelling waves in covarying sea-surface temperature (SST) and sea-level pressure (SLP) anomalies. These global SST/SLP modes/waves influence the GDR signals by altering the troposphere moisture flux converging onto the grazing districts from regional tropical and extra-tropical oceanic source regions. We construct statistical models to determine whether the evolution of these global SST/SLP modes/waves over the oceanic source regions can hindcast corresponding GDR signals from one year to the next. When these models allow for modulation of the modes/waves, they hindcast ,1/3 of the variance in the GDR indices at 1 year lead, including the drought episodes. We find drought resulting from the constructive interference of the dry phases of the quasi-decadal and interdecadal global SST/SLP modes/waves, accompanied by a weakening of year-to-year variability associated with either weak quasi-biennial and interannual modes/waves or their destructive interference. Copyright © 2003 Royal Meteorological Society [source] Changes in seasonal mean maximum air temperature in Romania and their connection with large-scale circulationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2002Rodica Tomozeiu Abstract This paper investigates the temporal and spatial variability of the seasonal mean of maximum air temperature in Romania and its links with the large,scale atmospheric circulation. The Romanian data sets are represented by time series at 14 stations. The large-scale parameters are represented by the observed sea-level pressure (SLP) and geopotential height at 500 hPa (Z500). The period analysed was 1922,98 for winter and 1960,98 for all seasons. Before analysis, the original temperature data were tested to detect for inhomogeneity using the standard normal homogeneity test. Empirical orthogonal functions (EOFs) were used to analyse the spatial and temporal variability of the local and large-scale parameters and to eliminate noise from the original data set. The time series associated with the first EOF pattern of the SLP and mean maximum temperature in Romania were analysed from trend and shifts point of view using the Pettitt and Mann,Kendall tests respectively. The covariance map computed using the Z500 and the seasonal mean of maximum temperature in Romania were used as additional methods to identify the large-scale circulation patterns influencing the local variability. Significant increasing trends were found for winter and summer mean maximum temperature in Romania, with upward shifts around 1947 and 1985 respectively. During autumn, a decreasing trend with a downward shift around 1969 was detected. These changes seem to be real, since they are connected to similar changes in the large-scale circulation. So, the intensification of the southwesterly circulation over Europe since 1933 overlapped with the enhancement of westerly circulation after the 1940s could be the reason for the change in winter mean maximum temperature. The slight weakening of the southwesterly circulation during autumn could be one of the reasons for the decrease in the regime of the mean maximum temperature for autumn seasons. Additionally, the covariance map technique reveals the influence of the North Atlantic oscillation in winter, East Atlantic Jet in summer and Scandinavian (or Euroasia-1) circulation pattern in autumn upon mean maximum air temperature. Copyright © 2002 Royal Meteorological Society. [source] Solar correlates of Southern Hemisphere mid-latitude climate variabilityINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 8 2002Ronald E. Thresher Abstract Atmospheric circulation in the southern mid-latitudes is dominated by strong circum-Antarctic zonal west winds (ZWW) over the latitude range of 35 to 60°S. These winds exhibit coherent seasonal and interannual variability, which has been related both to Antarctic (e.g. polar ice) and low-latitude climate (e.g. El Niño,southern oscillation) parameters. Historical and recent studies suggest that, at its northern margins, variability in the ZWW also has a marked quasi-decadal component. Analysis of sea-level pressure and rainfall data for the Australian region, South Africa and South America confirms frequent indications of quasi-decadal variability in parameters associated with the ZWW, which appears to be in phase around the hemisphere. This variation broadly correlates with the sunspot cycle, and specifically appears to reflect sunspot-correlated, seasonally modulated shifts in the latitude range each year of the sub-tropical ridge over eastern Australia. Sunspot-correlated variability in the southern mid-latitudes is likely to have substantial effects on temperate climate and ecology and is consistent with recent models of solar effects on upper atmospheric climate, though the mechanisms that link these to winds and rainfall at sea level remain obscure. Copyright © 2002 Royal Meteorological Society [source] United Kingdom and Ireland precipitation variability and the North Atlantic sea-level pressure fieldINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 8 2001Sarah J. Murphy Abstract The relationship between UK and Ireland (UK&I) precipitation variability and the North Atlantic sea-level pressure (SLP) field is examined. Strong positive correlations between the North Atlantic Oscillation (NAO) and precipitation in the northwest of the UK&I, particularly in winter, are confirmed but correlations are insignificant at the 0.05 level in the southeast during all months. This paper identifies new patterns of SLP associated with precipitation variability both for regions and months where precipitation variability is not strongly linked with the NAO and for patterns that appear to be more closely related to UK&I precipitation than the NAO. Two indices of monthly UK&I precipitation variability are calculated using empirical orthogonal functions (EOFs) of monthly UK&I precipitation anomalies. EOF1 represents precipitation variability for the UK&I as a whole and EOF2 the variability in the north,south precipitation gradient across the UK&I. Correlations between both these monthly EOF derived precipitation indices and SLP show a north,south (sub-tropical/mid-latitude) dipole, which is particularly strong in winter. These correlation patterns are then used to construct new SLP indices, which necessarily relate more closely to UK&I precipitation. The first index resembles the East Atlantic pattern from September to April. The second may be thought of as an alternative index of the NAO, such that it is optimized with respect to precipitation variability and is located northeast of those centres of action most commonly used to calculate the NAO index. Stepwise linear regression models, incorporating the two new indices and the original NAOI, suggest that over 25% of UK&I precipitation variability this century (1900,1994) in each month can be explained by a simple index representation of the North Atlantic SLP field. This rises to over 40% of variance explained in nearly all regions of the UK&I. Copyright © 2001 Royal Meteorological Society [source] Observations of atmosphere-ocean coupling in the North AtlanticTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 576 2001Arnaud Czaja Abstract Anindex of sea surface temperature (SST) variability, ,T, is introduced that measures the difference in SST across the separated Gulf Stream in late winter. By analysing a long observational record of SST and sea-level pressure (SLP), it is shown that ,T exhibits damped oscillations of decadal period, and covaries with the strength of a dipolar SLP anomaly reminiscent of the North Atlantic Oscillation (NAO). Analysis in the frequency domain shows a broad-band ,peak' at 10,20 years in ,T, with a continuous decrease of power on longer time-scales. Similar spectral signatures are found in the northern part of the SLP dipole (the Greenland-Icelandic Low region) but not in its southern part (the subtropical High region), whose power increases on long time-scales. The observations are interpreted in the framework of a delayed-oscillator model in which the ocean circulation introduces the delay, and modulates ,T on decadal time-scales. The decrease of power seen on long time-scales (>25 years) in the ,T index is captured by a model including wind-driven ocean circulation, and arises primarily as a passive response of the latter to the NAO forcing. Variability of the ocean's meridional overturning circulation could also play a role in modulating ,T on decadal time-scales. If a small feedback of ,T on the NAO pattern is introduced, the simple model can also reproduce the spectral structures seen in the SLP anomaly in the Greenland-Iceland region. [source] A statistical downscaling method for monthly total precipitation over TurkeyINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2004Hasan Tatli Abstract Researchers are aware of certain types of problems that arise when modelling interconnections between general circulation and regional processes, such as prediction of regional, local-scale climate variables from large-scale processes, e.g. by means of general circulation model (GCM) outputs. The problem solution is called downscaling. In this paper, a statistical downscaling approach to monthly total precipitation over Turkey, which is an integral part of system identification for analysis of local-scale climate variables, is investigated. Based on perfect prognosis, a new computationally effective working method is introduced by the proper predictors selected from the National Centers for Environmental Prediction,National Center for Atmospheric Research reanalysis data sets, which are simulated as perfectly as possible by GCMs during the period of 1961,98. The Sampson correlation ratio is used to determine the relationships between the monthly total precipitation series and the set of large-scale processes (namely 500 hPa geopotential heights, 700 hPa geopotential heights, sea-level pressures, 500 hPa vertical pressure velocities and 500,1000 hPa geopotential thicknesses). In the study, statistical preprocessing is implemented by independent component analysis rather than principal component analysis or principal factor analysis. The proposed downscaling method originates from a recurrent neural network model of Jordan that uses not only large-scale predictors, but also the previous states of the relevant local-scale variables. Finally, some possible improvements and suggestions for further study are mentioned. Copyright © 2004 Royal Meteorological Society [source] Mediated and direct effects of the North Atlantic Ocean on winter temperatures in northwest EuropeINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2003Martina M. Junge Abstract This study has used a multiple regression model to quantify the importance of wintertime mean North Atlantic sea-surface temperatures (SSTs) for explaining (simultaneous) variations in wintertime mean temperatures in northwestern Europe. Although wintertime temperature variations are primarily determined by atmospheric flow patterns, it has been speculated that North Atlantic SSTs might also provide some additional information. To test this hypothesis, we have attempted to explain 1900,93 variations in wintertime mean central England temperature (CET) by using multiple regression with contemporaneous winter mean North Atlantic sea-level pressures (SLPs) and SSTs as explanatory variables. With no SST information, the leading SLP patterns (including the North Atlantic oscillation) explain 63% of the total variance in winter mean CET; however, SSTs alone are capable of explaining only 16% of the variance in winter mean CET. Much of the SST effect is ,indirect' in that it supplies no more significant information than already contained in the mean SLP; e.g. both SLP and SST together can only explain 68% of the variance. However, there is a small (5% variance) direct effect due to SST that is not mediated by mean SLP, which has a spatial pattern resembling the Newfoundland SST pattern identified by Ratcliffe and Murray (1970. Quarterly Journal of the Royal Meteorological Society 96: 226,246). In predictive mode, however, using explanatory variables from preceding seasons, SSTs contain more information than SLP factors. On longer time scales, the variance explained by contemporaneous SST increases, but the SLP explanatory variables still provide a better model than the SST variables. Copyright © 2003 Royal Meteorological Society [source] | |||||||||||||