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1.
Summary ?A new procedure for grouping the circulation types occurring during autumn and spring over an area is introduced in this paper. The procedure combines surface air mass characteristics affecting the area with the prevailing synoptic conditions. Factor and Cluster Analysis are used to derive the circulation types, based on surface meteorological data, as well as on surface pressure grid data. The methods are applied to Athens, Greece, using data covering the period 1954–1999. Eight circulation types for the autumn (September, October, November) and eight for the spring (March, April, May) are derived. Clusters presenting low-pressure systems are found to be generally fast moving, while anticyclones generally remain over the area for more than two days. Finally an interesting seasonal distribution is found from some weather types. Received February 20, 2002; revised December 23, 2002; accepted January 9, 2003 Published online May 26, 2003  相似文献   

2.
Summary  Circulation types were identified by means of zonal and meridional indices calculated separately over ten different regions of 20°×20° over the Mediterranean and Europe. Seasonal temperature trends in 22 grid boxes of 5°×5° covering the entire Mediterranean, and at six stations Lisbon, Madrid, Florence, Luqa (Malta), Athens and Jerusalem, were calculated. A warming trend in the period 1873–1989 was detected. The warming is more evident in the western Mediterranean with an average rate of about 0.4 [°C/100 yr], than in the eastern Mediterranean with an increase of only 0.2 [°C/100 yr]. A cooling trend in autumn in the eastern Mediterranean with an average rate of −0.5 [°C/100 yr] was detected and attributed to an increase in northerly meridional circulation in that region. Warming trends at Lisbon, Madrid, Florence, Athens and Jerusalem, were more important than the trends in the grid boxes containing these stations. This rapid warming was attributed to urban effects. No such effects were found in Luqa due to its location and the lack of urban effects there. Temperatures at Luqa, Athens and Jerusalem are highly positively correlated. Likewise, temperatures at Lisbon and Madrid. Temperatures at Florence are either correlated with Madrid or with Luqa. Negative or no correlations were found between Lisbon or Madrid with Athens or Jerusalem, except during the winter. This was attributed to the fact that favourable circulation for high temperatures in the eastern stations was opposite to the favourable circulation for high temperatures in the western stations and vice versa. Finally, the above reinforces the concept of a Mediterranean Oscillation between the western and eastern basins. Received November 14, 1997 Revised June 2, 1998  相似文献   

3.
Summary  Based on the six-hourly re-analysis sea-level pressure data of the European Centre for Medium-Range Weather Forecast (ECMWF) a cyclone statistics for the Arctic region north of 60° is elaborated for the period 1 November 1986 to 31 October 1991. For each low pressure center on a weather map its location, central pressure and horizontal pressure gradients in E, W, N, and S direction are determined. Furthermore, cyclone centers are followed with time to calculate trajectories, pressure tendencies, and lifetimes. A horizontal grid of 300 km × 300 km is used as unit area for the statistical computations. A unit area experiences about 20 cyclone passages per year (range 5–40). On the average, six cyclones occur simultaneously in the Arctic region. Lifetimes vary from 6 h to 15 days. The annual cyclone activity over the 5-year period is nearly the same. Cyclones are more frequent in summer (about 94 per month) than in winter (77 per month). In general summer cyclones are weaker than winter cyclones. On the average, the minimum central pressure during the lifetime of a cyclone is about 1000 hPa (typical range 980–1020) in summer and about 988 hPa (typical range 940–1030) in winter. In winter, a zone of high cyclone frequency extends from the region near Iceland over the Greenland Sea, Barents Sea, and Kara Sea to the Laptev Sea while the interior of the Arctic shows little cyclone frequency. In summer, the region near Iceland and the interior of the Arctic are separate centers of high cyclone frequency. Both in winter and summer very high cyclone frequencies are observed over the northern Baffin Bay. The regional distribution of mean central pressures and maximum pressure gradients roughly follows the distribution of cyclone frequencies except for the Baffin Bay cyclones which are generally weak. Cyclolysis dominates cyclogenesis over largest parts of the Arctic. Regions of high cyclone frequency are also regions of frequent cyclogenesis and frequent cyclolysis. One third of all cyclones is generated in a region with an already existing cyclonic circulation. Cyclones in the Fram Strait are studied in more detail because of their special impact on the ice export from the Arctic Ocean to the Atlantic Ocean. On the average, there are 5 cyclones per month. the cyclone frequency in the Fram Strait is higher during the winter period than during the summer period. This is in contrast to the overall Arctic frequency which is higher in summer than in winter. Cyclogenesis predominates in winter and cyclolysis in summer in the Fram Strait. The most frequent direction of motion is from SW to NE. Received November, 1999 Revised June 22, 2000  相似文献   

4.
Summary This paper deals with the surface pressure covariability over the Altantic/European sector of the Northern Hemisphere, using monthly grid point data for the 100 year period 1890–1989. Factor analysis is applied to 90 grid point time series for January, February, July, and August. The initial 90 pressure variables can be reduced to 7–8 factors in winter and 10 in summer. A winter teleconnection was identified, known as the seesaw phenomenon, between the Icelandic low and the Azores subtropical anticyclone. In order to define the centers of action for temperature, winter precipitation and summer northerly wind frequency (etesian days) in Athens and in the Aegean sea, the variability of the factor scores and of these weather elements is compared. It is shown that the center of action for temperature in Athens is found to be in north and northwest Europe (centered over southern Scandinavia). For winter precipitation, the center of action is located in the west and southwest Mediterranean and northwest Africa. Finally, for the etesian winds frequency variability, this center of action is found over the northern Adriatic and northern former Yugoslavia, while there is no evidence of influence by the southwest Asia thermal low.With 12 Figures  相似文献   

5.
Summary The high-speed particle flux (solar wind) escaping from the Sun controls the geomagnetic activity at middle latitudes. The latter is found to be negatively correlated to the difference of atmospheric pressure between January and April averaged out in the area of the Northern Adriatic. This difference is again related to the water volume flowing from the South into the Northern Adriatic Sea and is a necessary (though not sufficient) condition for the occurrence of strong algal-blooms in summer. A physical model involving geomagnetic activity, mean atmospheric pressured difference between latitude 35 °N and 55 °N in the European area and atmospheric pressure difference from winter to spring in the Northern Adriatic basin is proposed. The possibility of predicting the long-term variations of geomagnetic activity allows one to obtain long-term predictions of winter minus spring pressure and therefore indications of the risk of strong summer-time algal-bloom episodes. Received March 29, 1996 Revised February 14, 1997  相似文献   

6.
Summary The Southern South America climatological 500 hPa relative vorticity mean state was examined using regional objective analyses of 500 hPa geopotential heights provided by the Servicio Meteorológico Nacional of Argentina. The dataset, covering the period June 1983 to July 1987, was stratified into two samples: the cold and warm seasons. Mean cyclonic vorticity south of 40° S results in a climatological trough over Patagonia with a northwest-southeast tilt. North of this latitude, mean anticyclonic circulation dominates with the exception of a centre of cyclonic vorticity over the Río de la Plata (35° S, 56° W). Seasonal changes appear to be small. Relative vorticity frequency distributions were also analysed. The association between precipitation and synoptic-scale features of the mid-troposphere circulation was investigated through vorticity fields. A particular distribution of vorticity anomalies associated with daily precipitation in Buenos Aires is revealed by biserial correlation coefficient fields. In winter, the strongest relationships are found between 35° S and 40° S over the Andes Mountains (minimum significant correlation coefficients indicating a cyclonic vorticity anomaly), and in the south of Brazil and east of Buenos Aires over the Atlantic Ocean down to a latitude of 40° S (maximum correlation coefficients related to anomalously anticyclonic circulation). This shows the preferential position of troughs and ridges that produce precipitation in Buenos Aires on the time scale of a day. In summer, centres of anomalously cyclonic and anticyclonic vorticity associated with precipitation shift slightly southward. For moderate or intense precipitation in Buenos Aires, advection of warm and wet air southwards appears to be more important in winter, while in summer the strong anomalous vorticity gradient north of the negative centre over the Andes Cordillera favours rainfall in Buenos Aires. Received April 17, 1997  相似文献   

7.
Summary The paper examines the annual cycle of the mid-tropospheric easterly jet (MTJ) over West Africa against the background of many reviews indicating different locations and characteristics of the jet and considering it as a summer feature. NCEP–NCAR reanalysis zonal wind datasets for the period 1971–2000 and upper air datasets over the region are used. The results exhibit realistic spatial structure of the easterly jet. The long-term mean of the datasets suggests that the jet over West Africa is not only a summer feature but can also be found in winter with the same order of magnitude in the wind velocity at the core. The jet axis is located at about lat. 2° N close to the Guinean Coast in winter and at lat. 14° N in summer. The meridional oscillation of the jet suggests that as it advances northward, it maintains an altitude of 700 hPa in winter and transits in mid-spring to 650 hPa and reaches 600 hPa in summer. In the retreat, it displaces to 650 hPa at the end of September rather sharply to reach 700 hPa in October. The jet’s core has been observed to have a northeast–southwest orientation from season to season, covering a longitude of 29° from its southernmost to the northernmost positions.  相似文献   

8.
Summary In this study, Principal Component Analysis (PCA) has been used to identify the major modes of the outgoing long-wave radiation data for the period (1979–2002) during the Indian monsoon period (June–September), using seasonal mean values over the Indian region covering 143 grid points (5° N–35° N and 70° E–95° E at 2.5° Longitude–Latitude intervals. The five principal components explain up to 98.0% of the total variance. The first principal component explains 60% of the total variance with a pronounced variation in the outgoing long-wave radiation over the region 10° N to 25° N. It appears that the major reason for the monsoon variability is the intensity and associated fluctuations in the two major semi-permanent seasonal systems. This is largely indicative of strong seasonal shift of the major area of cloudiness associated with convergence zone. The second principal component explaining 20% of the total variance exhibits higher positive component loadings along 25° N and east of 80° E. The possible reason for this could be the synoptic systems such as monsoon depression/lows over the north bay and trough/vortices off the west-coast in the Arabian sea.  相似文献   

9.
Summary ?A three-dimensional Ocean General Circulation Model has been developed in stretched coordinate from scratch. The same model has been used to perform some numerical experiments to simulate the basic circulation pattern and the model variability to atmospheric forcing. For numerical simulations 72 × 25 grid points in the horizontal directions and nine (10, 30, 75, 250, 500, 1000, 1500, 2000 and 3000 m) vertical levels are considered. The lateral boundaries are set at 60° N and 60° S. The basic focus of the paper is on the demonstration of the performance of the model and its assessment by employing appropriate forcing from the outputs of an atmospheric general circulation model. Hence, the model was forced with the forcing (wind and thermodynamic) derived from the ECMWF runs from the AMIP archives. The preliminary results show the realistic simulation of basic pattern of different fields. The model simulations show that the model is able to reproduce some of the general features of the ocean, such as surface currents, surface temperature and salinity, mass transport and meridional heat transport. It is also to be noted that the model is capable to capture the El-Ni?o and La-Ni?a type events. Received April 3, 2002; revised June 6, 2002; accepted July 24, 2002 Published online: February 20, 2003  相似文献   

10.
Upper-level cut-off lows in southern South America   总被引：2，自引：0，他引：2  
Summary This paper presents a statistical study of the spatial and seasonal distribution and duration of cut-off low systems over the southern South American region based on the NCEP- NCAR reanalysis data for the period 1979–1988. Cut-off lows were first objectively determined as minimum geopotential values at the 250 hPa level and then subjectively imposing a cut-off circulation and a cold core. A total of 171 cut-off low events were detected, being more frequent in austral autumn followed by winter, spring and summer. There is a preferential region of occurrence in spring and autumn located between 68°–80° W and 30°–45° S. The Pacific area showed the greatest frequency of occurrence followed by the Atlantic and the continental areas. Most of the cut-off lows last 2 or 3 days (around 90% of the cases) though there is a tendency of the continental events to be longer. The cut-off low event developed upwind the Andes on 22–28 September 1986 was selected as a case study. Low-level cold air advection was the main forcing of the deepening of the upper level low system.  相似文献   

11.
Summary  Monthly rainfall totals at 7 stations across Turkey and sea level pressure (SLP) in 16 grid points in the region delimited by the 20° E and 50° E longitudes and by the 30° N and 45° N latitudes were analysed. Data were available for a period longer than sixty years. The standard deviations of SLP at each grid point for each month, were calculated and mapped. For each station, months were defined as dry or wet according to their z scores: ≤ −1.0 or ≥ 1.0 respectively. Maps showing the SLP z scores of the corresponding dry or wet months for each station were prepared. The maps, enable to distinguish between SLP patterns associated with dry or wet conditions. Furthermore, correlations between monthly rainfall in each of the stations and SLP at each grid point were performed. The correlation coefficients were mapped. (a) The variability of the SLP decreases from the Balkans towards the Arabian Peninsula and is much larger in winter as compared with summer. (b) Relationship between rainfall in Turkey and the regional SLP is large in winter and non existing in summer. (c) Pressure patterns associated with dry conditions, show usually positive SLP departures, whereas, pressure patterns associated with wet conditions show usually negative SLP departures. (d) There is a great resemblance between pressure patterns associated with wet conditions and correlation maps of the same months. Received September 4, 2000 Revised January 15, 2001  相似文献   

12.
Summary An earlier developed multidecadal database of Northern Hemisphere cut-off low systems (COLs), covering a 41 years period (from 1958 to 1998) is used to study COLs interannual variability in the European sector (25°–47.5° N, 50° W–40° E) and the major factors controlling it. The study focus on the influence on COLs interannual variability, of larger scale phenomena such as blocking events and other main circulation modes defined over the Euro-Atlantic region. It is shown that there is a very large interannual variability in the COLs occurrence at the annual and seasonal scales, although without significant trends. The influence of larger scale phenomena is seasonal dependent, with the positive phase of the NAO favoring autumn COL development, while winter COL occurrence is mostly related to blocking events. During summer, the season when more COLs occur, no significant influences were found.  相似文献   

13.
Summary  A large number of atmospheric circulation classification techniques have been developed in the investigation of synoptic controls on regional rainfall. Often the rationale is to aid efforts to downscale GCM output for the purpose of producing more confident climate change impact scenarios. Discrete weather typing techniques, although proven to be successful do not capture weather type intensity and within-type variability can often be high. In this study an objective indexing method, developed for Egypt and the British Isles area is applied to the Iberian peninsula. Air flow index values are then used as predictor variables in simple linear regression models to estimate monthly mean grid point rainfall amounts. Separate models are evaluated for the winter and summer halves of the year and also for surface and mid-tropospheric flow (500 hPa). The models are evaluated and compared indicating that the index values provide good estimation of rainfall but variability in performance between season and site is noted. Received February 10, 2000  相似文献   

14.
 The influence of different vegetation distributions on the atmospheric circulation during the Last Glacial Maximum (LGM, 21 000 years before present) is investigated. The atmospheric general circulation model of the Bureau of Meteorology Research Center was run using a modern vegetation and in a second experiment with a vegetation reconstruction for the LGM. It is found that a change from conifer to desert and tundra causes an additional LGM cooling of 1–2 °C in Western Europe, up to −4 °C in North America and −6 °C in Siberia. An expansion of dryland vegetation causes an additional annual cooling of 1–2 °C for Australia and northern Africa. On the other hand, an increase of temperature (2 °C) is found in Alaska due to changes in circulation. In the equatorial region the LGM vegetation leads to an increased modelled temperature of 0.5–1.5 °C and decreased precipitation (30%) over land due to a reduction of the tropical rainforest, mainly in Indonesia, where the reduction of precipitation over land is associated with an increase of precipitation of 30% over the western Pacific. Received: 15 December 1999 / Accepted: 10 January 2001  相似文献   

15.
Summary Climatological characteristics associated with summer monsoon onset over the eastern Bay of Bengal (BOB) are examined in terms of the westerly-easterly boundary surface (WEB). The vertical tilt of the WEB depends on the horizontal meridional temperature gradient (MTG) near the WEB, under the constraint of the thermal wind balance. The switch in the WEB tilt firstly occurs between 90 and 100°E during the first pentad of May. At this time the 850 hPa ridgeline splits over the BOB and heavy rainfall commences over the eastern BOB, indicating the onset of the BOB summer monsoon (BOBSM). The area-averaged MTG (200–500 hPa) is proposed as an index to define the BOBSM onset. A comparison of the onset determined by the MTG, 850 hPa zonal wind, and outgoing longwave radiation (OLR) shows that the MTG index is the most effective in characterizing the interannual variability of the BOBSM onset. Strong precursor signals are found prior to an anomalous BOBSM onset. Composite results show that early (late) BOBSM onset follows excessive (deficient) rainfall over the western Pacific and anomalous lower tropospheric cyclonic circulation which extends zonally from the northern Indian Ocean into the western Pacific, and strong (weak) equatorial westerly anomalies in the preceding winter and spring. Prior to an early (late) BOBSM onset, significant positive (negative) thickness anomalies exist around the Tibetan Plateau, accompanied by anomalous upper tropospheric anticyclonic (cyclonic) circulation. The interannual variations of the BOBSM onset are significantly correlated with anomalous sea surface temperature related to ENSO. These occurs through changes in the Walker circulation and local Hadley circulation, leading to middle and upper tropospheric temperature anomalies over the Asian sector. The strong precursor signals around the Tibetan Plateau may be partly caused by local snow cover anomalies, and an early (late) BOBSM onset is preceded by less (more) snow accumulation over the Tibetan Plateau during the preceding winter.  相似文献   

16.
Summary The present study examines the long term trend in sea surface temperatures (SSTs) of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean in the context of global warming for the period 1901–2002 and for a subset period 1971–2002. An attempt has also been made to identify the relationship between SST variations over three different ocean areas, and All-India and homogeneous region summer monsoon rainfall variability, including the role of El-Ni?o/Southern Oscillation (ENSO). Annual sea surface temperatures of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean show a significant warming trend of 0.7 °C, 0.6 °C and 0.5 °C per hundred years, respectively, and a relatively accelerated warming of 0.16 °C, 0.14 °C and 0.14 °C per decade during the 1971–2002 period. There is a positive and statistically significant relationship between SSTs over the Arabian Sea from the preceding November to the current February, and Indian monsoon rainfall during the period 1901–2002. The correlation coefficient increases from October and peaks in December, decreasing from February to September. This significant relationship is also found in the recent period 1971–2002, whereas, during 1901–70, the relationship is not significant. On the seasonal scale, Arabian Sea winter SSTs are positively and significantly correlated with Indian monsoon rainfall, while spring SSTs have no significant positive relationship. Nino3 spring SSTs have a negative significant relationship with Indian monsoon rainfall and it is postulated that there is a combined effect of Nino3 and Arabian Sea SSTs on Indian monsoon. If the Nino3 SST effect is removed, the spring SSTs over the Arabian Sea also have a significant relationship with monsoon rainfall. Similarly, the Bay of Bengal and Equatorial South Indian Ocean spring SSTs are significantly and positively correlated with Indian monsoon rainfall after removing the Nino3 effect, and correlation values are more pronounced than for the Arabian Sea. Authors’ address: Dr. D. R. Kothawale, A. A. Munot, H. P. Borgaonkar, Climatology and Hydrometeorology divisions, Indian Institute of Tropical Meteorology, Pune 411008, India.  相似文献   

17.
 An improved concept of the best analogues method was used to reconstruct the Last Glacial Maximum (LGM) climate from a set of botanical records from the former Soviet Union and Mongolia. Terrestrial pollen and macrofossil taxa were grouped into broad classes – plant functional types (PFTs), defined by the ecological and climatic parameters used in the BIOME1 model. PFT scores were then calibrated in terms of modern climate using 1245 surface pollen spectra from Eurasia and North America. In contrast to individual taxa, which exhibit great variability and may not be present in the palaeoassemblages, even in suitable climates, PFTs are more characteristic of the vegetation types. The modified method thus allows climate reconstruction at time intervals with partial direct analogues of modern vegetation (e.g. the LGM). At 18 kBP, mean temperatures were 20–29 °C colder than today in winter and 5–11 °C colder in summer in European Russia and Ukraine. Sites from western Georgia show negative, but moderate temperature anomalies compared to today: 8–11 °C in January and 5–7 °C in July. LGM winters were 7–15 °C colder and summers were 1–7 °C colder in Siberia and Mongolia. Annual precipitation sums were 50–750 mm lower than today across northern Eurasia, suggesting a weakening of the Atlantic and Pacific influences. Reconstructed drought index shows much drier LGM conditions in northern and mid-latitude Russia, but similar to or slightly wetter than today around the Black Sea and in Mongolia, suggesting compensation of precipitation losses by lower-than-present evaporation. Received: 11 May 1998 / Accepted: 25 September 1998  相似文献   

18.
Summary  Anomalously wet and dry months in the Mediterranean basin were identified during the period 1860–1990 from observations at five stations located along the west-east axis of the Mediterranean basin (Barcelona, Florence, Malta, Athens and Jerusalem), supplemented by data from Madrid and Lisbon. Wet and dry months were characterized by hydric indices (HI) based on values of the standardized precipitation anomalies. Different patterns of anomalously wet and dry months were qualitatively identified on the basis of the spatial distributions of the hydric indices. The standardized sea level pressure values at 56 grid points in the domain 35° N–65° N, 30° W–40° E, for each of the anomalously wet and dry months, were subjected to T-mode Principal Component Analysis. The mean hydric indices associated with each principal component in each season are arranged in four distinct different spatial distributions for wet months and in three for dry months as following: (a) Mediterranean wide distribution of positive/negative anomalies; (b1) Strong positive anomalies to the west, but weaker to eastern Mediterranean; (b2) Strong negative anomalies to the west, but weaker or normal to the east; (c1) Strong positive anomalies to the west and to the east and weaker ones to the central Mediterranean; (c2) Negative anomalies to the west and east, but weaker, or normal, or positive to the central Mediterranean; (d) Relatively strong positive anomalies to the east and weaker ones to the western Mediterranean. Finally, monthly mean charts of standardized anomaly and mean sea level pressure are presented for each principalcomponent in each season. These charts are used to interpret the spatial distribution of the positive and negative precipitation anomalies in terms of mean circulation over the domain. Received December 10, 1998 Revised June 14, 1999  相似文献   

19.
In the paper the 5°×10°latitude-longitude grid point data of daily 500 hPa geopotential height over the NorthernHemisphere(NH)in summer(June—August)during 1980s are used.The base point(20°N,120°E)is selected to calcu-late point correlation between the base point and other grid points.We find that the summer heat source anomaly of thetropical western Pacific causes anomaly of summer general circulation over NH and teleconnection of general circula-tion similar to PNA pattern forms from East Asia to North America.The teleconnections show great interannualchanges.  相似文献   

20.
Summary The evolution of geophysical parameters over Indian Ocean during two contrasting monsoon years 2002 (drought) and 2003 (normal) were studied using TRMM/TMI satellite data. Analysis indicates that there was a lack of total water vapour (TWV) build up over Western Indian Ocean (WIO) during May 2002 (drought) when compared to 2003 (normal). Negative (positive) TWV anomalies were found over the WIO in May 2002 (2003). In 2002, negative SST anomaly of ∼1.5 °C is found over entire WIO when compared to 2003. Anomalously high sea surface wind speed (SWS) anomaly over the South West Indian Ocean (SWIO) and WIO would have resulted in cooling of the sea surface in May 2002 in comparison to 2003. In 2003 the wind speed anomaly over entire WIO and Arabian Sea (AS) was negative, whereas sea surface temperature (SST) anomaly was positive over the same region, which would have resulted in higher moisture availability over these regions. A negative (positive) TWV anomaly over Eastern Arabian Sea (EAS) and positive (negative) anomaly over WIO forms a dipole structure. In the month of June no major difference is seen in all these parameters over the Indian Ocean. In July 2002 the entire WIO and AS was drier by 10–15 mm as compared to 2003. The pentad (5 day) average TWV values shows high (>55 mm) TWV convergence over EAS and Bay of Bengal (BoB) during active periods of 2003, which gives high rainfall over these regions. However, during 2002 although TWV over BoB was >55 mm but it was ∼45–55 mm over EAS during entire July and hence less rainfall. The evaporation has been calculated from the bulk aerodynamic formula using TRMM/TMI geophysical products. It has been seen that the major portion of evaporative moisture flux is coming from southern Indian Ocean (SIO) between 15 and 25° S. Evaporation in June was more over AS and SIO in 2003 when compared to 2002 which may lead to reduce moisture supply in July 2002 and hence less rainfall compared to July 2003.  相似文献   

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