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1.
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   

2.
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   

3.
Spatial-temporal characteristics of temperature variation in China   总被引：5，自引：0，他引：5  
Summary Spatial-temporal characteristics of temperature variations were analyzed from China daily temperature based on 486 stations during the period 1960–2000. The method of hierarchical cluster analysis was used to divide the territory into sub-regional areas with a coherent evolution, both annually and seasonally. Areas numbering 7–9 are chosen to describe the regional features of air temperature in mainland China. All regions in mainland China experienced increasing trends of annual mean temperature. The trend of increasing temperature was about 0.2–0.3 °C/10 yr in northern China and less than 0.1 °C/10 yr in southern China. In the winter season, the increasing trend of temperature was about 0.5–0.7 °C/10 yr in northern China and about 0.2–0.3 °C/10 yr in southern China. The increasing trend of autumn temperature was mainly located in northwestern China and southwestern China including the Tibetan Plateau. In spring, the rising trend of temperature was concentrated in Northeast China and North China while there was a declining temperature trend of −0.13 °C/10 yr in the upper Yangtze River. In summer, the declining trend of temperature was only concentrated in the mid-low valley of the Yangtze and Yellow Rivers while surrounding this valley there were increasing trends in South China, Southwest China, Northwest China, and Northeast China. Rapid changes in temperature in various regions were detected by the multiple timescale t-test method. The year 1969 was a rapid change point from a high temperature to a low temperature along the Yangtze River and South China. In the years 1977–1979, temperature significantly increased from a lower level to a higher level in many places except for regions in North China and the Yangtze River. Another rapid increasing temperature trend was observed in 1987. In the years 1976–1979, a positive rapid change of summer temperature occurred in northwestern China and southwestern China while a decreasing temperature was found between the Yellow River and the Yangtze River. A rapid increase of winter temperature was found for 1977–1979 and 1985–1986 in many places. There were increasing events of extreme temperature in broad areas except in the north part of Northeast China and the north part of the Xinjiang region. In winter, increasing temperature of the climate state and weakening temperature extremes are observed in northern China. In summer, both increasing temperature of the climate state and enhancing temperature extremes were commonly exhibited in northern China. Present address: Linfen Meteorological Office, Linfen 041000, Shanxi Province, China.  相似文献   

4.
Mesoscale aspects of the Urban Heat Island around New York City   总被引：15，自引：1，他引：15  
Summary ?A mesoscale analysis of the Urban Heat Island (UHI) of New York City (NYC) is performed using a mesoscale network of weather stations. In all seasons the UHI switches on rapidly in late afternoon and shuts down even more rapidly shortly after dawn. It averages about 4 °C in summer and autumn and 3 °C in winter and spring. It is largest on nights with clear skies, low relative humidity through much of the troposphere, and weak northwest winds, when it may exceed 8 °C. The synoptic meteorological situation associated with the largest UHI occurs roughly two to three nights after cold front passages. During spring and summer, sea breezes commonly reduce and delay the UHI and displace it about 10 km to the west. Backdoor cold fronts, which occur most frequently in spring and early summer, reduce or even reverse the UHI, as cold air from the water to the northeast keeps NYC colder than the western suburbs. Cases documenting the sensitivity and rapidity of changes of the UHI to changes in parameters such as cloud cover, ceiling, and wind speed and direction are presented. Received August 16, 2001; revised October 6, 2002; accepted November 20, 2002 Published online March 17, 2003  相似文献   

5.
Summary Climatological statistics of extreme temperature events over Kenya are established from the analysis of daily and monthly maximum temperatures for a representative station (Nairobi Dagoretti Corner) over the period 1956–1997. The months of June to August were shown to be the coldest with a mean monthly maximum temperature of less than 22 °C. Seasonal (June to August) mean maximum temperature was 21.5 °C. Using this seasonal mean temperature for the period 1967–1997 delineated 1968 as the coldest year in this series and 1983 as the warmest year. Spectral analysis of the seasonal data, for both the coldest and the warmest years, revealed that the major periods were the quasi-biweekly (10 days) and the Intraseasonal Oscillations (23 days). Secondary peaks occurred at periods of 4–6 and 2.5–3.5 days. A temperature threshold of 16.7 °C during July was used to define cold air outbreaks over Nairobi. This threshold temperature of 16.7 °C was obtained from the mean July maximum temperature (20.9 °C) minus two standard deviations. Notable trends include a decrease in the frequency of station-days, between 1956 and 1997, with temperatures less than 16.7 °C during July. Surface pressure patterns indicate that the origin of the cold air is near latitude 25° S and to the east of mainland South Africa. The cold air near 25° S is advected northwards ahead of the surface pressure ridge. Received July 19, 1999 Revised January 11, 2000  相似文献   

6.
Summary This paper characterizes Mesoscale Convective Systems (MCSs) during 2001 over Iberia and the Balearic Islands and their meteorological settings. Enhanced infrared Meteosat imagery has been used to detect their occurrence over the Western Mediterranean region between June and December 2001 according to satellite-defined criteria based on the MCS physical characteristics. Twelve MCSs have been identified. The results show that the occurrence of 2001 MCSs is limited to the August–October period, with September being the most active period. They tend to develop during the late afternoon or early night, with preferred eastern Iberian coast locations and eastward migrations. A cloud shield area of 50.000 km2 is rarely exceeded. When our results are compared with previous studies, it is possible to assert that though 2001 MCS activity was moderate, the convective season was substantially less prolonged than usual, with shorter MCS life cycles and higher average speeds. The average MCS precipitation rate was 3.3 mm·h−1 but a wide range of values varying from scarce precipitation to intense events of 130 mm·24 h−1 (6 September) were collected. The results suggest that, during 2001, MCS rainfall was the principal source of precipitation in the Mediterranean region during the convective season, but its impact varied according to the location. Synoptic analysis based on NCEP/NCAR reanalysis show that several common precursors could be identified over the Western Mediterranean Sea when the 2001 MCSs occurred: a low-level tongue of moist air and precipitable water (PW) exceeding 25 mm through the southern portion of the Western Mediterranean area, low-level zonal warm advection over 2 °C·24 h−1 towards eastern Iberia, a modest 1000–850 hPa equivalent potential temperature (θe) difference over 20 °C located close to the eastern Iberian coast, a mid level trough (sometimes a cut-off low) over Northern Africa or Southern Spain and high levels geostrophic vorticity advection exceeding 12·10−10 s−2 over eastern Iberia and Northern Africa. Finally, the results suggest that synoptic, orographic and a warm-air advection were the most relevant forcing mechanisms during 2001.  相似文献   

7.
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   

8.
Summary The west coast of the Indian peninsula receives very heavy rainfall during the summer Monsoon (June–September) season with average rainfall over some parts exceeding 250 cm. Heavy rainfall events with rainfall more than 15 cm day−1 at one or more stations along the west coast of India occur frequently and cause considerable damage. A special observational programme, Arabian Sea Monsoon Experiment, was carried out during the monsoon season of 2002 to study these events. The spatial and temporal distributions of intense rainfall events, presented here, were used for the planning of this observational campaign. The present study using daily rainfall data for summer monsoon season of 37 years (1951–1987) shows that the probability of getting intense rainfall is the maximum between 14° N–16° N and near 19° N. The probability of occurrence of these intense rainfall events is high from mid June to mid August, with a dip in early July. It has been believed for a long time that offshore troughs and vortices are responsible for these intense rainfall events. However, analysis of the characteristics of cloud systems associated with the intense rainfall events during 1985–1988 using very high resolution brightness temperature data from INSAT-IB satellite shows that the cloud systems during these events are characterized by large spatial scales and high cloud tops. Further study using daily satellite derived outgoing longwave radiation (OLR) data over a longer period (1975–1998) shows that, most of these events (about 62%) are associated with systems organized on synoptic and larger scales. We find that most of the offshore convective systems responsible for intense rainfall along the west coast of India are linked to the atmospheric conditions over equatorial Indian Ocean.  相似文献   

9.
Summary By analyzing 12-year (1979–1990) 200 hPa wind data from National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis, we demonstrate that the intraseasonal time scale (30–60 days) variability of the Tropical Easterly Jet (TEJ) reported in individual case studies occurs during most years. In the entrance region (east of ∼70° E), axis of the TEJ at 200 hPa is found along the near equatorial latitudes during monsoon onset/monsoon revivals and propagates northward as the monsoon advances over India. This axis is found along ∼5° N and ∼15° N during active monsoon and break monsoon conditions respectively. Examination of the European Centre for Medium Range Weather Forecasts reanalysis wind data also confirms the northward propagation of the TEJ on intraseasonal time scales. During the intraseasonal northward propagations, axis of the TEJ is found about 10°–15° latitudes south of the well-known intraseasonally northward propagating monsoon convective belts. Because of this 10°–15° displacement, axis of the TEJ arrives over a location about two weeks after the arrival of the monsoon convection. Systematic shifting of the locations by convection, low level monsoon flow and TEJ in a collective way during different phases of the monsoon suggests that they all may be related.  相似文献   

10.
Summary The authors perform an exploratory analysis on the effect of the timing of the stratospheric vortex breakup in the occurrence of cut-off low systems (COLs) in the Northern Hemisphere. The first multidecadal Northern Hemisphere COLs database (Nieto et al, 2005) covering a 41 year-long period (1958–1998) was used in the analysis. The dates of stratospheric vortex breakup were obtained using two different approaches recently purposed in literature based in potential vorticity and zonal winds. An analysis of differences of COLs occurrences for the five earlier (later) breakup years showed that, at latitudes lower than 45° N, COLs are more frequent for earlier vortex years during the following spring and summer. The monthly analysis showed that, in general, the significant differences start in May lasting until September, being especially relevant for the European sector, the area with the highest rates of COLs occurrence in the Northern Hemisphere.  相似文献   

11.
Summary ?This paper presents an objective analysis of the structure of daily rainfall variability over the South American/South Atlantic region (15°–60° W and 0°–40° S) during individual austral summer months of November to March. From EOF analysis of satellite derived daily rainfall we find that the leading mode of variability is represented by a highly coherent meridional dipole structure, organised into 2 extensive bands, oriented northwest to southeast across the continent and Atlantic Ocean. We argue that this dipole structure represents variability in the meridional position of the South Atlantic Convergence Zone (SACZ). During early and later summer, in the positive (negative) phase of the dipole, enhanced (suppressed) rainfall over eastern tropical Brazil links with that over the subtropical and extra-tropical Atlantic and is associated with suppressed (enhanced) rainfall over the sub-tropical plains and adjacent Atlantic Ocean. This structure is indicative of interaction between the tropical, subtropical and temperate zones. Composite fields from NCEP reanalysis products (associated with the major positive and negative events) show that in early and late summer the position of the SACZ is associated with variability in: (a) the midlatitude wave structure, (b) the position of the continental low, and (c) the zonal position of the South Atlantic Subtropical High. Harmonic analysis of the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the rainfall dipole structure are associated primarily with variability in zonal wave 4. There is evidence of a wave train extending throughout the midlatitudes from the western Pacific into the SACZ region. During positive (negative) events the largest anomalous moisture advection occurs within westerlies (easterlies) primarily from Amazonia (the South Atlantic). In both phases a convergent poleward flow results along the leading edge of the low-level trough extending from the tropics into temperate latitudes. High summer events differ from those in early and late summer in that the rainfall dipole is primarily associated with variability in the phase of zonal wave 3, and that tropical-temperate link is not clearly evident in positive events. Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002  相似文献   

12.
Summary In order to explore the spatial and temporal variations of 500 hPa flow patterns and their relationship with the climate of Iran, monthly mean geopotential heights for the region 0° E to 70° E and 20° N to 50° N, at 5 degree resolution, were analysed. The study period covered the winter months October to March during the period 1961–90. The monthly height of the 500 hPa level was averaged along each meridian from 25° N to 45° N. The height of the mean monthly pressure pattern was mapped against the study years. The results showed that the characteristics of the 500 hPa flow pattern varied over monthly and annual time scales. Principal Component Analysis, with S-mode and Varimax rotation, was also used to reduce the gridded data to 5 (6 in October) significant factors. The factor scores for each month were then correlated with monthly Z-scores of precipitation and temperature anomalies over Iran. The results showed that troughs and ridges located close to Iran had more influence on the climate of Iran. Two troughs were identified and named the Caspian and Syrian troughs. Received April 12, 2001 Revised July 24, 2001  相似文献   

13.
 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   

14.
Summary  The relationship between European surface temperature and winds over the eastern North Atlantic are investigated for the years 1988 to 1997. Daily Special Sensor Microwave Imager SSM/I observations are used to evaluate a monthly surface wind index that quantifies the influence of southwesterly flow. Our wind index and the monthly-mean surface-air temperatures in late winter and early spring over France and northern-latitude Europe are highly correlated. In February, the year-to-year increases/decreases match every year for France (correlation of 0.82 with the Index); and every year with just one exception for Europe (correlation with the Index of 0.76 for a longitudinal strip through Europe 45–50° N, and 0.73 for the 50–60° N strip). In March, the increases/decreases of the wind Index and of the temperatures for France also match, but the correlation with the Index is lower, 0.65. The high correlation between our Index and the large interannual fluctuations in the monthly temperature in late winter and early spring indicate that the onset of the spring conditions in Europe is significantly influenced by the wind patterns over the eastern North Atlantic. Coinciding with the fluctuations from warm-Europe/high-Index winter to the opposite conditions, we observe “seesaw” effects, fluctuations over the North Atlantic, in opposite directions in the east (25–5° W), and the west (65–45° W). In the low-Index years we find that: (a) the surface-air temperatures in the west are appreciably higher than in the east (but slightly lower in the high-Index year), and (b) the difference between the 500 mb meridional wind in the west and that in the east is positive and large, exceeding 10 m s−1 (but it becomes negative and small in the high-Index years). The “seesaw” effects suggest that a positive feedback exits between these cross-Atlantic temperature differences and the surface winds. Received August 7, 1998 Revised April 23, 1999  相似文献   

15.
 This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall. It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific. This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations: anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level pressure over West Africa. Received: 14 April 1998 / Accepted: 24 December 1998  相似文献   

16.
A 680-year ring-width chronology of Sabina przewalskii Kom. was developed for Wulan area of northeastern Qinghai-Tibetan Plateau, China. Response function and correlation analyses showed that spring precipitation (May–June) is the critical limiting factor for tree-ring growth, and temperature in prior November may also play a role in affecting tree-ring growth. Excessive spring precipitation occurred during AD 1380s–1390s, 1410s–1420s, 1520s–1560s and 1938 to present. Dry springs occurred during AD 1430s–1510s, 1640s–1730s and 1780s–1890s most of which generally coincided with cold intervals of the Little Ice Age (LIA) on the plateau, suggesting that the LIA climate on the northeastern Qinghai-Tibetan Plateau might be characterized by three episodes of dry spring and cold autumn. The relatively driest spring and probably coldest autumn occurred in AD 1710s–1720s, 1787–1797, 1815–1824, 1869–1879 and 1891–1895. The extreme drought in AD 1787–1797 might result from little monsoon precipitation due to the failure of Asian monsoon in this period. The tree-ring data produced in this study contribute to the spatial expansion of proxy climate records for the Qinghai-Tibetan Plateau.  相似文献   

17.
 Monthly sea surface temperature anomalies (SSTA) at near-global scale (60 °N–40 °S) and May to October rainfall amounts in West Africa (16 °N–5 °N; 16 °W–16 °E) are first used to investigate the seasonal and interannual evolutions of their relationship. It is shown that West African rainfall variability is associated with two types of oceanic changes: (1) a large-scale evolution involving the two largest SSTA leading eigenmodes (16% of the total variance with stronger loadings in the equatorial and southern oceans) related to the long-term (multiannual) component of rainfall variability mainly expressed in the Sudan–Sahel region; and (2) a regional and seasonally coupled evolution of the meridional thermal gradient in the tropical Atlantic due to the linear combination of the two largest SSTA modes in the Atlantic (11% with strong inverse loadings over the northern and southern tropics) which is associated with the interannual and quasi-decadal components of regional rainfall in West Africa. Linear regression and discriminant analyses provide evidence that the main July–September rainfall anomalies in Sudan–Sahel can be detected with rather good skills using the leading (April–June) or synchronous (July–September) values of the four main oceanic modes. In particular, the driest conditions over Sahel, more marked since the beginning of the 1970s, are specifically linked to the warm phases of the two global modes and to cold/warm anomalies in the northern/southern tropical Atlantic. Idealized but realistic SSTA patterns, obtained from some basic linear combinations of the four main oceanic modes appear sufficient to generate quickly (from mid-July to the end of August) significant West African rainfall anomalies in model experiments, consistent with the statistical results. The recent negative impact on West African rainfall exerted by the global oceanic forcing is primarily due to the generation of subsidence anomalies in the mid-troposphere over West Africa. When an idealized north to south SSTA gradient is added in the tropical Atlantic, strong north to south height gradients in the middle levels appear. These limit the northward excursion of the rainbelt in West Africa: the Sahelian area experiences drier conditions due to the additive effect (subsidence anomalies+latitudinal blocking) while over the Guinea regions wet conditions do not significantly increase, since the subsidence anomalies and the blocking effect act here in opposite ways. Received: 26 June 1997 / Accepted: 3 October 1997  相似文献   

18.
We used neural network models to seasonally forecast the tropical Pacific sea surface temperature anomalies (SSTA) in the Ni?o 3.4 region (6 °S–6 °N, 120 °W–170 °W). The inputs to the neural networks (i.e., the predictors) were the first seven wind stress empirical orthogonal function (EOF) modes of the tropical Pacific (20 °S–20 °N, 120 °E–70 °W) for four seasons and the Ni?o 3.4 SSTA itself for the final season. The period of 1952–1981 was used for training the neural network models, and the period 1982–1992 for forecast validation. At 6-month lead time, neural networks attained forecast skills comparable to the other El Ni?o-Southern Oscillation (ENSO) models. Our results suggested that neural network models were viable for ENSO forecasting even at longer lead times of 9 to 12 months. We hypothesized that at these longer leads, the underlying relationship between the wind stress and Ni?o 3.4 SSTA became increasingly nonlinear. The neural network results were interpreted in light of current theories, e.g., the role of the “off-equatorial” Rossby waves in triggering the onset of an ENSO event and the delayed-oscillator theory in the development and termination of an ENSO event. Received: 31 October 1995 / Accepted: 25 July 1996  相似文献   

19.
Summary  The year 1997 witnessed one of the most severe El-Ni?o events of the century. However, the All-India Summer Monsoon Rainfall (AISMR) was 102% of its long period average. In view of recent studies (Tourre and White, 1995, 1997) of detection of ENSO signal over Indian Ocean, the Sea-Surface Temperature (SST) variation over Indian Ocean (20° N–10° S/50° E–100° E), concurrent to El-Ni?o event of 1997 is examined. It is observed that during the developing, mature and decaying stages of El-Ni?o, the North Indian Ocean was abnormally warm. This anomalous warming may be one of the factors responsible for anomalous precipitation over India during October to December of 1997. Received August 24, 1999/Revised February 15, 2000  相似文献   

20.
Summary This paper is to promote a further understanding of the interdecadal mode of the South Pacific. With this focus, we will specifically aim at better understanding the difference between interannual and interdecadal SSTA modes over South Pacific. We define the difference of the normalization area-averaged SSTA in the southern extratropical Pacific (160° W–110° W, 40° S–25° S) and the south subpolar Pacific (150° W–110° W, 60° S–45° S) as the South Pacific interdecadal index (I spd). It is found that the interannual mode is more coherent than the interdecadal mode in the central and eastern tropical Pacific, and the interdecadal mode is significant only during boreal winter (DJF). The interdecadal variation of SSTA firstly occurring in the extratropic South Pacific propagates to the western boundary of the South Pacific, then moves northeast to cross the equator, and finally reaches the central tropic Pacific. It takes about 8 years to propagate from southeast subtropical Pacific to the north hemisphere. The previous studies have suggested the mechanism of waves in the subsurface in the South Pacific. Our study also highlights the Rossby waves play important roles in linkage between the extratropics-tropics South Pacific SSTA on interdecadal time scales. Moreover, the paper shows that the interdecadal variability originated in the extrotropic southeast Pacific is mainly induced by interannual variability in the tropic Pacific.  相似文献   

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