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1.
S. D. Dahale N. Panchawagh S. V. Singh E. R. Ranatunge M. Brikshavana 《Theoretical and Applied Climatology》1994,49(1):27-39
Summary Daily rainfall observations during the principal rainy seasons over a large part of Tropical Asia and the equatorial Pacific are analysed for persistence by fitting Markov chains of various order. Daily rainfall data of 98 stations from India, Sri Lanka and Thailand falling in the monsoonal regime and 9 stations in the non-monsoonal regime of the equatorial Pacific are examined.The appropriate order of Markov chain is determined by analyzing wet and dry spell length characteristics and by applying the Schwarz Baysian Criterion to the arbitrary sequences of 5-day length. Markov chains of order greater than 1 are found to characterize the persistence in rainfall over India and to some extent over wet zones of Sri Lanka and central equatorial Pacific. Simple Markov chains are suggested for Thailand, the dry zone of Sri Lanka and the stations of central equatorial Pacific lying some what away from the equator.With 5 Figures 相似文献
2.
B. A. Malmgren R. Hullugalla G. Lindeberg Y. Inoue Y. Hayashi T. Mikami 《Theoretical and Applied Climatology》2007,89(1-2):115-125
Summary In this study, we have analyzed the temporal oscillations of precipitation in meso-scale zones of Sri Lanka to examine potential
existence of periodic oscillatory behavior in rainfall. Only a few statistically significant cycles were identified: a 3.5-year
cycle in most of central Sri Lanka during the January–March rainfall regime and a cycle of the same length in southwestern
Sri Lanka during the October regime. A 2.1-year cycle marks the northeastern parts of Sri Lanka during the December/April
contrast rainfall regime. This cycle is shown to be strongly related to Quasi-Biennial Oscillation. October and November rainfall
are found to be coupled with ENSO fluctuations, and on average, more than 900 mm more rainfall is observed per month over
all stations during El Ni?o than during La Ni?a years. Analysis of relationships between the observed meso-scale rainfall
regions and the Sea Surface Temperatures (SSTs) in the Indian Ocean north of the Equator showed that the northern Indian Ocean
can be divided into three particular regions based on similarity in the SST fluctuations: (a) a region with cool upwelling
water, (b) non upwelling water, and (c) the Indian Ocean Warm Pool. We found that there are no statistically significant relationships
between the observed SST regions in the Indian Ocean and the meso-scale precipitation patterns in Sri Lanka. 相似文献
3.
Summary The spatial organization of Monsoon rainfall over Sri Lanka is examined using Orthogonal Factor Analysis (OFA) on long-term mean monthly rainfall data. Three types of orthogonal structure of Monsoon regime in Sri Lanka have been identified. Interpretation of orthogonal factor scores revealed that a large amount of rainfall occurs from March to October in the southwestern parts of Sri Lanka, from December to February in the eastern parts, and in November in the northern and mid-western parts which are all represented by high positive factor scores. Orthogonal factor scores for the first three factors account for 93.6% of the total variance of mean monthly rainfall and clearly indicate that the southeast and northwest parts of the country with lowest rainfall, resulting from lack of Monsoons, are represented by negative factor scores. The three orthogonal factors identified different rainfall maxima in different time periods and, additionally, significant spatial differences between regions. Seasonal changes in the Monsoon wind system, ITCZ weather phenomena, and topography were the main factors which influence the spatial structure of Monsoon rainfall over Sri Lanka.With 4 Figures 相似文献
4.
This study applies the Ricardian technique to estimate the effect of climate change on the smallholder agriculture sector
in Sri Lanka. The main contribution of the paper is the use of household-level data to analyze long-term climate impacts on
farm profitability. Household-level data allows us to control for a host of factors such as human and physical capital available
to farmers as well as adaptation mechanisms at the farm level. We find that non-climate variables explain about half the variation
in net revenues. However, our results suggest that climate change will have a significant impact on smallholder profitability.
In particular, reductions in precipitation during key agricultural months can be devastating. At the national level, a change
in net revenues of between −23% and +22% is likely depending on the climate change scenario simulated. These impacts will
vary considerably across geographic areas from losses of 67% to gains that more than double current net revenues. The largest
adverse impacts are anticipated in the dry zones of the North Central region and the dry zones of the South Eastern regions
of Sri Lanka. On the other hand, the intermediate and wet zones are likely to benefit, mostly due to the predicted increase
in rainfall. 相似文献
5.
J. Vialard P. Terray J.-P. Duvel R. S. Nanjundiah S. S. C. Shenoi D. Shankar 《Climate Dynamics》2011,37(3-4):493-507
Most of the annual rainfall over India occurs during the Southwest (June?CSeptember) and Northeast (October?CDecember) monsoon periods. In March 2008, however, Southern peninsular India and Sri Lanka received the largest rainfall anomaly on record since 1979, with amplitude comparable to summer-monsoon interannual anomalies. This anomalous rainfall appeared to be modulated at intraseasonal timescale by the Madden Julian Oscillation, and was synchronous with a decaying La Ni?a event in the Pacific Ocean. Was this a coincidence or indicative of a teleconnection pattern? In this paper, we explore factors controlling rainfall over southern India and Sri Lanka between January and April, i.e. outside of the southwest and northeast monsoons. This period accounts for 20% of annual precipitation over Sri Lanka and 10% over the southern Indian states of Kerala and Tamil Nadu. Interannual variability is strong (about 40% of the January?CApril climatology). Intraseasonal rainfall anomalies over southern India and Sri Lanka are significantly associated with equatorial eastward propagation, characteristic of the Madden Julian Oscillation. At the interannual timescale, we find a clear connection with El Ni?o-Southern Oscillation (ENSO); with El Ni?os being associated with decreased rainfall (correlation of ?0.46 significant at the 98% level). There is also a significant link with local SST anomalies over the Indian Ocean, and in particular with the inter-hemispheric sea surface temperature (SST) gradient over the Indian Ocean (with colder SST south of the equator being conducive to more rainfall, correlation of 0.55 significant at the 99% level). La Ni?as/cold SSTs south of the equator tend to have a larger impact than El Ni?os. We discuss two possible mechanisms that could explain these statistical relationships: (1) subsidence over southern India remotely forced by Pacific SST anomalies; (2) impact of ENSO-forced regional Indian Ocean SST anomalies on convection. However, the length of the observational record does not allow distinguishing between these two mechanisms in a statistically significant manner. 相似文献
6.
Summary The rainfall patterns and regions of the monsoonal island of Sri Lanka have been investigated. On the basis of quality-controlled monthly rainfall data from 110 gauges for the period 1931–60, three major homogeneous rainfall regions were derived based on a variety of rainfall parameters using cluster and discriminant analysis. For each of these regions, identified as the South-west Monsoon (SWM), the Northeast Monsoon (NEM) and the Inter-monsoon II (IMII) regions, multiple regression methods were used to determine the geographical factors influencing the spatial patterns of rainfall. For the NEM and IMII regions, the level of explanation as indicated by the coefficients of multiple determination were reasonable with values generally above 0.60. For the SWM region, the level of explanation was much lower at about 0.20. Examination of the residuals of the regression equations demonstrated the significance of local factors in influencing the patterns of rainfall distribution in Sri Lanka.With 6 Figures 相似文献
7.
Intraseasonal oscillation of the southwest monsoon over Sri Lanka and evaluation of its subseasonal forecast skill 
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下载免费PDF全文 斯里兰卡的雨季发生于5-9月间,主要受西南季风的控制.本文发现该地区的西南季风降水存在很强的次季节变率,主导周期为10-35天.降水的季节内变化与西传的异常气旋有关.进一步,利用S2S比较计划中欧洲中心的数值预报模式(ECMWF)提供的回报试验数据,评估了当今动力模式对斯里兰卡西南季风次季节变化的预报技巧.结果显示,对季风指数的预测技巧超过30天,而对降水指数的预测技巧大约两周,且模式的预报技巧具有明显的年际差异.分析表明,能否正确模拟出大尺度环流对热带对流的响应是影响斯里兰卡降水预测的重要因子. 相似文献
8.
On the recent strengthening of the relationship between ENSO and northeast monsoon rainfall over South Asia 总被引：1，自引：1，他引：1
The southeastern parts of India and Sri Lanka receive substantial rainfall from the northeast monsoon (NEM) during October through December. The interannual variability in NEM rainfall is known to be significantly influenced by the El-Niño/Southern Oscillation (ENSO). Unlike the southwest monsoon (SWM), the NEM rainfall is enhanced during the warm ENSO events, and vice versa. In the context of the recent weakening of the inverse relationship between Southwest Monsoon (SWM) and ENSO, we examine the secular variations in the positive relationship between ENSO and NEM rainfall over South Asia, showing that their relationship has strengthened over the recent years. Based on the analysis of GISST, IMD/CRU precipitation and NCEP/NCAR reanalysis data, we suggest that this secular variation of the relationship is due to epochal changes in the tropospheric circulation associated with ENSO over the region. 相似文献
9.
本文主要研究了1979—2016期间斯里兰卡在西南季风期间降水的年代际、年际变化以及与印度洋海温的联系.首先用经验正交的方法分析了斯里兰卡以及周边地区降水的时空分布,发现前两个模态能够解释超过70%的方差.其中第一模态为均一模态,且其PC1以及斯里兰卡7 a滑动平均降水序列都有年代际变化,降水异常在2000年前后异常偏多和偏少.通过合成分析发现2000年之后降水的异常减少与热带西部、中部印度洋的暖海温异常有关.暖海温异常通过调整经向环流引起了斯里兰卡上空的下沉运动,抑制了降水.在第二模态中,负的信号出现在斯里兰卡大部分地区,只有在斯里兰卡北部海角很小地区出现了正的信号.PC2表现出了年际变化,且与热带东南印度洋海温异常有显著的关系.通过Gill-Matsuno响应,热带东南印度洋海温异常造成热带北印度洋上空的气旋性环流异常,引起了水汽的辐合,从而利于降水. 相似文献
10.
Summary Based on the regional division of another paper [39], the rainfall variations of Sri Lanka have been investigated for the respective regions by power spectrum analysis and filtering methods, making use of data for the period from 1881 to 1980. The 3–4 year periodicity was observed over the entire island, but other cycles differ from region to region. The 13–16 months oscillation arises in Regions A, D and E, which roughly correspond to the Wet Zone and Dry Zone. The 10 and 2 year oscillations emerge in Regions A, B and C, where the southwest monsoon dominates the fluctuation patterns. In particular, it was confirmed that the quasi-biennial oscillation is not only in Sri Lanka, but also in other low latitude countries. The quasi-five year oscillation is noticed in Regions D and E, where the northeast monsoon influences on the fluctuation patterns. Irregularities in amplitude and in phase changes were noticed in their longer period fluctuation.
With 4 Figures 相似文献
Variationen des Niederschlags in Sri Lanka. Teil 2: Regionale Fluktuationen
Zusammenfassung Aufgrund einer früher vorgenommenen regionalen Einteilung [39] wurden die Niederschlagsvariationen in Sri Lanka für die einzelnen Regionen mit Spektralanalyse und Filterungsmethoden unter Verwendung von Beobachtungsdaten aus der Periode 1881–1980 untersucht. Die 3- bis 4jährige Periodizität wurde auf der ganzen Insel festgestellt; aber andere Zyklen unterscheiden sich von Region zu Region. Eine 13- bis 16monatige Oszillation zeigt sich in den Regionen A, D und E, die ungefähr der Feuchtzone und der Trockenzone entsprechen. 10- bis 2jährige Oszillationen treten in den Regionen A, B und C auf, wo der Südwestmonsun die Fluktuationsformen beherrscht. Im besonderen wurde festgestellt, daß die quasi-zweijährige Oszillation nicht nur in Sri Lanka, sondern auch in anderen Ländern niedriger Breiten vorkommt. Eine quasifünfjährige Oszillation wurde in den Regionen D und E festgestellt, wo der Nordostmonsun die Fluktuationsformen beeinflußt. Unregelmäßigkeiten in der Amplitude und in Phasenänderungen wurden in ihrer längerperiodischen Fluktuation festgestellt.
With 4 Figures 相似文献
11.
北半球夏季,北印度洋环流主要受到西南季风流控制,将热带印度洋水体从西向东进行跨海盆输运,然而在斯里兰卡南部沿岸存在一支与西南季风流方向相反的西向沿岸流,即南斯里兰卡沿岸流（SSLCC）.本文主要利用ECCO2资料进行南斯里兰卡沿岸流的动力学特征研究.结果表明,SSLCC的形成和孟加拉湾局地环流密切相关.当斯里兰卡穹顶区（SLD）环流偏强时,斯里兰卡南部形成局地气旋式涡旋,斯里兰卡东部沿岸流在SLD西部向南流动,随着气旋式涡旋北部转向西流形成强的SSLCC.相比之下,SLD较弱时,沿岸流仅存在斯里兰卡东部沿岸,斯里兰卡东部沿岸流无法向西转向,SSLCC和西南季风流一起向东流动,其可能的主要原因是局地风应力对SLD产生的强度影响.研究还表明,SLD强度对SSLCC流向和强度有着重要影响. 相似文献
12.
L. Zubair 《Theoretical and Applied Climatology》2002,71(1-2):119-127
Summary Hourly measurements of surface winds at Sita Eliya (6° 58′ N, 80° 46′ E, 1860 m a.s.l.) located atop the North-South mountain ridge in Sri Lanka were analyzed to investigate the diurnal and seasonal variation
in the wind climate. Surface winds are dominated by the monsoon regimes, with Northeasterlies from November to January, Southwesterlies
from February to May and Westerlies and Northwesterlies from June to October. Through most of the year, the average wind speed
is around 6–8 m/s. However from June to August, it is around 10–14 m/s. Wind in June is gusty due the location of the low-level
Easterly jet over Sri Lanka. The wind undergoes a reversal in both zonal and meridional directions in March and November coincident
with the migration of the Inter-Tropical Convergence Zone. Notwithstanding the period from May to September being designated
as the Southwest monsoon, the wind is from West, South-West-West and North-North-West. During the Southwest monsoon, wind
speed during the night is nearly as high as that during the day. This anomalous diurnal variation in wind speeds may be related
to orographic influences. The high wind speeds at Sita Eliya, in conjunction with the moderate diurnal and seasonal variability
in wind speed, is suitable for wind-energy generation.
Received January 2, 2001 Revised May 26, 2001 相似文献
13.
Synoptic classification is a methodology that represents diverse atmospheric variables and allows researchers to relate large-scale atmospheric circulation patterns to regional- and small-scale terrestrial processes. Synoptic classification has often been applied to questions concerning the surface environment. However, full applicability has been under-utilized to date, especially in disciplines such as hydroclimatology, which are intimately linked to atmospheric inputs. This paper aims to (1) outline the development of a daily synoptic calendar for the Mid-Atlantic (USA), (2) define seasonal synoptic patterns occurring in the region, and (3) provide hydroclimatological examples whereby the cascading response of precipitation characteristics, soil moisture, and streamflow are explained by synoptic classification. Together, achievement of these objectives serves as a guide for development and use of a synoptic calendar for hydroclimatological studies. In total 22 unique synoptic types were identified, derived from a combination of 12 types occurring in the winter (DJF), 13 in spring (MAM), 9 in summer (JJA), and 11 in autumn (SON). This includes six low pressure systems, four high pressure systems, one cold front, three north/northwest flow regimes, three south/southwest flow regimes, and five weakly defined regimes. Pairwise comparisons indicated that 84.3 % had significantly different rainfall magnitudes, 86.4 % had different rainfall durations, and 84.7 % had different rainfall intensities. The largest precipitation-producing classifications were not restricted to low pressure systems, but rather to patterns with access to moisture sources from the Atlantic Ocean and easterly (on-shore) winds, which transport moisture inland. These same classifications resulted in comparable rates of soil moisture recharge and streamflow discharge, illustrating the applicability of synoptic classification for a range of hydroclimatological research objectives. 相似文献
14.
Anthropogenic aerosols in the lower troposphere increase the absorption and scattering of solar radiation by air and clouds,
causing a warmer atmosphere and a cooler surface. It is suspected that these effects contribute to slow down the hydrological
cycle. We conducted a series of numerical experiments using a limited area atmospheric model to understand the impacts of
aerosol radiative forcing on the rainfall process. Experiments with different radiative conditions under an idealized setting
revealed that increasing atmospheric forcing and decreasing surface forcing of radiation causes reductions in rainfall. There
was no relationship of top of the atmosphere forcing to the rainfall yield. The model was then used to simulate a domain covering
southern part of Sri Lanka, over for the period from November 2002 to July 2003. For a given radiative forcing, instances
with lower rainfall yields showed larger fractional reductions in rainfall. The trends in seasonal rainfall observed over
the site in past 30 years in a different study confirms this finding. We conclude that the negative impact of increase of
anthropogenic aerosols on rainfall would be more severe on regions and seasons with lower rainfall yields. The consequences
of this problem on the industries that critically depend on well-distributed rainfall like non-irrigated agriculture and on
the general livelihood of societies in low-rain areas can be serious. 相似文献
15.
Drawing attention to the production of vulnerability across scales in Sri Lanka, we contribute to knowledge of why certain people and social groups are vulnerable. We build our contribution on the theoretical application of ‘situated adaptation’. A situated analytical approach identifies, assesses, and responds to the everyday realities and politics of those living in climate changed environments. It highlights uneven geographies of vulnerability and opportunity, while identifying new imaginations and possibilities for transformative action that counter the production of vulnerability. We illustrate the utility of ‘situated adaptation’ by filling an empirical gap relating to experiences of political-economic and environmental change in Sri Lanka’s Dry Zone. We detail situated experiences by drawing on field research in the Anuradhapura District, revealing how the lives and livelihoods of farmer participants are structured by a productivity-vulnerability paradox. We demonstrate how a prevalent adaptation-development paradigm (whereby development and adaptation programs co-exist in theory and practice) is unable to address the structural drivers of vulnerability in Sri Lanka’s Dry Zone. A situated adaptation approach both explains why this is the case and highlights opportunities for alternative transformative actions, potentially identifying a more democratic and egalitarian politics of co-determining socionatural change. 相似文献
16.
A. K. Guèye Serge Janicot A. Niang S. Sawadogo Benjamin Sultan A. Diongue-Niang S. Thiria 《Climate Dynamics》2012,39(9-10):2251-2272
The aim of this work is to define over the period 1979–2002 the main synoptic weather regimes relevant for understanding the daily variability of rainfall during the summer monsoon season over Senegal. “Interannual” synoptic weather regimes are defined by removing the influence of the mean 1979–2002 seasonal cycle. This is different from Part I where the seasonal evolution of each year was removed, then removing also the contribution of interannual variability. As in Part I, the self-organizing maps approach, a clustering methodology based on non-linear artificial neural network, is combined with a hierarchical ascendant classification to compute these regimes. Nine weather regimes are identified using the mean sea level pressure and 850?hPa wind field as variables. The composite circulation patterns of all these nine weather regimes are very consistent with the associated anomaly patterns of precipitable water, mid-troposphere vertical velocity and rainfall. They are also consistent with the distribution of rainfall extremes. These regimes have been then gathered into different groups. A first group of four regimes is included in an inner circuit and is characterized by a modulation of the semi-permanent trough located along the western coast of West Africa and an opposite modulation on the east. This circuit is important because it associates the two wettest and highly persistent weather regimes over Senegal with the driest and the most persistent one. One derivation of this circuit is highlighted, including the two driest regimes and the most persistent one, what can provide important dry sequences occurrence. An exit of this circuit is characterised by a filling of the Saharan heat low. An entry into the main circuit includes a southward location of the Saharan heat low followed by its deepening. The last weather regime is isolated from the other ones and it has no significant impact on Senegal. It is present in June and September, and missing in July and August, meaning that this is a weather regime more specific of the intermediate seasons than the summer. It is included in a large-scale pattern covering the northern latitudes of Europe. The correspondence between these “interannual” synoptic weather regimes and the “pure” synoptic regimes defined in Part I has been established. By selecting a high statistical significance level for these correspondences, each of five out of nine “interannual” weather regimes has a close correspondence with one “pure” synoptic weather regime, one out of them have links with two “pure” regimes, and the last three regimes have no significant correspondence in terms of “pure” regimes. However when considering more moderate links, two out of these three regimes show a connection with a “pure” regime, and the last one remains isolated. The ensemble of the weather regimes occurrences can explain a significant part of interannual variability of summer rainfall amount over Senegal, especially linked to the driest and the wettest weather regimes occurrences. It is also shown that Senegal rainfall state is very sensitive to a small displacement or deformation of the weather regime patterns. 相似文献
17.
Hydrography and Circulation in the Eastern Tropical Indian Ocean during April-May 2011 总被引：2，自引：0，他引：2 下载免费PDF全文
下载免费PDF全文 The distribution of hydrography and circulation in the eastern tropical Indian Ocean(ETIO) during April-May 2011 were analyzed using cruise observations,satellite observations,and historic hydrographic data.It was observed that warm water(28℃) occupies the upper 50-m layer in the ETIO.Low-salinity surface water was observed at the mouth of the Bay of Bengal(BOB),which further extends to the Arabian Sea and off Sumatra via the Sri Lanka coast and the eastern bay mouth.Arabian Sea high-salinity water(ASHSW) is carried eastward along the equator to around 90°E by the equatorial undercurrent(EUC).It also runs south of Sri Lanka(north to 3°N) and in the western bay mouth(west to 87°E) but is much shallower than its counterpart at the equator.It is suggested to be the residual of the ASHSW,which intrudes into the BOB during the preceding southwest monsoon.Our results also show that,in the south of Sri Lanka,just below this subsurface high-salinity water,very-low-salinity water(about 34.8) occurs at depths of 100-200 m.Further analysis suggests that this low-salinity water comes from the BOB. 相似文献
18.
Increasing trend of northeast monsoon rainfall over the equatorial Indian Ocean and peninsular India 总被引：2，自引：1，他引：1
Satya Prakash Mahesh C. V. Sathiyamoorthy R. M. Gairola 《Theoretical and Applied Climatology》2013,112(1-2):185-191
Peninsular India and Sri Lanka receive major part of their annual rainfall during the northeast monsoon season (October–December). The long-term trend in the northeast monsoon rainfall over the Indian Ocean and peninsular India is examined in the vicinity of global warming scenario using the Global Precipitation Climatology Project (GPCP) dataset available for the period 1979–2010. The result shows a significant increasing trend in rainfall rate of about 0.5 mm day?1 decade?1 over a large region bounded by 10 °S–10 °N and 55 °E–100 °E. The interannual variability of seasonal rainfall rate over peninsular India using conventional rain gauge data is also investigated in conjunction to the Indian Ocean dipole. The homogeneous rain gauge data developed by Indian Institute of Tropical Meteorology over peninsular India also exhibit the considerable upward rainfall trend of about 0.4 mm day?1 decade?1 during this period. The associated outgoing longwave radiation shows coherent decrease in the order of 2 W?m?2 decade?1 over the rainfall increase region. 相似文献
19.
Summary The variability and extreme wet anomalies in the Greater Horn of Africa (GHA) climate are investigated based on a multi-year
National Center for Atmospheric Research (NCAR) AGCM ensemble data. While the GCM ensemble average reproduces realistic inter-annual
variability of rainfall pattern over the GHA sub-region compared to observations, there is a distinct northward shift in the
simulated regions of rainfall maxima throughout the season. However, in agreement with observations and many previous studies,
the inter-annual variability derived from leading mode of EOF analysis is dominated by ENSO-related fluctuations. On the other
hand, the spatial pattern corresponding to the second mode (EOF2) exhibits a unique dipole rainfall anomaly pattern (wet/dry
conditions) over the northern/southern halves of our domain during all the three months of the short rains season. When the
3–10 year periodicity is filtered out from the 40-year EOF2 time series of the ensemble mean data, three distinct quasi-decadal
regimes in the rainfall anomalies is exhibited for both monthly and seasonal mean data.
It is also evident from our results that a combination of anomalous surface and mid-tropospheric flow from northwestern and
eastern Atlantic Ocean and easterly flow from the Indian Ocean played a significant role in setting up the non-ENSO related
1961 floods. Coversely, during the ENSO-related 1997 floods, the mid-troposheric flow was characterized by anomalous westerly
flow originating from the Congo rainforest that converged with the flow from Indian Ocean along the East Africa coast and
over eastern/northeastern Kenya. The anomalous moisture flux convergence/divergence in both the ensemble and NCEP reanalysis
is also consistent with the mid-trospheric flow anomalies that are associated with the two wet events. 相似文献
20.
Cortesi Nicola Torralba Ver&#;nica Lled&#; Lloren&#; Manrique-Su&#;&#;n Andrea Gonzalez-Reviriego Nube Soret Albert Doblas-Reyes Francisco J. 《Climate Dynamics》2021,56(11):3933-3964
It is often assumed that weather regimes adequately characterize atmospheric circulation variability. However, regime classifications spanning many months and with a low number of regimes may not satisfy this assumption. The first aim of this study is to test such hypothesis for the Euro-Atlantic region. The second one is to extend the assessment of sub-seasonal forecast skill in predicting the frequencies of occurrence of the regimes beyond the winter season. Two regime classifications of four regimes each were obtained from sea level pressure anomalies clustered from October to March and from April to September respectively. Their spatial patterns were compared with those representing the annual cycle. Results highlight that the two regime classifications are able to reproduce most part of the patterns of the annual cycle, except during the transition weeks between the two periods, when patterns of the annual cycle resembling Atlantic Low regime are not also observed in any of the two classifications. Forecast skill of Atlantic Low was found to be similar to that of NAO+, the regime replacing Atlantic Low in the two classifications. Thus, although clustering yearly circulation data in two periods of 6 months each introduces a few deviations from the annual cycle of the regime patterns, it does not negatively affect sub-seasonal forecast skill. Beyond the winter season and the first ten forecast days, sub-seasonal forecasts of ECMWF are still able to achieve weekly frequency correlations of r = 0.5 for some regimes and start dates, including summer ones. ECMWF forecasts beat climatological forecasts in case of long-lasting regime events, and when measured by the fair continuous ranked probability skill score, but not when measured by the Brier skill score. Thus, more efforts have to be done yet in order to achieve minimum skill necessary to develop forecast products based on weather regimes outside winter season.
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