共查询到20条相似文献，搜索用时 15 毫秒
1.
We assess the likely changes in climate extremes under enhanced greenhouse gases over the southern extratropics, with emphasis in southern South America and sub-Antarctic seas, through the analysis of extreme indices measured from models participating in the IPCC 4th Assessment Report. We discuss how the anthropogenic climate change under A1B scenario influences both the patterns of mean change of extreme indices and the likelihood of occurrence of severe extreme indices. The likelihood of occurrence of a year with a large number of days with “warm” minimum temperatures is estimated to increase by a factor of 4 by the end of this century over most of the southern extratropics. By that time, the risk of “severe” precipitation intensity is projected to rise in most areas with the exception of the subtropical anticyclones, which experience particularly strong drying. Over the Southern Ocean this likelihood has increased to over 60%. Corresponding estimates of the changing likelihood for very long dry spells show a banded structure with positive ratios to the north of about 50° S and negative ratios in the sub Antarctic seas. In southern South America this risk about doubled between present and future climates. Then, we explore if the Southern Annular Mode influences the occurrence of severe extreme indices during the period 2070–2099. Its positive phase inhibits the extremely warm minimum temperatures in the Southern Ocean, with the exception of the eastern Bellingshausen Sea, and favors severe frost days to the north of the Ross Sea. Temperature indices show very little change induced by the SAM to the north of 50° S. Severe dry spells are inhibited during the positive phase along the sub Antarctic seas, while the mid-latitudes, including most of Patagonia, show the opposite behaviour. The Southern Ocean reveals a non-uniform distribution with both increases and decreases in the occurrence of heavier precipitation during positive SAM. 相似文献
2.
Severe heat waves in Southern Australia: synoptic climatology and large scale connections 总被引：1，自引：0，他引：1
This paper brings a new perspective on the large scale dynamics of severe heat wave (HW) events that commonly affect southern Australia. Through an automatic tracking scheme, the cyclones and anticyclones associated with HWs affecting Melbourne, Adelaide and Perth are tracked at both the surface and upper levels, producing for the first time a synoptic climatology that reveals the broader connections associated with these extreme phenomena. The results show that a couplet (or pressure dipole) formed by transient cyclones and anticyclones can reinforce the HW similarly to what is observed in cold surges (CS), with an obvious opposite polarity. Our results show that there is a large degree of mobility in the synoptic signature associated with the passage of the upper level ridges before they reach Australia and the blocking is established, with HW-associated surface anticyclones often initiating over the west Indian Ocean and decaying in the eastern Pacific. In contrast to this result the 500?hPa anticyclone tracks show a very small degree of mobility, responding to the dominance of the upper level blocking ridge. An important feature of HWs is that most of the cyclones are formed inland in association with heat troughs, while in CS the cyclones are typically maritime (often explosive), associated with a strong cold front. Hence the influence of the cyclone is indirect, contributing to reinforce the blocking ridge through hot and dry advection on the ridge’s western flank. Additional insights are drawn for the record Adelaide case of March 2008 with fifteen consecutive days above 35°C breaking the previous record by 7?days. Sea surface temperatures suggest a significant air-sea interaction mechanism, with a broad increase in the meridional temperature gradient over the Indian Ocean amplifying the upstream Rossby waves that can trigger HW events. A robust cooling of the waters close to the Australian coast also contributes to the maintenance of the blocking highs locally, which is a fundamental ingredient to sustain the HWs. 相似文献
3.
This paper investigates the impact of the Madden-Julian Oscillation (MJO) on the diurnal cycle of rainfall over the western Maritime Continent during the austral summer. For this purpose, cyclostationary empirical orthogonal function analysis is applied to the tropical rainfall measuring mission rain rate and the Japanese Reanalysis-25 data for the period 1998–2008. The real-time multivariate MJO index by Wheeler and Hendon (Mon Wea Rev 132:1917–1932, 2004) is adopted to define the intensity and the phase of MJO. It is demonstrated that the hourly maximum rain rate over the domain tends to increase when convectively active phase of MJO approaches the Maritime Continent. In contrast, the hourly maximum rain rate tends to decrease when convectively suppressed phase of MJO resides over the region. The changes in the rain rate due to MJO differ over the ocean and the land. This difference is the greatest when the MJO is in the mature stage. Throughout the day during this stage, terrestrial rain rates show minimum values while diurnally varying oceanic rain rates record maximum values. Thus, precipitation becomes more intense in the morning over the Java Sea and is weakened in the evening over Borneo and Sumatra during the mature stage of MJO. During the decaying stage of MJO over the Maritime Continent, the diurnal cycle of precipitation weakens significantly over the ocean but only weakly over land. Analyses suggest that the anomalous lower level winds accompanied by MJO interact with the monsoonal flow over the Maritime Continent. Westerlies induced by MJO convection in the mature stage are superimposed on the monsoonal westerlies over the equator and increase wind speed mainly over the Java Sea due to the blocking effect of orography. Mountainous islands induce flow bifurcation, causing near-surface winds to converge mainly over the oceanic channels between two islands. As a result, heat flux release from the ocean to the atmosphere is enhanced by the increased surface wind resulting in instability as described in the wind-induced surface heat exchange mechanism. This may contribute to heavy rainfall over the Java Sea in the morning during the mature stage. On the other hand, convergence and vertical velocity over the islands, which play important roles in inducing nighttime rainfall, tend to be weak in the evening during the mature stage of MJO. Strong westerlies arising from MJO and the seasonal flow during the mature stage tend to interrupt convergence over islands. This interruption of convergence by MJO gives rise to decreased rain rates over the land regions. 相似文献
4.
Occurrence of drought and dry periods in southeastern Australia has been linked to broad scale climate phenomena including the Southern Oscillation, Interdecadal Pacific Oscillation (IPO), Indian Ocean Dipole (IOD), Southern Annular Mode (SAM) and persistence of blocking high pressure in the Tasman Sea. We examine relationships between Athrotaxis tree ring chronologies from southern Australia extending over much of the past millennia and these broad scale indices. We also examine relationships between the chronologies, temperature, precipitation and a standardised precipitation and evapotranspiration index. Timing of significant correlations with maximum temperature varies between species. The responses of the species with broadscale indices vary with location: northern Athrotaxis cupressoides (Pencil Pine) are more strongly related to the Interdecadal Pacific Oscillation (IPO) and Southern Oscillation Index (SOI) than southern sites. As an exception to this, a site in the far south had significant correlations with both the SOI and IPO, opposite in sign to those observed for the northern sites. Significant spectral power at frequencies consistent with the SOI and IPO occur in all chronologies. Western and southern sites are more strongly related to a seasonalised index of SAM. These three systems have played important roles in determining moisture conditions in southeastern Australia over the past millennium. Results suggest that reconstructions of the SOI, IPO or SAM are unlikely based solely on this Athrotaxis network. The Athrotaxis network of tree ring sites, is however, likely to be an important input to multi-proxy models reconstructing the SOI, IPO or SAM in the Australian sector. The Athrotaxis network of sites is also an important extension of the existing network of Australian tree ring sites that could be used to reconstruct historical drought in southeastern Australia. 相似文献
5.
Impacts of the MJO on Winter Rainfall and Circulation in China 总被引：10，自引：0，他引：10
Impacts of the MJO on winter rainfall and circulation in China are investigated using a real-time multivariate MJO index.Composite results using the daily rainfall anomalies and "rainy day" anomalies according to eight different MJO phases show that the MJO has considerable influence on winter rainfall in China. Rainfall anomalies show systematic and substantial changes(enhanced/suppressed) in the Yangtze River Basin and South China with the eastward propagation of the MJO convective center from the Indian Ocean to the western Pacific.When the MJO is in phase 2 and 3(MJO convective center is located over the Indian Ocean),rainfall probability is significantly enhanced.While in phase 6 and 7(MJO convective center is over the western Pacific),rainfall probability is significantly reduced. MJO in winter influences the rainfall in China mainly through modulating the circulation in the subtropics and mid-high latitudes.For the subtropics,MJO influences the northward moisture transport coming from the Bay of Bengal and the South China Sea by modulating the southern trough of the Bay of Bengal and the western Pacific subtropical high.For the mid-high latitudes,the propagation of the low frequency perturbations associated with the eastward-propagating MJO convection modulate the circulation in the mid-high latitudes,e.g.the East Asian winter monsoon and the low trough over central Asia. 相似文献
6.
The simulation and prediction of extreme heat over Australia on intraseasonal timescales in association with the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) is assessed using the Bureau of Meteorology’s Predictive Ocean Atmosphere Model for Australia (POAMA). The analysis is based on hindcasts over 1981–2010 and focuses on weeks 2 and 3 of the forecasts, i.e. beyond a typical weather forecast. POAMA simulates the observed increased probabilities of extreme heat during El Niño events, focussed over south eastern and southern Australia in SON and over northern Australia in DJF, and the decreased probabilities of extreme heat during La Niña events, although the magnitude of these relationships is smaller than observed. POAMA also captures the signal of increased probabilities of extreme heat during positive phases of the IOD across southern Australia in SON and over Western Australia in JJA, but again underestimates the strength of the relationship. Shortcomings in the simulation of extreme heat in association with ENSO and the IOD over southern Australia may be linked to deficiencies in the teleconnection with Indian Ocean SSTs. Forecast skill for intraseasonal episodes of extreme heat is assessed using the Symmetric Extremal Dependence Index. Skill is highest over northern Australia in MAM and JJA and over south-eastern and eastern Australia in JJA and SON, whereas skill is generally poor over south-west Western Australia. Results show there are windows of forecast opportunity related to the state of ENSO and the IOD, where the skill in predicting extreme temperatures over certain regions is increased. 相似文献
7.
The regional influence of the Madden–Julian oscillation (MJO) on South America is described. Maps of probability of weekly-averaged rainfall exceeding the upper tercile were computed for all seasons and related statistically with the phase of the MJO as characterized by the Wheeler–Hendon real-time multivariate MJO (RMM) index and with the OLR MJO Index. The accompanying surface air temperature and circulation anomalies were also calculated. The influence of the MJO on regional scales along with their marked seasonal variations was documented. During December–February when the South American monsoon system is active, chances of enhanced rainfall are observed in southeastern South America (SESA) region mainly during RMM phases 3 and 4, accompanied by cold anomalies in the extratropics, while enhanced rainfall in the South Atlantic Convergence Zone (SACZ) region is observed in phases 8 and 1. The SESA (SACZ) signal is characterized by upper-level convergence (divergence) over tropical South America and a cyclonic (anticyclonic) anomaly near the southern tip of the continent. Impacts during March–May are similar, but attenuated in the extratropics. Conversely, in June–November, reduced rainfall and cold anomalies are observed near the coast of the SACZ region during phases 4 and 5, favored by upper-level convergence over tropical South America and an anticyclonic anomaly over southern South America. In September–November, enhanced rainfall and upper-level divergence are observed in the SACZ region during phases 7 and 8. These signals are generated primarily through the propagation of Rossby wave energy generated in the region of anomalous heating associated with the MJO. 相似文献
8.
The large sea surface temperature variations induced by the Madden-Julian Oscillation (MJO) on the northwest shelf of Australia and the remote influence of the MJO on the subtropical Western Australian coast are explored using the POAMA Ensemble Ocean Data Assimilation System reanalyses (PEODAS) for the period 1980–2010. The focus here is during the November–April extended summer season when the impacts of the MJO on and along the west coast of Australia are greatest. The MJO is well known to force equatorial Kelvin and Rossby waves in the Indian Ocean, and these are well depicted in the PEODAS reanalyses. When the downwelling Kelvin waves (forced by the westerly-convective phase of the MJO) reach the Indonesian region at the eastern boundary of the Indian Ocean, a coastally trapped Kelvin wave appears to propagate southeast along the Indonesian coastline. At the same time, the suppressed convection/easterly phase of the MJO arrives in the eastern Indian Ocean, with increased heat flux into the ocean due to reduced latent heat flux and increased insolation. The coastally trapped Kelvin waves do not appear to get onto the Western Australian coast. Rather, the increased heat flux and Ekman-induced downwelling onto the northwest (NW) coast in the suppressed/easterly phase of the MJO drive an increase in sea surface temperature on the NW Australian shelf. The piling up of warm water and associated sea level rise on the NW shelf is then communicated down the Western Australian coast as a coastally trapped wave, resulting in an increase in the Leeuwin current. Thus we conclude that the MJO signal in sea level along the west coast of Australia does not result from transmission of equatorial waves onto the Western Australian coast, but rather a southward-propagating coastal trapped wave that is directly forced on the NW shelf through Ekman-induced vertical advection and surface heat fluxes in the easterly phase of the MJO. Additionally, subtropical coastal sea level variability is reinforced locally via a teleconnection of the MJO to the local meridional wind off the southwest Australian coast. Considering the capability to predict the MJO to about 4 weeks lead time plus the 2 weeks taken for the MJO signal on the NW shelf to influence sea level at Fremantle, the use of MJO forecasts in management of the Western Australian marine environment should be considered for future application. 相似文献
9.
基于1979—2020年逐日的NOAA向外长波辐射资料、NCEP/NCAR再分析风场资料,以及全球CMAP再分析降水资料,探讨了气候态亚洲热带夏季风涌的传播过程及与我国夏季相应的降水联系。分析结果表明,主汛期亚洲热带气候态夏季风季节内振荡(CISO)活动是亚洲夏季风活动的主要特征,随时间北传的亚洲热带夏季风CISO称为亚洲热带夏季风涌,主要有南亚夏季风涌和南海夏季风涌。亚洲热带夏季风涌的传播可分为四个阶段。在亚洲热带夏季风涌的发展阶段,印度洋区域低频气旋与对流活跃,孟加拉湾和南海热带区域被低频东风控制,我国大部分地区无降水发生,降水中心位于两广地区。当进入亚洲热带夏季风涌活跃阶段,孟加拉湾和南海热带地区低频气旋和对流活跃,东亚低频“PJ”波列显著,我国降水中心北移到长江以南的附近区域。亚洲热带夏季风涌减弱阶段,孟加拉湾与南海低频气旋消亡,对流减弱,低频西风加强,日本南部附近为低频反气旋控制,我国长江中下游低频南风活跃,降水中心也北移到长江中下游地区,而华南地区已基本无降水,此阶段的大气低频环流场与亚洲热带夏季风涌发展阶段基本相反。进入亚洲热带夏季风涌间歇阶段时,孟加拉湾和南海热带地区低... 相似文献
10.
Neville Nicholls 《Climate Dynamics》2010,34(6):835-845
The 1958–2007 decline in March–August rainfall over southern Australia (south of 30°S) is very closely related to an increase in surface atmospheric pressure over Australia. Sea surface temperatures around northern Australia are strongly correlated with southern Australian rainfall but the recent warming of the ocean should have led to increased rainfall rather than the observed rainfall decline. The relationships between the rainfall and indices of several modes of the atmosphere/ocean system are investigated to determine a cause of the rainfall decline. Indices of the modes that only use data remote from the Australian region are used to avoid the possibility that a relationship between the mode and Australian rainfall is simply reflecting the behaviour of “local” portions of the index. Thus a climate mode index that incorporates Australian pressure would, of course, be related to southern Australian rainfall, even if the remote parts of the mode were unrelated to Australian rainfall. Unless the remote contributions to the mode index were also related to Australian rainfall it seems physically unrealistic to consider that the mode, per se, was affecting Australian rainfall (rather than simply reflecting the influence of the local pressure changes). The rainfall decline does not appear to be explainable by a change in the behaviour of the El Niño-Southern Oscillation (remote indices of this phenomenon do not exhibit a trend over this period) or the Indian Ocean Dipole (which is not strongly correlated with Australian rainfall on detrended data). The strong 1958–2007 trend in the southern annular mode (SAM) appears able to explain much of the rainfall decline since its year-to-year variations are correlated with year-to-year variations in southern Australian rainfall, and the sense of the correlation and the SAM trend would lead to a decline in rainfall (and an increase in pressure over Australia). The observed trend in SAM can reproduce over 70% of the observed rainfall trend. All these conclusions also apply to the rainfall declines in the southeast and southwest sub-regions. 相似文献
11.
利用NCEP/NCAR再分析资料和中科院大气物理研究所PIAP3大气环流模式,分析了印度洋偶极子对夏季中国南海西南季风水汽输送的影响。结果表明,印度洋偶极子正位相期间夏季中国南海西南水汽输送较强,负位相期间则较弱。原因可归结为以下:正位相期间,MJO（Madden-Julian Oscillation）多活动于热带西印度洋,其向东传播受到阻碍,但经向传播明显,通常可传播至孟加拉湾地区,同时PIAP3显示印度洋季风槽位置偏北,且印尼以西过赤道气流较强,从而使得这一地区气旋性环流得到建立与加强。孟加拉湾地区对应着较强的对流活动以及深厚积云对流加热,从而通过对流加热的二级热力响应使西太平洋副热带高压位置向北推进,进而使得南海地区西南季风水汽输送得到建立与加强。在此期间孟加拉湾、中南半岛至南海地区对流活动较强,而苏门答腊沿岸对流活动受到抑制,由此增强了Reverse-Hadley环流,使低层经向风较强,进而增强了南海西南季风的水汽输送,PIAP3大气环流模式证实了Reverse-Hadley环流的增强。负位相期间,MJO多活动于热带东印度洋,在东传过程中受到Walker环流配置影响,在140°E赤道附近形成东西向非对称积云对流加热热源,其东侧Kelvin波响应加强了东风异常并配合副热带高压南缘东风压制了中国南海的西南季风水汽输送。在此期间,MJO在南海地区的经向传播较强,但经向传播常止步于南海地区15°N附近,虽携带大量水汽,但深厚积云对流强烈地消耗水汽使大气中水汽含量降低,PIAP3大气环流模式证实负位相期间深厚积云对流对水汽消耗加大,从而使得负位相期间南海地区水汽含量与正位相期间大体相近,但由于经向风不足使水汽向北输送较弱。 相似文献
12.
利用再分析资料分析了MJO（Madden-Julian Oscillation）不同位相对春季中国东部降水的影响。结果表明：MJO处于位相3时对应长江及其以南地区降水增多,处于位相7时该区域降水减小。当热带MJO对流从位相1东传至位相4,与其相伴的北向辐散辐合流会在印度东北部—长江及副热带西北太平洋地区的对流层中低层产生明显的辐合异常,且在MJO位相3时中国东部的辐合异常达到最大。从Rossby波源角度分析,这种辐合异常会增强对流层中低层气旋性环流,导致MJO处于位相3时长江流域及其以南地区降水增多。同时,利用现代次季节和季节预报业务系统探讨了MJO与降水的关系对降水预报技巧的影响,发现预报降水和再分析产品的相关系数在MJO处于位相3和7时有所增加。 相似文献
13.
Deo Anil Chand Savin S. Ramsay Hamish Holbrook Neil J. McGree Simon Magee Andrew Bell Samuel Titimaea Mulipola Haruhiru Alick Malsale Philip Mulitalo Silipa Daphne Arieta Prakash Bipen Vainikolo Vaiola Koshiba Shirley 《Climate Dynamics》2021,56(11):3967-3993
Southwest Pacific nations are among some of the worst impacted and most vulnerable globally in terms of tropical cyclone (TC)-induced flooding and accompanying risks. This study objectively quantifies the fractional contribution of TCs to extreme rainfall (hereafter, TC contributions) in the context of climate variability and change. We show that TC contributions to extreme rainfall are substantially enhanced during active phases of the Madden–Julian Oscillation and by El Niño conditions (particularly over the eastern southwest Pacific region); this enhancement is primarily attributed to increased TC activity during these event periods. There are also indications of increasing intensities of TC-induced extreme rainfall events over the past few decades. A key part of this work involves development of sophisticated Bayesian regression models for individual island nations in order to better understand the synergistic relationships between TC-induced extreme rainfall and combinations of various climatic drivers that modulate the relationship. Such models are found to be very useful for not only assessing probabilities of TC- and non-TC induced extreme rainfall events but also evaluating probabilities of extreme rainfall for cases with different underlying climatic conditions. For example, TC-induced extreme rainfall probability over Samoa can vary from ~ 95 to ~ 75% during a La Niña period, if it coincides with an active or inactive phase of the MJO, and can be reduced to ~ 30% during a combination of El Niño period and inactive phase of the MJO. Several other such cases have been assessed for different island nations, providing information that have potentially important implications for planning and preparing for TC risks in vulnerable Pacific Island nations.
相似文献14.
A. G. Marshall D. Hudson H. H. Hendon M. J. Pook O. Alves M. C. Wheeler 《Climate Dynamics》2014,42(11-12):3271-3288
We assess the depiction and prediction of blocking at 140°E and its impact on Australian intra-seasonal climate variability in the Bureau of Meteorology’s dynamical intra-seasonal/seasonal forecast model Predictive Ocean Atmosphere Model for Australia version 2 (POAMA-2). The model simulates well the strong seasonality of blocking but underestimates its strength and frequency increasingly with lead time, particularly after the first fortnight of the hindcast, in connection with the model’s drifting basic state. POAMA-2 reproduces well the large-scale structure of weekly-mean blocking anomalies and associated rainfall anomalies over Australia; the depiction of total blocking in POAMA-2 may be improved with the reduction of biases in the distribution of Indian Ocean rainfall via a tropical-extratropical wave teleconnection linking blocking activity at 140°E with tropical variability near Indonesia. POAMA-2 demonstrates the ability to skilfully predict the daily blocking index out to 16 days lead time for the ensemble mean hindcast, surpassing the average predictive skill of the individual hindcast members (5 days), the skill obtained from persistence of observed (2 days), and the decorrelation timescale of blocking (3 days). This skilful prediction of the blocking index, together with effective simulation of blocking rainfall anomalies, translates into higher skill in forecasting rainfall in weeks 2 and 3 over much of Australia when blocking is high at the initial time of the hindcast, compared to when the blocking index is small. POAMA-2 is thus capable of providing forecast skill for blocking rainfall on the intra-seasonal timescale to meet the needs of Australian farming communities, whose management practises often rely upon decisions being made a few weeks ahead. 相似文献
15.
B. Pohl N. Fauchereau Y. Richard M. Rouault C. J. C. Reason 《Climate Dynamics》2009,33(7-8):1033-1050
After removing the annual cycle, a principal component analysis is applied to the daily outgoing longwave radiation anomaly field, used here as a proxy for atmospheric convection. The analysis is carried out over the southern African region (7.5°E–70°E, 10°S–40°S) for austral summer (November through February) for the period 1979–1980 to 2006–2007. The first five principal components (PC) are retained. The first two PCs describe spatial patterns oriented north-west to south-east from tropical southern Africa (SA) to the mid-latitudes. They are interpreted to be different possible locations for synoptic-scale tropical–temperate troughs (TTT), one dominant rainfall-producing synoptic system in the region. The phase relationship between these two PCs describes a tendency for these TTT to propagate eastwards from SA to the Mozambique Channel and southern Madagascar. The next three PCs describe convective fluctuations, respectively, located over the north-west, the south and the centre of SA. Their time series are significantly associated with Madden–Julian oscillation (MJO) activity in the tropics. However, we find that TTT systems are statistically independent of the MJO, i.e. they are equally liable to occur during any phase of the MJO. Three PCs out of five also show a significant association with El Niño southern oscillation, confirming that El Niño years mostly coincide with suppressed convection at the intraseasonal time-scales, a result consistent with its impact on seasonal averages diagnosed in previous studies. 相似文献
16.
Distinct influential mechanisms of the warm pool Madden–Julian Oscillation on persistent extreme cold events in Northeast China 
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下载免费PDF全文 This study investigates whether and how the Madden–Julian Oscillation (MJO) influences persistent extreme cold events (PECEs), a major type of natural disaster in boreal winter, over Northeast China. Significantly increased occurrence probabilities of PECEs over Northeast China are observed in phases 3 and 5 of the MJO, when MJO-related convection is located over the eastern Indian Ocean and the western Pacific, respectively. Using the temperature tendency equation, it is found that the physical processes resulting in the cooling effects required for the occurrence of PECEs are distinct in the two phases of the MJO when MJO-related convection is consistently located over the warm pool area. The PECEs in phase 3 of the MJO mainly occur as a result of adiabatic cooling associated with ascending motion of the low-pressure anomaly over Northeast Asia. The cooling effect associated with phase 5 is stronger and longer than that in phase 3. The PECEs associated with phase 5 of the MJO are linked with the northwesterly cold advection of a cyclonic anomaly, which is part of the subtropical Rossby wave train induced by MJO-related convection in the tropical western Pacific.摘要 本文利用高分辨率气温数据和热带季节内振荡 (MJO) 实时指数, 研究了1979–2015年冬季MJO活动对中国东北持续性极端低温事件 (PECE) 的影响特征和机理.结果表明:当MJO对流分别位于暖池地区的东印度洋 (位相3) 和西太平洋 (位相5) 时, 中国东北PECE的发生频率显著增加.利用温度方程诊断分析发现MJO两个位相所导致的冷却过程不同: 当 MJO处于位相3时, 中国东北地区为低压异常, 上升运动引起绝热冷却作用; 而位相5所形成的气旋性环流为中国东北地区带来西北风冷平流, 降温过程更强且持续更长时间. 相似文献
17.
Michael J. Pook James S. Risbey Peter C. McIntosh 《Theoretical and Applied Climatology》2014,117(3-4):521-533
Two distinct synoptic weather systems, cut-off lows and fronts, deliver most of the cool-season rainfall to the cropping regions of southern Australia. A comparative synoptic climatology of daily rainfall events over approximately five decades reveals both spatial and temporal variations of the dominant synoptic types. The rainfall characteristics and associated large-scale drivers differ between the two synoptic types. Understanding regional rainfall depends on understanding these differences. Cut-off lows contribute one half of growing season rainfall in southeast Australia, while frontal systems associated with Southern Ocean depressions contribute about a third. The proportions are reversed in the Central Wheat Belt (CWB) of Western Australia where Southern Ocean fronts are the dominant source of growing season rainfall. In the southern island state of Tasmania, topography strongly influences the outcome with cut-off lows contributing about half the rainfall near the east coast and fronts dominating a short distance to the west. Cut-off lows generally contribute their highest proportion of rainfall in the austral autumn and spring while frontal rainfall is at its maximum in late winter. Cut-off low rainfall contributes more strongly in percentage terms to the recent decline in rainfall. The distribution of synoptic types is explained by the dominant long-wave structure in the winter half of the year. The major trough near Western Australia favours frontogenesis to the southwest of the CWB but fronts moving out of the region encounter a persistent meridional ridge in the Tasman Sea where there is a high frequency of blocking events. 相似文献
18.
Anticipating and mitigating wave-related hazards rely heavily on understanding wave variability drivers. Here, we describe wave conditions related to concurrent Southern Annular Mode (SAM) and El Niño–Southern Oscillation (ENSO) phases during the austral summer. To identify such conditions, significant wave height (Hs) and peak wave period (Tp) daily anomalies were composited during different SAM–ENSO phase combinations over the last four decades (1979–2018). Surface wind anomalies were also composited to assist in the interpretation of wave conditions. The composites show significant wave variability across all ocean basins and in several semi-enclosed seas throughout the different SAM–ENSO phase combinations. The Southern, Indian, and Pacific Oceans generally experience the strongest Tp anomalies during combinations of SAM phases with El Niño, and the weakest Tp anomalies during combinations of SAM phases with La Niña. The anomalously large waves observed in the south-western Pacific, Tasman Sea, and the Southern Ocean, previously ascribed to ENSO conditions, seem to be instead associated with the SAM variability. SAM-related atmospheric conditions are found to be able to modulate the intensity of ENSO-related winds over the South China Sea, which, in turn, alter the magnitude of waves in that region. These and other wave anomaly structures described here, especially those contrasting the behaviour expected for a given ENSO phase, such as the one found along the California coast, stress the importance of understanding relationships between wave parameters and climate patterns interactions. 相似文献
19.
The Madden–Julian Oscillation (MJO)/Boreal Summer Intraseasonal Oscillation (BSISO) has been considered an important climate mode of variability on subseasonal timescales for East Asian summer. However, it is unclear how well the MJO/BSISO indices would serve as guidance for subseasonal forecasts. Using a probabilistic forecast model determined through multiple linear regression (MLR) with MJO, ENSO, and long-term trend as predictors, we examine lagged impacts of each predictor on East Asia extended summer (May–October) climate from 1982 to 2015. The forecast skills of surface air temperature (T2m) contributed by each predictor is evaluated for lead times out to five weeks. We also provide a systematic evaluation of three commonly used, real-time MJO/BSISO indices in the context of lagged temperature impacts over East Asia. It is found that the influence of the trend provides substantial summertime skill over broad regions of East Asia on subseasonal timescales. In contrast, the MJO influence shows regional as well as phase dependence outside the tropical band of the main action centers of the MJO convective anomalies. All three MJO/BSISO indices generate forecasts that yield high skill scores for week 1 forecasts. For some initial phases of the MJO/BSISO, skill reemerges over some regions for lead times of 3–5 weeks. This emergence indicates the existence of windows of opportunity for skillful subseasonal forecasts over East Asia in summer. We also explore the dynamics that contribute to the elevated skills at long lead times over Tibet and Taiwan–Philippine regions following the initial state of phases 7 and 5, respectively. The elevated skill is rooted in a wave train forced by the MJO convective heating over the Arabian Sea and feedbacks between MJO convection and SSTs in Taiwan–Philippine region. Two out of the three commonly used MJO/BSISO indices tend to identify MJO events that evolve consistently in time, allowing them to serve as reliable predictors for subseasonal forecasts for up to 5 weeks.
相似文献20.
Pascal Oettli Tomoki Tozuka Takeshi Izumo Francois A. Engelbrecht Toshio Yamagata 《Climate Dynamics》2014,43(5-6):1557-1573
The Madden–Julian Oscillation (MJO) is the major mode of intraseasonal variability (30–60 days) in the tropics, having large rainfall impacts globally, and possibly on southern Africa. However, the latter impact is not well understood and needs to be further explored. The life cycle of the MJO, known to be asymmetric, has been nevertheless analyzed usually through methods constrained by both linearity and orthogonality, such as empirical orthogonal function analysis. Here we explore a non-linear classification method, the self-organizing map (SOM), a type of artificial neural network used to produce a low-dimensional representation of high-dimensional datasets, to capture more accurately the life cycle of the MJO and its global impacts. The classification is applied on intraseasonal anomalies of outgoing longwave radiation within the tropical region over the 1980–2009 period. Using the SOM to describe the MJO is a new approach, complimentary to the usual real-time multivariate MJO index. It efficiently captures this propagative phenomenon and its seasonality, and is shown to provide additional temporal and spatial information on MJO activity. For each node, the subtropical convection is analyzed, with a particular focus on the southern Africa region. Results show that the convection activity over the central tropical Indian Ocean is a key factor influencing the intraseasonal convective activity over the southern African region. Enhanced (suppressed) convection over the central Indian Ocean tends to suppress (enhance) convection over the southern African region with a 10-day lag by modulating the moisture transport. 相似文献