Amazon River plume influence on Western Tropical Atlantic dynamic variability

This study focuses on analysing the potential impact of the Amazon and Pará Rivers on the salinity, temperature and hydrodynamics of the Western Tropical North Atlantic (WTNA) region between 60.5°–24 °W and 5 °S–16 °N. The Regional Ocean Model System (ROMS) was used to simulate ocean circulation with 0.25° horizontal resolution and 32 vertical levels. Two numerical experiments were performed considering river discharge and river input. Temperature and salinity distributions obtained numerically were compared with Simple Ocean Data Assimilation (SODA) and in situ observations from the Prediction Research Moored Array in the Tropical Atlantic (PIRATA) buoys located at 38 °W8 °N and 38 °W12 °N. Surface currents were compared with Surface Currents from Diagnostic model (SCUD). Once we verified that model results agreed with observations, scenarios with and without river discharges were compared. The difference between both simulations in the Sea Surface Temperature distribution was smaller than 2 °C, whereas the Sea Surface Salinity (SSS) changed by approximately 8 psu in the plume area close to the coast from August to December and reaching SSS differences of approximately 4 psu in the region of the North Equatorial Counter Current (NECC). The surface current velocities are stronger in the experiment with river discharge, mainly in the NECC area from September to December and close to the coast in June to August. The results show that river discharges also cause a phase shift in the zonal currents, anticipating the retroflection of the North Brazil Current by two months and enhancing eastward NECC transport, which is in agreement with observations. The Mixed Layer Depth and Isothermal Layer Depth in the presence of river discharge is 20–50 m shallower over the entire extension of the Amazon plume compared with the situation without continental inflows. As a consequence, stronger Barrier Layers develop in the river plumes, reducing the Oceanic Heat Content in the WTNA.     

Atmospheric teleconnection mechanisms of extratropical North Atlantic SST influence on Sahel rainfall

Extratropical North Atlantic cooling has been tied to droughts over the Sahel in both paleoclimate observations and modeling studies. This study, which uses an atmospheric general circulation model (GCM) coupled to a slab ocean model that simulates this connection, explores the hypothesis that the extratropical North Atlantic cooling causes the Sahel droughts via an atmospheric teleconnection mediated by tropospheric cooling. The drying is also produced in a regional climate model simulation of the Sahel when reductions in air temperature (and associated geopotential height and humidity changes) from the GCM simulation are imposed as the lateral boundary conditions. This latter simulation explicitly demonstrates the central role of tropospheric cooling in mediating the atmospheric teleconnection from extratropical North Atlantic cooling. Diagnostic analyses are applied to the GCM simulation to infer teleconnection mechanisms. An analysis of top of atmosphere radiative flux changes diagnosed with a radiative kernel technique shows that extratropical North Atlantic cooling is augmented by a positive low cloud feedback and advected downstream, cooling Europe and North Africa. The cooling over North Africa is further amplified by a reduced greenhouse effect from decreased atmospheric specific humidity. A moisture budget analysis shows that the direct moisture effect and monsoon weakening, both tied to the ambient cooling and resulting circulation changes, and feedbacks by vertical circulation and evaporation augment the rainfall reduction. Cooling over the Tropical North Atlantic in response to the prescribed extratropical cooling also augments the Sahel drying. Taken together, they suggest a thermodynamic pathway for the teleconnection. The teleconnection may also be applicable to understanding the North Atlantic influence on Sahel rainfall over the twentieth century.     

Tropical influence on Euro-Asian autumn rainfall variability

The connection between autumn rainfall variability in the Euro-Asian domain and tropical climate is documented using state-of-the-art global observational datasets and re-analyses. Results suggest a robust statistical relationship between the El Niño Southern Oscillation (ENSO) and autumn rainfall in parts of southwest Europe, northern Africa and southwest Asia. The correlation between area-mean anomalies over this region (P_ea) and the NIÑO3.4 index is 0.68, stationary over the last 50 years. Global ENSO-like tropical climate anomalies are observed in conjunction with P_ea anomalies confirming the relationship found with the NIÑO3.4 index. Overall, the connection with Indo-Pacific variability is stronger than that with the eastern Pacific.While rainfall anomalies in southwest Europe and southwest Asia appear to largely co-vary as one pattern under the influence of ENSO, our results suggest that different mechanisms may be contributing to the observed anomalies. In the North Atlantic/European region, it is speculated that while a PNA-like mode maybe the prevailing teleconnection mechanism for high P_ea, for low P_ea tropical Atlantic ENSO related SST anomalies may be playing a more relevant role forcing northeastward propagating Rossby waves. Over southwest Asia, a more direct connection to the Indo-Pacific region is suggested by the upper air anomaly observed over southern Asia, possibly the Rossby wave response to enhanced heating in the Indian Ocean.     

Interdecadal variability and trends of rainfall across the Amazon basin

Summary An analysis of decadal and long-term patterns of rainfall has been carried out using a combination of raingauge and gridded rainfall datasets, for the entire Amazon basin and for its northern and southern sub-basins. The study covers the period 1929–98. Rainfall variability and variations in circulation and sea surface temperature fields have been analysed in more detail for the period 1950–98. Negative rainfall trends were identified for the entire Amazon basin, while at the regional level there is a negative trend in northern Amazonia and positive trend in southern Amazonia. Decadal time scale variations in rainfall have been discovered, with periods of relatively drier and wetter conditions, with different behaviour in northern and southern Amazonia. Spectral analyses show decadal time scale variations in southern Amazonia, while northern Amazonia exhibits both interannual and decadal scale variations. Shifts in the rainfall regime in both sections of the Amazon basin were identified as occurring in the mid-1940s and 1970s. After 1975–76, northern Amazonia received less rainfall than before 1975. Changes in the circulation and oceanic fields after 1975 suggest an important role of the warming of the tropical central and eastern Pacific on the decreasing rainfall in northern Amazonia, due to more frequent and intense El Niño events during the relatively dry period 1975–98.     

The influence of geopotential heights on New South Wales rainfall

Summary An investigation of the relationships between New South Wales (NSW) seasonal rainfalls and fluctuations of geopotential height at four Australian radiosonde stations is presented. The connection between the Southern Oscillation Index (SOI) and the geopotential height was explored up to the mid-troposphere. The study determined that the 800 and 600 hPa heights at Woomera show stronger and more consistent correlations with winter and spring rainfalls respectively, than occur between SOI and rainfall. The 900 hPa height at Brisbane is also strongly correlated with autumn rainfall for much of coastal NSW. These correlations are found to be stable during high and low phases of the SO cycle. It was found that the effects of the considered geopotential data on rainfall are independent of the influence of the SO phenomenon. The study also found that the fluctuations of geopotential heights at Woomera are related to rainfall variability over a wide region of southern Australia. At Darwin the 800 hPa surface appears to be the highest altitude at which there is any influence from the Southern Oscillation during winter. Furthermore, airmass movement over inland NSW is quite strongly related to SOI but coastal airmass movement is only weakly related to SOI. A mechanism for the influence of the Southern Oscillation on NSW rainfall is suggested.With 9 Figures     

江苏不同强度降雨对能见度影响分析

利用江苏70个基本站多年逐时雨量、相对湿度、风向、风速以及同时段内最低能见度等观测资料,分析不同强度降雨对能见度的影响,并对比分析两种不同强度降雨造成的低能见度事件统计特征。结果表明:降雨是除雾以外,江苏低能见度的主要影响天气(14. 7%),其中稳定性弱降雨和短时强降雨影响最大。与低能见度雾事件不同,降雨造成的低能见度事件全天各时段均可能出现,发生时可伴随较强的风速(2 m/s),短强低能见度多见风速4 m/s(26. 6%)。江苏冬春两季为雨雾高发季,主要受降雨持续时间影响,对应的低能见度区间为500~1 000 m,有明显日变化。短强低能见度主要受雨强影响,多发生于6—9月,对应的低能见度区间为小于200 m,无明显日变化。两种降雨产生的低能见度事件有明显的空间分布差异,且雨雾低能见度发生时偏北风占主导,短强低能见度发生时则偏东风占主导。     

Contributions of various classes of convection to rainfall in the Atlantic Ocean

Summary Whilst the tropics comprise only 50% of the Earth's surface, 75% of the annual, global rainfall occurs there. Hence, the tropics are the latent heat engine for the general circulation of the atmosphere. In this environment, all manner of convective weather systems exist: intense and destructive tropical storms (TS), organized mesoscale convective systems (MCCs and CCCs here) and much weaker, short-lived convection (DSL). The relative importance of these differing convective weather systems to the hydrologic cycle of the Atlantic Ocean basin is considered here. An automated, satellite-based climatology and classification of these four different classes of convective weather systems is used to define system characteristics and contribution to basin-wide rainfall over an 18 month time period.It is found that short-lived thunderstorms (DSL class) are the largest contributors to the basin-wide rainfall, however their contribution represents only about half of the total diagnosed rainfall. Organized mesoscale systems contribute the balance. Hence, mesoscale organized weather systems seem to play an important rôle in the Atlantic Ocean hydrologic cycle.Due to the potentially large error bounds on the satellite rainfall climatologies used here, the results of this study are contransted with the recent climatology of Cotton et al. (1995), which incorporates some estimates of rainfall characteristics for mesoscale systems based on numerical model simulations. Comparison of these two climatologies showed good agreement in the relative magnitudes of rainfall determined for each class of convection.With 11 Figures     

Tropical versus high latitude freshwater influence on the Atlantic circulation

We investigate the model sensitivity of the Atlantic meridional overturning circulation (AMOC) to anomalous freshwater flux in the tropical and northern Atlantic. Forcing in both locations leads to the same qualitative response: a positive freshwater anomaly induces a weakening of the AMOC and a negative freshwater anomaly strengthens the AMOC. Strong differences arise in the temporal characteristics and amplitude of the response. The advection of the tropical anomaly up to the deep water formation area leads to a time delayed response compared to a northern forcing. Thus, in its transient response, the AMOC is less sensitive to a constant anomalous freshwater flux in the tropics than in the north. This difference decreases with time and practically vanishes in equilibrium with constant freshwater forcing. The equilibrium response of the AMOC shows a non-linear dependence on freshwater forcing in both locations, with a stronger sensitivity to positive freshwater forcing. As a consequence, competitive forcing in both regions is balanced when the negative forcing is about 1.5 times larger than the positive forcing. The relaxation time of the AMOC after termination of a freshwater perturbation depends significantly on the AMOC strength itself. A strong overturning exhibits a faster relaxation to its unperturbed state. By means of a set of complementary experiments (pulse-perturbations, constant and stochastic forcing) we quantify these effects and discuss the corresponding time scales and physical processes.     

Influence of the North Atlantic Oscillation on winter rainfall in Calabria (southern Italy)

Seasonal rainfall amounts, directly responsible for availability of water resources on a specified area, are strongly dependent on the climate system. In order to highlight some features of such dependence, generally circulation indexes based on the difference in the sea level pressure between two geographic areas are taken into account. In the present study, the relationships between winter rainfall series observed in the Calabria region (southern Italy) and the North Atlantic Oscillation Index (NAOI) have been analysed. Firstly, a correlation analysis between precipitation and the NAOI was performed. Subsequently, the influence of the different phases of the NAO on the winter precipitation has been detected by a composite analysis, and by identifying changes in the behaviour of the probability density functions (gamma distribution) fitted on monthly rainfall. The results evidence a clear link existing between the phases of the climatic index and the amount of winter rainfall.     

Large-scale circulation patterns and related rainfall in the Amazon Basin: a neuronal networks approach

This study describes the main circulation patterns (CP) in the Amazonian Basin over the 1975–2002 period and their relationship with rainfall variability. CPs in the Amazonian Basin have been computed for each season from the ERA-40 daily 850?hPa winds using an approach combining artificial neural network (Self Organizing Maps) and Hierarchical Ascendant Classification. A 6 to 8 cluster solutions (depending on the season considered) is shown to yield an integrated view of the complex regional circulation variability. For austral fall, winter and spring the temporal evolution between the different CPs shows a clear tendency to describe a cycle, with southern wind anomalies and their convergence with the trade winds progressing northward from the La Plata Basin to the Amazon Basin. This sequence is strongly related to eastward moving extra tropical perturbations and their incursion toward low latitude that modulate the geopotential and winds over South America and its adjoining oceans. During Austral summer, CPs are less spatially and temporally organized compared to other seasons, principally due to weaker extra tropical perturbations and more frequent shallow low situations. Each of these CPs is shown to be associated with coherent northward moving regional rainfall patterns (both in in situ data and ERA-40 reanalysis) and convective activity. However, our results reveals that precipitation variability is better reproduced by ERA-40 in the southern part of the Amazonian Basin than in the northern part, where rainfall variability is likely to be more constrained by local and subdaily processes (e.g. squall lines) that could be misrepresented in the reanalysis dataset. This analysis clearly illustrates the existing connections between the southern and northern part of the Amazonian Basin in terms of regional circulation/rainfall patterns. The identification of these CPs provide useful information to understand local rainfall variability and could hence be used to better understand the influence of these CPs on the hydrological variability in the Amazonian Basin.     

Simulating the influence of the South Atlantic dipole on the South Atlantic convergence zone during neutral ENSO

The South Atlantic Convergence Zone (SACZ) is an intrinsic characteristic of the South American Summer Monsoon. In a recent study, we verified that the main mode of coupled variability over the South Atlantic (South Atlantic Dipole (SAD)) plays a role in modulating the position of extratropical cyclones that affect the SACZ precipitation. In this study, we perform numerical experiments to further investigate the mechanisms between SAD and the SACZ. Numerical experiments forced with prescribed SST anomalies showed that, even though the Atlantic SST affects the position of the cyclone associated with the SACZ, the atmospheric response and precipitation patterns over land are opposed to the observations. On the other hand, experiments forced with prescribed anomalous driving fields showed that the atmospheric component of SAD plays a significant role for the right position and intensity of precipitation associated with the SACZ. SAD negative anomalies provide the low-level and upper-level atmospheric support for the intensification of the cyclone at surface and for the increase in precipitation over the land portion of the SACZ. Therefore, the numerical experiments suggest that, during El Niño Southern Oscillation neutral conditions, the SACZ precipitation variability associated with SAD is largely dependent on the atmospheric variability rather than the underlying SST.     

台风暴雨落区研究综述

台风暴雨最重要的两个因素是雨强和降雨分布,后者即为暴雨的落区。影响台风暴雨落区的因子主要有3个：1）台风涡旋内部结构;2）台风周围环境大气影响;3）台风下垫面强迫作用。本文对这3类因子的作用和影响作了总结。台风暴雨可分为台风环流内的暴雨和台风环流之外的暴雨两大类。本文把台风环流内的暴雨概括为5个落区,包括眼壁暴雨、螺旋雨带暴雨、小涡暴雨、倒槽暴雨、切变暴雨。把台风环流之外的暴雨分为台前飑线暴雨、远距离暴雨和变性下游效应暴雨。地形可能会改变两类暴雨的强度和落区。本文对每一个落区的暴雨特点和形成机理作了总结,对台风暴雨业务预报有一定的参考价值。     

The influence of Southern Hemispheric Equatorial Trough on rainfall during southwest monsoon

Summary Monsoon is a complex dynamical system. Interannual and intraseasonal fluctuations have been studied in the regional and global perspective. Southern Hemispheric Equatorial Trough and its activity during the northern summer appears inversely related to the rainfall activity over the Indian sub-continent in general and Central India in particular. During typical active SHET epochs the rainfall activity becomes deficient in the Central parts of the country. The activity of the SHET is fluctuating in nature. Its duration is prolonged and intensity is increased during the years of major failure.With 2 Figures     

热带气旋直接降水对中国的影响研究

本文基于RSMC台风最佳路径资料以及TRMM逐日降水资料,对1998—2013年间中国区域热带气旋(TC)直接降水的时空分布特征进行了统计研究。结果表明:约54.2%的西北太平洋TC在中国陆地上产生了直接降水。TC直接降水对中国的影响范围较广,未变性TC的降水决定了全部TC降水的西界,而潜在变性TC的降水决定了全部TC降水的北界。自每年的4月开始,TC直接降水开始影响中国,以8月最为频繁,潜在变性TC的影响以9月为最。降水首日,全体TC以台风及以上强度为主,中心主要分布在30°N以南。未变性的TC强度较弱,降水面积大、降水强度强;而潜在变性TC则以台风(TY)及以上强度为主,其单个TC的平均降水面积和降水强度以热带低压(TD)及以下强度TC为最大。本文工作为TC在中国直接降水的气候特征以及TC变性前期降水分布特征提供了一些有意义的结论。     

The influence of stochasticism on Indian summer monsoon rainfall and its impact on prediction

Output from a multi-millennial control simulation of the CSIRO Mark 2 coupled model has been used to investigate quantitatively the relation between the Indian summer monsoon rain and El Nino/Southern Oscillation events. A moving window correlation between these two features revealed marked interannual and multi-decadal variability with the correlation coefficient varying between ?0.8 and +0.2. This suggests that current observations showing a decline in this correlation are due to natural climatic variability. A scatter diagram of the anomalies of the Indian summer monsoon rainfall and NINO 3.4 surface temperature showed that in almost 40 % of the cases ENSO events were associated with rainfall anomalies opposite to those implied by the climatological correlation coefficient. Case studies and composites of global distributions of surface temperature and rainfall anomalies for El Nino (or La Nina) events highlight the opposite rainfall anomalies over India that can result from very similar ENSO surface temperature anomalies. Composite differences are used to demonstrate the unique sensitivity of Indian summer monsoon rainfall anomalies to ENSO events. The problem of predicting such anomalies is discussed in relation to the fact that time series of the monsoon rainfall, both observed and simulated, consist of white noise. Based on the scatter diagram it is concluded that in about 60 % of the cases seasonal or annual prediction of monsoon rainfall based on individual ENSO events will result in the correct outcome. Unfortunately, there is no way, a priori, of determining for a given ENSO event whether the correct or a rogue prediction will result. Analysis of the present model’s results suggest that this is an almost world-wide problem for seasonal predictions of rainfall.     

黔东南州一次成功联动增雨分析

通过对2005年7月10日在黔东南州进行的一次成功统一联合增雨作业行动的效果分析,得出对天气形势的正确分析和预测,特别是对雷达回波的连续观测分析,是成功实施人工增雨作业的保障,火箭炮配合高炮灵活机动的联合增雨作业是提高人工增雨作业效果最有效的途径之一。通过对该次联动增雨作业事例分析,总结出一些较为成功的联动增雨作业经验。