Extratropical Disturbances Research Articles

A survey of westward‐propagating mixed Rossby–Gravity waves and quantification of their association with extratropical disturbances

AbstractIn this study we have conducted a survey of westward‐propagating mixed Rossby–Gravity (MRG) wave events in the upper troposphere and quantified their association with the intrusions of extratropical disturbances for the period 1979–2019. MRG wave events are identified by imposing the meridional structure of theoretical MRG waves onto the equatorial meridional winds at 200 hPa. In all, 2390 MRG wave events are identified and the majority (61%) of them occurred during the months of May–October, and 65% of the total MRG wave events occurred over the central–east Pacific and Atlantic Ocean domains. Not only the frequency of occurrence but also the amplitude, wavenumber and trapping scale of the MRG wave events are found to exhibit a clear seasonality. MRG wave events associated with intrusions of extratropical disturbances are identified as when the potential vorticity on the 350‐K isentropic surface at 15° latitude exceeded 1 Potential Vorticity Unit (PVU) in the vicinity of the MRG wave events. We find that 37% of the MRG wave events are intrusion MRG wave events and a large majority (88%) of such events occurred over the central–east Pacific and Atlantic Ocean domains. It is also noteworthy that nearly 70% of such intrusions occurred in the winter hemisphere where the westerly wind ducts are well developed. Over the central–east Pacific during northern hemispheric (NH) winter, it is observed that the intrusion MRG wave events have a bigger amplitude and have a larger meridional extent compared to non‐intrusion MRG wave events. They also exhibit a similar spatial scale as the extratropical disturbances implying that resonant interactions may be a primary mechanism for the genesis of MRG wave events. During NH summer, on the other hand, MRG wave events are primarily triggered by convective processes and the extratropical disturbances may be instrumental in amplifying their amplitude.

Tropical Transition of Tropical Storm Kirogi (2012) over the Western North Pacific: Synoptic Analysis and Mesoscale Simulation

Abstract Tropical transition (TT) is a cyclogenesis process in which a baroclinic disturbance is transformed into a tropical cyclone. Many studies have analyzed TT events over the North Atlantic. This study assesses TT processes from a possible subtropical cyclone to Tropical Storm Kirogi at a relatively high latitude over the western North Pacific in an environment of enhanced baroclinicity in August 2012. Analyses based on satellite observations, the JRA-55 reanalysis, and a simulation with 2.5-km horizontal grid spacing demonstrate three stages during the TT: the baroclinic, intermediate, and convective stages. Over the baroclinic stage, Kirogi had an asymmetric comma-shaped cloud pattern with convection in the northern and eastern parts of the cyclone. This convection is attributed to quasigeostrophic forcing and frontogenesis associated with advection of warm and moist air. Vorticity locally generated by this convection was advected to the cyclone center by cyclone-relative northerly flow. Kirogi also had a shallow warm-core structure due to the interaction with an upper-level cold trough extending from the midlatitudes. In the intermediate stage, the warm and moist air in the lower troposphere and the cold trough in the upper troposphere wrapped around Kirogi. In the convective stage, Kirogi attained characteristics of a typical tropical cyclone with convection concentrated near the cyclone center and a deep warm-core structure. These results demonstrate that baroclinic processes can directly trigger formation of a tropical storm at relatively high latitudes over the western North Pacific in a similar manner to that over the North Atlantic. Significance Statement Tropical cyclogenesis is an important process for early identification of tropical cyclone hazards. Tropical transition is a tropical cyclogenesis process that is triggered by a subtropical or extratropical disturbance. It is unique to relatively high latitudes and has social importance particularly for midlatitude countries. There have been fewer studies on tropical transition over the western North Pacific than over the North Atlantic. This study demonstrates the dynamics of a distinct tropical transition event that led to the formation of Tropical Storm Kirogi (2012) at a relatively high latitude over the western North Pacific.

Rossby Wave Dynamics over South America Explored with Automatic Tropical–Extratropical Cloud Band Identification Framework

AbstractDuring austral summer, persistent tropical–extratropical cloud bands, such as the South Atlantic convergence zone (SACZ) over South America (SAm), link the tropical humid areas to the subtropics. In this study, we utilize an automatic object-based methodology to identify synoptic cloud band events occurring over SAm that are responsible for almost 60% of the precipitation during the rainy season (November–March). In addition to identifying SACZ events as cloud bands persisting 4 or more days, the framework also highlights the relevance of transient events (i.e., events persisting for 3 days or less) to the climatology. The location and persistence of the cloud band events are modulated by the propagation of synoptic-scale extratropical disturbances interacting with intraseasonal variability in the basic-state upper-level zonal wind. During persistent events (i.e., lasting 4 or more days), upper-level westerly anomalies over the subtropics favor the propagation extratropical disturbances deeper into the tropics. Conversely, transient events occur when the Bolivian high is displaced/expanded southeastward, bringing upper-level easterly winds into subtropical latitudes and blocking the propagation of Rossby waves into lower latitudes. Subsequent anomalous subtropical convection from the cloud bands result in sources of Rossby waves that interact with the basic flow, resulting in downwind enhancement or damping of the extratropical disturbances. The adopted methodology proved to be a powerful framework in demonstrating this interaction between scales, with the basic state influencing and being modified by the synoptic disturbances.

Three‐dimensional pathways of dust over the Sahara during summer 2011 as revealed by new Infrared Atmospheric Sounding Interferometer observations

AbstractWe present a new characterisation of the three‐dimensional (3D) distribution of dust over the Sahara during summer, exemplified for June 2011. Our approach, called AEROIASI, is based on the innovative retrieval of vertical profiles of the dust extinction coefficient from daily cloud‐free hyperspectral Infrared Atmospheric Sounder Interferometer (IASI) satellite observations. AEROIASI observations clearly agree with other widely used measurements (from lidar and radiometers). The 3D characterisation is focused on the dust maximum in June 2011, located in the central Sahara (17–23°N, 1–7°E) and linked to the major atmospheric dynamical drivers associated with the West African Monsoon (WAM) system. AEROIASI shows the near‐surface dust load to be dominated by five major emission events occurring every 3–4 days. These all occur when the study region is under the influence of northward bursts of the WAM and convection‐related cold pools, likely associated with orographic forcing by the Aïr Mountains. During the earliest (June 10) and the dustiest (June 17) cases, northward advection of moisture over the hotspot is favoured by the superposition of cyclonic circulations related to an extratropical disturbance northwest of the Sahara and to the Saharan heat low over Mauritania, respectively. Convection over the hotspot also triggers wave‐like disturbances that travel westwards. The three dustiest events are characterised by elongated dust fronts moving northwards, with a leading edge spanning 200–300 km horizontally and extending from the surface up to 2 km of altitude. Further south, the dust layer progressively elevates to 3.5 km along the slanted isentropes at the interface of the monsoon and the harmattan, increasingly losing contact with the ground. When northerlies blow over the study region, elevated dust layers at 3–5 km are observed, which are transported southwards within the Saharan air layer and westwards along the northern edge of the African easterly jet (after June 13).

The Tropical Transition in the Western North Pacific: The Case of Tropical Cyclone Peipah (2007)

AbstractThis study examines the transition of an extratropical disturbance to a tropical cyclone (TC), Peipah (2007), in the western North Pacific (WNP), using reanalysis and geostationary satellite data. Instead of regular diurnal fluctuations of deep convection, the pre‐TC disturbance accompanies deep convection only for short durations every other day. When the pre‐TC vorticity is traced back to 7 days prior to its formation, the traced‐back vorticity indicates a strong potential vorticity (PV) trough in the subtropical upper troposphere that originated from the midlatitude lower stratosphere. The quasi‐geostrophic forcing and reduced static stability at the leading edge of the PV trough result in the formation of an extratropical disturbance. The vertical structure of the extratropical disturbance shows maximum vorticity in the upper troposphere and cold temperature anomaly within it throughout the entire troposphere. As the extratropical disturbance moved into the tropical WNP, deep convection associated with quasi‐geostrophic dynamics over the warm ocean initiated tropical transition of the extratropical disturbance to a TC through diabatic redistribution of PV in the tropospheric column as well as transition of the cold anomaly into a warm anomaly within the vortex. With additional contribution of barotropic energy conversion in the lower troposphere, the warm‐core low system finally developed into a TC‐strength vortex. These results indicate that PV troughs of the stratospheric origin over the subtropical Pacific Ocean can contribute to TC formations in the WNP.

Representation of Western Disturbances in CMIP5 Models

AbstractWestern disturbances (WDs) are synoptic extratropical disturbances embedded in the subtropical westerly jet stream. They are an integral part of the South Asian winter climate, both for the agriculture-supporting precipitation they bring to the region and for the associated isolated extreme events that can induce devastating flash flooding. Here, WD behavior and impacts are characterized in 23 CMIP5 historical simulations and compared with reanalysis and observations. It is found that WD frequency has a strong relationship with model resolution: higher-resolution models produce significantly more WDs and a disproportionately high fraction of extreme events. Exploring metrics of jet strength and shape, we find that the most probable cause of this relationship is that the jet is wider in models with coarser resolution, and therefore the northern edge in which WDs are spun up sits too far north of India. The frequency of WDs in both winter and summer is found to be overestimated by most models, and thus the winter frequency of WDs estimated from the multimodel mean (30 per winter) is above the reanalysis mean (26 per winter). In this case, the error cannot be adequately explained by local jet position and strength. Instead, we show that it is linked with a positive bias in upstream midtropospheric baroclinicity. Despite a positive winter precipitation bias in CMIP5 models over most of India and Pakistan and a dry bias in the western Himalayas, the fraction of winter precipitation for which WDs are responsible is accurately represented. Using partial correlation, it is shown that the overestimation in WD frequency is the largest contributor to this bias, with a secondary, spatially heterogeneous contribution coming from the overestimation of WD intensity.

The Early Development of the 2015/16 Quasi-Biennial Oscillation Disruption

Abstract An unprecedented disruption of the quasi-biennial oscillation (QBO) started to develop from late 2015. The early development of this event is analyzed using the space–time spectra of eddies from reanalysis data. While the extratropical waves propagating horizontally into the tropics were assumed to be the main driver for the disruption, it was not clear why these waves dissipated near the jet core instead of near the jet edge as linear theory predicts. This study shows that the drastic deceleration of the equatorial jet was largely brought about by a single strong wave packet, and the local winds experienced by the wave packet served as a better indicator of the wave breaking latitude than the zonal mean winds. Surprisingly, tropical mixed Rossby–gravity waves also made an appreciable contribution to the deceleration of the equatorial westerly jet by the horizontal eddy momentum fluxes, especially before January 2016. The horizontal eddy momentum fluxes associated with the tropical waves arise from the deformation of the wave structure when background westerlies increase with height. These horizontal eddy momentum anomalies from the tropical waves are commonly observed in the reanalysis data but are typically much weaker than those in the 2015/16 winter. The possibility exists that exceptionally strong equatorially trapped waves precondition the flow to disruption by an extratropical disturbance.

The influences of El Nino and Arctic sea-ice on the QBO disruption in February 2016

The westerly phase of the quasi-biennial oscillation (QBO) was unexpectedly disrupted by an anomalous easterly near 40 hPa (~23 km) in February 2016. At the same time, a very strong El Nino and a very low Arctic sea-ice concentration in the Barents and Kara Sea were present. Previous studies have shown that the disruption of the QBO was primarily caused by the momentum transport of the atmospheric waves in the Northern Hemisphere. Our results indicate that the tropical waves evident over the Atlantic, Africa, and the western Pacific were associated with extratropical disturbances. Moreover, we suggest that the El Nino and sea-ice anomalies in 2016 account for approximately half of the disturbances and waves based on multiple regression analysis of the observational/reanalysis data and large-ensemble experiments using an atmospheric global climate model.

Strong Relationship between Dry-Season Rainfall over West Africa and Extratropical Disturbance

Strong Relationship between Dry-Season Rainfall over West Africa and Extratropical Disturbance

Extratropical Forcing Triggered the 2015 Madden\u2013Julian Oscillation\u2013El Ni\xf1o Event

In this paper, we report the triggering effect of extratropical perturbation on the onset of an atypical Madden–Julian Oscillation (MJO) and onset of the 2015–16 El Niño in March 2015. The MJO exhibited several unique characteristics: the effect of extratropical forcing, atypical genesis location and timing in the equatorial western Pacific, and the extremity of amplitudes in many aspects. The southward-penetrating northerly associated with the extratropical disturbances in the extratropical western North Pacific contributed to triggering the deep convection and westerly wind burst (WWB) and onset of the MJO over the anomalously warm tropical western Pacific in early March. The persisting strong WWB forced downwelling Kelvin wave-like oceanic perturbation that propagated eastward and led to the onset of the 2015–16 El Niño. The proposed novel extratropical forcing mechanism explaining the unique extratropics–MJO–El Niño association, based on both data diagnostics and numerical experiments, warrants further attention for a more detailed understanding of the onset of the MJO and its potential effect on El Niño.

The Dynamics of Australian Monsoon Bursts

Abstract The wet season of the Australian monsoon is characterized by subseasonal periods of excessively wet or dry conditions, commonly known as monsoon bursts and breaks. This study is concerned with the synoptic evolution prior to monsoon bursts, which are defined here by abrupt transitions of the area-averaged rainfall over the tropical parts of the Australian continent. There is large variability in the number of monsoon bursts from year to year and in the time interval between consecutive monsoon bursts. Reanalysis data are used to construct a lag composite of the sequence of events prior to a monsoon burst. It is determined that a burst in the Australian monsoon is preceded by the development of a well-defined extratropical wave packet in the Indian Ocean, which propagates toward the Australian continent in the few days leading up to the onset of heavy rainfall in the tropics. As in previous studies on the monsoon onset, the extratropical disturbances propagate equatorward over the Australian continent. These extratropical systems are accompanied by lower-tropospheric airmass boundaries, which also propagate into low latitudes. Ahead of these boundaries, relatively warm moist air is advected from the surrounding oceans, locally increasing the convective available potential energy. Commonly employed climate indices show that monsoon bursts are more likely to occur when the active phase of the Madden–Julian oscillation is in the vicinity of Australia. Neither El Niño–Southern Oscillation nor the southern annular mode has a significant impact on the occurrence of monsoon bursts.

Mechanistic analysis of the suppressed convective anomaly precursor associated with the initiation of primary MJO events over the tropical Indian Ocean

A primary Madden–Julian oscillation (MJO) event is one with no immediately preceding MJO event. Most primary MJO events initiating over the tropical Indian Ocean are preceded by a local suppressed convective anomaly (SCA). Based on daily outgoing longwave radiation and atmospheric circulation reanalysis data, composite analyses are performed to reveal the dynamical and thermodynamical mechanisms responsible for the generation of the precursor SCA associated with anomalous descending motion. During the developing stage, before the maximum SCA, the anomalous descending motion in the upper troposphere is dynamically forced by anomalous convergence over the eastern equatorial Indian Ocean. The anomalous convergent winds result from changes in subtropical circulation structures forced by extratropical disturbances in association with equatorward advection of positive potential vorticity. In the lower troposphere, the anomalous stable boundary layer with a strong thermal inversion is the dominant factor leading to descending motion. The sea surface temperature (SST) perturbations in the tropical Indian Ocean are an external forcing that generates the SCA through surface energy exchange. The negative SST anomalies in the northwestern portion of the tropical Indian Ocean act as a heat sink to cool the lower-tropospheric atmosphere, forcing strong descending motion and forming an anomalous anticyclone north of the equator. Meanwhile, the positive SST anomalies in the southeastern portion act as a heat source that forces anomalous divergent westerlies and northwesterlies toward the warmer SST area. Thus, anomalous descending motion is induced around the equator.

Synoptic and dynamical analysis of subtropical cyclone Anita (2010) and its potential for tropical transition over the South Atlantic Ocean

AbstractSubtropical cyclogenesis and tropical transitions (TT) over the South Atlantic Ocean only received attention after the first documented Hurricane Catarina occurred close to the southern Brazilian coast in March 2004. However, due to the lack of studies in this part of the Atlantic Ocean, it is still unclear what the main environmental conditions and dynamical processes associated with TT or even subtropical cyclogenesis are over the region. This study presents a synoptic and dynamical analysis of the subtropical cyclone Anita which occurred in March 2010 near the Brazilian coast. This system started as a pure subtropical cyclone, evolved to a condition favorable to TT, later developed into a cold‐core structure, and decayed as an extratropical cyclone. During the period favorable for TT, the turbulent heat fluxes (latent plus sensible) from the ocean decreased, and Anita started interacting with another extratropical disturbance, preventing the TT to happen. This interaction, in turn, increased the vertical wind shear, allowed the extratropical transition to occur, and promoted the westward displacement of Anita to colder waters, thus decreasing the turbulent heat fluxes. The results suggest that the combination of a dipole blocking pattern aloft, with contribution from barotropic energy conversions, and strong turbulent fluxes is an important ingredient for tropical storm development. Hybrid storms in such environmental conditions can be one form of precursors of hurricanes over the South Atlantic.

How well can CMIP5 simulate precipitation and its controlling processes over tropical South America?

Underestimated rainfall over Amazonia was a common problem for the Coupled Model Intercomparison Project phase 3 (CMIP3) models. We investigate whether it still exists in the CMIP phase 5 (CMIP5) models and, if so, what causes these biases? Our evaluation of historical simulations shows that some models still underestimate rainfall over Amazonia. During the dry season, both convective and large-scale precipitation is underestimated in most models. GFDL-ESM2M and IPSL notably show more pentads with no rainfall. During the wet season, large-scale precipitation is still underestimated in most models. In the dry and transition seasons, models with more realistic moisture convergence and surface evapotranspiration generally have more realistic rainfall totals. In some models, overestimates of rainfall are associated with the adjacent tropical and eastern Pacific ITCZs. However, in other models, too much surface net radiation and a resultant high Bowen ratio appears to cause underestimates of rainfall. During the transition season, low pre-seasonal latent heat, high sensible flux, and a weaker influence of cold air incursions contribute to the dry bias. About half the models can capture, but overestimate, the influences of teleconnection. Based on a simple metric, HadGEM2-ES outperforms other models especially for surface conditions and atmospheric circulation. GFDL-ESM2M has the strongest dry bias presumably due to its overestimate of moisture divergence, induced by overestimated ITCZs in adjacent oceans, and reinforced by positive feedbacks between reduced cloudiness, high Bowen ratio and suppression of rainfall during the dry season, and too weak incursions of extratropical disturbances during the transition season.

The predictability of precipitation episodes during the West African dry season

AbstractPrecipitation episodes in tropical West Africa (7–15°N, 10°W–10°E) during the dry season from November to March are rare, but can have significant impacts on human activities reaching from greening of pastures to spoiling harvests and health implications. Previous work has shown a link between these unseasonal rainfalls and extratropical disturbances via a decrease of surface pressure over the Sahara/Sahel and a subsequent inflow of moist air from the Gulf of Guinea. This paper supports the previously stated hypothesis that the extratropical influence leads to a high rainfall predictability through a careful analysis of operational 5 day forecasts from the European Centre for Medium‐Range Weather Forecasts' (ECMWF) ensemble prediction system (EPS), which are evaluated using Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) precipitation estimates for the 11 dry seasons 1998/99‐2008/09. The long‐term regional average of ensemble‐mean precipitation lies between the two observational datasets, with GPCP being considerably wetter. Temporal correlations between the ensemble mean and observations are 0.8. Standard probabilistic evaluation methods such as reliability and relative operating characteristic (ROC) diagrams indicate remarkably good reliability, sharpness and skill across a range of precipitation thresholds. However, a categorical verification focusing on the most extreme ensemble mean values indicates too many false alarms. Despite the considerable observational uncertainty the results show that the ECMWF EPS is capable of predicting winter rainfall events in tropical West Africa with good accuracy, at least on regional spatial and synoptic time‐scales, which should encourage West African weather services to capitalize more on the valuable information provided by ensemble prediction systems during the dry season. Copyright © 2012 Royal Meteorological Society

A seasonal study of the atmospheric dynamics over the Iberian Peninsula based on circulation types

A seasonal analysis of the atmospheric circulation over the Iberian Peninsula (IP) based on circulation types (CTs) obtained from sea level pressure and 500-hPa geopotential height is presented. The study covers the period of 1958–2008, when a high variability and important changes in winter and spring precipitation and temperature have been reported. Frequency, persistence, and the most probable transitions of the circulation types are analyzed. Among the clustering methods available in the literature, two of the most reliable classification methods have been tested, K-means and simulated annealing and diversified randomization. A comparison of both methods over the IP is presented for winter (DJF). The quality of the circulation types obtained through both methods as well as the better stability achieved by K-means suggest this method as more appropriated for our target area. Twelve CTs were obtained for each season and were analyzed. The patterns obtained were regrouped in five general situations: anticyclonic, cyclonic, zonal, summertime, and hybrid-mixed. The analysis of frequencies of these situations offers a similar characterization of the atmospheric circulation that others previously obtained by subjective methods. The analysis of the trends in frequency and persistence for each CT shows few significant trends, mainly in winter and spring with a general decrease of the cyclonic patterns and an increase of the anticyclonic situations. This can be related to the negative precipitation trends reported by other authors. Regarding the persistence, an interesting result is that there is a high interannual variability of the persistence in autumn and spring, when patterns can persist longer than in other seasons. An analysis of the most probable transitions between the CTs has been performed, revealing the existence of cyclic sequences in all seasons. These sequences are related to the high frequency of certain patterns such as the anticyclonic situations in winter. Finally, a clear seasonal dependence of the transitions between cyclonic situations associated with extratropical disturbances was found. This dependence suggests that the transitions of low-pressure systems towards the south of the IP are more likely in spring and autumn than in winter.

Roles of European blocking and tropical-extratropical interaction in the 2010 Pakistan flooding

[1] A sequence of monsoon surges struck Pakistan and Northwestern India during late July-early August 2010. The unusually heavy monsoon rainfall resulted in record-breaking floods, which affected 20 million people with a death toll of near 3000. Simultaneously, a long-lived blocking high appeared over Europe and Russia in middle June and persisted for nearly two months. Extreme flooding occurred when the southward penetration of extratropical potential vorticity in the deep trough east of the European blocking and the tropical monsoon surges arrived concurrently in Pakistan. This study demonstrates that the interaction between the tropical monsoon surges and the extratropical disturbances downstream of the European blocking was the key factor leading to the severe flooding in Pakistan. The 2010 La Niña condition contributed indirectly to the flooding by inducing a low-level easterly anomaly in South and Southeast Asia, which weakened eastward moisture transport and helped enhance moisture transport (convergence) to (in) the Northern Arabian Sea and Pakistan.

Evolution of extreme Total Water Levels along the northern coast of the Iberian Peninsula

Abstract. This paper assesses the evolution of storminess along the northern coast of the Iberian Peninsula through the calculation of extreme (1%) Total Water Levels (eTWL) on both observed (tide gauge and buoy data) and hindcasted (SIMAR-44) data. Those events were first identified and then characterized in terms of oceanographic parameters and atmospheric circulation features. Additionally, an analysis of the long-term trends in both types of data was performed. Most of the events correspond to a rough wave climate and moderate storm surges, linked to extratropical disturbances following a northern track. While local atmospheric conditions seem to be evolving towards lesser storminess, their impact has been balanced by the favorable exposure of the northern coast of the Iberian Peninsula to the increasing frequency and strength of distant disturbances crossing the North Atlantic. This evolution is also correctly reproduced by the simulated long-term evolution of the forcing component (meteorological sea level residuals and wave run up) of the Total Water Level values calculated from the SIMAR 44 database, since sea level residuals have been experiencing a reduction while waves are arriving with longer periods. Finally, the addition of the rate of relative sea level trend to the temporal evolution of the atmospheric forcing component of the Total Water Level values is enough to simulate more frequent and persistent eTWL.

Dust mobilization and aerosol transport from West Africa to Cape Verde—a meteorological overview of SAMUM–2

The second field campaign of the SAharan Mineral dUst experiMent (SAMUM-2) was performed between 15 January and 14 February 2008 at the airport of Praia, Cape Verde, and provided valuable information to study the westward transport of Saharan dust and the mixing with biomass-burning smoke and sea-salt aerosol. Here lidar, meteorological, and particle measurements at Praia, together with operational analyses, trajectories, and satellite and synoptic station data are used to give an overview of the meteorological conditions and to place other SAMUM-2 measurements into a large-scale context. It is demonstrated that wintertime dust conditions at Cape Verde are closely related to the movement and intensification of mid-latitude high-pressure systems and the associated pressure gradients at their southern flanks. These cause dust emission over Mauritania, Mali, and Niger, and subsequent westward transport to Cape Verde within about 1–5 d. Dust emissions often peak around midday, suggesting a relation to daytime mixing of momentum from nocturnal low-level jets to the surface. The dust layer over Cape Verde is usually restricted to the lowest 1.5 km of the atmosphere. During periods with near-surface wind speeds about 5.5 ms−1, a maritime aerosol layer develops which often mixes with dust from above. On most days, the middle levels up to about 5 km additionally contain smoke that can be traced back to sources in southernWest Africa. Above this layer, clean air masses are transported to Cape Verde with the westerly flow at the southern side of the subtropical jet. The penetration of extra-tropical disturbances to low latitudes can bring troposphere-deep westerly flow and unusually clean conditions to the region.

Influence of the tropical cyclone Omar on the weather in Europe

A case of the Atlantic tropical cyclone influence on the weather of Europe is considered. It is demonstrated on the basis of computations of the numerical mesoscale ETA model that one of the reasons of the positive air temperature anomaly in fall 2008 was the tropical cyclone Omar. The detailed analysis was carried out of the computation results of the movement and evolution of the tropical cyclone Omar and of the atmospheric structures surrounding it. The energy characteristics, the explicit and latent heat fluxes, and the precipitation in the tropical cyclone were computed. The computations revealed that starting since the certain moment, the tropical storm Omar actively interacted with the polar front which shifted to the low latitudes and with the extratropical disturbance of synoptic scale which originated on this front. It was obtained that this interaction resulted in the transfer of the huge tropical cyclone energy to the extratropical pole-front cyclone and in the intensification of the latter one. It is demonstrated that the air which circulated in the tropical cyclone and possessed high the thermohygrometric characteristics entered the warm sector of extratropical cyclone and was carried out to Europe.