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Coastal low-level jet and El Niño-like phenomenon at the Benguela coast
2017
The Benguela Eastern Boundary Upwelling System (EBUS) is characterised by intense coastal upwelling off the southwestern Africa, and is one of the most productive marine ecosystems in the world's oceans. Its highly nutrient-rich waters make this EBUS an essential habitat to many species and is crucial in supporting the livelihood of the local population. Forced by the wind-driven equatorward Benguela Current, the upwelling system is influenced by the surface winds. In turn, these are associated with the quasi-permanent Benguela Coastal Low-Level Jet (CLLJ), an atmospheric feature characterised by wind speeds superior to 10 m/s and especially intense during the austral spring. Every few years, an El Niño-like phenomenon affects the Benguela coastal region, disrupting the fragile upwelling ecosystems and the regional climate. This anomalous warming of the ocean surface is known as Benguela Niño, and may reach, in average, SST anomalies of 1.5ºC. While the El Niño and some CLLJs have been extensively studied in separate, little has been documented about their respective Benguela counterparts, and the relationship between the two features is rather unexplored. This study uses the high-resolution NOAA Optimal Interpolation Sea Surface Temperature V2 (OI SST V2) dataset and the Japanese 55-year Reanalysis (JRA-55) surface and model-level data for the time period between 1980 to 2016. For the first time, it is shown how the vertical structure of the marine atmospheric boundary layer (MABL) physical properties respond to the influences of both the Benguela Niño and the northern core of the CLLJ, establishing a connection between the two highly-impactful phenomena. Although the period studied is limited and the sampling for the Niño and Niña events is small (6 and 9 identified events for Niño and Niña, respectively), some characteristics of the Benguela jet for SST-based composites of Niño, Niña and "neutral" cases are presented. There is evidence that the physical background associated with the Benguela Niño (Niña) sustains weaker (stronger) manifestations of the Benguela CLLJ, and place it lower (higher) in the MABL. During Niños, the jet is also less frequent than Niñas. It is also shown that a horizontal spatial analysis of the surface wind field is insufficient to study the development of the Benguela CLLJ, and even the study of the vertical structure of the MABL properties cannot relay all the complex interaction between the lower atmosphere and the surface.
Tellus A, 2013
A B S T R A C T A low-level wind maximum is often found over the oceans near many coasts around the world. These Coastal Low-Level Jets (CLLJs) play an important role in the coastal weather and have significant impacts on regional climate and ecology as well as on a number of human activities. The presence of CLLJs is related to various local circumstances such as land-sea temperature contrasts, upwelling, coastal terrain, orientation of the coast, and so on, but also to the large-scale atmospheric dynamics. This makes studies of CLLJs not only interesting but also challenging. In this study, based on ERA-Interim reanalysis data, the global distribution, spatiotemporal structure and the seasonal variability of CLLJs are documented. Seasonal data from 1980 to 2011 are used to identify areas where CLLJs are frequently found in the lowest 2 km, following criteria based on the vertical profiles of wind speed and temperature. The results are analysed to highlight the fundamental aspects and distinctive features of the CLLJs across the globe, including their occurrence rate, jet height, maximum wind speed and horizontal extent. Global maps of CLLJs are constructed for the summer and winter seasons. The west coasts of North America, the Iberian Peninsula, north-western Africa and the south-eastern coast of the Arabian Peninsula make up the Northern Hemispheric CLLJ regions, while the west coasts of South America, Australia, and southern Africa comprise the South Hemispheric equivalents. The existence and characteristics of CLLJs along the southern coast of Oman and the western coast of the Iberian Peninsula regions are also discussed, not fully envisaged before in the context of CLLJs. The highest occurrence of CLLJs is found during the summer in both hemispheres, and the coast of Oman has the globally highest CLLJ frequency, with also the highest maximum wind speeds. The most commonly found CLLJ has a maximum wind speed between 9 and 15 m s (1 , and occurs at heights between 500 and 700 m a.s.l.
The Impact of a Warmer Climate on the Global Coastal Low-Level Wind Jets
2014
Coastal Low-Level Jets (henceforth referred to as "coastal jets" or simply as CLLJ) are low-tropospheric mesoscale wind features, with wind speed maxima confined to the marine atmospheric boundary layer (MABL), typically bellow 1km, and most of the times below 500 m. Coastal jets occur in the eastern flank of the semi-permanent subtropical mid-latitude high pressure systems, along equator ward eastern boundary currents, due to a large-scale synoptic forcing. In this study, based on a 2-member GCM EC-Earth simulations (the "ensemble"), the impact of a warmer climate on the global coastal low-level wind jets in the twenty-first century is briefly analysed, using the classification and filtering criteria of CLLJ detection proposed by . A twentieth century period from present climate is used as control run. The projected changes in the global CLLJ climate at the end of the twenty-first century are analysed for the 2071 to 2100 period with the RCP8.5 greenhouse gas emissions scenario. The projections show that the Iberian Peninsula and Arabian Peninsula are the two regions with a significant increase of the frequency of occurrence of CLLJ. There is also a projected expansion of the offshore extension to the west. In the future climate, the Iberian Peninsula Coastal Jet wind speed maximum is expected to occur at higher levels, with higher wind speeds.
Variability in satellite winds over the Benguela upwelling system during 1999–2000
Journal of Geophysical Research, 2004
Wind stress variability over the Benguela upwelling system is considered using 16 months (01 August 1999 to 29 November 2000) of satellite-derived QuikSCAT wind data. Variability is investigated using a type of artificial neural network, the selforganizing map (SOM), and a wavelet analysis. The SOM and wavelet analysis are applied to an extracted data set to find that the system may be divided into six discrete wind regimes. The wavelet power spectra for these wind regions span a range of frequencies from 4 to 64 days, with each region appearing to contain distinct periodicities. To the north, 10°-23.5°S, the majority of the power occurs during austral winter, with a 4-16 day periodicity. Further investigation of National Centers for Environmental Prediction reanalysis outgoing longwave radiation data indicates that the winter intensification of wind stress off the Angolan coast is linked with convective activity over equatorial West Africa. The summer activity appears to be linked with the intensification of the Angolan heat low. Convective activity over the Congo basin appears to impact upon wind stress variability, off the Angolan coast, throughout the year. Farther south, 24°-35°S, the majority of the power occurs in the summer. Here a bimodal distribution occurs, with peaks of 4-12 and 25-50 days. The southernmost regions appear to be forced at higher frequencies by both midlatitude cyclones (austral winter) and mesoscale coastal lows (austral summer). At lower frequencies, eastward propagating periodic wind events that originate over eastern South America appear to be important to the forcing of wind stress over the southern Benguela.
Linking wind and interannual upwelling variability in a regional model of the southern Benguela
Geophysical Research Letters, 2002
1] We quantify the wind contribution to the development of interannual sea surface temperature (SST) anomalies along the shelf of southern Africa. We compare numerical simulations that differ only in the amount of variability kept in the ERS1/2-derived surface wind forcing. Surprisingly, most of the cold and warm episodes over the Agulhas Bank are strictly related to local fluctuations of the forcing, whereas the shelf of the west coast extending 400 km north of Cape Columbine is equally sensitive to open-sea wind fluctuations. We diagnose the respective role of mesoscale eddy activity and of low frequency and intra-monthly wind fluctuations in generating interannual SST variability. The fair degree of correlation obtained at a few locations between the model and concomitant observations confirms the interest of a regional numerical tool to study anomalous events in the Benguela system.
2016
We quantify the wind contribution to the development of interannual sea surface temperature (SST) anomalies along the shelf of southern Africa. We compare numerical simulations that differ only in the amount of variability kept in the ERS1/2-derived surface wind forcing. Surprisingly, most of the cold and warm episodes over the Agulhas Bank are strictly related to local fluctuations of the forcing, whereas the shelf of the west coast extending 400 km north of Cape Columbine is equally sensitive to open-sea wind fluctuations. We diagnose the respective role of mesoscale eddy activity and of low frequency and intra-monthly wind fluctuations in generating interannual SST variability. The fair degree of correlation obtained at a few locations between the model and concomitant observations confirms the interest of a regional numerical tool to study anomalous events in the Benguela system.
Journal of Geophysical Research, 2005
The structure and patterns of variability of the southern Benguela coastal upwelling system are investigated with a high-resolution regional model forced by QuikSCAT winds over 1999QuikSCAT winds over -2003 The relevance of this global wind product is tested, at first, for the specific nearshore southeast Atlantic; then, the wind products are spatially or temporally degraded from the original 0.5° daily fluxes and are used to diagnose the main scales of the surface dynamical forcing variability. Time resolution appears as a crucial factor in the wind stress to retrieve the patterns of interannual anomalies in sea surface temperatures in a good agreement with independent NASA Pathfinder observations. Various upwelling indices are also calculated in the model to study specific warm or cold coastal events whose circulation schemes are detailed from a Lagrangian interpretation of the model time-varying threedimensional velocity field. Our study emphasizes the connections established between the Benguela upwelling and the large-scale circulation in relation with, for instance, the Agulhas Current retroflection and associated ring shedding.
A B S T R A C T Coastal low-level jets (CLLJ) are a low-tropospheric wind feature driven by the pressure gradient produced by a sharp contrast between high temperatures over land and lower temperatures over the sea. This contrast between the cold ocean and the warm land in the summer is intensified by the impact of the coastal parallel winds on the ocean generating upwelling currents, sharpening the temperature gradient close to the coast and giving rise to strong baroclinic structures at the coast. During summertime, the Iberian Peninsula is often under the effect of the Azores High and of a thermal low pressure system inland, leading to a seasonal wind, in the west coast, called the Nortada (northerly wind). This study presents a regional climatology of the CLLJ off the west coast of the Iberian Peninsula, based on a 9 km resolution downscaling dataset, produced using the Weather Research and Forecasting (WRF) mesoscale model, forced by 19 years of ERA-Interim reanalysis (1989Á2007). The simulation results show that the jet hourly frequency of occurrence in the summer is above 30% and decreases to about 10% during spring and autumn. The monthly frequencies of occurrence can reach higher values, around 40% in summer months, and reveal large inter-annual variability in all three seasons. In the summer, at a daily base, the CLLJ is present in almost 70% of the days. The CLLJ wind direction is mostly from north-northeasterly and occurs more persistently in three areas where the interaction of the jet flow with local capes and headlands is more pronounced. The coastal jets in this area occur at heights between 300 and 400 m, and its speed has a mean around 15 m/s, reaching maximum speeds of 25 m/s.
Climatic determinants of Benguela SST variability
Continental Shelf Research, 1995
Climatic determinants of summer sea surface temperature (SST) variations in the Benguela upwelling zone of the SE Atlantic are studied through statistical associations with field variables of SST and surface winds in the ocean basins surrounding SW Africa. Shelf SST indices are formulated from COADS monthly ship data for the southern and central Benguela for February and October respectively, months of active upwelling. Linear correlation maps are produced at lags-4 and 0 months and provide some unexpected results, February, southern area SST trends are downward over the period 1950-1988. Correlations between the shelf SST index and SST in the rest of the Benguela zone are positive and suggest that interannual signals are widespread in the longshore direction, but out of phase with the central ocean basins, Correlations between SST and meridional winds at 0 lag are-0.4 in the south and-0.8 in the central Benguela. This confirms the expected relationship between southerly wind and upwelling. The lower correlation in the south indicates that SST variability is often advectively forced whereas, in the central Benguela, SST variability is driven by local upwelling. To assess the impact of an El Nifio on winds in the SE Atlantic, a general circulation model (GCM) simulation is performed. A fixed +2°C SST anomaly is imposed in the central Indian Ocean for a period of three years. Resultant surface layer winds in February are compared with climatology and inferences are made with regard to upwelling. Southerly wind stress is nearly doubled in the central Benguela. On the other hand, the Cape Town area obtains increased onshore flow from mid-latitude westerlies, and a reduction in upwelling is inferred. The GCM simulation is consistent with the observed pattern of an enhanced anticyclonic gyre in the SE Atlantic and cooler SST in the central Benguela. The GCM and statistical results demonstrate regional response patterns with respect to coastal upwelling, and offer deterministic inputs to environmental long-range forecasts.