Abstract
The North African Coastal Low-Level Jet (NACLLJ) is a semi-permanent feature offshore the north western African coast, linked to the cold nearshore upwelling of the Canary Eastern Boundary Current system. Its main synoptic drivers are the Azores Anticyclone over the ocean and the inland Sahara thermal low. The coastal jet events occur in one of the world’s most productive fisheries region, thus the evaluation of the effects of global warming in its properties is imperative. This study proposes an analysis of the annual and intra-annual attributes of the NACLLJ for two time periods 1976–2005 (historical) and 2070–2199 (future), resorting to coupled and uncoupled atmosphere–ocean simulations with the ROM model, as well as near surface offshore wind speed from the CORDEX-Africa ensemble. The future simulations follow the RCP8.5 greenhouse gas emissions scenario. Overall, the ROM coupled simulation presents the best performance in reproducing the present-climate near surface wind speed, offshore northwest Africa, compared to the remaining RCM simulations. The higher SST resolution in the coupled simulations favours much localised colder upwelling strips near the coast and consequently stronger jets. In future climate, a small increase in the surface wind speed is projected, mainly linked to the regions of coastal jet presence. The NACLLJ is projected to be more frequent and intense, encompassing larger areas. An increase of the jet seasonal frequencies of occurrence is projected for all seasons, which is larger from spring to autumn (up to 15, 16 and 22% more frequent, respectively). However, in some offshore areas the winter NACLLJ persistency is likely to double, relatively to present-climate. Higher inter-annual variability is also projected for the future NACLLJ seasonal frequencies. The strengthening of the coastal jet speeds is also significant, between 5 and 12% in all seasons. Additionally, the jet’s diurnal cycle shows an increase in jet occurrence across the day, particularly in the mid and late afternoon.
Highlights
Regional and local climates along Eastern Boundary Current System (EBCS) regions are highly influenced by coastal atmosphere-land–ocean interaction processes, namely by the land–ocean temperature contrast and coastal geomorphology1 3 Vol.:(0123456789)(Bakun et al 2015)
In the North African Coastal Low-Level Jet (NACLLJ) region, both simulations project rises in the frequency of occurrence and, in general, these increases are larger in the coupled run
The EBCS are the most productive fisheries areas in the global Ocean (Wang et al 2015), the evaluation of climate change effects in these areas is of the upmost importance
Summary
Regional and local climates along Eastern Boundary Current System (EBCS) regions are highly influenced by coastal atmosphere-land–ocean interaction processes, namely by the land–ocean temperature contrast and coastal geomorphology1 3 Vol.:(0123456789)(Bakun et al 2015). The aloft subsidence compresses the MABL, that associated to the lowest sea surface temperature (SST) at the coast, forces the onshore tilting of the MABL height. The MABL inversion layer, and due to the decreasing temperature with height, a thermal wind in opposite direction of the synoptic flow develops. A thermal wind in the direction of the synoptic flow develops in association to the increasing temperature with height. This in conjunction with the surface friction and the decreasing wind above the MABL generates a maximum wind speed in the vicinities of the inversion layer which is designated by coastal low-level jet (CLLJ). The flow must turn to follow the coastline the thickness of the MABL decreases and the flow accelerates, forming an expansion fan
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