A Climatological Analysis of the Benguela Coastal Low-Level Jet

The Benguela coastal low-level jet (CLLJ) is characterized by intense winds that occur around 400 m above sea level, within or at the top of the marine atmospheric boundary layer. The semipermanent St. Helen high-pressure system, over the South Atlantic Ocean, and the inland thermal low-pressure system, over the Namib Desert, are the synoptic forcing behind the Benguela CLLJ. This coastal jet is an important mesoscale feature in the Namibia and Angola coastal areas, since it is present virtually all year round, with a marked seasonal cycle. This study investigates the climatology of the frequency and the intensity of the Benguela CLLJ and its relationship with synoptic and local forcing's, using high-resolution modeling. An uncoupled ROM (REMO-OASIS-MPIOM) hindcast simulation, with 25-km horizontal resolution, for the period 1980-2014, is used to analyze the features of the Benguela CLLJ. It is shown that Benguela CLLJ is characterized by two local maxima of frequency of occurrence at around 26 degrees S and 17.5 degrees S. During austral summer, the jet has a frequency of occurrence of about 60% and it is in its southern core. During autumn and winter, the frequency of occurrence decreases to nearly half and migrates equatorward. During spring, the jet has a frequency of occurrence of 45% and is found at the northern core. The jet wind speed is higher in the south core but is found at higher altitudes in the north core. The seasonal cycle of horizontal momentum budget defines the seasonality of Benguela CLLJ frequency of occurrence.