Ahaa – this is not about low flying aircraft but fast moving currents of air in the lowest 2000m of the atmosphere that follow coastlines with mountainous terrains. While generally narrow in nature they can extend several hundred nm offshore in width and can cause periodic heavy seas for shipping. They have also become of increasing interest in terms of positioning and operating wind farms as they generate high wind shear and turbulence.
Known as Coastal Low Level Jets (CLLJs), they require three main ingredients:
1. Mountainous terrain that runs parallel along the coast, which acts as a wall preventing winds near the surface to move inland and so are deflected by Coriolis force (to the right/left in the northern/southern hemispheres). Three typical locations are shown in Figure 1.
Figure 1: Oman, the Horn of Africa and California showing elevation along the coasts with narrow hinterlands. Source: Google Earth
2. The presence of a marine boundary layer (MBL) in the lower atmosphere which results from wind mixing and coupled ocean-atmospheric dynamics and thermodynamics which is more complex than over land. Generally, water has less friction than land, meaning winds over water will be stronger than winds over land, creating a deeper (as in height above sea level) of mixed maritime moist air capped by an inversion as depicted in Figure 2. This results in a strong large-scale horizontal pressure gradient that is driven by subtropical high pressure over the ocean and thermal low pressure over the land.
Figure 2: Left – vertical cross section of potential temperature along an idealised mountainous coastline where the temperature gradient is tight and sloped along the coastline close to the mountains and becomes near horizontal further offshore: Right – vertical cross section showing wind speeds with height in the same idealised mountainous coastline. The core of CLLJ is around and just below 925 hPa, roughly where the tightest temperature gradient is within the marine boundary layer. The winds extend downwards from the core towards the coastline, albeit with less strength. Source: The COMET Program.
3. The third ingredient is cooler surface waters associated with local upwelling along coastal regions creating a much greater land-sea thermal contrast and stronger temperature gradients over water than on land that collectively helps form the MBL capped by a strong inversion. Coastal winds exert stress on the ocean surface, displacing coastal waters offshore by Ekman transport which brings cooler waters to the surface as depicted in Figure 3. Essentially, the greater the land-sea thermal contrast then the stronger the wind. Equally, upwelling can also favour low cloud and extensive fog formation.
Figure 3. Left – schematic diagram of (Ekman spiral) net water transport for wind along a coast in the northern hemisphere, and Right – illustration showing upwelling and cooler surface waters that result for an example in the southern hemisphere.
As shown in Figure 2, as this temperature gradient approaches land it slopes down to ground level inland. The associated isotherms (lines of equal temperature) become more tightly packed together due to the terrain creating the wall effect and establishing a current of air where the tightest temperature gradient exists in the MBL. The strongest wind speeds will always be in the jet’s core, but the jet itself can extend down to the water’s surface, enhancing the local wind conditions and posing a potential hazard to shipping.
Some of the best known examples around the globe are associated with the five permanent eastern boundary current (EBC) system regions, i.e. the ocean currents that flow toward the equator off California, Peru-Humboldt, Benguela, Canaries and West Australia. Figure 4 shows these regions occur on the eastern flanks of the mid-latitude oceans and are synoptically associated with the presence of the semi-permanent sub-tropical anticyclone and inland thermal low-pressure systems. These jets occur within the MBL capped by a strong inversion and are locally enhanced by the land–ocean thermal contrast and the subsiding air from the anticyclone above it. In addition, local funnelling effects caused by coastal features such as peninsulas, capes and bays can also have a significant impact on the jet flow.
Figure 4. Map showing main global locations for Coastal Low Level Jets. Source: Globe image from Coperniscus.
The frequency of occurrence of coastal jets is stronger in the northern hemisphere than in the southern hemisphere. In the southern hemisphere, the Peru-Humboldt and Benguela coastal jets occur during the entire year, with lower frequencies of occurrence in the austral winter. In the northern hemisphere, during the intermediate seasons the CLLJ regions display lower frequencies of occurrence, with the notable exception of the North African CLLJ which is present all year round, with frequencies of occurrence above 20% in winter increasing to 50%-60% in summer as depicted in Figure 5. This jet can have a significant impact on northbound shipping.
Figure 5. Maps of seasonal coastal jet frequency of occurrence (%), DJF-winter, MAM-spring, JJA-summer and SON-autumn. Source: Soares 2018
Other, non EBC examples occur in the Caribbean (Figure 6: Right) and Arabian Sea where the set-up is slightly different. The easterly jet in the Caribbean Sea off Columbia and Venezuela is present throughout the year and is at a maximum in February and July and a minimum in May and October linked to the seasonal drift of the North Atlantic subtropical high pressure. The S-W Somalia jet (also known as Findlater) and W-SW Oman jets are linked to the SW Monsoon season and result from synoptic forcing associated with the Southeast Asia monsoon circulation which plays the same role as the sub-tropical high-pressure circulation does along mid-latitude continental western coasts. In particular, the Oman jet has strong seasonality with up to 70% occurrence during peak monsoon season during the boreal summer (Jul-Aug). Other non EBC examples include jets off New Zealand and the Bohai/Yellow Sea/Taiwan St (indicated with dashed white boxes in Figure 4) when strong pressure gradient differences occur, helped by local orography.
Figure 6: Mean Sea Level Pressure and wind speeds off the coasts of (left) California and Oregon and (right) off Colombia / Venezuela for 9 Feb 22. Note how the high pressure is oriented in a way that the winds follow the coastline. Also note the broad area of 20-30 kt and locally 30-35 kt winds hugging the coastlines.
CLLJs occur in many coastal regions around the globe and can give rise to strong to gale force alongshore winds and associated heavy seas. Local effects such as funnelling around peninsulas and capes can add to these effects. Upwelling of cooler waters in such regions can also lead to the formation of extensive fog and low cloud.
Stay connected and safe.