In May 2017, MODIS imagery from the Terra satellite revealed a rarely observed movement of water – internal waves or solitons in the eastern Strait of Gibraltar.
13 November 2020
22 May 2017
By Stephanie Ball (MeteoGib) and Jose Prieto (EUMETSAT)
While in meteorology, the use of satellite imagery may be more often associated with the study of clouds and the development of weather systems, high resolution imagery can also reveal other important information to meteorologists, such as the flow of dust or volcanic ash, algae blooms, and even the movement of water.
On Monday 22 May, the NASA Terra MODIS satellite revealed one such image, a rarely observed movement of water — internal waves or solitons in the Strait of Gibraltar, see Figure 1 and 2 (enhanced).
So, what are internal waves? Internal waves are gravity waves which take place within a body of a fluid, rather than on the surface, and which exist due to the fluctuations in density within a fluid caused by changes, for example, in temperature or salinity with depth across a fluid.
The Strait of Gibraltar provides the perfect ingredients for the presence of internal waves. It connects the Atlantic Ocean with the Mediterranean Sea, acting as an exchange point in the flow between both bodies of water, and, hence, consisting of two different layers — a deep layer of salty water from the Mediterranean and an overlying layer of Atlantic water with lower salinity.
The mean flow is composed of the difference in flow between these two bodies, with an upper layer of Atlantic water flowing into the Mediterranean Sea and known as the Atlantic jet, which can become enhanced during the summer months; while a lower layer of Mediterranean water trickles west in to the Atlantic Ocean.
The internal waves are generated by the tidal flow eastwards as it flows over the shallow Camarinal Sill within the Strait of Gibraltar (Figure 3), generating the waves which are not normally visible.
However, on occasions such as the 22 May, they can become visible to satellites due to sun glint off the water’s surface where they can appear like rings exiting the Eastern Strait (Figure 4).
These waves can, at times, go on for many kilometres, although they are not always so coherent and interference patterns can exist due to interaction with other waves.
An example of this occurrence can be seen in the image taken on the International Space Station on 3 June 2004 at 12:21 UTC (Figure 5).
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