Friday, December 8, 2023

High-amplitude internal waves, generated at the Camarinal Sill, propagate eastward through the Strait of Gibraltar.

From Eumetsat by Marina Bolado-Penagos, Águeda Vázquez, Miguel Bruno, Aida Alvera-Azcárate, Noemi Marsico, Ben Loveday
High-amplitude internal waves, generated at the Camarinal Sill, propagate eastward through the Strait of Gibraltar.
These waves, which effect surface roughness, can detected by both radar and optical satellite missions.

What are internal waves and why do we care about them?

Waves on the sea surface are a familiar phenomenon for anyone who has sailed or been to a beach. However, waves also occur in the depths of the oceans. 
They are the result of changes in density of ocean waters, as a result of variable water temperatures and salinity. 
They play a crucial role in how energy is transferred in the ocean and can affect the movement of nutrients and other essential chemical species, including those related to the ocean carbon cycle.
Internal waves can reach scales of over 100m vertically and, as a result, are a concern for underwater operations, eg by submarines.
They can have very little surface expression, especially compared to their scale at depth, however, they have also been associated with interesting features by sailors, including 'dead water'.
Internal wave generated in the Strait of Gibraltar?

In the region near the Strait of Gibraltar, where the Atlantic Ocean meets the Mediterranean Sea, there is a complex interaction of different water mass layers.
The lighter, upper layer of water, derived from the Atlantic, flows eastward into the Alboran Sea in a twisting current, known as the Atlantic Jet.
This current undergoes seasonal changes and plays a significant role in the upper layer dynamics of the region.
The more dense and salinated Mediterranean water flows westward at greater depths towards the Atlantic Ocean.
This interaction in the Strait of Gibraltar is often simplified as a two-layer exchange, and it creates a region called the Atlantic-Mediterranean Interface (AMI).
The AMI is warmer, more shallow, with higher salt levels, on the eastern side of the Strait than on the western side.

The AMI is subject to different coastal and sea-floor influences at different levels.
In the upper layers, the flow of Atlantic water is controlled by the narrowest part of the Strait, called the Tarifa Narrows, where the AMI is thicker.
The outflow of Mediterranean water is controlled by certain underwater sills or ridges in the Strait, especially the Espartel and Camarinal Sills.
The Camarinal Sill has been extensively studied, because it generates large waves beneath the surface, known as High Amplitude Internal Waves.
These waves form when the strong westward tidal currents pass over the Sill, creating an internal wave on the lee side of the ridge.
They remain near the Camarinal Sill for about three hours, until the water flow slows down, after which they mostly move eastward into the Mediterranean Sea.

Internal waves are studied using a dimensionless number called the internal Froude number (G2), which takes into account the average velocity, density, and thickness of the layers where internal waves form. Internal waves stay trapped at the Sill when this number equals or exceeds one, and are released when it is lower than one.
Even though these waves are created deep underwater, they can also be seen from space, because they effect the sea surface roughness.
This is visible in radar and optical imagery. Satellites have tracked these waves within the Alboran Sea as far as 200km from the Camarinal Sill (35.93 N, 5.75 W), showing that they can persist up to 50 hours after they start moving.
Do these internal waves always move at the same speed?

The answer is no. The speed at which they travel can change, based on different factors.
These include: how the water is layered in terms of density; the direction of winds (east to west or west to east), and the strength of tides or currents.
The tidal cycle influences the energy of these wave processes, and, while the most energetic waves occur during spring tides, a less intense process happens during neap tides.
During spring tides the very powerful process causes significant vertical movement of water layers, with changes in current speed, and neap tides involve moderate vertical water movement and less disturbance in current speed.

Vázquez et al. (2008) observed that the generation of High Amplitude Internal Waves often depends on the condition of flows which are primarily influenced by changes in atmospheric pressure over the Mediterranean region.
As a result, the occurrence of internal wave events can be suppressed or triggered depending on the specific weather conditions over the Mediterranean.
Recently, Bolado-Penagos et al. (2023) confirmed that atmospheric forcing must be taken into account along with the tidal cycle to characterise the travel time of internal waves towards the Mediterranean.
They observed that those waves took between 14 and 20 hours to reach the northwest of the Alboran Sea.
In terms of speed, this means that the internal waves were moving at a rate of approximately 0.9 to 1.3ms-1.
Estimating wave speed from Sentinel-1 SAR-C and Sentinel-3 OLCI

In this case study, the presence of an internal wavefront on 14 May 2023, is observed using images from the Copernicus Sentinel-1 SAR-C (synthetic aperture radar) sensor and Sentinel-3 OLCI (optical) sensors.
From the Sentinel-1 radar image (shown in the left side of Figure 1), we can see the presence of the wave train to the east of the Strait of Gibraltar.
This front was detected at 06:28 AM, while Sentinel-3 OLCI captured the same train at 10:49 AM (Figure 1, right panel).

Sentinel images comparison : Sentinel-3
Figure 1: Internal wave patterns in the Eastern Strait of Gibraltar, as extracted from Sentinel-1 SAR-C and Sentinel-3 OLCI on 14 May 2023. Left panel contains modified Copernicus Sentinel-1 data 2023, processed by Sentinel Hub EO-Browser.

From the optical OLCI imagery, it is evident that waves disperse as curved wavefronts when waves enter the Alboran Sea, due to the effects of diffraction and tidal advection processes.
But when did these waves originate?
Were they released from the Camarinal Sill?
It is worth noting that changes in surface roughness are often hard to detect in optical imagery, but they are often most visible in the 'glint' patterns that occur on the eastern edge of images, as shown in our example.
Usually, glint makes ocean colour data hard to work with, but in this case, it is actually helpful to us!

According to the methodology by Bolado-Penagos et al. (2023), these waves would have been released from the Sill around 18:45 PM on 13 May, by the time that the internal Froude number is lower than one (Figure 2a).
This estimation is calculated from the tidal current prediction over the Camarinal Sill at 45m depth (Figure 2b). 
Figure 2: (a) Internal Froude number (G2) estimation based on the (b) tidal current prediction (m s-1) over the Camarinal Sill at 45 m depth (Vázquez et al., 2008).
The horizontal black line delimits the positive/negative current directed toward the Mediterranean/Atlantic side.
The vertical gray rectangle indicates the time during which the internal waves would have been released (13 May 2023, 18:45 UTC).
Credit: Marina Bolado-Penagos, GHER.

The wavefront seen in the Sentinel-1 image (Figure 1, left hand side) is located around 50-55km from the Camarinal Sill.
This is observed about 12 hours after the possible release of these waves (Figure 2).
As a result, the travel speed of this front would be around 1.2ms-1.
This speed aligns with less energetic tide conditions (neap tides) and the presence of strong easterly winds in the area.
Regarding the propagation speed of internal waves within the Alboran Sea, it depends on the stratification of the area.
In less than four hours, the wave train now detected in the Sentinel-3 image (Figure 1, right hand side) appears to have travelled a distance of 20km, which implies a propagation speed of approximately 1.4ms-1.
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