Archive for August 8th, 2011

Thermal Wind

Monday, August 8th, 2011

Hi all,

we are trying to estimate the heat transportation along the SILBO path. To this end we are using essentially the thermal wind equations. Thermal wind is the most fundamental and significant dynamical balance controlling the large-scale circulation of the ocean. It is a consequence of hydrostatic and geostrophic balance, and appears as variations of geostrophic vertical velocity if density varies horizontally.

At first, in order to determine the scale at which processes take place, we took Rossby Radius as our reference (10 km in our case), so the processes will be on that scale. It's a good criteria to quantify all the mesoscale data. Every change in lower scales will be insignificance in our estimation.

After, we made an harmonic analysis, applying the Fourier Transform in order to estimate at which lengths (Δy) the changes of density gets more important for every range of depths (Δz = 10m, Figure 1). As we can see, most of the energy (90%, Figure 2) is concentrated above 100km (The red line indicates 100 km, and all the lengths at the left of this line are larger lengths). These analysis indicate that we are able to make different averaging among 10 and 100 kms in the direction of the glider path.

Figure 1

Figure 2

Finally, we applied the Thermal Wind equations with 10m as Δz and 10km as Δy (Figure 3). As we can see the variation of velocity gets higher while we get the surface. All these velocities are around 0 - 0.3 m/s in the surface along the glider path. These results seem to agree with real values.

Figure 3

Next step will be comparing these current velocities with the measured currents from the glider in order to check these results, before estimating the heat transportation combining these results with the measured temperature from glider.

Ruben & Alberto

The mystery eddy… Solved ????

Monday, August 8th, 2011

Hey guys

just complementing the post sent by Nislen, the current pattern follows flowing to the NW all the weekend (Figure 1).

Figure 1.- SIlbo surfacings on weekend 5-7 august 2011.

This has confused to the Silbo team this weekend, since we expected SW current associated to the central cold sun of the solar system that we actually cross. The solution to the mistery may come moving a step back and look the problem with some perspective. When we did it, solution appeared nitid and clear. It came observing the altimetric landscape given by the update of the SSHa today.  Data say that there are 2 guilties to this NW strong and regular current flow.

1.- If we observe the SSHa, there are a strong anticlockwise pattern that seems to be the responsible of such a NW flow. The central cold eddy (the sun of the solar system that we cross) targeted by a negative SSHa (green color), has been opened in his SE side and has been converted in an "altimetric channel". This channel seems to be built by a strong current flowing to NW (to silbo) like a river (Figure 2).

Figure 2a.- SSHa 8 august 2011.

This channel/river continues sorrounding the warm eddy South Silbo. It is a mesoscale pattern of 500 km of longitude (24-29 W), and 200 km of latitude (54.5-52.5 N) aproximately (Figure 2b).

Figure 2b.- SSHa 8th august 2011.

This mesoscale anticlockwise pattern would explain the NW flow. But whats its origin ? Who is the builder/guilty ?

2.- The thermohaline general pattern is the main characteristic of this particular N Atlantic ocean domain. Thus, the global pattern of the salinity is a good indicator of the global circulation.  The N arm of the gulf stream is flowing to NW (Southwestern Iceland) and NE (Eastern Iceland). Silbo is crossing the SW limit (strong salinity front) of this global salted river flowing north (Figure 3).

Figure 3.- Salinity field and thermohaline flow. 8th august 2011.

It could explain such a mesoescale anticlockwise flow associated to the thermohaline NW strong flow (Figure 4).

Figure 4.- Mesoscale anticlockwise pattern showed in the SSHa and Salinity fields (8h august 11).

As Nilsen proposes, we would have to move the WP to get the strong warm eddy located SE to silbo (we would have to be at 25 W, 53 N to find good and constant South oriented currents). Apparently, the NW river would flow for a while. So the course of silbo would be S-SW with the actual WP. We would have to fly to silbo to SE (100 km in longitude E, 200 km in latitude to S).

For that it would be neccessary to move the WP to the North and to the East to head it course to the SE, to 25 W, 53 N.

greetings all

Antonio Ramos

RCO Division, ULPGC.