Archive for July 11th, 2011

Sun and planets liquid constellation…

Monday, July 11th, 2011

Hey... Buenos días a todos ¡

Silbo has generated an intense job during these first 2 weeks of mission... However, we are learning a lot. We could say that now,

1.- we know where we are,

2.- where we want to go,

3.- why, and, most important,

4.- how to do it.

1.- Where we are ? Some posts ago, we showed that silbo was crossing an eddy field that seemed to organize as a liquid meccano formed by different cold (warm) eddies (PLANETS), gyring around a warm (cold) eddy (SUN) in the center of each gear of this incredible gear (figure 1).

Fig 1.- SSHa. Liquid constellation of cold and warm eddy planets and suns..

 

2.- Where we want to go ?. Our main objective was to fly the cold eddies to pass the first sector and heading the South...But we suffered two effects. The first, reported on 3-4 july, consisted in a inertial oscillation that seemed to be associated to 3 july storm pass. It changed the current field 360 degrees around silbo during a period of 24-36 hours. However, one day and a half later (5th july), the inertial perturbation disappeared and we returned to the periodic NW-W current field observed at the beginning of the mission (figura 2).

Fig 2.- Inertial Oscillation.

We got dense discussions in the core group and we concluded that, first, we knew now what changed the current field (storm pass) around silbo. Second, we designed new protocoles in order to minimize this effect.
3.- Why ?. However, on 6-8 july, we had another change on current field that did not gyre around silbo but that changed the current from WNW to N-N !!  We were crossing a anticlockwise cold eddy (a planet). We were trying to head the warm eddy in the center (the sun) of this incredible liquid meccano/constellation (figure 3).

Fig 3.- SSHa. Translation movement.

Fig 3.- SSHa (3D). Translation movement of the cold eddies around the warm eddy.

Looking to the ground current field targeted by silbo, we thought in another inertial wave, but the current field did not change 360 degrees around him. That would mean that another effect has to be got into account. The answer came with the analysis of the Sea Surface Height anomaly for these 3 days. The three SSHa model forecasts sent from Australia (Dave/Shannon team), USA (Nilsen team) and Las Palmas (our team) showed the same landscape. Silbo  that we were suffering a change of the position of the eddy that we were crossing…(figure 4).

 

Fig 4.- Translation movement reported by Silbo.

The eddy moves..to the W. Independently of the ROTATION (clockwise the warm eddy, anticlockwise the cold eddy) this system had (has) a complemen- tary TRANSLATION movement. Figure 5 shows the position of the eddy in the yellow line located at 60 N. He has drifted to the N. We calculated the component of the translation drift of the eddy on 8-12 km/day. If we kept to silbo fixed to the 24 W (7-8 july), the eddy would cross us and we would be sailing the eastern side (NNW current fields).

Fig 5. SSHa fields on 6th and 8th july.

4.- how to do it ?. For that, we decided for the last weekend to move the WP a little bit W. In spite of the storm passed yesterday, silbo looked to relax his flight moving to the SW with the same pattern of 5-5.5 km by stint (0.59-0.64 km/h) during 3 days !!.

Our primary target is the green line on figure 5, located in the 24.7 -25 W.

Fig. 6.- Silbo location update on 11th july 11.

We want to fly the warm eddy (SUN) located in the centre of this “liquid planetary”. The reason is really simple and fascinant. He does NOT TRANSLATE !. We want to fly him using the same protocole described by Scott et al with RU17 and RU27: Flying the eastern side looking for currents going to South. At this moment, we would move the WP again, keeping the nose of silbo heading the warmer south...

Finally, the landscape today (figure 6). The last night storm moved the current field direection again to the right side (inertial oscillation again ?). There are 2 possibilities:

1.- We will wait for the next surfacing, but it looks that we will not suffer any inertial perturbation since the current of the last surfacing become to the original NW position before the storm.

2.- Our translation speed (u component of silbo movement) showed 8-10 km/day.. a little bit lower than the meccano does (10-12 km/day). May be that we are being passed by the eddy translating W again.

We would have the answer to this question in the next surfacings. If the current field becomes to flow NW again, that would mean that current changes observed are associated to the storm. If so, we would be retarding our translation movement since our U speed is slower that the u speed of the cold eddy that we are crossing right now...

Cheers all and thanks to my incredible ULPGC team again.
Antonio G.Ramos (Robotic and Computational Oceanography division, Univ of Las Palmas de Gra Canaria).