Archive for August 19th, 2011

The end of the first chapter…

Friday, August 19th, 2011

 

"This one is neither the end, nor even the beginning of the end... but the end of the beginning"... (W. Churchill)

Hey guys...

Silbo is ending the first chapter (30 %) of his great history ... He has flown 1500 km at 0.3 m/s (1.1 km/h) during 57 days. He is crossing right now the green and magic western coast of Ireland.

Figure 1.- Silbo crossing (23 jun-19 aug 2011).

During these 1500 kms we have tried to fly the eastern side of the warm eddies. When we plot the synoptic profile of salinity and Temperature clockwise warm cores appear clearly above 400 m (in the epipelagic domain).

 

Figure 2.- Silbo profiles of Temperature and Salinity (23 jun-19 aug 2011).

Now, he is heading the main Gulf Stream branch located on 40s parallels. The first change that we would observe on this second chapter are poorer oligotrophic waters, at warmer SSTs and higher salinities.  However, the most important remark is that we left the thermohaline branch of the gulf stream flowing N (to Iceland) and we would begin to observe currents flowing E (to the French-Spaniard Gulf of Biscaine) and SE (Madeira-Canary islands).  It can be observed in the northern border of the main gulf stream branch in the map of salinity (figure 5). They are all warm eddies formed in the border resulting of an eastern flow of the general current path.

Figure 3.- Chl a field MODIS (19 aug 2011).

Figure 4.- SST field MODIS (19 aug 2011).

Figure 5.- Salinity field (19 aug 2011).

However, during the last week Silbo has reduced his speed. We expected intense current flowing SE-S and all the model (the last post) forecasted that silbo would cross the eastern side of warm eddies at higer speeds than observed on the ground.

Figure 6.- SSHa field (19 aug 2011).

Figure 7.- Current field (17-18-19 aug 2011).

The explanation to this extrange behaviour of this current fields was suggested in the last post, and, two days later, confirmed by Silbo on the ground. Storms (22 in 57 days) become stronger and more active at this latitudes (SST increases). The first effect of  storm is a significant change on current path. Since the 23 of june we have suffered two strong inertial oscillations that turned the direction of the current 360 degrees in 14-18 hours.

We think that we are actually suffering one of this inertial perturbation as a result of our 22th and 23th respective storms crossing at Silbo Latitude domain. It could explain the incongruence of his ground true current with these obtained from global models. Daily Global Models output would not explain the hourly variation of the current speed. To report such a feature, it would be neccessary to use ROM data (Regional Ocean Model outputs). But this, are only available in the European Domain eastern 20 W (we are at 24.5 W).

Figure 8.- Inertial oscillation (17-18-19 aug 2011).

We would keep you updated the next days.

have a nice weekend

Antonio Ramos & Nilsen Strandskov