05.30.2013: Various analyses for HOE-DYLANΒΆ
Fig. 1 shows the time series along 158W of AVISO SSH (the mean SSH is from the Mean Dynamic Ocean Topography) over the period of HOE-DYLAN observations.
Figure 1: AVISO SSH along 158W.
Fig. 2 shows the life history (track, size and amplitude) of the small cyclonic eddy. There are two different histories because around May 2012, two cyclones have merged and two slightly different algorithms capture the track of one eddy or the other. The size of the circle is the width of the Gaussian that fits the eddy locally in time and space. The portion of the circle that is filled corresponds to the percentage of parcels “initially” released during the HOE-DYLAN observations of July 2012 and that are still inside the eddy (defined by the circle) when we follow the trajectory of the eddy back in time. It tells us how far back in time the water that is inside the eddy during July 2012 was already inside that same eddy. We see that most of the water sampled is from the light-blue eddy with no or little contribution from the blue eddy. 10-month before July 2012, the light-blue eddy already contained about 20% of the water that is inside the eddy during July 2012, suggesting that the eddy may have some waters that originates far from the east. [Note: If the percentage of waters goes up and down, it is because, given the chosen definition of the eddy, some waters go in and out of the eddy, without never going away too far from the eddy. This is the case for the light-blue eddy].
The insert panel shows the evolution of the amplitude of the eddy with time running from right to left (to help to figure out how the eddy’s amplitude changes with time along its course, from east to west).
Figure 2: Tracks, size and amplitude of the cyclonic eddy that is near the location of HOE-DYLAN in July 2012. See text for details.
Fig. 3 is similar to Fig. 2 except that we show the life history of the large cyclonic eddy located north-east of HOE-DYLAN sampling location in July 2012. I stopped the track in March 2012 because this is where and when the eddy originates [its amplitude was then about 25% of the amplitude it has in July 2012]. The eddy thus grew quickly in size and amplitude over the next 6 months capturing more and more of the water that will be inside the eddy in July 2012.
Figure 3: Same as Fig. 2 but for the anticyclonic eddy.
My grand conclusion of this mirrors what I have done for BIOLINCS. Most eddies that are passing at station ALOHA are only partially coherent. That is, they do not always carry the same waters and they exchange a significant amount of waters with its surrounding. For the eddies shown above, at best 20 to 25% of the water that is inside the eddy in July 2012 was already there about 6 to 10 months ago. Also, except maybe for the light-blue cyclonic eddy, the life time of most eddies is short. The ones that are passing near station ALOHA have not been generated that far away. There are exceptions as Roger Lukas and his colleagues have shown but they just confirm the rule.