5 July 2016

New perspective on the MOC variability

A bundle of circle plots of the MOC at 26N

Circle plots

For datasets with an annual cycle that may obscure an overall tendency, it can be useful to view the plot as circle, where each clock direction represents a month of the year. @ed_hawkins has made a number of these with long time series of temperatures, and of Arctic sea ice.

The MOC time series is now available through 2015. You can download the data here: www.rapid.ac.uk

While the MOC time series at 26N is not nearly so long (now from April 2004 through October 2015), it’s interesting to view it in the same way. Ok, the GIFs are below (and you can click through the gallery), but if you’re tired of staring at spinning circles, the final images are here. Click the figures for captions.

So what can be seen in circle plots? From the Ekman and UNADW transports, we see that these two are mostly not very variable. Ekman transport did have some excursions (where the size of the circle shrunk), particularly in the winter months of December - March. These were associated with the strongly negative periods of the North Atlantic Oscillation index in Dec-Mar 2009, Dec 2010 and Mar 2013.

From the UMO transports, we see an oblong sort of shape, with peaks (less negative thermocline transports around Sep-Nov. This shape is associated with a somewhat pronounced seasonal cycle, not present in, for instance, the UNADW transports, which are mostly circular.

The Florida Current transports may be somewhat elongated, with a stronger transport in the summer months of June - September. However, when plotted with the full record (since 1983), these transports show a sort of pulsating indicating stronger then weaker periods on timescales of a couple of years.

The remaining two figures, LNADW and MOC, both seem to indicate that the bigger circles were in the earlier (blue) years, and smaller circles in the later (red) years. In the case of the LNADW transports, this means that the southward transport in the deep layer (3000 to 5000 m deep, between Florida and Africa) used to be more southward (note, signs were flipped) with a magnitude between 5 and 10 Sv. In the later period, this strength has decreased, hovering around 5 Sv with some excursions crossing zero.

For the MOC, the decreasing tendency identified in @Smeed-etal-2012 and extended in @FrajkaWilliams-etal-2016 is apparent with the stronger overturning (around 20 Sv) in the earlier (blue) years, and a weaker overturning (around 15 Sv, with smaller excursions) since. Curiously, MOC shows greater variability than any individual component (the other 5 plots), which means that the individual components do not cancel out when producing the MOC as a sum (FC+Ekman+Umo) or (UNADW+LNADW).

Here are the animated gifs. Click the figures for captions (though then you’ll only be able to see one at once).