100 years of Walter Munk ... and the role of lunar tides in ocean circulation

A piece celebrating the 100 year birthday of Walter Munk.

" ... a body of work that is wide-ranging and tremendously practical. His decades of discoveries have shaped what humanity knows about the nature of waves, currents, tides, global ocean circulation and deep-sea drilling. "

Which raises the interesting issue of the largely ignored paper by Munk's colleague Carl Wunsch and his paper "Moon, tides, and climate" in the 15 June 2000 issue of Nature.

"But Munk and I concluded that about half of the power required to return the deep waters to the surface was coming from mixing driven primarily by dissipation of tidal energyprincipally lunar, but with a minor solar component — in the deep ocean (Fig. 1)."

What's interesting about this is that there are three scales and modes at which tidal forces can have an impact on the ocean circulation

  1. Conventional ocean tides
    (provably forced by moon + sun, with tiny but measurable amount by wind)
  2. Thermocline sloshing, i.e. ENSO, as per Clarke [1]
    (believed to be forced by wind)
  3. Deep ocean mixing described by Wunsch & Munk  [2]
    (forced by mix of moon and wind)

This is what I find perplexing. For some reason tidal energy is excluded from contributing to #2 even though the periods of the ENSO cycle are precisely commensurate with the lunar fortnightly and monthly long periods.

The analogy is like saying that both rain and hail are caused by coalescence of water in the atmosphere, but snow isn't. I know that's an extreme analogy, but trying to get wide acknowledgment or to even get this lunar mode considered for #2  will take a huge amount of effort.  It would be interesting to ask Munk what he thinks of the ENSO lunar forcing model.

BTW, that paper by Wunsch suffered from what I considered an ill-advised attempt at humor. This is Fig.1 that Wunsch referred to in the excerpt above:

There was no content, only a lame joke worthy of Yakov Smirnoff.  That was not a good decision if you wanted other scientists to take the article seriously, IMO.

The joke was repeated here by Munk and Wunsh, in a slight reworking. This may have been a precursor draft to the Nature article, of which Munk did not appear as a co-author.

Conclusion

As per Munk and Wunsch on the influence of the moon on the pelagic (open ocean) zone:

"Our very tentative conclusions are that l) tidal dissipation plays a dominant role in pelagic mixing processes"

References

[1] A. J. Clarke, S. Van Gorder, and G. Colantuono, “Wind stress curl and ENSO discharge/recharge in the equatorial Pacific,” Journal of physical oceanography, vol. 37, no. 4, pp. 1077–1091, 2007

[2] Munk, W. & Wunsch, C. Deep-Sea Res. 45, 1976–2009 (1998).

 

One thought on “100 years of Walter Munk ... and the role of lunar tides in ocean circulation

Leave a Reply