Sometimes there are little questions that niggle at you. Occasionally they do that a very long time. On some special occasions folks point out a ‘look here’ that gives a pretty good explanation ‘right quick’.
In an earlier posting we saw that the Saros Cycle of 18.03 years had solar-lunar-earth alignments (and thus eclipses and tides) repeating every 3 rd cycle (due to an 8 hour ‘remainder’ term on the alignment frequency, each eclipse cycle is 120 degrees shifted on the surface of the earth. That means every 3rd one puts the gravitational forces back on the same patch of the surface of the tropical oceans). All well and good, and it can explain why there is a 50 something to 60 ish year cycle to some weather phenomenon like the PDO. (The weather right now in California is remarkably like what it was in the 1950s, much as the 1990s were remarkably warm as were the 1930s)
But that left a ‘loose end’ for me. That loose end is that the Lunar Orbit has an 18.6 year period for when apogee / perigee finish a cycle.
The lunar nodes precess around the ecliptic, completing a revolution (called a draconitic or nodical period, the period of nutation) in 6793.5 days or 18.5996 years (note that this is not the same length as a saros).
So there is a ‘longer than a Saros’ and ‘shorter than a Metonic’ precession of the nodes (and thus apogee / perigee).
See also: Lunar standstill
The lunar orbit is inclined by about 5 degrees on the ecliptic: hence the Moon can be up to about 5 degrees north or south of the ecliptic. The ecliptic is inclined by about 23.4° on the celestial equator, the plane that is perpendicular to the rotation axis of the Earth. As a consequence, once during the 18.6-year nodal period, when the ascending node of the Moon’s orbit coincides with the vernal equinox, then the Moon reaches extreme northern and southern declinations. Then it also has its extreme northern and southern azimuth points of rising and setting on the horizon; its extreme lowest and highest altitude when crossing the meridian; and potentially extreme late first sightings of the new moon. Furthermore, occultations by the Moon of the bright star group the Pleiades, which are over 4° North of the ecliptic, occur during a comparatively brief period once every nodal period.
So every 18.6 years we get a more extreme north / south range of lunar tidal forces. This gives a somewhat better description of it:
but the basic point is just that if you are looking for water being pulled toward / away from the poles, that will be happening on an 18.6 year cycle, while if you are looking for water to be pulled toward / away from the equatorial areas (eclipse times) that will be on an 18.03 year Saros cycle. Those two will interact. I get a 335.35 year product of the two. It seems to me I’ve seen some weather events with about that kind of period. Three times that at 1006.05 years is looking rather close to the periodic ‘spikes’ seen in temperatures in the GISP2 ice core, while 1341.4 is not too far off from the lower bound on Bond Events and 1676.8 is near the upper bound (Bond Events have an average of about 1500 years, but the actual events are spaced each side of that number). It looks to me like there are opportunities here to further investigation. An examination of exactly when how much water goes where seems to be in order (though likely beyond my means with ‘one guy and a laptop’ and no funding). There does look to be a reasonable possibility of a ‘beat frequency’ of water slopping around the globe on a kilo-year scale that could explain some of the longer term cycles.
The Moon’s maximum and minimum declination also varies because the plane of the Moon’s orbit around the Earth is inclined by about 5.14° to the ecliptic (the plane of the Earth’s orbit around the Sun), and the direction of lunar orbit inclination gradually changes over an 18.6-year cycle, alternately adding to or subtracting from the 23.5° tilt of the Earth’s axis. As a consequence, the maximum declination of the Moon varies from roughly (23.5° − 5°) = 18.5° to (23.5° + 5°) = 28.5°. As a result, at minor lunar standstill, the Moon will change its declination during the nodal period from +18.5° to −18.5°, which is a total movement of 37°. Then, 9.3 years later, during the major lunar standstill, the Moon will change its declination during the nodal period from +28.5° to −28.5°, which is a total movement of 57°, which is enough to take its culmination from high in the sky to low on the horizon in just two weeks (half an orbit).
I note in passing that this presents a 9.3 year cycle as well. Also note the speed with which some of these degree offsets can shift. Quite fast enough to be the trigger for ‘sudden climate change’ events, like the onset of droughts or heavy rains. Very little of the ancient records of dramatic climate events is accurate to single digit years. (Not the Egyptian calendar and not even the year of the birth of Christ).
So the upshot of all this is that I would expect to see a Saros influenced period to tropical weather and shifts of things like the PDO; yet an 18.6 year cycle influence on what happens at the poles. Then the two of them interacting to make for even more extreme events on very long time cycles.
This is an interesting article from Jo Nova:
Which finds that the “Lunar Tides and the Long-Term Variation of the Peak Latitude Anomaly of the Summer Sub-Tropical High Pressure Ridge over Eastern Australia” has a connection to the Earth orbit perigee vs new moon / full moon. That, I think, ought to be reflective of the lunar influence on weather periods.
This study looks for evidence of a correlation between long-term changes in the lunar tidal forces and the interannual to decadal variability of the peak latitude anomaly of the summer (DJF) subtropical high pressure ridge over Eastern Australia (LSA) between 1860 and 2010. A simple “resonance” model is proposed that assumes that if lunar tides play a role in influencing LSA, it is most likely one where the tidal forces act in “resonance” with the changes caused by the far more dominant solar-driven seasonal cycles. With this type of model, it is not so much in what years do the lunar tides reach their maximum strength, but whether or not there are peaks in the strength of the lunar tides that re-occur at the same time within the annual seasonal cycle. The “resonance” model predicts that if the seasonal peak lunar tides have a measurable effect upon LSA then there should be significant oscillatory signals in LSA that vary in-phase with the 9.31 year draconic spring tides, the 8.85 year perigean spring tides, and the 3.80 year peak spring tides. This study identifies significant peaks in the spectrum of LSA at 9.4 (+0.4/
That 9.4 is well inside the error bars for being a 9.3 lunar period.
But that’s not what got me started on this…
No CO2, nor even a mention of “Climate Change”
I found this paper remarkably refreshing. No mention of CO2. Not even the obligatory ‘kiss the ring’ of sprinkling the words “Climate Change” in somewhere. Just straight meat-and-potatoes science. Data, thesis, analysis, conclusions.
I was pointed at it by someone, somewhere… I’ll sort that out and put a h/t here later (as I’m still coping with fever, cough, haven’t eaten in 2 days – something I do when feverish – and generally I’m very low on energy. And can’t remember at all where I saw the pointer; but likely will in a day or two.)
The influence of the lunar nodal cycle on Arctic climate
Yndestad, H. 2006. The influence of the lunar nodal cycle on Arctic climate. e ICES
Journal of Marine Science, 63: 401e420.
The Arctic Ocean is a substantial energy sink for the northern hemisphere. Fluctuations in its energy budget will have a major influence on the Arctic climate. The paper presents an analysis of the time-series for the polar position, the extent of Arctic ice, sea level at Hammerfest, Kola section sea temperature, Røst winter air temperature, and the NAO winter index as a way to identify a source of dominant cycles. The investigation uses wavelet transformation to identify the period and the phase in these Arctic time-series. System dynamics are identified by studying the phase relationship between the dominant cycles in all time-series. A harmonic spectrum from the 18.6-year lunar nodal cycle in the Arctic timeseries has been identified. The cycles in this harmonic spectrum have a stationary period, but not stationary amplitude and phase. A sub-harmonic cycle of about 74 years may introduce a phase reversal of the 18.6-year cycle. The signal-to-noise ratio between the lunar nodal spectrum and other sources changes from 1.6 to 3.2. A lunar nodal cycle in all time-series indicates that there is a forced Arctic oscillating system controlled by the pull of gravity from the moon, a system that influences long-term fluctuations in the extent of Arctic ice. The phase relation between the identified cycles indicates a possible chain of events from lunar nodal gravity cycles, to long-term tides, polar motions, Arctic ice extent, the NAO winter index, weather, and climate.
The paper essentially looks at things as diverse as ice, temperatures, tides, and sea level and finds them modulated on harmonics of lunar cycles in the Arctic.
There’s some ‘tricky bits’ in it. Like finding that there are some phase shifts at times, and that the cycle is not amplitude proportional. Frankly, just the sort of thing I’d expect to see if there are both 18.6 and 18.03 year cycles interacting over the entire globe, but with the 18.6 being more important to the poles
Here are a couple of graphs from the paper to give you an idea what they show:
This one looks at a specific time series of temperatures from Kola:
I note that the peak temperature in Kola was in the late 1930s. Interesting what shows up when you get the data straight from the source and not laundered through NOAA / NCDC and the data diddlers…
So here we can see a couple of their examples. The paper is long and has many more. It is complex, but still straight forward in what they do. I think they have it right (and demonstrably so).
Their “Figure 13” for Greenland and Ice Extent shows all the wavelets down near a low in about 1998, but turning up. I would expect that this indicates a large ice recovery in the next few years. (Especially since that 74 year cycle will be pushing ‘one way only’ for the next 32 years…)
I also like the way they pull back and look at some very long history markers for a bit of a ‘sanity check’ on how likely it might be that the cycle is persistent.
The 74-year cycle in the NAO winter index represents a long-term indicator of climate change. The 74-year NAO cycle has a phase delay, exactly the mean between the 74-year cycle of Barents Sea ice cover and the 74-year cycle of Greenland Sea ice. Therefore, the total extent of Arctic ice cover is a potential source of this important 74-year climate indicator. Schlesinger and Ramankutty (1994) analysed long-term temperatures from the northern hemisphere continental regions bounding the northern Atlantic. There they found a dominant temperature cycle of about 76 years, with a maximum in 1945, the same year during which the identified 74.4-year cycle of Barents Sea ice cover was at its minimum.
The Nile flood in Egypt is an indicator of rainfall in Africa. Records from the periods 3150 BC to 2400 BC, and 622 AD to 1470 AD, show a peak with periods of 18.4, 53, and 77 years. Greenland ice cores show periodic cycles of 20, 78, and 181 years, and temperature records from central England from 1700 to 1950 show periodicities at cycles of 23 and 76 years (Borroughs, 1992; Currie, 1995). This indicates that the 74-76-year cycle is probably a long-term, stationary oscillation.
The 74-year NAO cycle explains the climate shift in 1960 and a possible new shift in the 1990s. From 2000, we may expect the 18-year NAO winter index cycle to turn positive over the next 10 years, and the 74-year cycle will turn in a negative direction during the next 30 years. This suggests perhaps a temporary period of cooler climate in the northern hemisphere. However, the long-term relationship between the step in the polar motion and the reduction in Arctic ice cover suggests that, in the long run, the climate would be expected to be warmer.
So it looks like we’ve got about 30 years of cooler, that one hopes is enough to put a spike in the heart of the CO2 / Warmer idea; then we can get back to a more pleasant warm world. Nice. Very nice. None of it too far out of the ordinary or recent history. Just enough cyclical turn to ‘frost the shorts’ of some folks who desperately need a cooling off…