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"The forgotten Milankovitch effect-Tides" del collaboratore nella ricerca Dr.Clive Best In evidenza

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The forgotten Milankovitch effect – Tides

The obliquity of the earth’s spin axis varies over a 41000y cycle between 24.5 and 22.2 degrees. The canonical work on calculating Milankovitch cycles has been done by J. Laskar and his team at the Observatoire de Paris[1]. Changes in obliquity have been the main driver for glacial cycles for over 2 million years. Larger obliquity increases summer insolation to both poles. Eccentricity and precession just modulate the seasonal balance at each pole. For the last 800,000 years obliquity alone has been insufficient to end glaciations, and the reason for this is still not fully understood.

Total-compare

Top graph: Obliquity and Eccentricity of the earth’s orbit for the last million years ( Laskar2010 ). Bottom graph shows just how small eccentricity affects net annual insolation.

However, increased obliquity has another effect. It changes the amplitude of the lunar orbital precession. The lunar orbit is inclined at 5 degrees to the solar ecliptic and precesses with

Lunar_standstilla period of 18.6 years. The maximum declination of the moon at ‘lunar standstill is currently 28.5 degrees and this occurs every 18.6 years, as shown in the diagram. However 7000 years ago the earth’s tilt was 24.3 degrees compared to the current 23.5 degrees.  The obliquity is also decreasing  so that in 12000 year time it will be only 22.6 degrees.

 

This means that lunar standstills reached 29.1 degrees around LGM, while in 12000 years time they will reduce to only 27.4 degrees. This significantly changes the maximum strength of tidal flows at large latitudes during a lunar standstill. It is the tractional (horizontal) component of the moon’s gravitational tide which draws the oceans into a tidal bulge. This maximum tractional force occurs at about 45 degrees to the tidal bulge, so increases the average traction acting on polar regions. I have calculated this effect using the lunar ephemeris published by IRME back to about 8000 years ago. A clear reduction of the amplitude of high latitudes, Lunar standstill tides with decreasing obliquity is observed.

Av-Traction

The graph shows the 28 day average of daily calculations of lunar tractional acceleration acting at 65N over the last ~ 6000y. The ephemeris is provided by ELPC82 (Laskar et al.) [1]

Extreme tides at high latitudes have been  decreasing for the last 8000 years  in coincidence with the decreasing obliquity of the earth. The solar tidal component at perihelion has also reduced by about 5% due to the  decrease in eccentricity. At the LGM sea levels were 120m below current levels. As a result the edge of ice sheets were grounded around Northern Europe and the Arctic which strongly affecting  tidal dynamics.

 

Bathymetry of the Arctic Ocean curtesy NOAA. The 100m contour shows the sea level 20,000y ago and the 1000m contour shows the maximum depth of sea ice.

Bathymetry of the Arctic Ocean curtesy NOAA. The 100m contour shows the approximate sea level 20,000y ago and the 1000m contour shows the maximum depth of grounded sea ice. Note the much narrower access channel to the North Atlantic. Tidal flows were about 5 times greater than today [2]

The energy dissipation in the North Atlantic and Arctic have been estimated to be  five times stronger than today[2].  Furthermore the tidal tractional force at maximum lunar standstill was significantly stronger combined with enhanced spring tides from the solar component. Enhanced tides also drive the  meridional overturning circulation MOC which is sensitive to increased enhanced tidal dissipation in the deep ocean [3].

 

Are these ‘Milankovitch’ tides the primary cause of the rapid melt back of the ice sheets once Arctic summer insolation had peaked ~15,000 years ago?

[1] La2010: a new orbital solution for the long-term motion of the Earth,  J. Laskar et al. A&A 532, A89 (2011)

[2] The evolution of tides and tidal dissipation over the past 21,000 years, Wilmes, Sophie-Berenice; Green, Mattias,  Journal of Geophysical Research. 2014

[3] Green, J. A. M., and M. Huber (2013), Tidal dissipation in the early Eocene and implications for ocean mixing, Geophys. Res. Lett., 40, 2707– 2713

Dr. Clive Best

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