The turn of the earth

Christopher Eccelstone is often overlooked amongst the Doctors of the 21st century — probably because he only appeared in one season — and the first season of the reboot to boot (sorry, could not resist!) But he has some memorable lines.

Rose: If you’re an alien, how come you sound like you’re from the North?

Doctor: Lots of planets have a North.

In that first episode, Rose keeps pressing the Doctor to tell her who he really is. And this is where we first hear about him “feeling the turn of the earth”. It is a very amazing idea to entertain.

One summer evening, I lay down on the grass in a park in Seattle and watched the sun going down over the Olympic Mountains. The light was filtered enough by haze that the sun was easy to look at. And as it sunk slowly down behind the mountains, I was able to imagine I was feeling the turn of the earth.

In graduate school, I took a class in physical oceanography which was my first mathematical encounter with the Coriolis force. The strength of this force is dependent on latitude (zero at Equator, maximum at Poles) and velocity. Fast-moving objects, such as baseballs and artillery shells, can see measurable deflection from the Coriolis force in just a matter of seconds. However, slow-moving currents of air or water may require at least one full rotation of the earth in order to register a deflection. But as long as the earth keeps turning, the Coriolis force affects how air and water move, deflecting transfer of fluid to the right, north of the equator, and to the left, south of the equator.

It was also in that class that I first encountered the thermohaline circulation of the global oceans, whereby differences in density of ocean water, driven by differences in temperature and salinity, cause water to sink — especially in the North Atlantic, where this sinking event helps support a poleward flow of relatively warm water towards Europe. The Gulf Stream, as it is known, is credited for keeping the winters of the British Isles remarkably mild given the high latitude (despite Britons insisting that their winters are cold.) Back in 1991, this flow driven by the sinking of cold water was known as the Atlantic Conveyor. Nowadays, it has earned a fancier name — Atlantic Meridional Overturning Circulation — with an obligatory acronym, AMOC. (An old joke at the National Oceanic & Atmospheric Administration is that NOAA actually stands for the National Association for the Advancement of Acronyms. And is true that earth/ocean/atmospheric science is awash with acronyms.)

This week, we learned that AMOC might be showing signs of weakening due to global warming. Warmer ocean temperatures and lower salinity due to runoff of glacial melt from Greenland threatens to reduce the rate of sinking. And this could disrupt the global thermohaline circulation of the oceans, with concurrent disruption of climate systems in many parts of the world.

The global conveyor of the ocean moves very slowly. A parcel of water sinking in the North Atlantic will eventually start to drift south along the ocean floor, crossing the Equator, and continuing towards Antarctica from where it may continue its journey into the Indian or Pacific Oceans. At some point in its journey, it will find its way back to the surface. But on average, this requires around a thousand years. Quite simply, water sinking off Greenland today would not be expected to surface again until around the year 3000.

In that class, I also learned of the two viewpoints from which we may consider fluid flow. For example, with water flowing in a river, we can view from the perspective of someone standing on a bridge above the river (Eulerian); or we can take the perspective of someone in a boat being carried along by the river (Lagrangian). The latter is more applicable when developing systems of equations from basic principles; but the Eulerian viewpoint is more convenient when it comes to solving the equations,

So why am I writing about this? In a post with the #autism tag? Well, this is the kind of stuff that really tickles the happier parts of my autistic brain. Right now, ocean water is surfacing, somewhere far from the British Isles, that might have sank in the North Atlantic the very same day when Harold took that arrow in the eye at the Battle of Hastings. From the Lagrangian perspective, that parcel of water has a memory of the last thousand years.

Feeling the turn of the earth is Lagrangian. And that must be very profound for someone like the Doctor who operates mainly in the Eulerian framework (setting coordinates in time and space).

Scientists are not expecting AMOC to suddenly shut down anytime soon — although they can not be sure. But I have rather enjoyed thinking about — after all these years.