Halocline swimming


The speed of light is the absolute fastest limit that anything can travel in our universe. Its speed is assigned the constant c, as in Einstein’s E=mc^2 fame. Its declared speed of 186,282 miles per second is achievable only in the empty vacuum of space. Any medium through which light passes will slow down its velocity; some enterprising and possibly bored scientists have in fact managed to slow light down to 38 miles per hour by beaming it through a cloud of super cold sodium vapor. The ratio of c to the velocity at which light travels in a transparent medium known as its refractive index. This index of refraction is also an indication of how much the propogation direction of light changes as it enters or exits a transmission medium. This can be seen in the everyday phenomena of dipping a broom stick into a swimming pool and having the broom stick appear to bend at an angle as it enters into the water. This refractive property of transparent materials gives us lenses that bend light allowing the construction of microscopes and telescopes. All this theory is most easily summed up as light changes speed and direction as it enters and exits different materials.


Things that are less dense float above things that are more dense. Fresh water can float on top of sea water because the salt in the sea water has brine denser. That is why you float easier in sea water than in fresh water. The cenotes along the Riviera Maya peninsula discharge enough freshwater into the Caribbean Sea to visibly affect the salinity of  the seawater. On calm days, when the outflow from the karst formations is not induced by wave action to mix with the sea water, the freshwater stratifies itself above the denser seawater. The boundary layer between the salty seawater and the less salty freshwater is called a halocline. Ironically, and contrary to scuba divers’ experience, the thermocline at this salinity boundary is reversed with the solar warmed seawater lying below the cool waters just discharged from the sunless cenotes.


Some two and a half feet below the surface of the Caribbean Sea at Tankah Bay, Mexico lies the undulating boundary between saltwater and freshwater. This boundary is visible as shimmering layer of water, similar to what cooking oil floating on top of water in a container might look like. This halocline is visible due to the different refractive indices of saltwater and freshwater.

Swimming on the surface, the submerged halocline can be seen as a glassy underwater layer, wavy like antique window panes before the perfection of modern flat glass manufacturing technique of floating molten glass on top of a sheet of melted, liquid tin. The visibility of the halocline is due to its refractive properties and how light bends as it crosses the freshwater to saltwater boundary.  Since the halocline was relatively shallow, it was easy to dip a hand or arm through the boundary layer while finning along on the surface, resulting in a visible plume of disturbed saltwater, like a cloud of fine dust kicked up by a car driving on a rural dirt road. Both the temperature and salinity differences stayed distinct within the region of disturbed water and did not homogenize, leading to an effect of veins of saltwater wrapped around veins of freshwater, forming a visual cloud of waters having two different refractive indices. Diving through the halocline resulted in being surrounded by an indistinct cloud of the disturbed halocline waters. Within the cloud, being made up of a weave of tendrils of different refractive indices, it was impossible for eyes to focus. It was very much like a dive mask fogging up completely. Actually, the first couple of times this happened to me, and before I realized what was going on, I did think that my mask had fogged up or somehow suffered a failure of some sort.

Drifting within the cloud, and concentrating on focusing and making sense of the visual field, the impression was that the cloud was made up of a grid of whorls of haloclines. It was actually somewhat indescribable. Only the limits of my breath holding capacity forced me to come up from this submerged playground.

This was a most unexpected, and most unique swimming experience, and I spent much time playing with the halocline boundary layer, just to watch the almost magical show that would emerge.