Many divers are familiar with the abrupt temperature change that occurs at the thermocline, a natural boundary that’s often seen as a thin, shimmering layer marking the interface between warmer surface waters and the deeper and colder waters below.
Because the cold water column below the layer is denser than the warmer water above, a pronounced thermocline acts as a natural barrier capable of reflecting sound waves; a characteristic that played a significant role in submarine warfare during the Cold War era.
In 1966, I was a member of a small, Admiralty-based naval unit tasked with establishing a viable forecast system for predicting the depth and strength of the thermocline; an ability that, among other things, would allow submarines operating in certain areas avoid detection from surface-based SONAR arrays.
In that same year, I signed a, ‘I-will-pay-for-them-if-I-lose-or-damage-them’ chit for two of the original Calypso cameras (pre-cursor to the Nikonos series) before being packed off to Malta to spend the summer months diving and photographing – using fluorescene dye packs – the thermocline and internal wave structures.
Keeping the in-water disturbances of exhaust bubbles and over-active swimming movements to a minimum, the several hundred original colour slide images that I took during my debut as an underwater photographer showed the fluorescent dye on a deep, dark blue background and looked – to my untrained eye – spectacular. (Sadly, the mono images that were later produced in a leading science journal are very poor reproductions.)
Forty-Nine years later and the cold war has taken on a whole new dimension – particularly on these early winter morning swims in Dee Why’s ocean pool. Fortunately, the innovative dive equipment company, Fourth Element, have taken the sting out of the ‘abrupt temperature change’. And while I may not be the ideal ‘poster-boy’, I’m very grateful for the long-sleeved, neutrally buoyant ‘Thermocline’ top.