Upwellings and Downwellings

Riding along in a current is a great diving thrill, but when it suddenly yanks you upward or downward, the thrill can become frightening and sometimes tragic. In the summer of 1998, dangerous currents near Columbia’s Malpelo Island trapped many divers in a blow hole for several days, with injuries ranging from broken ribs to an eyeball torn from its socket. Seven recreational divers off New Zealand’s coast were unexpectedly pulled to nearly 300 feet last March. Three died. Last September we reported about six divers in Cozumel who required treatment for both Type I and Type II DCS after being tossed about by violent vertical currents on the Santa Rosa Wall. How you react is a matter of life or death.

Oceans move constantly, with currents coursing through them like giant rivers. Currents are the result of winds, tides and thermally unstable water columns, which 12. enter rivers and seismic events, often in various combinations. Most currents run horizontally to the earth’s surface, but especially dangerous ones run vertically, toward the bottom or toward the surface.

Such currents are often found when a horizontal current strikes the face of a wall and then moves down, up, or both. Downcurrents can also appear when a horizontal current runs perpendicular to a drop off, or where two opposing currents run into or over each other. Marked differences in water temperature and salinity in the water column can also produce vertical currents, but these are generally sluggish and pose no threat to divers.

A downwelling can unexpectedly pull a diver deeper than his dive plan. It may happen so gradually, he may not notice it. Sometimes, however, the current will rapidly drive the diver deeper, occasionally much deeper. In Tobago several years ago, the current yanked a nearby diver from 20 feet to 85 feet in the blink of an eye.

While most divers fear downcurrents more than upcurrents, keep in mind that a downcurrent will eventually release you, sometimes surprisingly quickly, without taking you below recreational limits. If you have air, equalize quickly enough and don’t panic, you can probably ride it out uneventfully. A violent upcurrent, however, causes a diver to ascend far too rapidly, perhaps missing a safety stop if not planned, and precipitating DCS. Additionally, if surprise at the sudden ride upward results in breath-holding, an embolism is possible.

Unless you are properly trained, physically fit, and intentionally seeking out the challenges of strong vertical currents, it is best to avoid areas where they are known to be fierce. Divemasters and guides should be aware of problem sites, and will avoid or at least advise the diver about them. Of course, vertical currents aren’t always predictable. There may be surface manifestations, like circumscribed areas of water showing varying patterns of wave frequency, height and direction interspersed with eerie mirror smooth areas, but don’t count on this. Often surface conditions tell the diver nothing about currents at depth.

If caught in a vertical current, what should you do? One strategy is to swim out from the wall immediately, drop off or any other apparent source of the current. Vigorously fin away, but do not exhaust yourself. It’s helpful to orient yourself to bubbles or the direction and angle of any fish you observe. As reader Barry Lipman (Brookfield, CT) observed last year aboard the Galapagos Aggressor, “currents ranged from nothing to three or four knots. Sometimes your bubbles would leave you traveling up at a ten-degree angle; sometimes you’d watch as your bubbles travel down into the depths.” Many divers, however, do not have time to appreciate such details.

If you’re caught in an upcurrent, swim away and down. For a downcurrent, swim away and upwards. It is best if you can make adequate upward progress without BC inflation, as this both provides more surface area for the current to push against and raises the risk of a poorly controlled ascent once you are released. However, you may choose to inflate if rapidly descending or lacking adequate gas to ride out a downwelling that does not show rapid signs of weakening.

Do monitor depth and keep a hand on the deflator valve as you must avoid shooting to the surface when the current relents. Consequently, it is also best not to release your weights if possible, but again this may be necessary. In any vertical current, remember to breathe normally.

Reader Josef D. Prall (Carrollton, TX), aboard the Baruna Adventurer in Indonesia last year, says, “I was swept off a reef down and into the blue for a few minutes before I decided to end the dive at 95 feet after eight minutes including five minutes hang time. A beginner could easily panic when he finds himself being swept down in a current and drop weights when the appropriate response is to add air to the BC first, and then dump judiciously to control the ascent. Or maybe even drop weights one-at-a-time.”

An alternate strategy if you’re near something graspable, is to fin to it, grab on and pull yourself in the desired direction until you are free. (We get reports of autocratic divemasters grounding divers for grabbing coral in stiff currents — don’t let them intimidate you.)

Whichever method you employ, use surge to your advantage. When it propels you in the desired direction, go with it; conserve your energy when it is working against you. While sudden vertical currents are anxiety provoking, you can frequently negotiate them. As in other stressful scuba situations, remain calm and take rational problem solving action.

One final note: Cozumel is notorious for its vertical currents. While the diving there is exciting, it can often be intimidating for beginners or those who lack confidence in their skills. Reader Pat Wikstrom ( Warne, N.C.), in Cozumel last year, wrote: “On a dive at La Francesca Wall, we jumped into a ripping current that quickly broke our group into three separate pieces. Weird downcurrents caused my wife and me to abort the dive. Nevertheless, the poor DM chased the other two sections of our party all over the wall, brought up half the others after ten minutes, and went back down to find the last few folks and get them back safely. He really earned his tip that day.”

Be forewarned.