Downwellings

[japan-diver]

Up or downwellings can occur were ever there are currents. Normally seen on walls with strong tidal changes or standing currents. As others have said they are normally narrow and can be swum out of by trying to fight it while swimming at an angle away from the wall.

Downwellings:
I would suggest attempting to swim against it and inflating the BC if this does seem to be working angle the swim away from the wall. If your rate of descent continues drop weights- DCS is curable drowning isn't. Once the downwelling is escaped from be prepared to dump the BC and flair out if weights had to be dropped to slow the ascent

Upwellings:
Same procedure in reverse swim down and away, dump any air from BC-- Remember to breath The most dangerous thing in an upwelling would be an embolism from rapid ascent

I have seen many dangerous guides in my years of diving and some very good ones. A diver has to rely on themselves to make decisions on if its safe to dive and to handle problems underwater, a good dive guide will minimize problems and make dive more pleasant unfortunately there are many poorly trained guides that contribute to the problem instead of avoiding it.

 

[Rick Murchison]

While vertical currents due to salinity differences, temperature differences and even seismic activity exist in the ocean, upwellings and downwellings in the places we normally dive are almost always the result of current interacting with the bottom - and as such can be anticipated. And if anticipated, then avoided or dealt with.
For simplicity (and as a starting point) let's assume we have a vertical, straight wall that crests at 60 feet and runs due north/south, with the abyss to the west. In this case, any current flowing over the wall from the east will tend to flow downward as it goes over the wall, while any current from the west will tend to flow up and over the wall - so easterly currents will create downwellings and westerly currents will create upwellings.
The "standard" downwelling mishap reads "We were supposedly dropped on the xyz wall, but passing 60' we could see the captain had actually dropped us over the abyss. Suddenly we were in a downwelling... etc" No, the captain didn't drop over the abyss... the current was running off the top of the wall and then down; the divers were being carried off the reef into the abyss.
--------
Now, in the real world, reefs and walls are not straight and smooth, and so it's not always so simple. But in general, if you're "going with the flow" and you see yourself approaching bottom with vertical development going above you, you can expect an upwelling; if you see deep water ahead, you can expect a downwelling.
There are exceptions.
Because of the roughness of a reef, the flow across it will not be laminar (smooth) but will have quite a bit of turbulence, and this turbulence can create eddies - very localized strong currents running counter to the general flow of things. So a diver could find himself starting down as the current carries him over a wall only to be suddenly "grabbed" and thrown back up the face of the wall, then down again.
---------
What's the bottom line?
(1) blind faith in a DM or dive op can bite you. If you see that you've been "dropped over the abyss" realize you've drifted there - assume a downwelling's coming, and take corrective action early by either aborting or by swimming to the crest of the wall where you can either find shelter or grab onto something.
(2) throw the "do not touch" rule out the window. Get to the reef and find something to either "hide behind" or grab hold of. (grab the dead stuff if you can, but don't sacrifice your safety to the eco gods)
(3) The reef is your friend. My personal experience is that whether it be upwelling or downwelling, if I can get to the reef I can find a place to hide, to get out of the flow - and nine times out of ten I can do it without having to grab any coral
(4) if you can't get to the reef, then apply the "rip current" procedure of swimming horizontally (and angled a bit against the up/downwelling) - away from the wall if there is one - and you should clear the vertical current fairly quickly.
(5) abort early. If vertical currents are causing you to lose control of your depth, don't stay and fight - abort, surface and try again another day.
Rick