26 March, 2015

The Rise of the Catalina Anti-Eddy

At 12AM yesterday, Wednesday, March 25, 2015, I was watching several episodes of Why Planes Crash, one of my favorite Weather Channel series. You know how, at the bottom of every TWC screen, there's this changing strip that shows the conditons in the local cities? Anyhow, the wind directions shown in Laguna Beach and San Clemente were quite extraordinary to say the least. What were they exactly? San Clemente was showing due-easterly winds at 11mph. Water is transported toward the north in response to winds blowing from a direction like that. Meanwhile, in Laguna Beach, the winds were from the SSW at 6mph. That is a wind pattern that tends to favor anticyclonic gyrogenesis through Ekman transport, as the below diagram demonstrates:


Fast-forward to last night, and the pattern changed. Instead of blowing out of different directions, both were showing due-easterly winds of 11mph and 15mph, respectively, while winds in Irvine were blowing from the WNW. Using Weather Underground's awesome app, I decided to check to see if there was any boundary separating the differing wind directions (usually marked by a dashed line on the map). Sure enough, there was, and it was retreating westward while continuing to intensify.

Then, I checked the sea surface temperature map this morning. Sure enough, the wind vector boundary, combined with the gyre in question, ended up retreating to that region just south of Catalina Island that tends to favor the formation of gyres:


Whereas cyclonic gyres — which is what the famous Catalina gyre typically is — tend to upwell in the middle and downwell around the edges, anticyclonic gyres do the opposite. They downwell in the middle, and this sea surface temperature profile reflects that.

What makes this so peculiar is that when it comes to gyres, whether cyclonic or anticyclonic, it doesn't matter what direction winds come from, they intensify regardless. If the winds blow from the east or southeast, they induce rear-flank downwelling, which speeds up the subsurface rotation, intensifying the gyre, which in this case warms up the ocean through gyre downwelling. If they blow out of the west or northwest, the water is transported past the eastern side of the gyre, speeding up the rotation on the surface, intensifying the gyre, again causing ocean warming. If they blow out of the southwest, the water is transported past the northeast side of the gyre, again speeding up the rotation, intensifying the gyre and causing more ocean warming. And finally, if the winds blow out of the northeast, like they do ever so often during Santa Ana season, the water passes the gyre on the western flank, once again intensifying the gyre's rotation, AND, since those winds are typically dry, they induce more evaporation of the water in the middle of the gyre, resulting in anomalously high gyre salinity, and thus, an ocean warming double whammy.

So now we've got a runaway feedback on our hands. Combine this with a potentially highly active El Niño hurricane season in the eastern Pacific once again, and, yeah, this could get interesting.