What happened to the warm water?


Hopefully you were one of the lucky ones who received advanced notice last week regarding the sudden and precipitous fall of the ocean’s temperature. Regardless of how much rubber you may have decided to don, the question is universal: What happened?

In a word, “wind.”

We have wind all the time, and for much of the summer we have still enjoyed unusually balmy water temperatures, so what makes this past week so unique?

When hot air rises, cooler air rushes in to fill the empty space, thus creating wind at the surface. At height, the air goes from the warmer place to the cooler place. For example, near the coast on a hot, summer day, the inland air temperatures heat up more than the air over the sea and begin rising, the cooler air from the ocean rushes in to fill that space, creating an onshore flow (moving from west to east, away from the ocean) or “sea breeze.”

These typical wind patterns in San Diego generally mean calm winds in the morning while it is still somewhat cool outside. There will be a steady increase of onshore wind throughout the day as it heats up, returning to eventual calm in the evening . . . thus the term “evening glass-off.”

However, what we have had lately is a relentless convergence of high- and low-pressure systems. Simply put, way up in the atmosphere, above San Diego, we have a high-pressure system trying to fill in the space of a low-pressure system. Thinking in terms of a liquid, if you have an area of “high” water next to an area of “low” water, the deeper water will flow into the shallower water. This movement of air is wind. The greater the differences in pressure between the low and the high, the stronger the wind will blow.

In large part, this is due to the warm, moist air flowing in from the likes of Hurricane Dean and other tropical disturbances colliding with the cooler air flowing down from the north. This has increased the duration of wind over the past several days, causing it to blow continuously all day and throughout the night. Sustained winds will eventually cause upwelling, a literal rising up of colder water from the lower depths as the warmer surface water is blown away.

Swells can also cause a temporary drop in water temperature. A large, powerful swell can, and often will, move and displace large amounts of water, both on the surface and underneath. This creates a mixture of warm and cold water, causing a noticeable dip in the ocean’s temperature.

Before you go cursing at the wind however, think again. While we’ve enjoyed exceptionally warm water for an abnormally long period of time this summer in San Diego, we haven’t had much in terms of quality swells either. Wind is also the primary generator of waves. And obviously, there hasn’t been enough in the North or South Pacific. As the wind blows over the surface of the ocean, small ripples are created. If the wind continues to blow, these ripples will eventually be transformed into bands of waves, essentially transferred energy from air to water, wind into waves. Surfable waves generally require three elements of wind working together; velocity (speed), duration (sustained time) and fetch (distance). What we surfers prefer is that these winds occur in faraway places like Alaska and New Zealand!

But not all local winds are bad. If it weren’t for the local, short-interval windswells, we would have had nothing to surf on for most of this summer. Generally weaker and of poorer quality than their long-interval, groundswell counterparts, the windswell nonetheless has been our savior this summer.

And then there are the local, offshore breezes (winds blowing from the east to the west). These winds have the ability to turn an otherwise dumping beachbreak into a ripping, hollow-wave tube fest. Some waves, both here and abroad, rely heavily on offshore winds to give them their shape, quality and length of ride.

And not unlike the yen and yang of wind, cold water also has some advantages; besides thinning out the line-up, it helps our outer water kelp beds grow strong and healthy, protecting us from the dregs of a typical onshore flow. The kelp beds act like a giant sieve through which wind-affected waves pass, coming through thoroughly groomed and glassy… and thus the circuitous relationship between wind, water and wave is complete.