April 10, 2010: Wind on the Water

By Dallas Murphy

A Black & White Gallery of Photos by Brett Kuyper

It was rough last night, severe rolling in a beam sea (against the side of the ship) with wind gusts in the 40s as squalls marched through. Though this was far from radical weather by Knorr’s standards, it made for bad sleeping. Everyone was talking about that at breakfast. As I mentioned earlier, when wind blows against current—especially big current like the Agulhas—it stacks up distinctly uncomfortable seas with vertical faces and short “periods,” as they say, referring to the distance from crest to crest. When wind blows with the current, the wave character changes to a plunging sea; the waves sort of sag instead of break, and the period tends to lengthen. This makes for a much more comfortable ride, particularly if the ship is going the same way as the wind/current. Now, late morning, the waves are beautiful, at least to us ocean romantics, the sun glowing through the crests in translucent turquoise. And those of us with less romantic attachments to the ocean are happier in the gentler motion.

When sailors say “weather,” they mean wind. Wind determines everything at sea. Quality of life decreases as the wind speed increases. We walk as if walking were new, staggering from one handhold to the next. Nothing stays put. Wind velocity depends on the pressure of the atmosphere. When the atmospheric pressure decreases (a “low” or a “disturbance”), the wind speed increases. Conversely, when the pressure is high, the weather is usually fine, sunny, with light breezes.

weather map
Weather map provided by Brett Kuyper

See the accompanying weather map. Notice those concentric circles around particular points. The circles are “isobars,” or lines of equal atmospheric pressure. When the isobars are spread out—when there are few concentric circles around some center—there will be little wind. The atmosphere is relaxed, undisturbed, and you will have a nice beach day. But when there are many isobars, when the concentric circles begin to look like a fingerprint, watch out. The wind will blow hard. This is usually the case in a deep low-pressure area, a sort of a hole in the atmosphere. Nature doesn’t like holes in the atmosphere and tries to fill them up—with wind. A hurricane is essentially a very deep low. The wind will blow along the isobars around the center of the low, and in a hurricane, that center of low pressure is the “eye.” The direction of the wind depends on the location of the low (or high) in relation to your position.

In the Northern Hemisphere (north of the equator), the wind spins counterclockwise around a center of low pressure. Down here in the Southern Hemisphere, the wind spins clockwise around the center of the low. (It gets kind of confusing for those used to the Northern Hemisphere; looking at the daily weather map, we find ourselves making circles in the air with our hands to figure out what direction the wind will blow.)

We’ve had pretty good weather so far, because the lows have remained south of our position. This time of year, high pressure over southern Africa “blocks” the lows keeping them, to everyone’s pleasure, down south. Bret, a South African scientist with “local knowledge,” tells us that as winter comes on, the high-pressure regions will move north, allowing the lows to do the same, and the weather in Cape Town will turn very blustery. (Remember that winter in the Southern Hemisphere begins around May-June.) So generally Knorr has snuck in under the gun. And the reason why we had relatively heavy wind last night is because one of those highs fell down on the job and allowed the upper edge of a low to brush over us.

The above is very coarse-grained meteorology, of course, and unless you take up sailing, only the general concepts matter. But even the raw concepts of the behavior of wind suggest another concept worth keeping in mind. The air and the ocean are directly related. Someone likened them to two coats of paint on the same croquet ball. Scientists call them a “coupled system.” The ocean warms/chills the air, and vice versa. However, the temperature of the air changes much faster than an ocean of water, but both in collaboration determine our climate. The standard example of this is the Gulf Stream’s impact on the climate of Northern Europe.

An offshoot of the Gulf Stream, the North Atlantic Current, transports enormous quantities of warm water past Ireland and the British Isles up into the Nordic Seas. London lies at about 50 degrees North latitude, but its climate is far more moderate than northern Labrador at the same latitude on the west side of the Atlantic; and then there are those famous palm trees flourishing in Cornwall and western Scotland where no palms belong. Still farther north, the coast of Norway at the Arctic Circle remains ice-free year around. Sometimes people citing this example say that these places are graced with a climate more moderate than they deserve given their latitude due to the warmth of the Gulf Stream. Well, yes, their climate would be far colder absent the North Atlantic’s warmth, but warm water alone wouldn’t be enough. The cold Canadian winds blowing from the west are warmed as they pass over the surface of the Gulf Stream water. They then carry the ocean warmth onto the shores of Northern Europe. It’s not the ocean nor the wind alone that moderates the climate—but both together.

The South African Navy checking us out
Credit: Scott Loweth

A similar thing happens in South Africa. Western boundary currents such as the Agulhas and the Gulf Stream have particularly potent effect on local weather and climate because they carry warm waters away from the tropics to higher latitudes. Over here on the east coast where the hot Agulhas flows close to the coast, the climate is generally warm, moist, semi-tropical, with about three times as much rainfall as on the west coast of South Africa, where the climate is generally cool and dry.

Now at 1400 (2:00 PM) the wind is down to 14 knots. But I hear that mooring operations have been cancelled for the day due to big swells still running. Swells are waves that live on after the wind that created them has died. Wind imparts its energy to the water; the ocean responds with waves, but when the wind slackens, it takes hours for the ocean to shed that energy, longer in a dynamic environment like that in the Agulhas. That’s a distinctly temporal aspect of the interaction between air and ocean. But it matters a great deal to people in ships and boats.