Science, Surfing, Stratification, Overturning, and Timing

Guest Post by Willis Eschenbach

Surfing

When I was a kid, swimming in the ocean was a rarity. We kids spent summers with my Dad, and once, maybe twice over the summer we’d go to Stinson Beach. I loved the waves, cold as it was. We bodysurfed them over and over, emerging at the end of the afternoon blue with cold. And when my grandmother and my Aunt and my cousins moved to Santa Cruz when I was about ten, we visited them a couple times and went bodysurfing.

When I really started learning about the ocean, though, was the six months that my friend David and I spent in 1970 living on Makena Beach in Maui. Back then, there were maybe thirty people or so in semi-permanent residence on the beach. I have no idea where the owners were. People had “sugar shacks” made out of every conceivable material. David and I took the roof of an old car we’d found. We built up some metre (yard) tall rock walls and put the car roof on it … instant house, good for six months.

I had brought fins and mask and snorkel, and I had little money for food. So I spent hours and hours in the ocean, finding and eating opihi (illegal now, I understand), and trying to talk a fish into joining me for dinner. Plus, of course, bodysurfing whenever the swell came from the south. Way too poor to have a board, I would just play for hours in the waves, letting them toss me around, learning their ways, understanding their ebb and thrust. I spent hundreds of hours in and under the tropical ocean during this time.

The Pipeline on a small day … SOURCE

The next year I moved to Honolulu, and a friend showed me how to ride a board. I got adequate, not good, but I surfed Pipeline on small days and sat on the beach and watched the good guys on big days. Great fun. Plus I continued my snorkel explorations of the reefs and waters around the island. We’d put on mask, snorkel, and fins, and jump off of sheer cliffs into the ocean … swim around, check out the underwater world. Then get out by timing the wave on a vertical wall. On that kind of vertical wall, the sea doesn’t really break. Instead, it kind of heaves up and down, often a long ways. We’d ride the wave up to the top, then grab on to the ledges and cracks like a limpet, and then work our way back up to the top. Sometimes a bigger wave would pick us off the wall on the way up, but we’d just ride it down and back up again and grab on once more and keep climbing …

A decade later, around 1984 I did a lot of scuba diving in the harbors and on the outer reefs in Fiji, without benefit of a scuba license as you might expect … don’t try this at home, kids. I was already totally at ease in the wild ocean, and I’d already worked for hours and hours underwater as a gold miner. Fiji was my introduction to night diving as well, a whole different world from the ocean during the day.

During that time in Fiji, I made my first money underwater since I’d dived for gold. A man needed a diver to check his anchor chain, to make sure that another boat hadn’t laid their anchor right over the top. I said I’d swim the anchor for seventy-five dollars … I’d been down there. It was in Suva Harbour, but the visibility wasn’t bad. I figured I’d just swim carefully above the bottom over the chain, not stirring up the mud, and watch for cross chains. I asked how much chain he had out. He said about a hundred feet (30m), maybe a bit more. Easy seventy-five dollars, I figured … you’d think I would have been smarter by the age of thirty-seven about the lure of easy money, but noooo …

So I put on my gear, strapped on a tank, and went in the water. The visibility was indeed good. I followed the anchor chain down from the boat as it out curved through mid-water out towards the anchor.

I followed it down to the bottom. But instead of seeing a chain laying on the bottom as it was in my imagination … the chain hit the bottom and disappeared completely in the mud. There was absolutely no sign of it on the surface of the mud, it had sunk and been completely hidden by the currents. I couldn’t see the chain, or any possible cross chains. The surface of the mud was flat.

Well, at that point I had a choice. I could go back up topside and admit failure. Or I could go forwards and check the triple-damned chain as I had signed on to do … gingerly, I stuck my hands into the mud. Immediately a cloud of muddy water formed around me. I started forwards, hand over hand along the chain, feeling carefully along the links to make sure there was no chain going across them. I found that if I moved steadily and slowly I could stay with my face in clear water … although I didn’t like seeing jets of water coming up out of the mud as unknown creatures pulled themselves below the surface as I approached. For much of the time, the chain was about elbow deep in the thin mud, although at times it was deeper. At one point it was almost a full arms length below the surface. I kept moving, refusing to think about what creatures were living under the mud.

Suddenly, right in front of my face mask, one of the creatures I’d been fearing popped up out of the mud right in front of my face mask … the most perfect little sea horse, hanging in front of me and giving me the old fish eye. I put on the brakes, and we stared at each other until the cloud of mud caught up and swirled around me and he disappeared and I started moving forwards again … a hundred feet is a long, long way to be hand over handing along a chain with your arms moving constantly through mud deeper than your elbows. After what seemed like about eleven hours of creepy mud-whacking but wasn’t more than an hour, I can’t tell you how happy I was to finally come upon the anchor. I checked the anchor itself, job’s not over until it’s done, and went up to give him a clean bill of health and collect my well-earned $75. He winched in his anchor with no problems at all … I greatly enjoy making money underwater, but I could have passed on that one.

Then a few years later, the gorgeous ex-fiancee and I spent six years in the Solomons, where we both got our scuba licenses. The diving there is outrageous, I spent hours under the ocean both during the day and at night. Wreck diving, shallow and deep diving, amazing stuff.

We moved from there to Fiji, where for a couple of years Ellie and I and our two-year old daughter owned and lived on a houseboat in the Tradewinds Bay part of the Suva Harbour (British and therefore Fijian spelling.) We continued boating and diving.

A decade later I returned to Fiji. In addition to diving there as before, this time a friend introduced me to the modern bodyboard. They are both more fun and much easier to use than a stand-up board. And the beauty part is that if you never have to stand up on your board … you’ll never fall off your board. So I became totally immersed in bodyboarding the various breaks of the south coast of Viti Levu, where I was living, as well as lots of time surfing Frigates Passage, which is about seventeen miles (27km) out in the ocean on the fringing reef around Beqa Island.

Most recently, in the late 2000’s I spent another two years in the Solomons, where I was occasionally one of the dive team that attached the hoses to the fuel ships when the came into harbor, and I supervised their further dive training. I also continued with my sport diving and boating, and (at the age of 62) I got my Openwater II and my Rescue Diver’s certificates.

I bring this up to establish my credentials regarding how I know what’s going on in and under the tropical ocean. As usual, I have no credentials at all. I’ve never taken a single class in oceanography or marine biology, none of that.

However, as a result of thousands and thousands of hours spent boating and surfing and diving in the tropics, I am intimately acquainted with the upper 30 metres (100′) or so of the tropical ocean, from on top of it and from underneath the surface. And this practical knowledge has been aided and abetted by my endless curiosity and my reading about the ocean. So based on that, let me describe for you what happens to the tropical ocean during the day, and the totally different situation that obtains during the night.

Stratification

On a calm sunny day, in the morning a layer of heated water typically forms at the surface which is fairly thin. Sometimes if it’s really calm and with little waves or current, the layer can be so thin that, as I paddle my bodyboard across the surface, I can feel each hand dipping through a thermocline (the thermal interface between two water bodies containing warmer and cooler water) into colder water below. More commonly, in the presence of say 10-15 knots of trade winds (5-7 m/sec), this warm layer is maybe 1 – 3 m (yards) deep, again with a distinct thermocline, and colder water underneath.

The main characteristic of the daytime is the stratification of the ocean into a stable, non-circulating state. The top layer is the warmest. Because the warmest water at the surface is lighter than the cooler water below, the layer tends to persist. And the same is true for each layer beneath it. During the day, the ocean is stably ordered by temperature, warmest at the top, coolest at the bottom.

This situation builds up during the day, and by then end of the day, the ocean is normally totally stratified by temperature.

Overturning

During the night, however, something unexpected happens. At some point, an efficient emergent circulation pattern is established, which rapidly brings deeper water to the surface to radiate and cool. Here’s how that circulation pattern unfolds.

Most nights, at some time after dark, the daytime thermal stratification is upset. Without the sun, a cooler layer forms at the surface. This is the surface water which has been cooled by a combination of radiation and evaporation, so it is both colder and saltier than the water below.

Because this cooler, saltier layer is denser than the warmer water below, the night-time ocean is thermally unstable. At first, water just sinks randomly as it cools. Soon, however, we see the unexpected emergent phenomenon—a type of Rayleigh-Bénard circulation is rapidly (over the course of an hour) established.

Figure 1. Rayleigh-Bénard circulation.

This circulation features distinct vertical columns of descending water at various points in the ocean, separated by areas of slowly rising water. At the surface, the cool surface water runs horizontally to the nearest column of descending water, and then drops until it reaches the corresponding density.

This thermally driven vertical mixing constantly brings the warmest water to the surface, where it can radiate and evaporate the most efficiently. As the night goes on and more and more heat is lost to evaporation and radiation, this thermal circulation extends deeper and deeper into the ocean.

This lovely nocturnal mixing machine generally leaves the tropical ocean, by the dawn, without much vertical temperature change below the surface, and generally without any perceptible shallow thermocline such as develops during the day.

How do I know about this curious circulation? Well, by diving in it and snorkeling in it. I’ve read about it since, but that’s where I first noticed it. These areas of cold descending water are quite evident if you go diving at night in the tropics. They are at their strongest two or three metres below the surface on a calm night. When you swim through them, the colder, denser, rapidly descending water is perceptible immediately. They are much smaller than the surrounding area that they drain, and since what goes up must come down in equal amounts, the vertical current is quite evident.

There’s another curiosity to this circulation, an important one. The descending circulation extends deeper into the ocean than just the layer of equal temperature. You’d think that the cooler water would only extend as deep as the corresponding temperature layer, but this is not the case for a couple of reasons. First, simple mechanics. The descending column of water has inertia, so it tends to persist deeper than you’d expect from simple equality of temperatures.

The more important reason the mixing goes deeper than the level where the temperatures equalize is that the surface water is dense for two reasons—decreased temperature, and increased salinity. So it is denser than underlying water of an equal temperature.

As a result, the circulation is constantly stirring up waters that are colder than the cooled surface water. And this ever-deepening circulation is one thing that allows for the rapid nocturnal heat loss. The surface water brings colder water to the surface, which cools further and when it sinks, brings colder water yet to the surface. This overshoot is critical, because otherwise the surface wouldn’t continue to be fed with the deeper and deeper water.

In understanding this system, it is helpful to remember that, by and large, whatever energy is added to the ocean during the day is lost during the night … if not, the sucker would be boiling by now. We think of the ocean heating up a lot in the day, that’s easy to understand, we can see the sun, we can feel the warm surface layer that builds up during the day.

We forget that the ocean has to and does cool about the same amount each and every night, through thermally driven mixing which supports increased evaporation and radiation. The way that it does this is by efficiently bringing the warmest water to the surface to cool and radiate and evaporate. And the way that it regulates how much it cools is through the timing of the onset of the overturning.

Timing

In natural control systems containing emergent phenomena such as thunderstorms and the Rayleigh-Bénard nocturnal oceanic overturning, often the control is not imposed via the amplitude of the counteracting phenomena.

Instead, it is imposed by way of the timing of the onset and ending of the phenomena.

Let me take the onset of cumulus clouds as an example. These form as the day warms. It takes about an hour or less to go from no cumulus clouds at all to a fully formed cumulus regime. After that, coverage doesn’t change a whole lot.

So how much sun the earth gets doesn’t depend on the amount of the cumulus cover. That doesn’t change much. It depends on what time the cumulus form.

And in a beautiful regulatory manner, the warmer the morning is, the sooner the cumulus form, cutting down the heating. And on cooler mornings, the cumulus form later, allowing in more energy. This warms the earth when it is cool, and it cools the earth when it is warm … a lovely piece of engineering.

The same thing is true about the emergence of the night-time Rayleigh-Bénard circulation in the ocean. If it kicks in right after dark, the ocean loses a lot of energy. On the other hand, if it doesn’t emerge until 2 AM, there’s much less energy lost.

Now, consider the effect of the water temperature on the timing of the start of the nocturnal overturning.

When the water is warmer than usual, the top layer will cool faster than usual—radiation, evaporation, and sensible heat loss all go up as some power of water temperature in kelvins. This combined with warmer water below the surface will speed the onset of overturning, making it occur sooner after dark.

Cooler than average water, on the other hand, will lose heat more slowly. This combined with cooler water below the surface will delay the onset of nocturnal overturning.

This means that overall the warmer water loses heat more efficiently, and cooler water loses heat less efficiently. And this, of course, acts in a thermostatic manner. It tends to reduce deviations in temperature by speeding the cooling when the ocean is warm, and slowing the cooling when it is cooler..

My points here are simple.

1. The emergent phenomena that control the temperature do so by regulating the timing of the emergence of the phenomena, not just by regulating the strength of the phenomena.

2. This kind of control, as far as I know, is not even hinted at in the climate models.

3. In part, this is because the phenomena are way below grid scale.

4. The emergence of Rayleigh-Bénard overturning in the ocean is merely one of the many interlocking homeostatic phenomena that rule the climate. These include cumulus, thunderstorms, El Nino alterations, dust devils, cyclones, and as this post demonstrates, ocean overturning. These all work together to keep the planet from ever getting too hot, or too cold.

5. Please note one final, critical point. None of the emergent mechanisms have emergence times that are ruled by CO2. Instead, the emergence times are temperature based, and work to cool overly warm temperatures and to warm overly cool temperatures. As such, they are not  affected by a change in CO2.

Best regards,

w.

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I don’t think Willis will mind if I put up my favorite related song. It’s my favorite because it has a cool upbeat tempo that paints a picture in your mind, plus being instrumental I never had to worry about figuring out the lyrics with challenges presented by my lifetime hearing loss. – Anthony