Ah, the South Pacific at last! Warm trade winds, sunny skies, an occasional afternoon squall to cool things off - sounds lovely, doesn't it? We've done some sailing in the Northern Trades, and thought we knew what to expect down south: lighter winds, on the average, and maybe not quite so consistent, which sounded great. We were ready for it, because the Northern Trades sometimes get, well, almost boring in their consistency.
Well, I don't think anybody can say that the South Pacific weather has been boring! So what about these famous Southeast Trades? The average winds in the South Pacific Tropical Area are indeed southeasterlies in the 15-20 knot range, but to fully appreciate that, we need to consider for a moment what this word 'average' means. The concept is simple, just accumulate a bunch of observations over some period, do some arithmetic, and voila! Suppose, for instance, that it blew half the time from the northeast at 5 knots, and the other half from the south at 30. The average would indeed be our mythical southeast 15-20 knot winds. But it never blew 20, and it never blew from the southeast. The situation isn't quite that bad, but beware of anyone speaking of averages.
Actually, the weather on the passage from Mexico was mostly fine, and about what we expected: light to medium northeasterlies tending to easterlies north of the InterTropical Convergence Zone (ITCZ), generally light and goofy in the ITCZ with occasional squalls, some quite nasty, and becoming medium east-southeasterlies south of the equator.
The weather in the Marquesas was also mostly gorgeous, but we had a taste of things to come on 'The Day the Trough Came Through Nuku Hiva'. We were sailing a lovely reach in a nice 12-knot northeast breeze that was shifting north as we headed west around the north side of the island for Daniel's Bay. Then we sailed right into a black wall of cloud.
"No problem," I said. "Just a big squall."
"How about a reef?" Sue asked.
"Good idea, but let's at least wait to see if it gets up to 15 knots, don't you think?"
Dumb! The wind went right past 15 knots and we went right past the first reef. Our 10-knot northeasterly turned into 25-30 knots from the southwest at the edge of the cloud bank. With cold, hard rain. As we sailed a wet, sloppy beat around the island, the wind backed to the south and then southeast to stay right on our nose!
"What the heck was that, Almighty Weather Wizard?" asked a soggy Suzy after we had the hook down in Daniel's Bay.
"Beats me, honey, but it sure acted like a cold front. There's been nothing on the fax charts, but as they always say, 'If it walks like a duck, and talks like a duck…'"
We had been mostly watching the Pacific Surface Analysis charts from KVM70 at that point, which are put together by the National Weather Service (NWS) in Honolulu. They were pretty good at showing general wind flow, but as we were to find out, not too good with those silly discontinuities such as troughs, fronts and other minor disturbances that we learned to take such an interest in. The Tropical Analysis charts from NMC at Point Reyes, put together by NWS at the National Hurricane Center in Florida (another building with no windows), had been a great help tracking the ITCZ during the crossing from Mexico, but the are mostly useless south of the equator. We tried to get the Kiwi charts from ZKLF, but our schedule was out of date and the charts were fuzzy. We couldn't get anything that made sense until we were west of Papeete.
So where does this goofy weather come from? To understand what really happens, we first need to talk about the South Pacific High, specifically the fact that there's no such thing. Well, there is, but it's way over next to South America, anchored there for the same reason that the Pacific High is anchored off the coast of California: namely a bunch of mountains and no easy way to get across the summit. To move an area of high pressure across a mountain range requires lifting the entire vertical column of air over the mountains or finding a surface-level gap big enough to squeeze the air through. The Golden Gate isn't nearly big enough no matter how hard it blows, so the Pacific High remains anchored a few hundred miles off the California coast, providing stable northeast trade winds and blocking most of the low pressure troughs and fronts, to the north. There are exceptions, of course, and when the Pacific High breaks down, the low pressure systems can move south. This creates Hawaii's well-known Kona winds, westerlies associated with a low pressure system north of the islands.
The same thing happens in the South Pacific - backwards of course - except the stationary high is well east of the area that we are interested in. The weather books show a broad ridge of high pressure about 30 degrees S across most of the central South Pacific, but that's another misleading 'average'. What really happens is a series of traveling highs, which, like wandering minstrels, are here today and gone tomorrow, steadily shuffling along from Australia to South America with nothing much to slow them down. North of each high are east-southeasterly winds, the famous trades. If the high is strong, then the trades will also be strong. They'll usually be called 'reinforced trades' which we've always thought was a bit of a misnomer.
Separating the highs are areas of low pressure, troughs which are connected to the deep lows that march across the Southern Ocean at latitudes of 40-50 degrees S. These troughs, or valleys of low pressure, often extend into the tropics, disrupting the orderliness of the easterly trades. They bring a northerly shift as the first sign of their approach, and a southerly shift as they pass (i.e. to the west of the trough).
What we have so far are moderately variable winds, building and shifting from the southeast to the east as a high approaches to the south and then getting lighter and shifting to the north or northeast as it passes by and a trough approaches. Finally, they back to the south or southeast as the trough passes and the next high approaches. This is the underlying 'beat' of the Southeast Trades and repeats itself on a 5-10 day cycle all across the South Pacific. in fact, the Fijian name for a strong easterly wind translates to 'eight days'.
What makes the situation a bit more interesting is that the troughs are not simply areas of low pressure. They usually contain a weather front - just the remains of an old Southern Ocean front, but a front nonetheless. These can sometimes be quite strong. The fronts lie in the troughs, and have the effect of making the transition from the north-somethings to the south-somethings quite sharp and rowdy. Frequently, they are accompanied by quite a bit of moisture.
The actual wind directions will depend on the axis of the trough and its associated front. If it lies north-south, then the wind will be north to northeast as the front approaches, and south or southeast as it passes.
"Great, Mr. Wizard," Sue says. "I understand that it's going to get goofy, but why does the goofy weather come from the west when the wind is blowing from the east?
"Ah, humunahumuna, good question. Let me think for a minute..."
This is an important detail that most people initially miss: These weather systems - highs, troughs, fronts and the lot - move from the west to the east, against the prevailing easterly surface winds. That's right, backwards. The reason is that the systems are carried along by upper level winds, not the surface winds. And the upper level flow is from the west in a broad band from the southern latitudes as far north as 20 degrees S and usually 10-15 degrees S. The surface winds in the Tropics are actually rather thin, only a few thousand feet, and the movement of the weather systems is dominated by the strong upper level flow.
Concluding that all this stuff was interesting but mostly harmless, we took off for the Tuamotus with gorgeous weather. It was sunny and warm, and the weather chart looked ideal. It had showed a weak trough a few days previous, which we figured must have gone away. We had a lovely sail in 15 knots of easterly wind that shifted northeast and got lighter.
"Hmm, judging by the lighter winds and the shift to the north, I'd say the trough is still out here. But the barometer's not doing anything, so not to worry," I advised.
"Then why Is the sky so black up there?" she inquired.
"Don't panic," I replied. "Larry reported on the radio that they'd found some lousy weather west of us, so maybe it's some sort of front. Let's get rid of this big headsail, just In case."
As the rain started, the wind got all weird. It finally decided on 25-30 knots from the south, putting us on a miserable close reach with the #4 jib and two reefs. We charged into the night with no relief in sight.
"Where did this crap come from?" snarled my lovely mate as she gave me the evil eye.
"This makes no sense, unless we've found the Convergence Zone," I replied laconically.
I have no problem with a windshift at a frontal - line kinks in the isobars and all that. Isobars are usually kinked sharply at a front, causing a sharp wind shift (actually vice-versa). But here we had two winds blowing towards each other, not just shifting. The axis of this thing apparently was lying east to west - assuming it was the same stuff that Larry had and we had a meeting of northeasterly and southerly winds. The improbability factor must be huge - could this be the infamous South Pacific Convergence Zone that we had read about?
"There's a lot of air in all this breeze. Where's it going, straight up?" Suzy asked:
"You got it," I replied.
The South Pacific Convergence Zone (SPCZ) is a poorly kept secret. It's not mentioned in any of the weather books, at least not In those written in the Northern Hemisphere. Maybe that's because it is not considered a significant player in the world's weather, but for those that have met The Beast, it can be quite significant indeed. Further, it's related to virtually all of the South Pacific 'horror storles' that we have all heard from time to time. You know the ones, the headlines that read "Yachts Savaged by 70-knot Winds in Bora Bora," or something like that.
The best previous information that we had on the SPCZ was a letter from Sandy and Sidney Van Zandt of Soquel in the January '93 issue of the SSCA Bulletin. The Van Zandts described a couple of horror stories and gave a very brief description of the SPCZ including the normal position, from the Solomons in the Western South Pacific, to a point somewhere east of the Socleties and then south.
But it still didn't make any sense in terms of what causes the convergence, where it comes from, why it doesn't go away, how you know where it Is, how to avoid it and other important stuff like that.
To understand South Pacific weather, however, we need to digress and talk some more about some general weather theory, specifically air masses. 'Air Mass' is not what happens at dawn on Easter, but is a term that applies to a large chunk of atmosphere that is relatively homogeneous and uniform with respect to temperature and moisture content. These air masses develop over areas of uniform conditions, such as the tropical ocean, the polar regions, large land masses, etc. Marine air masses which develop over the sea tend to be moist and whatever the temperature of the ocean is. On the other hand, continental air masses developed over land tend to be dry and cold. Polar air masses are obviously colder than tropical air masses.
All air masses take a while to form, and as they move away from wherever they were formed, they change temperature and density only very slowly because air is a pretty good insulator. Any good weather book does a reasonable job of describing air masses, but they generally miss the mark a bit when it comes to describing the role they play aside from the classic storm tracks.
The basic theory of high and low pressure systems is well understood and does a pretty good job explaining where wind comes from it least in theory. Surface winds in the northern hemisphere circulate clockwise and diverge around an area of high pressure, and circulate counter-clockwise and converge around low pressure. These are the so-called gradient winds. The prevailing trades, for example, are a result of the pressure gradient between the highs that lie around 30-40 degrees north or south and the lower pressure near the equator.
In reality, however things aren't so simple. At each air mass boundary there is a change of air density due to a difference in temperature and moisture content. Air masses are always moving, propelled by the rotation of the earth, and will either converge or diverge at their boundaries. Divergence is a pretty benign phenomenon, and is associated with an area or ridge of high pressure which is sinking upper-level. air that subsides to the surface. This air becomes drier as it subsides and warms.
A convergence, on the other 'hand, involves rising air in a zone of low pressure. As this warm, moist surface air rises, it cools and moisture condenses, causing a lot of
Classic weather fronts are also converging air masses and things get rather boisterous along these fronts especially if the temperature differences are large. Rather than sedately mixing. these air masses interact violently. There is a lot of vertical convection and instability as well as condensation and rain. Fronts also relate to low pressure troughs, and the circulation of troughs. Depressions start as a shearing motion between two air masses and the low pressure convergence feeds the process by drawing the edges of the air masses together. In spite of the instability and violence at the boundary, this process is stable and will persist until the cold air mass warms up, the warm air mass cools off or both.
"So shouldn't the temperature differences even out and make the front go away?" Sue asked.
"I think it would," I said, "except that the converging nature of the front keeps it going by continually pulling In new cold and warm air and sending it aloft. As a result, the air at the boundary never has a chance to reach equilibrium."
"All right, I'll buy that, but how long is this stuff going to last?" she said.
"It's hard to tell, but probably not until we sail out of it. I think it's stationary and thus waiting for it to go away would be hopeless," I replied. "But there's got to be more to the story, to explain why things are so boisterous."
Vertical instability is the key to the potential boisterousness of the convergence zone, and has to do with the vertical temperature gradient or 'lapse rate' of the atmosphere. The lapse rate describes how fast the atmosphere cools with increasing altitude. A rising pocket of air will naturally cool due to falling pressure. And if the atmosphere cools faster aloft than the pocket of rising air does, then the rising air will be warmer than the surrounding atmosphere, causing it to rise faster yet!
Surface air over the ocean will also be quite moist, causing condensation into cloud and rain as the air rises. This slows the cooling rate - evaporation cools, condensation warms - causing the air to rise even faster. The rising air also tends to gather into cells of particularly high thermal gradients and intense activity, which accounts for the bursts of especially nasty weather imbedded In areas of generally nasty weather.
This is also the mechanism responsible for isolated disturbances like squalls and thunderstorms. As cool air moves over warmer waters - say a cold polar air mass moving into the tropics - the lower levels of the atmosphere will be warmed by the ocean while the upper levels remain cool. This increases the lapse rate, making the atmosphere unstable, and causing the warm air at the lowest levels to start rising. Cool air over warm water is unstable, warm air over cool water is stable.
"Great," said Sue, trying to balance her coffee against the motion of the boat. "It's a nice theory, but you can't call this cold air!"
"Well, it's all relative. And it's certainly not as warm as it was back in the Marquesasl"
That's the nice theory of convergence. But the piece that kept eluding us was the answer to the question: a convergence between what? It takes two to tango and all that, and a front at the edge of the cool air mass as it moves into the tropics is (relatively) easy to understand. But why does a stationary convergence zone form? That's the question we kept asking without success until we found Bob McDavitt at the Auckland Met office.
Bob explained that the key is the South Pacific High - the one which we dismissed earlier as being not important. This is the high that is stationary off the South
As each new trough moves eastward across the Tasman Sea from Australia, it brings a new mass of cold polar air behind a cold front and ahead of another high. In the higher (more southern) latitudes, these fronts move pretty much due eastward. But in the more tropical latitudes, they slow and move slowly northwards. The surface winds behind or to the westward of the front are initially southerlies in advance of the next high. These become southeasterlies as the front moves north.
Winds in advance to the east of the front will generally be northerlies of some flavor. As the trailing part of the front moves north, it becomes oriented more east-west and will eventually meet the warmer easterlies we just talked about. This new load of cold or at least cooler air behind the old front will interact with the warmer air of the easterlies to the north, forming an active convergence. The farther north this cooler air mass drifts, the slower it goes. That's because the high-level westerlies get lighter in the lower (more northerly) latitudes. The convergence becomes relatively stationary and sits there until it is joined by the next front with a new load of cool air.
So the South Pacific Convergence Zone is really just a 'graveyard' for old Southern Ocean fronts. The reason they stop here and don't continue on to the ITCZ is that they converge with the warm easterlies that flow from the stationary high west of Chile. The Convergence Zone will become less active when there haven't been any fronts for a while. This Is because the air mass behind each successive front warms up
It's important to remember that the SPCZ is a relatively stable beast in the geographic sense. Once a new front meets the CZ, it starts another 'feeding frenzy' as the cool air gets pulled into the stationary convergence. The convergence zone will wander back and forth slowly, but has little impetus to move. The predominate change is for it to become stronger and weaker with each new cooler air mass. The basic cycle is the 510 day weather period of the Southern Ocean. On the other hand, it may take an entire season for the CZ to move along to some other island group!
The SPCZ usually starts somewhere near the Solomon Islands say 5 degrees S 16 degrees E and extends eastward to the north of Fiji, Tonga and most of the Cooks, crossing 160W between 10 and 15S. It then curves a little south, usually passing north of Bora Bora and the Societies, then curves southeast between the Tuamotus and the Marquesas.
This year the SPCZ was pretty active, and early in the season was pretty much where it was expected. Later In the season it was farther south and spent a lot of time over Bora Bora, Suwarrow Island in the Cooks, and Vava'u, Tonga.
"Wonderful," Sue said, stifling another yawn. "Is it going to rain tomorrow?"
So why is there this almost universal misconception about the nature of South Pacific weather? Maybe the answer can be found in a conversation that we had sitting under the convergence zone at the Bora Bora Yacht Club. We were talking about the weather with some friends and waiting for the rain to stop. Sharing the table was a professional photographer on assignment to do some travel shots of Bora Bora. The point is not what was said, which was the usual "Howya doin? Whereya from?" sort of stuff, but simply that he was hanging around the bar talking to yachties instead of being out In the rain taking pictures. Ever seen a picture of Bora Bora on a rainy day? We haven't either, even amongst our own.
Next month, Part II.
- jim corenman
Please note: If the actual issue is no longer be available, we will still be able to make photocopies or PDFs of it.
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