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WHAT'S AHEAD – WEATHERWISE FIVE ZONES MARK OUR OFFSHORE WEATHER BY BOB MCDAVITT I n late spring, cruising yachts migrate across the South Pacific, leaving the tropics before the cyclone season kicks in. These voyages take yachties into new territory and this article is a brief introduction describing what weather features are found where, and when they are worth avoiding. The prime movers of our air are fire and ice. The sun's fire updates the warmth of the tropics every day, but the two polar regions have six continuous dark months each year to grow icy cold. This varying temperature difference between the equator and the poles drives our weather. On the daily weather map of the South Pacific, between the warm equator and the icy polar regions, five identifiable features are normally present. A flow of divergent nor'easterly winds is found over and about the equator. This flow starts on the northern side of a large semi- permanent area of high pressure found just west of the South American Andes. It spreads out over the Pacific Island groups just south of the equator. This flow often runs out of puff, especially around or just east of the Solomon Islands, which is called a doldrum area, with light winds but scattered squalls. The trade winds is the name for the winds found in the tropics between the band of low pressure around the equator and the band of high pressure that lies roughly along 30 degrees latitude. These are the most reliable winds on earth, and sailing ships use them as trading routes. The South Pacific Convergence Zone, also called the Trade Wind Front in French Polynesia, is where the equatorial easterly winds converge with and bump into the trade winds. Some of the clouds in the zone contain occasional heavy and squally downpours, with wind gusts reaching 40 knots in the squalls. The output from global weather models is made available to mariners in GRIB files, but these average things out and cannot resolve the details of the convergence zone. Avoid. A zone of transient troughs and disturbed westerlies, also known as the Roaring Forties, is in the mid-latitudes. This is where the main clash of air masses takes place and where any air flowing out of the tropics meets air coming from the polar regions. These airflows are of different density so they do not mix. They clash, twisting around each other to make low-pressure troughs and bumping into each other to make fronts. Between the troughs are ridges of high pressure, so the mid- latitudes have a varying procession of high and low-pressure systems. Troughs and fronts normally contain strong winds and zones of rain. Avoid. The average strength of the disturbed westerlies is related to the temperature difference between the tropics and the polar regions. This is greatest when Antarctica is at its coldest, soon after the spring equinox (when the sun starts shining there after six dark months). So the winds of the disturbed westerlies are strongest, and reach their northernmost point during September, October and November. Some people call this season of extra wind the equinoctial gales, but I like to think of them as the gales of the Antarctic dawn. The fifth zone is the sub-tropical ridge dividing the disturbed westerlies of the Southern Ocean from the trade winds in the 56 Professional Skipper November/December 2011 tropics. This is where high-pressure areas (anticyclones) are found. In our part of the world these highs tend to travel generally from west to east. When they reach the eastern end of the Pacific Ocean, the Andes inhibits any further travel. The anticyclone track in summer (say November to April) is usually across the Southern Australian Bight and the New Zealand latitudes. This track migrates northwards in winter, "following the sun". We can now introduce the squash zone. This is my name for a weather feature that often appears in the trade winds or in the Roaring Forties. The isobars in this zone are squashed closer together by surrounding features, so the winds are increased. Waves tend to be pushed closer together, making for rough conditions. Avoid. Weather features in the South Pacific displayed on a Google Earth map Energy from the sun powers our weather engine and one of the main drivers for this is evaporation. This occurs fastest over the equator where the seas are warmest. Recent sea temperatures over much of the equatorial Pacific Ocean have been nearly normal, producing a neutral balance between El Nino and La Nina conditions. However, a cooler than normal area is slowly growing near the Galapagos Islands and this is helping to produce a brief resurgence of La Nina at times. The disturbed westerly winds that ring Antarctica were weaker than normal in July and August, and this contributed to some notable polar outbreaks for New Zealand. These winds returned to normal during September and may be up and down during October and November. The sub-tropical ridge normally skirts northern New Zealand during spring. One trait of La Nina is to tug this zone southwards, and some highs have been lingering in the central and southern Tasman Sea. If this trend continues it may weaken the rain-bearing fronts as they approach from the Tasman Sea. Some of these highs may stall near Tasmania and help to direct a burst of polar-chilled south or sou'westerly winds onto southern New Zealand. Bob writes a weekly blog at http://weathergram.blogspot.com nominating which areas in the South Pacific are worth avoiding.