Southern Ocean

Chart 2 Pacific Ocean pressure and wind patterns for January. Based on information in Ocean Passages for the World NP136.

• Warmer than normal surfacc water extending into the central and eastern tropical Pacific can cause tropical cyclones to move much farther east than normal. This would certainly be a good reason to delay your arrival in Polynesia until late May or June if a moderate El Niño was in progress.

Should a major El Niño such as in 1982-3 be in progress, it might be prudent to enjoy a season in Central America and leave a Pacific crossing until the following season. Back-to-back El Niños are not common, although this occurred as recently as the early 1990s. Furthermore, El Niño seasons are often interspersed with 'La Niña' such as in 1988, when the opposite effects prevail: high pressure, strong trade winds and a diminished chance of tropical storms moving east - all of which might prompt an early start, with the aim of getting to Polynesia in March or April.

The Doldrums

Dealing with the Doldrums (Inter Tropical Convergence Zone - ITCZ) is discussed in various sources on passage planning. Leaving Panama, one can wait in the Las Perlas Islands (which are teeming with wildlife due to the cold, nutrient-rich waters brought north by the Peru current) until the ITCZ has moved north, which hopefully will provide favourable winds for heading westward.

The south-east Pacific high

This almost stationary system will, as mentioned, be substantially affected by an El Niño. In a 'normal' season, the higher the pressure, the stronger the trade winds and vice versa. The pilot charts give some indication of historical normal pressure for each month of the year. If there is a weatherfax aboard, then some idea of what to expect can be gained, but on a passage of 20 days or more, there is not much you will do differently anyway.

The passage of lows and highs moving eastward As you sail farther west, quite often the wind will go into the cast and north-east, usually getting lighter. It may go

The Southern Oscillation (SO)

This was first identified (in scientific literature) by C T Walker in the 1920s, but well known by many generations of mariners and fishermen in the Pacific and named 'El Nino' because its effects were often observed around the New Year. Essentially, ever)' few years a surplus of water builds up in the western Pacific, pushed there by the trade winds and their associated currents. Equilibrium is restored by the warm surface water flowing all the way back to the eastern Pacific. The severity of an el Nino depends mostly on the temperature and quantity of the warm surface water moving eastwards.

In 1982-3 a very severe SO occurred. Five cyclones moved as far cast as French Polynesia. There were dramatic changes in the fish population and movement in the entire eastern Pacific. This event really got the attention not only of those economically affcctcd, but also the scientific community. Much observation and study has been done on the subject.1

1 For those who would like some well presented (although somewhat technical) reading, the Spring 1995 issue of Engineering and Science, the Alumni magazine of the California Institute of Technology. is recommended. Since the ramifications of the SO arc worldwide, we will be hearing a great deal more about it in years to come.

It has now become clear to meteorologists and clima-tologists that the SO is a major element of global weather dynamics. Now that the El Nino (SO) is so well known and observation and prediction made much easier and far more accurate by satellite, most meteorological offices, certainly the major ones, will be a good source of information on its status.

Its impact on planning from a yachtsman's point of view depends on the severity of an SO. • Weak, even fluky. trade winds caused by a weaker than normal high pressure area between South America and the central Pacific will equate to a slow passage west.

Fig 1 Southern Hemisphere low with trailing cold front, a mirror image of the Northern Hemisphere.

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