## The Roaring Forties sometimes purr

The ‘Roaring Forties’ and the ‘Furious Fifties’ are terms often applied to the strong westerly winds which are experienced over the mid-latitudes of the Southern Hemisphere. But while this image of raging gales is accurate for much of the time the strength of these winds varies greatly from day to day, across the seasons and also from year to year.

When Aurora Australis sailed into the Southern Ocean in July of 1995 (Voyage 1, 1995–96) many expected very difficult weather conditions. However, on the voyage south, reports received from the vessel indicated that despite traversing approximately 2000km of ocean between Tasmania and the coast of Antarctica in winter, the weather was remarkably kind with relatively light winds and unexceptional sea conditions. Expeditioners did eventually encounter some intense storms near the Antarctic coast, but the question remains: How can a ship experience gentle wind and sea conditions at a time of year when southern Australia is lashed by rain and cold changes from Southern Ocean depressions and cold fronts?

Measuring the actual wind in remote areas such as the Southern Ocean is very difficult. It can be achieved by making measurements on isolated ships and from some instrumented buoys. More recently it has been accomplished over larger areas from satellites equipped with an instrument known as a scatterometer, which measures average wave height — closely related to wind speed.

But estimating wind speeds and directions over the ocean has traditionally meant making inferences from carefully constructed atmospheric pressure charts. This involves assumptions that the analysed distribution of pressure is realistic and that the airflow above the friction layer (about the lowest kilometre of atmosphere) is in geostrophic balance. That is, the force due to the pressure gradient (as indicated by the isobars) is balanced by the apparent force due to the rotation of the earth (the coriolis force).

Application of a simple mathematical formula gives the wind speed at a given latitude for a specified pressure gradient. The wind is normally resolved into two components, one in the west-east direction and the other in the south-north sense. The wind direction above the lowest few hundred metres (the friction layer) is parallel to the isobars with low pressure to the right of the wind vector in the Southern Hemisphere. Close to the surface, the wind is slowed by the effect of friction and the wind vector turns slightly across the isobars towards lower pressure.

To understand the seasonal variation of wind over the Southern Ocean we compare the seasonal pressure patterns over the Southern Hemisphere. Figure 1 (see pdf file) shows the average pattern of pressure at mean sea level (MSLP) over the Southern Ocean in the four seasons from the NCEP (National Centres for Environmental Prediction) reanalysis program (Kalnay et al., 1996). The most obvious variation in the appearance of the maps from season to season relates to the north-south migration of the subtropical ridge of high pressure. In summer, the ridge is at its maximum southwards extent and in winter it moves northwards over Australia.

A more subtle change occurs in the trough of low pressure encircling the Antarctic coast south of 60°S, known as the Circumpolar Trough — the region of minimum barometric pressure where the effects of the intense depressions over the Southern Ocean are most felt. It reaches peak intensity and moves closer to the coast in autumn and spring, intensifying the pressure gradient and strengthening the westerly winds to its north. Hence strong winds are associated with the equinoxes (misleadingly called equinoctial gales — great variations in wind speed can happen at any location within these seasons as well as from year to year).

In Figure 1c (see pdf file) the average MSLP for winter indicates that the westerly winds are on average established across southern Australia but the orientation and spacing of the isobars over the Tasman Sea and New Zealand region suggest that there is a ‘split’ in the westerly winds in this area. This divergence of the westerly flow occurs because there is a high incidence of anticylonic activity in winter, the so-called ‘blocking highs’ which are found well to the south of the subtropical ridge. It was such a situation in July and August of 1995 that resulted in the spell of light winds enjoyed by Voyage 1. Figure 2 (see pdf file) reveals the average mean sea level pressure over five days in late July 1995. It features an extensive high pressure ridge extending southwards of Tasmania between major depressions to its east and west.

The seasonal variation of the strength of the westerly component of the wind is demonstrated in Figure 3 (see pdf file) which plots the wind speed at the 1000 hPa level for the months of January, March, June and October. On average, the belt of strongest winds over the Southern Ocean is found between 50°S and 60°S at all times of the year, but within that zone maxima occur in autumn and spring. The strongest westerly winds occur in the Indian Ocean sector. The area of lightest westerly winds is apparent in the Tasman Sea and New Zealand region in winter, but relatively light winds also extend across the South Pacific Ocean at this time of year (Figure 3c).

In summary, although there is a well understood seasonal variation of the ‘westerlies’ there is great variability in the day to day strength of the wind over the Southern Ocean in response to the position and movement of the pressure systems. There is also variation in the strength and frequency of systems within seasons and from one year to the next.

Michael Pook,
Antarctic Cooperative Research Centre

Reference

Kalnay, E., and co-authors (1996). ‘The NCEP/NCAR 40-year reanalysis project', Bull. Amer. Met. Soc., 77, 437-71.

on