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Weather Conditions in the South Pacific

"This 26-27 November [1972] gale was barely over before, on the night of the 27th, the barometer starting dropping again. These repeated gales were seriously beginning to get me down I could hardly remember when my storm clothes had last been removed On the 28th the bottom fell out of the glass The pointer moved right off the scale and continued downwards to about 950 mb during the night ... something altogether new had burst upon us - a storm of hurricane intensity The waves increased in height with unbelievable rapidity. Nothing in my previous experience [including North Atlantic autumn gales, a Coral Sea cyclone, gales off Iceland, Cape of Good Hope, and in Magellan Straits] had prepared me for this By evening the estimated wind speed was over 60 knots; the seas were conservatively forty feet high and growing taller Came a roar, as of an approaching express train The tottering breaker exploded right over us ...".

This description of stormy weather (near 60°S, 140°W) is one of many in solo yachtsman David Lewis' book "Ice Bird", describing his journey across the Southern Ocean (Lewis, 1975).

The "second voyage" of James Cook (1772-75) encountered similar weather events. In addition, Cook's experience graphically illustrates the extent of high latitude cloudiness over the oceans. In 1773-74, in the high latitude portion of the New Zealand-Pacific Sector (the same general area covered by Lewis' description), only 1% of weather observations indicate fine, clear weather. Cook also concluded (correctly) that ice in the Southern Ocean is further north in the Atlantic and Indian Ocean sectors than in the Pacific Sector.

Although adverse weather conditions are not encountered all the time in the Southern Ocean, it is as well for travellers through the region to be prepared. Typically, as one travels south from 40°S, conditions progressively deteriorate and the frequency of severe weather increases. South of 50°S in the South Pacific one finds, according to the U.S. Navy Marine Climatic Atlas (1979), low cloud covering more than half the sky at least 70% of the time; precipitation is mentioned in weather observations about 25% of the time; winds are stronger, with speeds above 34 knots (gale force) 10-20% of the time; and wave heights exceed 6 metres (20 feet) 10-15% of the time. Winter conditions are generally worse than in summer, with the additional factor of sea ice to be considered. Greatest stormi-ness occurs in the central Southern Pacific, far from New Zealand and South America. For example, on David Lewis' circumnavigation of Antarctica, the boat capsized twice within a space of two weeks near 60°S between longitudes 140°W and 120°W.

In the latter part of the 19th century, there were sporadic attempts by meteorologists to draw charts showing daily weather conditions over limited geographic regions of the high latitude Southern Hemisphere. At the time, there was little sea level information available apart from very isolated land stations, whaling ships, and other ships of opportunity. This limited flow of information actually declined further with the opening of the Suez Canal in 1869 and the Panama Canal in 1914, which diverted much of the ship traffic from the Southern Ocean. Information on atmospheric conditions above the surface (pressure, temperature, humidity, wind) was not available at all until the development of the balloon radiosonde and radar after World War II. Convincing weather analysis of the Southern Ocean has only been possible since the International Geophysical Year (IGY) in 1957-58, and subsequently with the development of automatic weather stations, buoys and satellites, coupled with the long-term maintenance of a modest network of ground level and upper air weather observing stations in sub-Antarctic and Antarctic latitudes.

Weather Observations in the Southern Hemisphere since 1957

The 18-month period July 1957 to December 1958, known as the International Geophysical Year, was a time of intensive meteorological observations around the world. For Southern Hemisphere meteorologists, the high latitude oceanic areas and continental Antarctica had long been regions where data coverage was very sparse in space and time, and one of the aims of the IGY programme was to improve this situation. A special effort was made to co-ordinate all reports from land stations and ships to produce daily sea level and upper air charts for the entire globe. Nevertheless, coverage of the South Pacific remained poor. Few ships crossed this area and there were no island stations south of 40°S, except for those close to New Zealand (Campbell Island at about 52°S and Macquarie Island at 55°S) and the Antarctic Peninsula.

Thirty-nine stations operated in Antarctica during the IGY (Taljaard, 1972), but their distribution was uneven, with 15 of these clustered close to the Antarctic Peninsula. In spite of these deficiencies, the resulting map series was the best available up to that time and, as the results were analyzed and published during the ensuing decade, much was learned about the general circulation of the Antarctic region, and possible influences of Antarctica on the atmospheric circulation at lower latitudes.

The launching of the first meteorological satellite (TIROS-l) on 1 April 1960 ushered in an exciting new era in which it became possible to receive daily cloud photographs of vast areas of the Southern Ocean. This was also a time of rapid advances in computer technology and in the development of numerical modelling of atmospheric behaviour. The first authoritative treatments of the large-scale circulation in southern latitudes began to appear (e.g., van Loon, 1967; Taljaard, 1972), but most analyses continued to be based on case studies for limited areas and short periods. Then, in May 1972, the Australian Bureau of Meteorology, which assumed the responsibility of World Meteorological Centre for the Southern Hemisphere, began to produce daily numerical analyses of the broadscale features of the hemispheric flow. In the absence of land-based and ship observations in much of the South Pacific and South Indian Oceans, these analyses relied heavily on the interpretation of satellite cloud photographs as pioneered by Australian meteorologists (e.g., Guymer, 1978).

In the last ten years, there have been considerable improvements in the way observational data are assimilated into the computer models, as well as increases in the number of types of data available. Satellite products now include cloud movement vectors and temperature profiles through the atmosphere, in addition to global cloud photographs several times daily. Another special meteorological observation programme, which began in December 1978, was known as the First GARP Global Experiment (GARP = Global Atmospheric Research Programme), or FGGE for short. A particular feature of this 12-month programme was the use of drifting ocean buoys to measure surface air pressure, a parameter not available from satellite observations. These ocean buoys were of great value in defining the intensity of synoptic* weather systems in the Southern Ocean, and generally resulted in observations of lower mean pressures and greater daily variability in the low pressure belt encircling Antarctica than had previously been thought to occur (Guymer and Le Marshall, 1980). Reduced numbers of buoys have continued to be used since the end of the FGGE period. Southern Hemisphere meteorologists working on aspects of the general circulation have therefore had a reasonably uniform and complete set of hemispheric analyses since 1972, although surface pressures in very high latitudes are probably more realistic from 1979 onwards. Satellite measurements of the extent of Antarctic sea ice are also fairly complete from about 1972 and form a second valuable time series.

The temperature of the ocean surface layer is another quantity of value to meteorologists but unfortunately a long homogeneous time series for the Southern Ocean is not yet available. Although satellites are ideal instruments for determining the spatial pattern of sea-surface temperature, obtaining absolute values is not as easy. Water vapour in the atmosphere absorbs radiation of the same wavelengths that satellites use to detect the surface emissions so that a correction factor must be applied to the calculated temperature; liquid water in the form of clouds or rain is a further complicating factor in deducing the temperature from the radiation measurements.

Figure 2.1 shows a typical hemispheric surface pressure analysis and accompanying meteorological satellite cloud mosaic. The satellite images are photographed in the infrared part of the spectrum, which allows changes in cloud features to be followed during hours of darkness. (For daylight satellite passes, photographs are taken in the visible part of the spectrum.) Low pressure centres circulating around the Antarctic continent (Fig. 2.1b) can, in most cases, be identified with distinctive cloud features (Fig. 2.1a); the correspondence may not

* In meteorology, the term "synoptic" refers to the use of data obtained simultaneously over a wide area so as to give a nearly instantaneous picture of the atmospheric conditions.

always be exact because the satellite mosaic is a composite of a number of passes over a 24-hour period, whilst the surface pressure analysis is an instantaneous "synoptic" view.

Chapter Outline

Although there are still many gaps in our knowledge of physical processes occurring in the atmosphere at high southern latitudes, the existing data base is sufficient to provide a broad view of the climatology of the region. In the following sections, we describe the main features of the atmospheric circulation over the Pacific south of 40°S. Readers are referred to the World Survey of Climatology series for further discussion of the climate of the South Pacific Ocean (Streten and Zillman, 1984) and the Antarctic (Schwerdtfeger, 1970). Both these volumes contain many useful maps of climatic elements, such as surface pressure, directional frequencies of wind, sea temperature etc. (see also the U.S. Navy Marine Climatic Atlas of the Antarctic, 1965, and of the South Pacific Ocean, 1979).

We are concerned here with the meteorology of the South Pacific over the longitude range from Tasmania eastwards to the Antarctic Peninsula. However, many statements about the Pacific circulation can apply equally well to any longitude of the Southern Ocean. This is hardly surprising considering the relative uniformity of the underlying surface. The sharp contrasts between land and sea that drive the seasonally varying regional circulations of the Northern Hemisphere are absent in the south. The constriction at Drake Passage, which has important consequences for ocean currents, is much less significant for the atmosphere.

The approach taken is therefore to describe the atmospheric circulation over the Southern Ocean from a hemispheric point of view, and to highlight results peculiar to the Pacific Sector wherever possible. The following sections of this chapter describe the meteorology at increasingly higher latitude bands. Firstly, weather systems over the oceanic latitudes are examined. Secondly, we consider the complications of sea ice and thirdly the effect of the Antarctic continent itself. A complete separation of these interacting factors is, of course, not possible, as will quickly become obvious to the reader. Indeed, a synthesis of the various feedbacks between the polar energy balance, seasonally varying sea ice and north-south heat transport by atmosphere and ocean is the ultimate aim of much high latitude climate research. Current research efforts are reviewed briefly in the concluding section of this chapter.

Fig. 2.1. (a) Hemispheric cloud mosaic from NOAA-9 satellite over period 22-23 January 1987, and covering entire Southern Hemisphere from Equator to South Pole. The photographs are taken at infrared wavelengths, and whiter features on the image indicate lower temperatures (usually associated with higher-topped cloud); (b) Mean sea-level pressure analysis at 1200 GMT (or midnight New Zealand Standard Time) on 22 January 1987drawn to same scale and orientation as (a), but only covering the area from 20°S to South Pole. Isobars drawn every 5 hectoPascals (hPa) and high/low pressure centres denoted by H/L.


The main feature of the middle and high latitude atmospheric circulation over the Southern Ocean is a wide belt of persistent westerly winds. On the northern side of this belt, at about 30°S, the westerlies give way to a more variable wind pattern coinciding with relatively high atmospheric pressures. Far to the south of the main westerly belt, at about 65°S, winds again become variable, coinciding in this case with the frequent passage of the centres of low pressure that circle Antarctica from west to east (often with a southward component as well).

The 10-year mean surface pressure pattern is shown in Fig. 2.2 for January and July. The middle latitude westerlies are bounded to the north by the subtropical high pressure belt, and to the south by the low pressure region known as the Antarctic Circumpolar Trough or Subantarctic Trough. The location of the circumpolar pressure minimum oscillates with a half-yearly cycle, being furthest north in June and December. Within the trough, there is a tendency for distinct pressure minima to persist in three or four geographic areas, which include the Ross and Weddell Seas. Poleward of the Antarctic Circumpolar Trough is a narrow ring of easterlies around coastal Antarctica, which originates primarily as an outflow of air from the high central plateau, but is reinforced by easterly winds on the southern side of the travelling low pressure systems in the Circumpolar Trough. The high pressure centre depicted over the Pole is unreal, and results from an attempt to estimate the sea level pressure from observations made on the ice surface more than 3 km above mean sea level. Note how much more zonally symmetric the long-term mean flow field is (Fig. 2.2) compared to a daily one (Fig. 2.1. (b)).

The higher altitude airflow is very different from that at sea level. At a pressure altitude of 500 hectoPascals* (approximately 5 km above the surface), where the pressure has reduced to about half that at sea level, the circulation is dominated by a circumpolar westerly flow throughout the year. This westerly flow is not perfectly symmetrical about the Pole but contains large-scale standing-wave perturbations known as "long waves", which are more prominent in winter than in summer. However, even in winter the long waves are of much smaller amplitude than those found in comparable latitudes in the Northern Hemisphere.

An average north-south cross-section of the flow over the sector 130°E-140°W during the austral winter (June-August) 1979 is shown in Fig. 2.3. At very high altitudes (above 100 hPa, or about 16 km) at 60°S, a marked westerly maximum occurs, known as the polar-night jet stream. It is reflected through all levels down to sea level. A second and stronger westerly wind maximum, the subtropical jet

* A "hectopascal" (abbreviated "hPa", and equal to 10"2 Pa) is the same as the older unit "millibar". The name commemorates Blaise Pascal (1623-62), a French mathematician, who made significant contributions to the theory of hydrostatics and the development of the barometer.

Fig. 2.2. Monthly mean sea-level pressure for (a) January, and (b) July, based on ten years of daily numerical analyses from September 1972 to August 1982. Contour interval 5 hPa (from Le Marshall et al., 1985).

stream, occurs at about 28°S at 200 hPa (12 km). Unlike the maximum at 60°S, that near 28°S is not reflected in sea level winds.

This mean jet stream pattern in the central and western Pacific is different from that further westwards over the Indian Ocean sector of the Southern Hemisphere (Mullan et al., 1986). In winter, the subtropical jet is appreciably stronger and the polar-night jet considerably further south in the Pacific than in the Indian Ocean sector. A further difference occurs in the temperature field. In the Pacific, a zone of enhanced north-south temperature gradient lies immediately polewards of both the subtropical and polar-night jet stream axes. In the Indian Ocean sector, there is only one zone of enhanced north-south temperature gradient. These zones produce conditions favourable for the development of cyclones, and we will see later that the patterns of cyclone development in the Indian and Pacific Oceans are quite different

It is worth emphasizing here that the double jet structure that we see in Fig. 2.3 is unique to the Southern Hemisphere winter season. In summer, the jets merge


Fig. 2.3. Height-latitude cross-section showing zonal wind and potential temperature fields, averaged over the period June-August 1979 and the sector 130°E-140°W. Isotachs are shown as solid lines, with contours every 5 m.s"1 (and hatched regions indicating easterlies). Potential temperature shown as dotted lines, with contours every 5°K. Vertical scale indicates pressure in units of hPa (from Mullan et al., 1986).


Fig. 2.3. Height-latitude cross-section showing zonal wind and potential temperature fields, averaged over the period June-August 1979 and the sector 130°E-140°W. Isotachs are shown as solid lines, with contours every 5 m.s"1 (and hatched regions indicating easterlies). Potential temperature shown as dotted lines, with contours every 5°K. Vertical scale indicates pressure in units of hPa (from Mullan et al., 1986).

into one. In the Northern Hemisphere, there is only a single westerly wind maximum at all times of the year. The main difference between the westerly wind regime in the two hemispheres is therefore one of pattern in winter (with a double jet in the Southern Hemisphere) and one of intensity in summer (with the stronger westerly jet in the Southern Hemisphere).

Half-yearly Oscillation

Time series of many variables in middle and high southern latitudes show a marked semi-annual oscillation. An example is given in Fig. 2.4, showing the seasonal variation of the mean atmospheric sea-level pressure difference between latitudes 40°S and 60°S. Since the pressure difference is directly related to wind speed, Fig. 2.4 implies that the high latitude westerlies also undergo a semiannual oscillation in strength, being strongest in autumn and spring. The north-south temperature gradient and the position of the Antarctic Circumpolar Trough exhibit 6-monthly oscillations too. These variations have been recognized for many years, and the explanation put forward by van Loon (1967) is still generally accepted.

Van Loon (1967) noted that at a pressure level of 500 hPa the temperature contrast between middle and high latitudes had a semi-annual oscillation, with maximum latitudinal gradients occurring in March and September. The seasonal temperature cycle at both middle and higher latitudes, however, displays a single

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