Environmental Steering

Nascent tropical disturbances in the South Pacific may stay more or less stationary while developing. Once formed, the majority of tropical cyclones migrate generally eastwards and polewards from their original position. This means that in spite of being relatively small features at synoptic scales, cyclones may cause great damage on any islands they encounter, throughout a belt that is several hundred kilometres wide and more than a thousand kilometres long. Yet predicting the movement of an individual storm remains one of the most difficult tasks in tropical forecasting (McGregor and Nieuwolt 1998). This is because tropical cyclones are notorious for displaying erratic behaviour in their speed and direction.

Environmental steering is the primary influence on the motion of a tropical cyclone. A commonly used analogy is to liken the motion to a leaf being steered by the currents in a stream, except that in the atmosphere the 'stream' has no set boundaries. While an incipient cyclone system is still forming within the SPCZ, and does not yet possess a tall vertical structure, its motion has a tendency to be more erratic compared to later stages of maturity. This is because a young system is steered mainly by surface layer winds, and these winds often show irregular patterns over relatively short distances, owing to the uneven nature of convergence and disturbance of air masses within the SPCZ.

Once a tropical cyclone attains a mature vertical structure reaching high into the tropopause, and if it manages to break free of the influence of the SPCZ by moving southwards, then steering is controlled by atmospheric steering winds. The important steering winds are those that occur at the 850-500 hPa level in the mid-troposphere, which are referred to as the deep layer winds. The mid-tropospheric wind field tends to be well defined over large areas of the South Pacific. This makes the job of track forecasting somewhat easier through the middle and latter phases of a storm's life cycle (Fiji Meteorological Service 2005, pers. commun.). The example of Tropical Cyclone Tam in January 2006 (Fig. 4.5) illustrates the environmental steering effect. As a result of steering by the mid-tropospheric wind field (the mean deep layer shown in Fig. 4.6), TC Tam tracked in a southeasterly direction between the islands of Fiji and Samoa.

Synoptic-scale influences have important effects on the environmental steering currents. A ridge of high pressure may keep a tropical cyclone entrenched in the trade wind belt, whereas an advancing trough of low pressure can cause a storm track to recurve. Recurvature is the term given to

Fiji Trade Winds
Fig. 4.5. Isobaric chart of Tropical Cyclone Tam at 6 a.m. on 13 January 2006, Fiji Standard Time. Courtesy of the Fiji Meteorological Service.

a deflection in track direction. In tropical latitudes above 15°S, cyclones often move towards the west with only a slight poleward component. This is because of the existence of the subtropical high-pressure ridge farther south. The subtropical high is a large anticyclone situated in the subtropics. It is elongated along a west to east axis, thus forming a ridge of high pressure. It is a persistent feature and rarely absent. On the equatorward side of the ridge, easterly winds prevail, pushing cyclones westwards. In the absence of strong steering currents or if the subtropical ridge is weak, then Coriolis acceleration causes storms to turn in a more poleward direction. On the poleward side of the subtropical ridge, westerly winds steer the path of tropical cyclones back towards the east.

In rare cases of two tropical cyclones either forming adjacently or moving close to one another, the steering flow is affected by their interaction. The interaction is subject to the Fujiwhara effect, resulting in some atypical behaviour as the two storms swirl towards each other. The Fujiwhara effect is when two vortices are mutually attracted, and as a result have a tendency to spiral around a central point, sometimes merging together. If the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. Therefore, if two cyclones approach each other, their vortices begin interacting in this way. Tropical Cyclone Nancy underwent a minor Fujiwhara effect with Tropical Cyclone Olaf in February 2005 (Fig. 4.7).

Prevailing Wind Direction Fiji

Fig. 4.6. Chart of mean deep winds at the 850-500 hPa level in the mid-troposphere, over the South Pacific at 5 a.m. Fiji Standard Time on 13 January 2006 (5 p.m. GMT on 12 January 2006), during the early life of Tropical Cyclone Tam (shown in Fig. 4.5). Wind feathers are drawn to illustrate the direction from which the wind blows: short barb - 5 knots, long barb - 10 knots, solid triangular barb - 50 knots (1 knot = 1.852 km h-1). Note the northwesterly wind-field pattern south of 10°S and east of 180°, in the vicinity of Samoa. As a result of steering by this wind field, TC Tam followed a southeasterly course in this region. Courtesy of the Australian Bureau of Meteorology.

Fig. 4.6. Chart of mean deep winds at the 850-500 hPa level in the mid-troposphere, over the South Pacific at 5 a.m. Fiji Standard Time on 13 January 2006 (5 p.m. GMT on 12 January 2006), during the early life of Tropical Cyclone Tam (shown in Fig. 4.5). Wind feathers are drawn to illustrate the direction from which the wind blows: short barb - 5 knots, long barb - 10 knots, solid triangular barb - 50 knots (1 knot = 1.852 km h-1). Note the northwesterly wind-field pattern south of 10°S and east of 180°, in the vicinity of Samoa. As a result of steering by this wind field, TC Tam followed a southeasterly course in this region. Courtesy of the Australian Bureau of Meteorology.

Was this article helpful?

+1 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment