Equatorial Westward Drift and Rainfall in Australia

Equatorial Westward Drift

and Rainfall in Australia

By Graham Dull

The Equatorial Westward Drift refers to the wide band of westward moving air in the tropical zone sitting across the equator.

The relative strength of the Equatorial Westward Drift (EWD) affects rainfall patterns across Australia.

The area to which this article directly relates is inland NSW and southern Queensland, although the greater portion of Australia is affected (in some way) by the EWD.

Fig. 1

The MAP highlights the focus area of NSW and Qld.

With a stronger EWD, a lower rainfall is experienced in these areas -- while a moderate EWD gives greater rainfall. (And an 'extremely weak' EWD may cause fast moving fronts to cross the country -- likewise restricting rainfall.)

A 'Strong EWD' indicates reduced rainfall (and drought).

A 'Moderate EWD' indicates a greater likelihood of good rainfall.

Equatorial Westward Drift

As already stated, the Equatorial Westward Drift refers to the wide band of westward moving air in the tropical zone sitting across the equator. This wide band of air moves gently westward. The EWD may strengthen and weaken. it may drift further to the north or south, it may become broader and wider, and these changes will affect the weather patterns across Australia. The EWD includes the north-east trade winds (northern hemisphere), and the south-east trade winds (southern hemisphere).

There are easily identifiable signs which can be used to monitor the strength of the EWD such as 'tropical cyclones tracking in either a westerly or an easterly direction' -- but we will consider these indicators in detail later.

Traveling in the opposite direction to the Equatorial Westward Drift are the predominant Westerlies in the middle latitudes (between 30 -- 60 degrees latitude) which obviously occur in both the northern and southern hemispheres. The prevailing westerlies comprise powerful and fast-moving air masses traveling from west to east.

Fig. 2

The MAP defines the EWD influencing northern Australia, and the Westerlies affecting southern Australia.

Coriolis Force

Air moving from the equator towards the poles is affected by the 'Coriolis force.' And the air moving from the poles towards the equator is affected likewise.

The Coriolis effect causes tropical air which is moving south from the equator to deflect to the left, and polar air moving north will deflect to the left also.

Australian Climate

Australia lies in the southern hemisphere in the zone of generally High Pressure. The descending air typical of this zone gives Australia the largely clear blue skies and warm climate for which we are well noted. So, a high-pressure system dominates Australia's climate. It is a natural aspect of our weather that we receive a lower rainfall than many other parts of the world.

This noted, we will look at the influence the Equatorial Westward Drift has on our rainfall patterns.

Fig. 3

Diagram showing the most significant Atmospheric Circulation patterns.

As mentioned earlier, a 'Strong EWD' indicates reduced rainfall (and drought).

This occurs because a stronger Equatorial Westward Drift strengthens the dominant high-pressure system over the continent. Rain events find it hard to penetrate the system of high-pressure.

Examples

(1) A southerly low touching the lower portion of Western Australia -- and having sufficient moisture to bring decent rain across Australia -- is forced down into the Southern Ocean by the powerful high.

(2) A southerly low reaching South Australia is held to the south by the rigid high-pressure system -- and may bring rain to Victoria but it does not penetrate NSW.

(3) Huge pools of moist air off the north-west coast of WA fail to move across into southern Queensland and NSW because the predominant high-pressure system forces the rain band south into the Southern Ocean.

We have no choice but to accept these weather patterns. But what may be done to monitor them? What indicators can we observe to give us a clue as to whether rain is likely or not?

The Equatorial Westward Drift

The relative strength of the Equatorial Westward Drift can be monitored by numerous weather events around the globe. Because the EWD extends completely around the earth, weather events globally can be used to monitor Australia's climate.

The EWD is not a fast-moving air mass like the Westerlies. It drifts from east to west -- but it can increase in persistence and strengthen. Besides persistence, it can become broader -- hence more powerful.

Local weather events which indicate the strength of the EWD

(STRONG indicates dry conditions -- MODERATE indicates rain)

(1) STRONG/DRY -- Cyclones and rain depressions to Australia's north tending to travel from east to west or stagnate. MODERATE/RAIN -- Cyclones travel from west to east.

(2) STRONG/DRY -- Cyclones do not become 'southerly moving' rain depressions but remain in northern latitudes. MODERATE/RAIN -- Cyclones become 'southerly moving' rain depressions.

(3) STRONG/DRY -- Rain and substantial rain events in Cairns, and dry in Darwin. MODERATE/RAIN -- Wet in Darwin, dryer in Cairns.

These indicators are observed right around the world because the Equatorial Westward Drift encircles the globe.

International weather events which indicate the strength of the EWD

The hurricanes which affect the Americas commence life in Africa. The Equatorial Westward Drift carries these rain systems westward while they build in strength as they cross the Atlantic Ocean to the Caribbean Islands and to mainland America. An increase in the number, and an increase in the power of the hurricanes indicate a stronger EWD. With a weaker EWD much of this moisture would dissipate to the north and south over the Atlantic into numerous smaller storm systems.

This principle applies to all Cyclones, Hurricanes, and Typhoons which approach continental landfall from an easterly direction. Massive rain depressions generated from the east and affecting these same areas are also included in the data. So, whether it is the east coast of northern Australia, the east coast of Asia, the east coast of India, the east coast of Africa, or the east and Gulf coasts of America -- an increase in intensity of such rain events indicates a 'Strong EWD.' And a strong EWD indicates a dryer season for much of Australia.

With a strong EWD, storm systems build into larger, more powerful storms, and with a weaker EWD, the storm events are dispersed into numerous rain events which are more evenly distributed over wider areas.

It is not just the El Nino in the Pacific, and a positive Indian Ocean Dipole which indicate dryer conditions for much of Australia, but the related global Equatorial Westward Drift.

(4) STRONG/DRY -- An increase in number, and strength of Cyclones, Hurricanes, and Typhoons affecting any land close to a continental east coast. MODERATE/RAIN -- A decrease of the same.

(5) STRONG/DRY -- As above relating to an extensive rain depression. MODERATE/RAIN -- A decrease of the same.

These are indicators which are easy and inexpensive to observe.

An example -- the Townsville flood of 2019

This example relates to reference (1). (Note that a strong EWD -- indicates dry conditions for the target area.)

(1) STRONG/DRY -- Cyclones and rain depressions to Australia's north tending to travel from east to west, or to stagnate.

The example given here relates to the Townsville flood of Jan-Feb 2019. On this occasion, a monsoonal trough stagnated over northern Queensland.

A rainfall of 1,455mm was recorded at Townsville airport for the 14 days from Jan 27 to Feb 9. The long-term annual average is 1,128mm. The 14-day flood event far exceeded the annual average rainfall. The daily totals were 30, 52, 80, 109, 153, 216, 142, 172, 181, 42, 110, 16, 118 and 1 mm.

The flooding not only affected Townsville. The rain extended from the coast to hundreds of kilometres inland to the west. Besides the extensive flood damage, and the human deaths in Townsville, an estimated 500,000 cattle were swept away or otherwise died because of the flood waters many kilometres inland.

While so much rain fell in northern Queensland -- inland New South Wales and southern Queensland remained drought stricken. It should be noted also that Darwin and many areas of north-west Australia did not receive the expected monsoonal rain for the summer and are drought stricken. The extreme wet occurring in one area, is an indication of dire drought conditions for others.

Other indicators

Besides rainfall indicators (which have been the topic of this article), a 'lack of rain' in certain parts of the globe also indicate a strong EWD. And a prolonged lack of rain can lead to grass, scrub, and forest fires. Features such as drought, heatwave, and wildfire conditions are related to each other. One needs to be careful though, to ensure that the drought, heatwave conditions, and fires are exceptional -- and do not just indicate a regular (year in, year out) weather pattern for that part of the world.

Inland NSW and Southern Queensland

The weather patterns of inland NSW and southern Queensland tend to remain stable through extended periods -- there are periods of above average (good) rainfall, and periods of low or extremely low (poor) rainfall. At times the wet or dry cycle will persist for years. It is hard (or perhaps impossible) to predict the change from one pattern to the other.

The observations made in this article are not put forward to predict a major change -- from wet to dry, or from dry to wet. The observations are intended to indicate the overall strength of the global system which is either generating rain -- or providing ideal holiday weather under clear blue skies.

In Closing

(The high-pressure system) over Australia can perhaps be likened to a pulley rotating anticlockwise with a belt running across the top (the Equatorial Westward Drift) and a belt running along the bottom (the Westerlies). Each one feeds and strengthens the other -- so if one is fast and strong, then the other is strong. If one is slow and weak, so is the other.