Home  > Status Report  > Chapter 2 - Physical Environment  > 2.1 Weather & Climate of Cleveland Bay

2.1.4  Variability of climate and El Niño-Southern Oscillation (ENSO) events

Climate of Cleveland Bay varies naturally on a range of timescales – from year-to-year, to decadal and longer. The major source of global year-to-year climate variability is the El Niño-Southern Oscillation (ENSO). ENSO is the term used to describe large-scale fluctuations of the ocean-atmosphere climate of the tropical Pacific (see Allan et al. 1996 for a detailed description and history of our understanding of ENSO). The strength of ENSO is commonly monitored by the Southern Oscillation Index (SOI) - the difference in sea-level pressure between Tahiti and Darwin. ENSO has two opposite phases, both of which can result in significant climate anomalies in Cleveland Bay: 1) ENSO, El Niño, negative SOI or “warm” events - when the eastern equatorial Pacific is warmer than normal and 2) anti-ENSO, La Niña, positive SOI or “cool” events - when the eastern equatorial Pacific is cooler than normal.

During ENSO events the summer monsoon of north Queensland tends to be weaker than during anti-ENSO events which gives rise to substantial differences in surface climate conditions between these sets of years. By averaging climate conditions over several ENSO events and over several anti-ENSO events we gain a picture of climate anomalies typical of ENSO and anti-ENSO events in the region. It should be noted, however, that no two ENSO or anti-ENSO events are alike and climate conditions in any given ENSO and anti-ENSO year may differ from the following average or “composite” pictures.

In ENSO years, SSTs tend to be cooler in winter and warmer in late summer compared to anti-ENSO years in the vicinity of Cleveland Bay (Figure 1). The weaker summer monsoon typical of ENSO years leads to below average rainfall at Townsville compared to anti-ENSO years (Figure 2). The median total annual rainfall (over the water year October-September) for ENSO years is 796 mm compared with 1,350 mm for anti-ENSO years. ENSO years are also characterised by substantially reduced river flows and tropical cyclone activity compared with anti-ENSO years when the summer monsoon tends to be more vigorous (see Lough, 1994; in press).

Figure 2.1         Average differences in monthly sea-surface temperatures at 19.5oS, 147.5oE between years of ENSO and anti-ENSO events over the two-year period typical of evolution of such events. Solid blue bars indicate months when SSTs are significantly cooler and red bars significantly warmer during ENSO events compared to anti-ENSO events.

Figure 2.2         Median monthly rainfall for Townsville from July to June for ENSO years (red line) and anti-ENSO years (blue line).


This page was last updated on Monday 8 April 2002
by Joanna McIntosh