El Nino has crept up on us. Sea temperatures have warmed over the eastern tropical Pacific, both at the surface and at depth. The easterly trade winds that blow across the tropics have weakened and the atmospheric pressure has decreased in the eastern Pacific around Tahiti, while at the same time it has increased in the west, over the area around Darwin.
Weather around the Pacific and beyond is already showing the influence of El Nino. Hurricanes and tropical storms have also been partially suppressed over the Atlantic. An active season had been expected, with the number of storms forecast to be well above average including those making landfall on the coast of the United States. As we head towards the end of the season, the number of storms is close to average and only a few, mostly weak storms, have struck the United States. Instead, the hurricanes have tended to stay out to sea, curving away towards Europe, where their remains have brought welcome rain to drought stricken areas such as Spain and Portugal.
El Nino has this effect because the warmer waters near Ecuador and Colombia cause increased upward motion in the atmosphere there. As the air rises, it spreads outwards, thereby increasing the westerly winds at mid-levels of the atmosphere over the Caribbean. The stronger winds shear off the top halves of developing tropical storms near the Americas before they can fully develop. Those hurricanes that develop further to the east tend to be steered away from the United States by the stronger westerlies.
Over the western Pacific, El Nino has contributed to the serious drought that has affected parts of southeast Australia, where many farmers have had to sell off their stock at low prices, and bush fires have been much worse than normal this early in spring.
Lack of rain over Indonesia has contributed to the worst fires there since the El Nino of 1998. Smoke produced by the fires in Sumatra and Kalimantan has drifted over neighbouring countries such as Malaysia and Singapore. Visibility has been reduced to 50 metres at times, disrupting aviation and shipping and causing road accidents. Drinking water has been contaminated in places and the air has been dangerous to breathe, causing schools to close. The fires are started deliberately to clear land for agriculture, not only for small farmers but also for new plantations of palm oil trees. Lighting the fires is illegal but the law is poorly enforced. Some of the fires get deep into peat soils and can burn for years.
Worldwide, it has been a bad year for fires. The USA has had its most destructive and costliest fire season in 50 years. Around 9 million acres have been destroyed at a government-estimated cost of 1.5 billion dollars. Poland, Greece, Portugal and Spain were badly affected during the summer’s droughts and record heatwaves. In Spain, where the cost of the fires was estimated at 100 million euros, dozens of people have been arrested for arson. Motives range from personal vendettas against neighbours to forest clearance for grazing and employment opportunities for casual fire fighters who are only paid when fires are burning.
The strength of this El Nino, as measured by the departure from average of the sea-surface temperatures, is weak going on moderate. Although it seems to be strengthening, it is not expected to become strong, as was the El Nino of 199798. This is partly based on computer projections of ocean temperatures and partly on decadal cycles in El Ninos performance. Long-term fluctuations in the behaviour of El Nino have been identified using palaeoclimate indicators, such as tree rings, which can extend records of weather patterns back hundreds of years. In the years from about 1977 to 1999, El Ninos were much more frequent and stronger than La Ninas. Based on past patterns of behaviour, it seems likely that we are now entering several decades where the El Ninos will be a lot less frequent than they were and not as strong.
So what can New Zealand expect over the coming months? El Ninos correlate with more frequent south-west airstreams over New Zealand, which bring more rain than average to southern Westland, Fiordland, Southland and south Otago, while conditions are drier than normal over the top of the North Island and east of the main ranges from Gisborne to north Otago. On the whole, the country tends to be cooler than average but eastern areas can be warmer because of more frequent fohn winds blowing down from the mountains. These winds bear very little moisture and further dry out vegetation already suffering from lack of rain, thereby adding to the fire risk.
New Zealand has already had an early start to its fire season. Severe gale north-west winds in eastern areas of the South Island have caused a number of fires to burn out of control. In Dunedin, a fire in pine forest threatened the suburb of Ravensbourne. Around a hundred houses were evacuated in the middle of the night as five helicopters with monsoon buckets struggled to contain flames whipped on by gale force winds.
In the South Pacific, during an El Nino, tropical cyclones tend to form further east, and once formed mostly move southeastwards. This increases the cyclone risk for Fiji,Tonga, Niue, and the Southern Cook Islands. For New Zealand, however, the risk of the remains of a tropical cyclone affecting the country is reduced for the same reasons.
However, it is as well to remember that, over the long term, El Nino only explains around 20 per cent of the yearly variation of our rainfall and we probably won’t get the extremely dry conditions in eastern areas that we saw during the very strong El Nifios of recent memory.
There is another factor that could influence our summer. Satellite measurements of the ozone hole over Antarctica showed a record loss of 40 million tonnes of ozone on October 2. Research a few years ago linked the springtime ozone hole in the stratosphere to increased summer westerlies at sea level in the Southern hemisphere (see New Zealand Geographic, Issue 68).
Although these stronger westerlies tend to affect latitudes south of New Zealand, it is likely that fronts moving through them will sometimes shift the strong winds over us.
Paradoxically, although New Zealand tends to have cooler than average summers during El Nino events, the global average temperature tends to be warmer than average because of the large area of tropical ocean that is warmer than normal and flow on effects such as milder winters over North America.
The strong El Nino of 1997-98 helped 1998 become the warmest year on record, although that was pushed very close by 2005. Given the many record temperatures that have already occurred in various regions this year, now that we have an El Nino on board we may have another shot at that macabre record of the hottest year in recorded history.