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let’s talk about the weather

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Well there’s much politicking going on currently about the carbon tax, or carbon pricing, and its impact on agw and on the economy, it’s likely impact, or non-impact, on our energy and environmental choices, and also, of course, whether we need worry about climate change generally. In Australia, for those concerned about agw, the local weather has become a mite uncooperative over the past few seasons.

We’ve experienced a double La Nina event since 2010, with memories of the long drought preceding it fast receding, at least for city slickers. 2011 was Australia’s coolest year in a decade. 2010, the drought-breaking year, was Australia’s third wettest on record overall, and the following year, 2011, was our second wettest. The only year, in the past century, that has beaten these, was 1974, also a La Nina year. 2011 was the wettest year on record for the Murray-Darling basin.

The Australian government’s Bureau of Meteorology has produced a handsome volume detailing the 2010-2012 La Nina events, and it’s available free in PDF form from this site.

The El Nino Southern Oscillation [ENSO] is a semi-regular cycle which affects weather conditions in countries and islands within and around the rim of the Pacific Ocean. ENSO events generally last from one southern hemisphere autumn to the next, but it’s not unusual for a La Nina event to last two or three years, as for example the 1998-2001 event, from autumn 1998 to autumn 2001.

Now I’ll take a deep breath and try to explain how these cycles work. Between ENSO events, which of course vary in strength and complexity, the Pacific is described as being in a ‘neutral phase’. But even in this state, temperatures vary throughout the Pacific. For example, in the western tropical Pacific, above New Guinea, there’s an area known as ‘the warm pool’ which regularly has some of the highest ocean temperatures in the world. This has something to do with the thermocline, the region separating warm, well-mixed surface water and cool, deep ocean water [which is richer in nutrients]. The thermocline is deeper in the western Pacific than in the eastern. These temperature gradients, and the way they vary across the Pacific, are the single most important drivers of ENSO events.

In normal conditions, trade winds blow from east to west across the Pacific. These winds are a feature of what is known as the Walker circulation, a current and convection cycle in which the winds drive warm moist air and warm surface waters westward, while the central Pacific remains relatively cool. These warm western waters pump heat and moisture high into the atmosphere, where moist air forms clouds and rain, and the air moves eastward, and then falls in the eastern Pacific. So we have eastward-moving air in the upper atmosphere and westward-moving air at the sea surface.

The Southern Oscillation Index [SOI] is a measure of the endlessly varying strength of the Walker Circulation:

The SOI measures the difference in surface air pressure between Tahiti and Darwin. The index is best represented by monthly (or longer) averages as daily or weekly SOI values can fluctuate markedly due to short-lived, day-to-day weather patterns, particularly if a tropical cyclone is present. Sustained positive SOI values above about +8 indicate a La Niña event while sustained negative values below about –8 indicate an El Niño.

The 2010-11 La Nina event was a particularly powerful one. Although it produced only 11 tropical cyclones during the cyclone season [November to April] – which, though it sounds like a lot, is actually less than the average number of cyclones [12] in any particular year, La Nina or not – it produced some 29 tropical depression systems, one level below a cyclone, and well above the usual number of such systems. More importantly for Australia’s north-east coastal residents, 5 of those 11 cyclones were in the severe category, which was well above average. Three of them, Tasha, Yasi and Anthony, crossed the Queensland coast, with Yasi being the most severe cyclone to hit Queensland since 1911, when there was a double cyclone event, again during a La Nina year. Double or multiple cyclone crossings have always been associated with La Nina events, at least as far as our brief recorded history can tell.

The factors affecting the strength and duration of La Nina and El Nino events are, of course, enormously complicated, and further complicated by other weather patterns such as the Indian Ocean Dipole [IOD] and the Southern Annular Mode [SAM]. The IOD, as its name suggests, is a measure of the difference in ocean temperatures at two ‘poles’, in the eastern and the western Indian Ocean. This shows another complex pattern of temperature and pressure variability, both oceanic and atmospheric, with probable throughflow from the Pacific leading to an association between positive IOD events and the El Nino and negative events with La Nina. The SAM, a weather pattern in the Southern Ocean, also known as the Antarctic Oscillation, obviously impacts our southern coastal areas in particular, either enhancing or diminishing ENSO events regionally:

In recent years, a high positive SAM has dominated during autumn–winter, and has been a significant contributor to the ‘big dry’ observed in southern Australia from 1997 to 2010.

The south-west region of Australia, a vital agricultural region, missed out on the wet conditions experienced by the rest of the country as a result of the 2010-11 La Nina. In fact this region experienced its driest year on record in 2010, and this appears to have been at least partially caused by a record high positive SAM event of that year.

Of course the point of this post is to show how complex weather patterns are as a background to climate change. Overall oceanic warming due to climate change increases convection which leads to higher rainfall, air turbulence and storm conditions, none of which are precisely predictable, though our monitoring has greatly improved over the years. I’m sure that most farmers, whose livelihoods are so greatly affected by these conditions, are keeping a ‘a weather eye’ on cycles and oscillations, but city-dwellers are naturally less sensitive to them. For many of them, well, the long drought and the water restrictions are over, the temperatures are far from unbearable, and now the money-grubbing government is slapping on a carbon tax after promising not to. My response would be – inform yourself, learn more about the causes of local, short-term weather conditions as well as global, long-term climatic ones, examine the evidence, and only then start to draw conclusions. And thanks a lot to the Bureau of Metereology for their contribution to informing our understanding.

the height of the 2010-11 La Nina – cold surface waters in the mid-Pacific, warmer waters in the western Pacific

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Written by stewart henderson

August 27, 2012 at 1:06 pm

2 Responses

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  1. like the way i can read this on my email and not bother going to another site 🙂

    Sarahen

    August 28, 2012 at 10:34 am

    • Discovered this comment in a round-about way. Hope you’re still following.

      luigifun

      January 17, 2013 at 9:26 am


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