Why we had a drought
SATURDAY APRIL 06, 2013
The three factors that drive climate and consequently the weather patterns that a region can expect are the angle of the sun's rays (latitude), the direction of prevailing winds, and the topography of the location. The first sets the amount of heat reaching you which manifests as rain amounts because of evaporation, and the second moderates how quickly you will dry out and how slowly or quickly your weather patterns may change.
Topography, the third factor means the shape of the land and the shape and nature of land masses near to you. As well as monitoring the previous two, topography can be a function of elevation, proximity to coastlines, size of landmass and the size, nearness and shape of neighbouring land masses.
Accordingly, one of the biggest factors driving NZ’s weather and climate is the shape and climate of the bottom half of Australia. For NZ it means that whatever Australia gets, especially Victoria and Tasmania, we get also, because we are downwind of them and share the same latitude.
It should then come as no great surprise that if the whole bottom half of Australia receives intense heat for most of their summer, then we will cop that too.
The reason for the extra heat has been the lateness of the monsoon season. From November to April monsoons migrate from India across the top of Australia. Normally they generate powerful winds which, when they reach intensity charge down across the hot interior of Australia sometime during summer, and the combination of hot desert air meeting cooler wet airflows can generate thunderstorms, flooding, gales, and tropical depressions and cyclones.
This time around, the monsoon season was late and weak, and along with that, tropical cyclones that normally develop out of monsoonal troughs were fairly nonexistent. When they did finally arrive in March, they were mostly rain systems that resulted in some flooding, but were fizzers in terms of cyclones and caused little wind damage.
Meanwhile the middle of Australia was heating up due to the normal summer sun. When there is little or no monsoonal activity to blow it away, the heat in the interior builds unabated. When winds cooled by the Southern Ocean race through the Bight across the bottom of Australia with associated cold fronts, heat from regions further to the north are sucked south. Heat always flows to cold, as we all know to our annoyance when a door of a cosy warm room is opened on a winter's day and the inside temperature instantly plunges.
The subsequent unusual heat ended up covering the whole of the southern half of the Australian continent from Perth to Tasmania, with nothing to move it away.
The second factor of climate is the direction and strength of prevailing winds. This season the cooler westerlies across the Bight were weaker and slower moving, resulting in heat lingering for a longer period over the bottom half of Australia during summer and similarly affecting the North Island of NZ which is in direct line with the bottom half of Australia. Slow winds mean weather patterns are slower to change, and excessive heat can persist.
One might ask, why was the monsoon season so weak? It is the sea currents that drive weather patterns. The monsoon season was weak because the sea currents were weak. There is a strong link between solar activity which is the engine of circulation, lunar activity which is the driver and determines the timing of weather events, and wind direction which is moderated by sea currents.
So why were the ocean currents weak? That was because the sun's power is weak at the moment as it nears the peak of its present sunspot cycle, Cycle 24. It is almost at a very low peak in an already weak cycle. Sea currents drive winds and pressure gradients, and are moderated by sun and moon. A weaker sun diminishes the seasonal current flows in the oceans.
We got our dollops of unusual heat for the same reason that the UK got their recent unusual coldness. What moderates the UK winter is the Gulf Stream which normally brings warmer waters up from the equator. A slower and weaker Gulf Stream can allow for less Gulf Stream extension. If warmer waters don't reach the UK then cold Arctic temperatures can make more of their presence felt. A lack of solar activity causes cold east winds to blow-in colder conditions because it disrupts the Gulf Stream's normal influence over their weather.
Therefore we can blame their cold and our heat on the sun. The moon was in there too, because anything to do with the ocean has a huge lunar component. We had closer lunar perigees from November to the start of February, which means the moon was closer to earth over the summer three months than for the seasons on either side. Perigees exaggerate whatever season they occur in, and closer perigees even more so, so closer moon times exaggerated the effects of our summer.
This was why we predicted all last year that summer for NZ would start around the last 10 days of November as the closer perigees kicked in, and right on cue summer conditions did arrive on or near 20 November. Because of the 9-year cycles of weather, it was the turn of a slower monsoonal season and hence the greater heat.
The 2013 almanac mentioned the word 'drought' no fewer than six times, on pages 27, 62, 83 and 86. On page 27 the words were “by mid-January there may be talk of drought.” On page 62 the word appears three times, viz “there may be talk of drought”, “Northland may be bracing itself for drought”, and page 86 “drought covering much of the North Island”, repeated on page 86. On page 100, we wrote “long hot summer days may frustrate dairy farmers but not nashi pear growers.”
We could not have warned more, nor in any clearer way.
Because the book was available in bookstores from September 2012 it gave farmers time to prepare.
The 2014 Weather almanac for NZ is now completed and at the publishers for processing. It will be released into bookshops in September. It may interest dairy farmers that the 2014 book does not mention the word drought at all.