El Ni or No
THURSDAY OCTOBER 01, 2009
We have in fact had two of the wettest months for ages and few, if any, frosts. Columnist Mathew Torbit of the Dominion Post has called it the spring of “wet and misery”. So when are the general public and the do-good element among politicians going to wake up to the fact that the "experts" can't get the next few months' weather right? On 6 Sept Metservice announced there was a 50% chance of El Nino this summer, based on cooling sea temperatures. The signature of an El Nino is warmer water off the west of South America with heavy rain and floods there and corresponding cooler water off Australia leading to drought. So is El Nino coming? And what does it mean for NZ?
An El Nino is the result of changes in the atmospheric pressure balance across the southern Pacific Ocean, known as the southern oscillation index(SOI). The weather experts cannot get a proper grip on the El Nino/ La Nina oscillation because they are too busy looking at ocean temperatures as measured by buoys. Their belief is that sea temperatures heat the air and then drive the atmospherics in what they see as a closed system. However it is not a closed system and it is worth looking at what is actually going on. When it comes to El Nino the atmospherics come first. In other words the air above the sea heats the surface of the sea and not vice versa. The air temperature is driven by the heat from the sun. After the air is heated the ocean follows later due to a time delay for the extra solar heat to be absorbed by and distributed in the water, which only then may reinforce the atmospherics. This is why the SOI is a far better indicator of the EN/LN oscillation than the Sea Surface Temperature (SST) parameters that the mets use. The SSTs are more useful in terms of seeing whether they are reinforcing or inhibiting the atmospherics (SOI). SSTs are also more useful to mariners than isobaric pressure maps because mariners are at water level and any changes in air temperature which warn of weather change are more quickly detectable.
The EN/LN is bistable, meaning that the climate tends to be in one regime for a while before flipping over into the other. It should be called an oscillation, rather than each a single weather pattern. Calling something a "weak El Nino” is just hedging bets and meaningless, like calling a set of 12 consecutive hours a weak day or night. The trigger mechanism for the oscillation is increased solar activity, in the form of sunspots, solar flares and coronal mass ejections that send plasma storms hurtling towards Earth. When solar storms arrive to batter our planet the climate system responds by pushing further into the current regime until the next big outburst triggers a reversal which sends it in the other direction for subsequent outbursts. The net result is that for a while one regime may dominate, with the SOI being mostly +ve or mostly -ve for that period, while quiet solar periods see the SOI relaxing back towards neutral. Because atmospheric changes occur first, the SOI usually rises after it rains, so labels like El Nino that climatologists love to come out with to make themselves appear knowledgeable are usually descriptive of conditions after the fact.
On average, the warmest part of the equatorial Pacific ocean is in the area around Indonesia, which is geothermal with many active volcanoes and undersea volcanic vents, while the coolest part of the equatorial Pacific is off South America, where cooler deep water rises to the surface.
When EN (-ve SOI) dominates, prevailing winds blowing to the east push warmer surface water towards South America drawing heat away from Indonesia, which after crossing the Pacific slows or stops the flow of cooler water rising up from the depths off South America, so sea surface temperature becomes higher than average around Tahiti and lower than average around Darwin.
When LN (+ve SOI) dominates, prevailing winds blowing to the west push cooler water away from South America towards Indonesia allowing a greater upwards flow of cool water from below off South America, and causing ocean heat to build up around Indonesia, so sea surface temperature becomes lower around Tahiti and higher around Darwin.
The sea surface temperature either dampens the wind regime or reinforces it, depending on where in the cycle one is. However the atmosphere responds first, with the ocean at first lagging behind and damping down the situation, then some months later reinforcing it as the ocean temperature catches up. Exactly why the wind regime and hence EN/LN phase reversals occur is connected to global atmospheric circulation patterns which are higher-order lunar air tides subject to the larger cycle of solar storms. When solar storms hit the Earth the atmospheric energy bursts start from both poles and then take about a week to propagate to the equator along the geomagnetic force field lines that encompass Earth. These affect prevailing wind and pressure patterns and from there climate regimes around the globe. The last EN took place in 1997-1998 and was severe. In the U.S. it was marked by such conditions as flooding rains in California and along the Gulf Coast. We have a 9 to 16 year sunspot cycle (av 11 years) that has within it a faster rising phase (average 4.5 yrs) and a slower falling phase (average 6.5 yrs), which each have midpoints, thus defining the cycles of EN followed by LN.
Where are we now? The last solar minimum peaked in May 2006. We have entered an El Nino phase which will peak around May 2007. We have just left a warmer drier phase sunspot cycle to move into a cooler wetter phase sunspot cycle. Previous phase reversals in this regime occurred in late 1913 (EN dominated phase), early 1947 (LN dominated phase), late 1976 (EN dominated phase), and the next occurs in mid 2007 (LN dominated phase). What often complicates matters is that you can still get significant EN events in a LN phase and vice versa. Each of these cycles has warmer and cooler phases that tend to add together or cancel each other depending on phase relationships. When longer term cycles reverse their phase they can also reverse the phase relationship of shorter term cycles, making predictions more difficult.
As to whether this summer will see El Nino conditions is not yet clear. The season will see a normal number of rain days, although rain will not be in great amounts. Mean temperatures should be higher but mainly because of higher minimums, and minimum temperatures alone do not qualify for El Nino. Between December and next March, maximums may average 23degC and minimums average about 11degC, and no one should be too surprised if between now and the beginning of next April Ashburton has over a dozen days with maximum temperatures reaching 30degC or above, with a significant number of these hot days coming before the end of this year.
Call it El Nino 50% or whatever, call it SOI, or even cite global warming. One thing is for sure; warmer days are coming and we used to call it summer.