Weather this Summer - La Nina explained

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Bob McDavitt writes:

 

LA NINA and the coming cyclone season

 

First, a recap on our weather zones: The weather engine starts with energy from the sun. The warmest seas are near the equator and sun on them causes evaporation which rises to form the Intertropical convergence zone. Air rises as far as the tropopause and then travels pole-wards. In the southern hemisphere a lot of this air sinks around 30S and returns along the surface as trade winds back to the equator--- this is the Hadley cell. The zone of sinking air is called the subtropical ridge. Further south are the westerly winds of the roaring 40s. These weather zones move about, causing seasons. By the time we get to the longest day, around 22 Dec, the subtropical ridge is usually "following the sun" southwards and gets to around 40S. This is all part of the annual cycle.

 

The second strongest cycle for seasonal weather is the ENSO = El Nino Southern Oscillation (there are others). When the seas along the equatorial Pacific Ocean are cooler than normal, as the are now, we call it a La Nina episode. The weather engine turns over more slowly. The peak upward motions in the Pacific occur over the Australian side rather than the Peru side. This has the impact of, in the Southern Hemisphere, encouraging the weather zones to go further south than normal. Already the "anticyclones of summer" are reaching northern NZ, and there are signs of an early start to the wet season in northern Australia. This also tugs the South Pacific Convergence zone SPCZ to the south and west.

 

When we look at the sea surface temps, the Oceanic Nino Index ONI for Sep was -1.0 and when we look at the weather maps the SOI (Tahiti to Darwin normalised pressure difference) was +2.5. Both these point to a La Nina that is already moderate to strong. The sub-surface temps in the central Pacific are cool as well, so the consensus is that this La Nina will continue through the coming South Pacific Cyclone centre.

 

NIWA has been able to come up with 8 similar cyclone seasons: Nov 1970 to April 1971 or 70/71, also 71/72, 73/74,75/76, 88/89, 98/99, 99/00, and 07/08. Using these years as a guide, the forecast for the Nov 2010 to April 2011 season is for 9 to 12 named storms (9 is average). 3 are forecast to reach at least category 3, and 1 is forecast to reach at least category 4 (average winds near centre of 64 knots or more). The Coral Sea and surrounding places west of 180 have an elevated risk of Cyclone impact.

 

Although there appears to be a reduced risk for places east of 180, all communities should remain alert and prepared. In the 8 similar seasons chosen there have been some cyclone impact in southwest parts of French Polynesia and in the Southern Cooks. During previous moderate to strong La Nina's, cyclones have been able to leave the tropics and cross the Tasman Sea onto southern NZ.

 

NIWA cyclone report and seasonal climate outlook:

http://www.niwa.co.nz/our-science/climate/publications/all/seasonal-climate-outlook

 

MetService seasonal weather outlook:

http://metservice.com/rural/seasonal-forecast-north-island

 

MetService description of seasonal forecasts:

http://vaac.metservice.com/default/index.php?alias=month_feature_july_2007