Posted by: Carl | February 3, 2009

The Evolution of La Nina

The European Centre for Medium-Range Weather Forecasts (ECMWF) provides a great resource for following daily developments in the equatorial Pacific: http://www.ecmwf.int/products/forecasts/d/charts/ocean/.

I’ve made a video using images produced by the ECMWF to show how La Ninas may evolve.  The images being cycled through are zonal sections of temperature anomaly in the equatorial Pacific.  The images are weekly, beginning in February 2007 and ending in January 2008.

Weekly Zonal Sections

The video illustrates that at least the 2007/2008 La Nina was driven by periodic jolts of anomalous cold water that originate between 50 and 150 meters down from the ocean surface, and between 160 and 100 degrees West.  This cold water takes a path upward and Eastward until it reaches the surface.  The cold water has the most powerful effect when it is first forming, causing cooling on the surface far from the coast.  As the cold water takes its upward and Eastward path, it weakens, and the cold surface anomaly begins to disappear.  The best illustration of this begins at 53 seconds.  The origin of this cold water is not the Humboldt Current pumping exceptionally cold water periodically from the Southern Ocean for the current follows too close to the coast.  Instead, it is likely that the transfer of warm water back-and-forth between the East and West Pacific noted by Bob Tisdale here is responsible for the cold jolts.

Below is a Hovmoller Diagram of Sea Level in the equatorial Pacific.  The Hovmoller extends over a 12-month period, covering the 2007/2008 La Nina.  As you can see, the La Nina was not a continually growing event, but rather an event sustained and strengthened by the periodic intensification that would occur every couple of months.  It is apparent that the rising regions of anomalous cold water shown in the video coincide with the intensified periods during the recent La Nina.

sea32level_anomaly_1232months

Here is one more video, with the same format as the previous video, except using daily data over the past week.

Daily Zonal Sections

A very large amount of cold water is approaching the surface, so we may see a resurgance of La Nina conditions over the next week or two.  However, no new pockets of cold water appear to be forming, and unless that changes, the La Nina conditions in the Pacific may soon fade.

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Responses

  1. Hi Carl,
    Very interesting post as usual and a great movie. Would it be possible to have a voice over chanting the months as they pass? Its difficult for old eyes to read the fine print. I see a future for you in the world of movie animation.

    I see that the hovmoller relates to waters along the equator. There is no doubt that the upwelling of cold water intensifies the swings that occur along the equator.

    There is also no doubt that the upwellings in the east are in part related to the strength of the easterlies. I wonder too whether the changing temperature of the water between the surface and 300 metres reflects the incidence of sunlight because that is its depth of penetration in reasonably clean water.

    There is also no doubt that the water will warm when the upwellings cease.

    However, my data shows that the water starts to warm up thirty degrees away from the equator, not at the equator. Upwellings are not a factor at 20-30°S or on the western side of the oceans.

    My data also shows that the warming is not simultaneous but close to it when you consider the average interval of four years between warming events and I have looked at both hemispheres in many locations in the Pacific, the Indian and the Atlantic where upwellings are not always a factor.

    So, my guess is that while upwellings play a significant role in generating the temperature extremes that we see in the eastern Pacific the ultimate driver of tropical warming is cloud loss consequent upon upper troposphere warming and the episodic loss of atmospheric humidity on year to year, decadal and 100 year timescales.

    The upper troposphere warming is also important in changing surface pressure and thereby the strength of the easterlies that drive the upwellings.

    So, I imagine that to explain tropical warming events and the associated change in global temperature over longer time scales we need to look at a wider field of activity than the cockpit of the ENSO observation areas.

    As to whether the La Nina conditions will fade or intensify I think we need to look at surface pressure off Chile and off California. It has been dropping off Chile in the 12 month moving average and is currently bumping along at a level just below median. In the past there has frequently been a lag in the response of surface temperature of up to a year and that lag period should close off by March. The SOI has shown some signs of a backtrack in recent weeks.

    The recent sudden stratospheric warming in the Arctic tells us that a small amount of solar activity (five days of sunspots between 10 and 20 between the 9th and the 13th January) can have a large effect on a strongly ozone charged upper atmosphere five days later. The warming extended all the way to the surface and it therefore upsets the Arctic Vortex.

    The atmosphere is now considerably de-humidified by the long La Nina, the upper atmosphere charged with ozone because it is cooler and drier because of the cool waters along the equator and the long weakness in convection that is the source of stratospheric moisture (ozone soluble in water) and so it is primed for change as soon as significant sunspot activity appears to drive up temperature in the upper troposphere.

  2. Concerning the stratospheric warming:
    http://weatherclimatelink.blogspot.com/
    Ed Berry has been discussing the SSW event in depth.

  3. […] to Carl Wolk of Climate Change: https://climatechange1.wordpress.com/ Carl’s post The Evolution of La Nina gave me the idea for this […]


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