El Niño Southern Oscillation Open Discussion

At WUWT, this thread engendered the following string of comments.

  • Steven Mosher | August 18, 2011 at 8:02 pm

ENSO has a thrust?

  • Steve Garcia | August 19, 2011 at 2:15 am

    Who knows? It might. I would love it if anyone here could tell me what causes El Niño – where is the extra energy coming from, and why isn’t it there all the time? What makes the heat plume move westward, then back? With movement there must be something akin to thrust.

  • Chief Hydrologist | August 19, 2011 at 5:27 am

ENSO originates in more or less upwelling of cold and nutrient rich in the region of the Humboldt Current. I have a review here -http://www.earthandocean.robertellison.com.au/

The thermal evolution of the Humboldt Current is best understood in terms of ENSO. ENSO is an oscillation between El Niño and La Niña states over a 2 to 7 year cycle. An El Niño is defined as sustained SST anomalies greater than 0.5O C (in the Nino 3 region) over the central pacific. Conversely, a La Niña is defined as sustained SST anomalies less than -0.5O C. The oscillations (more correctly chaotic bifurcation – but we will come to that) are driven by complex interactions of cloud, wind, sea level pressure, sea surface temperature, planetary rotation and surface and subsurface currents. The short explanation is that the Pacific trade winds set up conditions for a La Niña. Trade winds, south-easterly in the Southern Hemisphere and north-easterly in the Northern Hemisphere, pile up warm surface water against Australia and Indonesia. Water vapour rises in the western Pacific creating low pressure cells that strengthen the trade winds piling yet more warm water up in the western Pacific. Cool, subsurface water rises in the eastern Pacific and spreads westward. At some point the trade winds falter and warm water spreads out westward across the Pacific.

In the region of the Humboldt Current – there is a balance between upwelling where deep ocean currents emerge and suppression of those currents by a warm surface layer.

The reasons for more or less upwelling may hinge on the state of the Southern Annular Mode. Here is some up to date info on SAM – http://www.antarctica.ac.uk/met/gjma/sam.html

Here we are very at the edge on the known. I went looking for some connection between UV and SAM – and found myself quoted at – http://forum.weatherzone.com.au/ubbthreads.php/topics/28657/3/The_Southern_Annular_Mode_SAM. There is a good discussion there.

Here is an SST anomaly thermally enhanced satellite image from October last year.


SST Anomalies 10/04/2010

You can see the PDO in the north Pacific and La Nina in full swing in the central Pacific. You can also see the potential for cold water being pushed up from the Southern Ocean and onto the western coast of South America in the area of the Humboldt Current. The region of the Humboldt Current is the most biologically productive area on Earth because the cold southern water is joined there by upwelling frigid water. The upwelling (or not) in turn determines the thermal evolution of ENSO. ENSO is many things but starts in upwelling – or not – in the eastern Pacific.

There is a direct physical link between UV and the SAM in the ozone layer of the middle atmosphere – and thus in storm tracks spinning of the Southern Ocean, pooling cold water off the western coast of South America and diluting the warm surface layer that suppresses upwelling in the eastern Pacific.’

ChiefHydrologist’s comments are welcome, and yet I see inconsistencies and have questions.  It clearly appears to me that well into the year 2011 the cause of El Niño is explained, but not explained to my own standards.

Comments follow:




One response to “El Niño Southern Oscillation Open Discussion

  1. My first comment on this would be to mention the parts of what CH added that I agree with, as far as my understanding goes. Perhaps one at a time would be a good way to list them.

    1. I totally agree with the term “chaotic bifurcation” having to do with the El Niño phase of ENSO. Further in that ‘Earth and Ocean article, is this:

    Dynamics of Pacific Ocean climate states may well involve small changes in ‘top down’ solar UV forcing (e.g. Lockwood 2010, Lean 2008) initiating chaotic bifurcation of Pacific Ocean climate states (Tsonis et al 2007, Swanson and Tsonis 2009).

    I would suggest also that La Niña is the non-bifurcated portion of the oscillation. That seems like what the paper says.

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