Feature article
World Meteorological Organisation Expert Team meeting on ‘El Niño and La Niña’, Calverton, USA, August 13–15 2007
El Niño conditions.
La Niña conditions.
Luc Maitrepierre, New Caledonia
This WMO’s Expert Team (ET) was formed in November 2005 during the fourteenth session of the Commission of Climatology (CCl), with Luc Maitrepierre as its leader. The first ET meeting was held in Calverton, USA, August 13-15 2007.
This followed the report of the previous ET. El Niño was first referred to as the seasonal appearance of a current of warmer than usual water off the Peruvian coast. El Niño sea surfacewarming is typically followed by either near-average sea surface temperatures (SST) or colder than average SSTs (La Niña).
El Niño and La Niña events have now been clearly identified as perturbations of theocean-atmosphere system. There are typically changes in convection and the strength and direction of the trade winds across the equatorial Pacific basin. These changes are represented by the Southern Oscillation Index (SOI), which is defined as the normalised difference in surface pressurebetween Tahiti, French Polynesia, and Darwin, Australia.
As a result of such atmospheric perturbations associated with ENSO events, various impacts are felt at different locations around the globe, and these regional impacts are dependent on the location, degree, and timing of the onset of the anomalous SSTs, and the intensity of the atmospheric response. Because of the potential confusion associated with the differing definitions in existence, CCl is addressing the issue of El Niño and La Niñadefinitions.
The aims of the meeting were to initiate the work of the ET on two main actions from the Terms of Reference given by CCl:
- To produce a first version of an atlas of regional ENSOimpacts.
- To work on common language about ENSO.
The main issues about the atlas of regional ENSO impacts are:
- ET decided not to use one unique definition for El Niño or La Niña: ENSO variability should be seen as a continuum, and for practical reasons it was suggested that the ENSO cycle should be categorised by quintiles (0–20%,21–40%, etc) of the chosen ENSO index, where it is based on sea surface temperature anomalies (NINO3.4), the Multivariate ENSO Index, Southern Oscillation Index or another.
- Impacts on climate should be compared to normal and
expressed in probabilities. The seasonal variability should
be highlighted. - The final atlas will be available on the web but the ET
recommended that a hard copy should also be developed.
The ET decided that a workshop on communicating about ENSO (‘Toward Developing a Common Understanding’) will be held in Hawaii from 8–10 April 2008. The goals of this workshop will be to:
- come to a common understanding of terminology used in
describing the state of ENSO and its impacts, and - develop best practices for communicating information on
ENSO and its impacts to a wide range of users.
The agenda and the list of participants (about 50 people) are currently under consideration by the ET and will be available bythe end of 2007.
Source: http://www.pmel.noaa.gov/tao/elnino/nino_normal.html
The ET’s terms of reference and the Catalogue of Indices and Definitionsof El Niño and La Niña in Operational Use can be found at:
http://www.wmo.int/pages/prog/wcp/ccl/opags/opag3/et3.3/et3.3_members_tors.htm