El Niño : Ocean Mechanisms
Channel: David Webb
Date: 19/12/2018

The talk is about the ocean mechanism involved in the development of an El Niño.  It is based on results from a high resolution (1/12 degree) global ocean model.

The results emphasise the importance of the North Equatorial Counter Current (NECC).  This current, which is comparable in strength to the Gulf Stream, transports water from the warm western Pacific to the cool eastern Pacific.  In principal such as transport of warm water can move the centre of Deep Atmospheric Convection away from Indonesia towards the central and eastern Pacific and result in major changes of the global atmospheric circulation.

The model results show that in most years this does not happen because of the action of tropical instability waves and the near surface Ekman layer.  These features act to dilute the warm core of the NECC, replacing it by cooler water from the Equator.  However once an El Niño has started the strength of these processes is reduced in regions of deep atmospheric convection.  This allows the NECC to carry warm water further east where it may trigger a further region of deep atmospheric convection.

The model results also show that the strong El Niño years are triggered by a stronger than normal annual Rossby wave.  This wave is generated in the first few months of each year by changes in the winds of the eastern Pacific north of the Equator.  The wave is actually a series of waves which move slowly across the Pacific, depressing sea level, with the fastest waves reaching the western Pacific in mid-year.  Here they enhance the strength of the NECC, drawing heat eastwards and starting the El Niño process.  As the warm water moves eastward it is helped by the slower moving Rossby waves, so that it reaches the far western Pacific around Christmas.  Here it is given the name "The Christ Child" or El Niño.

The references for the two papers reporting on the study are given below.   Another YouTube video showing the currents and rainfall when the ocean model is coupled to a high resolution atmospheric model can be found at "https://youtu.be/yyFgiq1MwyU".

References

Webb, D.J. (2018) On the role of the North Equatorial Counter Current during a strong El Niño.  Ocean Science, 14, 533-660.  https://www.ocean-sci.net/14/633/2018/
Webb, D.J. (2016) A comparison of sea surface temperatures in the Equatorial Pacific Niño regions with results from two early runs of the NEMO 1/12° Ocean Model.  National Oceanography Centre Research and Consultancy Report, 55, 31pp.  http://nora.nerc.ac.uk/id/eprint/513264/

Notes:

1.  In the talk it is said that deep atmospheric convection is not found along the Equator.  What should have been said is that in the central and eastern Pacific the Equator is not a significant area of deep atmospheric convection, the main areas being found to the north and south.