On the role of the North Equatorial Counter Current during a strong El Niño

Abstract

Abstract. An analysis of archived data from the NEMO 1∕12th degree global ocean model shows the importance of the North Equatorial Counter Current (NECC) in the development of the strong 1982–1983 and 1997–1998 El Niños. The model results indicate that in a normal year the core of warm water in the NECC is diluted by the surface Ekman transport, by geostrophic inflow and by tropical instability waves. During the development of the 1982–1983 and 1997–1998 El Niños, these processes had reduced effect at the longitudes of warmest equatorial temperatures and to the west. During the autumns of 1982 and 1997, the speed of the NECC was also increased by a stronger-than-normal annual Rossby wave. The increased transport of warm water by the NECC due to these changes resulted in warm water reaching the far eastern Pacific and appears to have been a major factor in moving the centre of deep atmospheric convection eastwards across the Pacific.

A Comparison of Ocean Model Results with Satellite Observations during the Development of the strong 1997–98 El Niño

“…In mid-1997, they both show the larger than normal drop in sea level that occurs between 160°E and 180°E. In Webb (2018) this was found to be connected with an increase in the strength of the NECC, such that it carried warm surface water further east than was normal.…”

“…The model data used in this paper is from the five day average datasets generated during run 6 of the Nemo 1/12°model. This is the same data as used for Webb (2016) and Webb (2018), where detailed information on the model, the model run and the archive datasets can be found. The model data is on a 1/12 degree grid but, for the comparisons reported here, sea surface values were averaged onto the same grid as used for the processed satellite data.…”

“…The El Niño is usually characterised in terms of the average sea surface temperatures between 5°N and 5°S. However the model results reported by Webb (2018) indicate that the most significant changes occur further north.…”

“…4 Sea Levels along 6°N and the Equator Webb (2018) found that the increased strength of the NECC during the growth of an El Nino was in part associated with an increased sea level difference between the Equator and 6°N. It is thus of interest to see how the observations and model compare at both these latitudes.…”

“…Observed and model sea surface temperatures and their difference, averaged between 5°S and 5°N, for the equatorial Pacific between 140°E and 280°E (80°W). Showing (a) Tropical Instability Waves, strongest in second half of non-El Niño years, (b) initial midocean El Niño, discussed inWebb (2018), (c) strong El Niño, starting in the west around mid-year and arriving in the east around new year.Temperature units are degrees C.…”