A comparison of ocean model data and satellite observations of features affecting the growth of the NECC during the strong 1997–1998 El Nino

David Webb (National Oceanography Centre, United Kingdom)

CoAuthors

Andrew Coward (National Oceanography Centre, United Kingdom); Helen Snaith (National Oceanography Centre, United Kingdom)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Science II: Large Scale Ocean Circulation Variability and Change

Presentation type: Type Forum

Descriptions of the ocean’s role in the El Nino usually focus on equatorial Kelvin waves and the ability of such waves to change the mean thermocline depth and the sea surface temperature (SST) in the central and eastern Pacific.

In contrast, starting from a study of the transport of water with temperatures greater than 28C, sufficient to trigger deep atmospheric convection, Webb (2018) found that, during the strong El Ninos of 1983–1984 and 1997–1998, advection by the North Equatorial Counter Current (NECC) had a much greater impact on sea surface temperatures than processes occurring near the Equator.

Webb’s analysis, which supports the scheme proposed by Wyrtki (1973, 1974), used archived data from a 1/12 degree version of the Nemo ocean model. Previously the model had been checked against SST observations in the equatorial Pacific, but, given the contentious nature of the new analysis, the model’s behaviour in key areas needed to be checked further against observations.

The poster outlines how this was done for the 1987–1988 El Nino, using satellite observations of SST and sea level. SST is used to check the movement of warm water near the Equator and at the latitudes of the NECC. Sea level is used to check the model results at the Equator and at 6N in the North Equatorial Trough. Sea level differences between these latitudes affect the transport of the NECC, the increased transport at the start of each strong El Niño being associated with a drop in sea level at 6N in the western Pacific. Later rises in sea level at the Equator increase the transport of the NECC in mid-ocean.

The variability of sea level at 6N is also used to compare the strength of tropical instability waves in the model and in the observations. The model showed that in a normal year these act to dilute the temperature in the core of the NECC. However their strength declined during the development of the strong El Ninos, allowing the NECC to carry warm water much further than normal across the Pacific.
 
David Webb
National Oceanography Centre
United Kingdom
djw@noc.ac.uk