the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Nov 2024
On the response of the Equatorial Atmosphere and Ocean to changes in Sea Surface Temperature along the Path of the North Equatorial Counter Current
Abstract. The CESM climate model is used to test the hypothesis that the changes observed during El Niños are, at least in part, a response of the coupled ocean/atmosphere system to changes in sea surface temperature along the path of the North Equatorial Counter Current.
The model results show that increased temperatures at the latitudes of the NECC produce more deep atmospheric convection within the ITCZ. This has a local effect on the ocean's surface pressure field. The increased deep atmospheric convection also appears to affect the longitude structure of the Hadley Circulation. This results in less sinking air in the south-east Pacific and produces changes in surface pressure similar to those of a Southern Oscillation. Both mechanisms reduce the zonal component of the surface pressure gradient and wind stress along the Equator, and produce an El Niño type response in the ocean.
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RC1: 'Comment on egusphere-2024-3560', Anonymous Referee #1, 17 Dec 2024
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This study uses the CESM coupled ocean-atmosphere climate model to test the hypothesis that strong El Niños are due, at least in part, to warmer than normal water along the path of the North Equatorial Countercurrent (NECC). The model setup involves comparing the results of a control run with no anomalous SST warming in the vicinity of the NECC to one with a prescribed warm anomaly of ~1C along the path of the NECC between 120W and the dateline. The differences in coupled ocean-atmosphere anomalies between these two runs provide a measure of NECC warming impact on the tropical Pacific. The NECC roughly coincides with the location of the ITCZ, a region of ascending air masses that significantly affects the oceanic and atmospheric circulation of the Pacific basin. So the imposition of anomalous SSTs in this region has significant impacts on atmospheric and oceanic fields that look like they could contribute to the development of an El Niño.
I have no particular issues with the results or their interpretation. My main criticism is that the prescribed SST anomalies are unlikely to occur in the real world in isolation of a broader ENSO warming across the Pacific basin. The author finds support for his hypothesis (lines 65-66) that "strong El Niños are due, at least in part, to warmer than normal water along the path of the North Equatorial Countercurrent." What is unstated and crucially important though, is that warmer than normal water along the path of the North Equatorial Countercurrent is due to strong El Niños. The processes that generate ENSO events involve, first and foremost, ocean-atmosphere interactions and ocean dynamics in the equatorial waveguide. A broader range of latitudes is involved but is it highly unlikely that the SST anomalies prescribed in this study would be present without the presence of a developing El Niño. As as long as these points are made clearly in the paper, I am comfortable with its acceptance.
Minor points:
1) Line 55. "extracted" is not a conventional dynamical term. Explain what is meant.
2) Using east longitudes > 180 degrees for west longitudes requires the reader to stop and do math to figure out what the real longitude is. Use west longitudes for west longitudes.
Citation: https://doi.org/10.5194/egusphere-2024-3560-RC1
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