Abstract
We look at changes in the El Niño Southern Oscillation (ENSO) in a high-resolution eddy-permitting climate model experiment in which the Atlantic Meridional Circulation (AMOC) is switched off using freshwater hosing. The ENSO mode is shifted eastward and its period becomes longer and more regular when the AMOC is off. The eastward shift can be attributed to an anomalous eastern Ekman transport in the mean equatorial Pacific ocean state. Convergence of this transport deepens the thermocline in the eastern tropical Pacific and increases the temperature anomaly relaxation time, causing increased ENSO period. The anomalous Ekman transport is caused by a surface northerly wind anomaly in response to the meridional sea surface temperature dipole that results from switching the AMOC off. In contrast to a previous study with an earlier version of the model, which showed an increase in ENSO amplitude in an AMOC off experiment, here the amplitude remains the same as in the AMOC on control state. We attribute this difference to variations in the response of decreased stochastic forcing in the different models, which competes with the reduced damping of temperature anomalies. In the new high-resolution model, these effects approximately cancel resulting in no change in amplitude.
Highlights
The net transport of heat from the equator to higher northern latitudes in the Atlantic caused by the flow of upper water layers in the Atlantic ocean, part of the Atlantic meridional overturning circulation (AMOC), is responsible for the mild winters in western Europe
We find the same eastern shift and weakened annual cycle in HadGEM3 they differ in El Niño Southern Oscillation (ENSO) period response, going to longer periods in HadGEM3 and the amplitude remains approximately the same (HadCM3 ENSO amplitude increased by about a third)
Variability in global annual mean surface air temperature (SAT) is increased in the Atlantic Meridional Circulation (AMOC) off run the reverse is true in the annual mean taken over the tropical Pacific region
Summary
The net transport of heat from the equator to higher northern latitudes in the Atlantic caused by the flow of upper water layers in the Atlantic ocean, part of the Atlantic meridional overturning circulation (AMOC), is responsible for the mild winters in western Europe. This net ocean flow is part of a larger ocean mode known as the thermohaline circulation. Even though a shutdown is thought to be unlikely by the end of the 21st century, a weakening in AMOC was assessed to be very likely by the IPCC AR5 (Collins et al. Vol.:(0123456789)
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