Abstract
Abstract. Using the ECHAM5/MPI-OM model, we study the relation between the variations in the Atlantic meridional overturning circulation (AMOC) and both the Pacific sea surface temperature (SST) and the El Niño-Southern Oscillation (ENSO) amplitude. In a 17-member 20C3M/SRES-A1b ensemble for 1950–2100 the Pacific response to AMOC variations on different time scales and amplitudes is considered. The Pacific response to AMOC variations associated with the Atlantic Multidecadal Oscillation (AMO) is very small. In a 5-member hosing ensemble where the AMOC collapses due to a large freshwater anomaly, the Pacific SST response is large and in agreement with previous work. Our results show that the modelled connection between AMOC and ENSO depends very strongly on the frequency and/or the modelled amplitude of the AMOC variations. Interannual AMOC variations, decadal AMOC variations and an AMOC collapse lead to entirely different responses in the Pacific Ocean.
Highlights
Decadal to multidecadal modulations of the amplitude of El Nino-Southern Oscillation (ENSO) have been found in observations of sea surface temperature (SST), sea level pressure and rainfall (e.g., Torrence and Webster, 1999)
We compare the effect of Atlantic meridional overturning circulation (AMOC) changes on the tropical Pacific SST and ENSO amplitude using two ensembles of runs performed with the coupled ECHAM5/MPI-OM model
With the hosing ensemble the Pacific response to a large, long lasting variation in the AMOC is investigated, whereas with the standard ensemble we study the influence of relatively small amplitude natural variability in the AMOC on the Pacific
Summary
Decadal to multidecadal modulations of the amplitude of ENSO have been found in observations of SST, sea level pressure and rainfall (e.g., Torrence and Webster, 1999). Modelling studies in which a substantial weakening of the AMOC is induced by large freshwater anomalies indicate that changes in the AMOC can affect the tropical Pacific mean state, as well as ENSO amplitudes (Vellinga and Wood, 2002; Zhang and Delworth, 2005; Timmermann et al, 2005, 2007). We compare the effect of AMOC changes on the tropical Pacific SST and ENSO amplitude using two ensembles of runs performed with the coupled ECHAM5/MPI-OM model. Using a 17-member ensemble of climate runs for the period 1950–2100 under the 20C3M/SRES-A1b scenario, we first study the response of the tropical Pacific to AMO and related AMOC variations. With the hosing ensemble the Pacific response to a large, long lasting variation in the AMOC is investigated, whereas with the standard ensemble we study the influence of relatively small amplitude natural variability in the AMOC on the Pacific. We do not use data before 1900 because of the large uncertainties
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