Abstract. This study aims to determine the role of the tropical ocean dynamics in the
response of the climate to extratropical thermal forcing. We analyse and
compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first
configuration the AGCM is coupled with a slab ocean model while in the second
a reduced gravity ocean (RGO) model is additionally coupled in the tropical
region. We find that the imposition of extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere
with zero global mean) produces, in terms of annual means, a weaker response
when the RGO is coupled, thus indicating that the tropical ocean dynamics
oppose the incoming remote signal. On the other hand, while the slab ocean
coupling does not produce significant changes to the equatorial Pacific sea
surface temperature (SST) seasonal cycle, the RGO configuration generates
strong warming in the central-eastern basin from April to August
balanced by cooling during the rest of the year, strengthening the seasonal
cycle in the eastern portion of the basin. We hypothesize that such changes
are possible via the dynamical effect that zonal wind stress has on the
thermocline depth. We also find that the imposed extratropical pattern
affects El Niño–Southern Oscillation, weakening its amplitude and
low-frequency behaviour.
ENSO dynamics and seasonal cycle in the tropical Pacific as simulated by the ECHAM4/OPYC3 coupled general circulation model