Impact of intra-daily SST variability on ENSO characteristics in a coupled model
Résumé
This paper explores the impact of intra-daily Sea
Surface Temperature (SST) variability on the tropical largescale
climate variability and differentiates it from the
response of the system to the forcing of the solar diurnal
cycle. Our methodology is based on a set of numerical
experiments based on a fully global coupled ocean–atmosphere
general circulation in which we alter (1) the frequency
at which the atmosphere sees the SST variations and (2) the
amplitude of the SST diurnal cycle. Our results highlight
the complexity of the scale interactions existing between the
intra-daily and inter-annual variability of the tropical climate
system. Neglecting the SST intra-daily variability results, in
our CGCM, to a systematic decrease of 15% of El Nino—
Southern Oscillation (ENSO) amplitude. Furthermore,
ENSO frequency and skewness are also significantly modified
and are in better agreement with observations when SST
intra-daily variability is directly taken into account in the
coupling interface of our CGCM. These significant modifications
of the SST interannual variability are not associated
with any remarkable changes in the mean state or the seasonal
variability. They can therefore not be explained by a
rectification of the mean state as usually advocated in recent
studies focusing on the diurnal cycle and its impact. Furthermore, we demonstrate that SST high frequency
coupling is systematically associated with a strengthening of
the air-sea feedbacks involved in ENSO physics: SST/sea
level pressure (or Bjerknes) feedback, zonal wind/heat
content (or Wyrtki) feedback, but also negative surface heat
flux feedbacks. In our model, nearly all these results
(excepted for SST skewness) are independent of the amplitude
of the SST diurnal cycle suggesting that the systematic
deterioration of the air-sea coupling by a daily exchange of
SST information is cascading toward the major mode of
tropical variability, i.e. ENSO.
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