The annual cycle of SST in the eastern tropical Pacific,
diagnosed in an ocean GCM
W.S. Kessler, L.M. Rothstein, D. Chen, 1998
Journal of Climate, 11, 777-799
The annual onset of the east Pacific cold tongue is diagnosed in a
ocean GCM simulation of the tropical Pacific. The model uses a
recently-developed mixed layer scheme that explicitly
simulates the processes of vertical exchange of heat and momentum with
the deeper layers of the ocean; comparison with observations of
temperature and currents shows that many important aspects of the
model fields are realistic. As previous studies have found, the
heat balance in the eastern tropical Pacific is notoriously complicated,
and virtually every term in the balance plays a significant role at
one time or another. However, despite many complications, the three-
dimensional ocean advection terms in the cold tongue region tend to
cancel each other in the annual cycle, and to first order the variation
of SST can be described as simply following the variation of net solar
radiation at the sea surface (sun minus clouds). The cancellation is
primarily between cooling due to equatorial upwelling and warming due
to tropical instability waves, both of which are strongest in the second
half of the year (when the winds are stronger). Even near the equator
where the ocean advection is relatively intense, the terms associated with
cloudiness variations are among the largest contributions to the SST balance.
The annual cycle of cloudiness transforms the semi-annual solar cycle at
the top of the atmosphere into a largely one cycle per year variation of
insolation at the sea surface. However, the annual cycle of cloudiness
appears closely tied to SST in coupled feedbacks (positive for low stratus
decks and negative for deep cumulus convection), so the annual cycle of SST
cannot be fully diagnosed in an ocean-only modeling context as in the present
study. Zonal advection was found to be a relatively small influence on
annual equatorial cold tongue variations; in particular there was no direct
(oceanic) connection between the Peru coastal upwelling and equatorial
annual cycles. Meridional advection driven by cross-equatorial winds has
been conjectured as a key factor leading to the onset of the cold tongue.
Our results suggest that the SST changes due to this process are modest,
and if meridional advection is in fact a major factor then it must be
through interaction with another process (such as a coupled feedback with
stratus cloudiness). At present, it is not possible to evaluate this
feedback quantitatively.
Download the paper as it appeared in J.Climate (pdf, 507 Kb)
Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA
