GFDL - Geophysical Fluid Dynamics Laboratory

Publication 8303

Lanzante, J., and R. Harnack, 1983: An investigation of summer
sea-surface temperature anomalies in the eastern North Pacific Ocean.

Tellus, 35A, 256-268.


A study of summer sea surface temperature anomalies in the eastern North
Pacific Ocean was undertaken to examine some of the processes that could
affect their evolution and which may be important for their prediction. An
empirical approach was utilized. The factors considered include sea surface
temperature (SST) persistence (due to the relatively large heat capacity of
the water), oceanic thermal advection, and wind mixing (which presumably
acts through changes in the mixed layer depth of the ocean). The primary
variable studied was the summer SST anomaly for each of 25, 5o
latitude by 10o longitude boxes in the region
140oW-170oE, 30-55oN. Thirty years of data
were used (1947-76).

Diagnostic analyses using data from four Ocean Weather Stations (C, D, P and V)
were performed prior to the analyses described above in order to examine
wind mixing more closely. Only at the OWS’s are high frequency (3 hour),
measured wind speeds available at the same location as SST observations for
a long period of time. These analyses indicated a statistically significant
lag relationship between average April and May monthly wind speed and
subsequent summer SST at Station P.

The monthly mean SLP gradient was used as an estimate of average monthly wind
speed over the 25 box domain where measured winds are not routinely available.
In addition, winds derived from daily sea level pressure (SLP) analyses were
averaged to form monthly means. At most boxes, no significant lag relationship
between summer SST anomalies and spring (April and May) winds was found
using either data set.

The box-averaged data were employed to specify the summer SST from the
components of the total horizontal thermal advection (based on the mean
and anomalous components of the surface current and SST fields). The anomalous
summer surface current advecting the mean summer SST field was found to be
the dominant horizontal advective term affecting the local time rate of
change of SST, particularly for 40-50oN. Substitution of derived
May anomalous currents for those of summer greatly diminished the strength
of the relationship.

In the predictive portion of the study, skill relative to persistence was
marginal based upon dependent sample testing. Thus, in the
predictive mode neither wind mixing (as parameterized here) nor
oceanic thermal advection added very much information not contained in the
initial SST anomaly field. However, persistence alone exhibited considerable
skill, particularly in the northeastern portion of the domain.