(12)U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
The off equatorial thermal measurements available in boreal spring allow estimation of the meridional terms in the mixed layer temperature equation (1).
(12)
(13)
where
where the SST at latitude Y is T
(Y)
and 
Y = 5°.
The form of the meridional diffusive heat flux (equation
13) is the same as that used in numerical models of the tropical ocean [Philander
and Pacanowski, 1980]. The value of the eddy coefficient K
was chosen based on the results of Hansen
and Paul [1982] and Bryden
and Brady [1989]. These studies estimated meridional heat transport
associated with the tropical instability waves and inferred an eddy coefficient.
This coefficient likely changes seasonally and interannually (in boreal spring
and during ENSO warm events the instability waves are weaker or disappear) and
is probably a function of latitude. In the estimation of Q
these possible variations were ignored. The value of K
used corresponds to the Hansen
and Paul [1982] estimate and is about a factor of three larger than
the mean value at 20 m found by Bryden
and Brady [1989].
The meridional temperature gradient in equation (11) was estimated from the moored data by differencing the 5°N and equatorial records. This probably overestimates the actual gradient at the equator. The second derivative was obtained by second differencing the 5°N, 0°, and 5°S records. In order to obtain as long a record as possible in spite of data gaps, the time series were filtered using only a 45-day low pass Hanning filter instead of the 91-day filter used in Figure 6. The 45-day filter length was chosen in order to reduce the influence of the tropical instability waves which have an average period of about 20 days [Halpern et al. 1988] which is close to the zero of the Hanning filter. The effects of these waves are then included in the eddy flux.
Time series of the mixed layer heating, Q
;
the meridional advective heat flux, Q
;
the meridional diffusive heat flux, Q;
and sum of all heat flux terms on the right hand side of equation
(1), Q
,
are shown in Figure 11 for all three
years.
Fig. 11. The top panel for each year shows time series (45-day low-pass
Hanning filter) of Q (dashed)
and Q (solid;
see text for definition). The bottom panel for each year shows meridional diffusive
heat flux Q (dashed) and
meridional advective heat flux Q
(solid). Records for boreal spring 1986, 1987, and 1988 are shown.
The meridional diffusive heat flux had a characteristic pattern each year.
It was largest in December, weakest in March, and increased again in May. This
pattern simply reflects the strength of the equatorial cold tongue and hence
the meridional curvature of SST at the equator. Maximum estimated magnitude
was about 50 W m
. The diffusive heat flux
always tends to warm the equator.
Estimates of the meridional advective heat flux can be quite large because
of the strong front just north of the equator. The resolution provided by the
moorings is not adequate to accurately resolve this front and establish the
meridional temperature gradient right on the equator. It is likely that our
estimates of Q are often
too large. Adding this term improves the agreement between the mixed layer heating
and the heat flux into the mixed layer during the warming (December-January)
of all three years. Interestingly, the rapid rise in temperature in January
1987 which appears to be associated with the passage of the Kelvin wave event
(Figure 9) is seen in the meridional but not
the zonal advective heat flux. Geise
and Harrison [1990] speculated that Kelvin pulses can modify the background
instability wave field in the eastern equatorial Pacific and lead to relatively
large changes in meridional velocity even though the Kelvin signal itself has
no meridional velocity component. Perhaps this effect is responsible for the
nearly 100 W m meridional heat advection
in January 1987.
A major discrepancy between the temperature change and the estimated heat flux
developed in May 1986 (compare Figure 6e
and 11). At that time the cold tongue was recovering
and the mixed layer was deepening. A weak southward velocity led to a large
apparent warming of 150 W m. The discrepancy
in May 1987 continued to be present even with inclusion of meridional terms.
The meridional advection enhanced the erroneous cooling seen in Figure
6 e. It appears that during the period when the equatorial front
is intensifying, the moored data, with coarse spacial resolution, do not adequately
resolve the fluctuations.
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