U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Up to now, the individual contributions of Q,
Q
, Q
and Q
to the mixed layer
temperature change have been estimated and no single term has been found which
accounts for the observed variations of Q
.
Estimation of the remaining terms in equation (1),
meridional advection and diffusion, require off-equatorial information. Such
data are not available continuously throughout the study period (
Figure 1) because of deployment schedules and instrument failures. Coverage
is best in boreal spring of each year and these records are discussed in the
next section. First, however, we consider how well the sum of the terms available
throughout the entire record can balance the heating.
Figure 6e shows the comparison of the
sum of the surface heat flux, penetrative radiation, zonal advection, entrainment
and vertical diffusion (Q
= Q
+ Q
+ Q
+ Q
)
and the observed mixed layer heating (Q
).
Over the entire 29-month record the correlation coefficient between the two
series was 0.71; limiting the record to 24 months in 1986-87 increases the correlation
to 0.80. Both of these correlations are significant at the 95% level. The linear
regression coefficient (1.1) between the two complete series indicates that
the fluctuations in Q
are about 10% larger than those observed in surface heating. The offset between
the two series showed that the mean surface heating was about 30 W m
larger than Q
.
Thus an additional heating source (perhaps meridional diffusion) is required
to balance these terms.
Observed and computed heating agreed reasonably well in 1986. During the initial
development of warm conditions in the eastern Pacific in August through November
1986 [MH], the mixed layer was relatively deep and was warming by up to nearly
100 W m. This heat was provided by the net
solar radiation (which was enhanced at that time by the semiannual cycle in
Q
and the reduced Q
associated with the deep mixed layer), and variations in all the oceanic terms.
Changes in latent heat flux do not appear to contribute to this warming. The
large increase in heating in mid-September 1986 was associated with reduced
oceanic cooling as the zonal advection was near zero, vertical entrainment was
reduced, and vertical diffusion was near its minimum.
The phase of the seasonal cycle from February through July 1987 was also rather
well represented by the computed heat flux terms. Ignoring the rapid rise in
January, the general spring warming in 1987 required about 50 W m
in February. This heat was provided by the net solar radiation and reduced vertical
diffusion. Reduced zonal advective cooling is nearly balanced by increased entrainment.
The subsequent cooling in May-July appears to result primarily from a large
increase in vertical turbulent diffusion out of the mixed layer. This diffusion
and increased entrainment is sufficient to counteract the increased mixed layer
heating associated with zonal advection.
Relatively large (>50 W m) discrepancies
between the surface heating and the heat flux occur throughout the record although
the phase of the two terms generally agree. In January 1987 the observed mixed
layer heating was nearly 60 W m
larger than
the estimated heat flux into the mixed layer. In May 1987 the estimated cooling
was nearly 100 W m
too large. The period
March-May 1988 also remains anomalous. At that time changes in heating and heat
flux were out of phase. As the mixed layer cooled by about 10 W m
(and SST dropped nearly 8°C), the heat flux was trying to warm the ocean by
up to 50 W m
. This period is considered
in the context of boreal spring 1986 and 1987 in the next section.
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