Warning: This writeup was done to help organize
my thoughts prior to the presentation.
The actual presentation may have been quite different!
Before getting started, I would like to thank Roger Lukas, Joel Picaut
and David Tang for providing data from nearby moorings. This data
enables us to evaluate the horizontal advection of heat and freshwater
The outline of the talk is as follows:
* I'll then show DATA from the central buoy at 0,156E as well as
temperature and salinity data from nearby moorings.
* The heart of the presentation will be the discussion of the
FRESHWATER Balance AND its relation to the HEAT BALANCE.
To get oriented, here are 3 panels showing the mean SST, SSS and
rainfall in the tropics. The study location is here at 0,156E, in the
western equatorial Pacific Warm Pool. The WP is a region in which
T>28C. From the bottom panel, we see that this is also a region of
intense rainfall (accumulations of 2-5 m/yr) and fresh SSS. In order
to determine the processes responsible for the variabiltiy of surface
salinity, we will evaluate the surface layer salinity budget.
The surface layer salinity tendency rate depends upon 3 terms:
1) Evaporation - Precipitation, 2) advection and 3) mixing and
diffusion. Likewise, as shown in the bottom half of this figure, the
surface layer temperature tendency rate depends upon surface heat flux
(which includes incoming shortwave radiaiton, longwave
radiation,latent heat of evaporation and sensible heat loss due to
conduction), horizontal heat advection, entrainment mixing and
diffusion. As I will show, during the Coupled Ocean Atmosphere
Response Experiment, we were able to measure nearly all the terms in
both budgets. In particular, the surface turbulent heat flux and
evaporative freshwater flux were computed using the TOGA-COARE bulk
algorithm. While data gaps exist, in general, the only term we could
not directly measure was mixing and diffusion. These terms we estimate
as residuals of the budget.
[OPTIONAL: A major objective of the TOGA-COARE was to describe and understand the principal processesresponsible for the coupling of the ocean and atmosphere in the western Pacific warm pool system.]
[OPTIONAL: A major goal of the Coupled Ocean Atmosphere Response Experiment was
to describe and understand theocean's response to the combined (heat
and freshwater) buoyancy and wind forcing.]
This figure shows the COARE enhanced monitoring array. The budget analysis I will be describing uses data from this central mooring at 0,156E as well as temperature data from nearby ATLAS moorings and salinity data from nearby ATLAS moorings with SEACATS.
This slide shows surface data collected from the central 0,156E
mooring. Winds, rain, Qrad, SSS, SST. RH and Tair are not shown
here. For clarity I'm showing the 5-day smoothed winds rather than the
hourly. However all surface and subsurface data were collected at 1
hourly sample rate at this mooring.
In this presentation, I will focus on a 3-month period case study from
Sep-Dec 1992. This is one of the most well resolved periods, and
includes the beginning of the COARE intensive observational
period. The heat balance analysis for this period is written up and is
currently under review.
In October 1992 there was a moderately strong westerly windburst, which was associated with a reduction in the shortwave radiation and increase in the rain rate.
The mooring also measured subsurface salinity and temperature, from which we can compute density. For this analysis, I define the mixed layer depth to be the depth of the 21.8 isopycnal.
[OVERLAY WITH MLD SLIDE]
As you can see, this is near the top of the thermocline and is above
the high salinity core.
[OVERLAY WITH PERIODS]
At the beginning of the study period, the pycnocline is shallow. Then
during the WWB, the surface layer deepens and remains deep until about
June of the following year.
This slide shows the 5m salinity and temperature time series from
moorings along the equator from 0,154E (green) to 0,165E (red). So
the color scheme is colder colors in the west and warmer colors in the
east. The bottom panel shows the 20 m zonal velocity at the study
location, 0,156E. As you can see, for much of the record, the SST and
SSS are zonally homogenous and we might expect that zonal advection
would be small. However, at other times, there are strong gradients as
for example prior to and during the early stages of the October 1992
wind burst. During this period the zonal velocity is westward and
strong (70 cm/s). During the windburst, the surface current changes
direction and becomes eastward.
Likewise, this slide shows the 5 m Salinity and temperature along the
156E longitude and the 20 m meridional velocity at 0,156E. The off
equatorial records are much shorter, unfortunately. However, because
the meridional velocity is both smaller in magnitude then the zonal
velocity and generally has variability on timescales of 2 weeks and
less, we find that after applying a 15-day triangular low-pass filter
to the budget, the horizontal advection is dominated by the zonal
I'll begin by discussing the heat balance.The top panel shows the
surface layer temperature tendency rate. Positive values, as for
example, prior to and following the wind burst, indicate the layer is
warming, negative values, as for example during the wind burst period,
indicate the layer is cooling. This tendency rate is repeated as a
dashed line in all the other panels.
The second panel shows warming and cooling due to the surface heat
flux. Notice the close correlation between the observed warming and
cooling and the surface heat flux forcing.
As shown in the next panel, the advection of warm water from the east
in early to mid-October provided one of the largest terms in the
The bottom panel shows the residual of the budget, which we will
interpret as being due to entrainment mixing as well as errors.
Notice that prior to the wind burst, when the mixed layer is shallow,
there is strong entrainment of cold deep water into the surface layer.
Now for the salinity balance. As in the heat balance, the top panel
shows the surface layer salinity tendency rate, which is repeated in
each of the panels as a dotted line. Positive values indicate that the
layer salinity is increasing, negative values (as during the later
portion of the WWB) indicate that the layer is freshening.
As can be seen in this second panel, precipitation dominates over
evaporation so that the affect of E-P is to freshen the layer. Notice
that unlike the heat balance, there is not much of a correlation
between the observed salinity tendency rate and the surface E-P
The strong rainfall in fact occurs during a period when the surface
salinity is increasing due to advection from the east and entrainment
of deep salty water into the surface layer.
[OPTIONAL: OVERLAY BOTH BUDGETS]
[OPTIONAL: While I've been advised against overlaying these figures
as there are too many lines, ....it is interesting to note that:
I'll summarize with some statistics computed using data from the full
2yr monitoring period. The mean Precipitation measured by the optical
rain gauges is 4.7 m/yr, while evaporation, computed using the COARE
bulk algorithm, is 1.3 m/yr. The mean Surface heat flux is relatively
small (20 Watts) and positive (i.e. a warming effect). These values
are relatively consistent with other climatologies.
Now looking at the cross-correlations, the surface salinity
variability is not correlated with E-P forcing. Variability in SSS is
contolled predominately by zonal advection.
While zonal advection also affects the SST variability, it is a
secondary process. As you can see from the large correlation
coeficient, the SST variability is primarily controlled by variability
in the surface heat flux.
These statistics computed over the 2 year period, are consistent with
the results of the Sep-Dec 1992 case study.
Meghan F. Cronin
Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Seattle, WA 98115 USA
Meghan Cronin's Home Page
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