Warning: This writeup was done to help organize
my thoughts prior to the presentation.
The actual presentation may have been quite different!
Heat balance analysis: Cronin and McPhaden (JGR 1997)
Salinity balance analysis: Cronin and McPhaden (JGR submitted)
Part of the Warm Pool's heat balance analysis that I'll be discussing today was published in JGR last year. The salinity balance analysis is currently under review. In today's presentation, I'll compare the balances by converting the heat and freshwater fluxes into buoyancy fluxes. By doing so, we will see how the two budgets are coupled.
To get oriented, here are 3 panels showing the longterm mean SST, SSS and rainfall in the tropics. The study location is here at 0,156E, in the western equatorial Pacific Warm Pool. The Warm Pool is a region, usually in the western Pacific, where SST>28C. From the bottom panels, we see that this is also a region of intense rainfall (accumulations of 2-5 m/yr) and low SSS. One of the major goals of TOGA COARE was to understand how these fields are related.
This figure shows the COARE enhanced monitoring array which was in place from August 1991 through April 1994. The budget analysis I will be describing uses data from this central mooring at 0,156E as well as temperature and salinity data from nearby ATLAS moorings with SEACATS.
At the central location, the surface data included hourly wind speed and direction, air temperature, relative humidity, incoming shortwave radiation, rain rates from an optical rain gauge, and the 1-M sea surface temperature and sea surface salinity. This surface met data was used with the COARE bulk algorithm to compute the turbulent fluxes of heat, moisture and momentum.
The hourly subsurface data included temperature to 500m, salinity to 200m, and zonal and meridional velocity to 250m from an ADCP.
These temperature and salinity data, shown in the top two panels, can be combined to compute density, which is shown in the bottom panel.
In the following layered heat and freshwater budgets I define the upper layer as the layer above the 21.8 density surface or 20m, which ever is deeper. This surface is overlaid on the 3 fields.
Notice that this surface is at the top of the thermocline and above the high salinity core. Within this "Upper Layer", the temperature and salinity are weakly stratified and for low frequency variability, is similar to the vertical average.
Here is a cartoon showing how some of the processes work. The surface layer is warmed by incoming shortwave radiation and cooled by the net incoming and outgoing longwave radiation, turbulent latent and sensible heat loss. Of course the latent heat will be associated with evaporation and clouds associated with precipitation will affect the net shortwave and longwave radiation absorbed by the layer.
Likewise, turbulent entrainment will lead to cooling if the density gradient defining the layer depth is controlled by the temperature stratification, and will lead to an increased surface salinity if the layer stratified by freshwater. As discussed by Lukas and Lindstrom, in this case, the halocline provides a "barrier layer" to the turbulent mixing of heat.
[OPTIONAL: During the Coupled Ocean Atmosphere Response Experiment we were able to measure nearly all the terms in both budgets. 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.]
In order to compare the effects of these heat and freshwater fluxes on the stratification, we will scale the budgets by gravity and the thermal and haline expansion coefficients. The buoyancy field can thus be viewed as a reduced gravity. Surface buoyancy flux is:
B0 = -g*alpha* (Q0/rho_0 c_p) - g*beta* S*(P-E)
= B0_T + B0_S
The red line shows the time series of the net surface heat flux. Positive values indicate a stabilizing (warming) heat flux, negative values indicate (cooling) destabilizing heat flux. The record length mean value is 21 W/m2.
The blue line shows the time series of Precipitation minus Evaporation. It's axis is on the far right. The record length mean value is 9 mm/day, which in terms of a buoyancy flux (far left axis), corresponds to approximately 35 W/m2 heat flux. Thus precipitation is extremely important to the buoyancy of the warm pool over full study period.
[OPTIONAL: The Optical Rain Gauges tend to be higher than other measurements (e.g. satellite and radar estimates). Thus precipitation should be considered as an upper limit.]
The other important thing to notice here is that the buoyancy flux due to E-P is strongly anticorrelated with the buoyancy flux due to Q0 (the correlation is -0.5). Essentially, during westerly windbursts (see top panel), precipitation tends to stabilize the layer and the net surface heat flux tends to cool/destabilize the layer.
If the ocean was a slave to the meteorology, then we would expect that a similiarly high correlation should be observed between the variability of the SSS and variability of the SST.
Curiously, the observed local change in the upper layer temperature and salinity fields seen here red and blue, are sometimes positively correlated (these are periods when the layer is getting both warm and fresh or cold and salty) and sometimes are negatively correlated (getting warm and salty,...). Over the record length, the correlation is zero. Obviously the ocean is responding to more than just the net surface fluxes.
I'll begin by discussing the heat balance. The top panel shows the surface layer temperature tendency rate. Positive temperature tendencies indicate the layer is warming and becoming more stable, negative temperature tendencies indicate the layer is cooling and destabilizing. 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 (0.7) 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 balance.
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 (destabilizing), negative values
indicate that the layer is freshening (stabilizing).
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, the observed salinity tendency rate and
the surface E-P forcing are essentially uncorrelated (rxy=0.2).
Zonal advection appears to be the controlling mechanism for local
salinity variability (rxy=0.6).
The residual is nearly always positive, consistent with a turbulent mixing increasing the surface salinity.
[OPTIONAL: 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, although the ORG rainfall is generally higher than other rain observations.]
I'll summarize with some statistics computed using data from the full 2yr monitoring period. The net surface heat flux is anticorrelated with Precipitation and zonal wind stress: westerly wind bursts are associated with a net surface heat loss and increased precipitation.
Other terms in the salinity and heat balances are also anticorrelated: Zonal advection of low temperatures is often associated with advection of high salinity. Likewise the residual is anticorrelated: turbulent mixing tends to increase the salinity and decrease the surface temperature. Yet the observed temperature and salinity tendency rates are uncorrelated.
What I believe is happening is that the turbulent mixing during the convective periods causes a near steady state balance between precipitation and mixing. Consequently, zonal advection is the primary process responsible for salinity variability. In contrast, since the turbulent mixing tends to occur during periods of surface cooling and rainfall, the freshwater buoyancy can act as a barrier to the mixing of heat. Consequently, the WWB mixing will erode the near surface freshwater stratification, causing locally generated barrier layers to be short lived. Once the barrier layer is eroded, the WWB turbulent mixing will tend to cool the surface, accentuating the WWB cooling due to the net surface heat flux.
Over the record length, the surface heat and moisture fluxes tend to stabilize the warm pool.
Convection and strong winds during Westerly Wind Bursts, cause precipitation, a net surface cooling flux, and turbulent mixing (although rain tends to stabilize the surface).
The WWB mixing tends to bring cold-salty water into the surface layer. Mixing balances precipitation freshening and adds to surface cooling. Local salinity variability is controlled by zonal advection.
Freshwater buoyancy due to rain can act as a barrier to the mixing of heat.
WWB mixing erodes the near surface freshwater stratification causing locally generated barrier layers to be short-lived.
Meghan F. Cronin
Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Seattle, WA 98115 USA
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