Fall AGU 1996: The Surface Layer Salinity Balance in the Western Equatorial Pacific at 0,156E

Warning: This writeup was done to help organize my thoughts prior to the presentation.
The actual presentation may have been quite different!

Fall AGU 1996 San Francisco, CA

Presented in the salinity special session (15 min slot)

  • I. TITLE SLIDE. 1min (1min)

    The Surface Layer Salinity Balance in the Western Equatorial Pacific at 0,156E

    Meghan Cronin
    Michael McPhaden

    NOAA Pacific Marine Environmental Laboratory

    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 at 0,156E.

    The outline of the talk is as follows:

  • II. TROPICAL SST, RAIN, SSS 1minute (2min)

    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.

  • IV. COARE ENHANCED MONITORING ARRAY. 1 minute (4.5 minute)

    [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.

  • V. FULL SFC MET RECORD FROM 0,156E 1.5 minute (6 minutes)

    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.

  • VI. SUBSFC TSD FULL RECORD 1 minute (7 minute)

    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.


    As you can see, this is near the top of the thermocline and is above the high salinity core.


    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.

  • VIII. T&S ALONG 156E & V AT 0,156E 1 MIN (9MIN)

    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 advection.

  • IX. DTDT 1.5 minute (10.5 minutes)

    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 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.

  • X. DSDT BUDGET 1 minute (11.5 minutes)

    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 forcing.

    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: While I've been advised against overlaying these figures as there are too many lines, ....it is interesting to note that:


  • XI. STATS [SUMMARY SLIDE ON 2ND SCREEN] 1 min (12.5 min)

    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.

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    Meghan F. Cronin
    Pacific Marine Environmental Laboratory
    7600 Sand Point Way NE
    Seattle, WA 98115 USA
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