Summary of Work | |
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Strong disagreements exist
regarding the mechanisms which regulate Sea Surface Temperature
(SST) over the Western Pacific Warm Pool. As these waters
have a critical impact on convection in the
western Pacific which in turn influences the planetary scale Walker and Hadley
circulations, the climatic implications of the warm pool are
extensive (Webster and Lukas, 1992) .
Here we examine fluctations in Warm Pool SST as it relates to shifts in the the surface energy balance during strong intraseasonal variations in SST in an effort to understand the warming and cooling of the oceanic mixed layer. Portions of both the Indian and Pacific Ocean Warm Pools are considered. The reanalysis products (NCEP and ECMWF) are used to assess the surface turbulent flux over an extended climatology. Recognizing the potential presence of significant error in these reanlaysis fields, TOGA COARE flux data are cited for error estimation. Large errors are found in the surface shortwave flux from the reanlaysis products and alternative methods for diagnosing surface shortwave radiation (Bishop et al., 1997; Shinoda and Hendon, 1997) are also used. | |
Four results from this study are worthy of mention. | |
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A TOGA CASE STUDY | |
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Measurements from the Moana Wave ship during the TOGA COARE Intensive Operating Period Capture a period of exceptional SST variation from Nov 1, 1992 to Jan 11, 1993. | |
SST |
SST A comparison between Reynolds SST and ship measurements is generally good. Note that the first half of our period corresponds to an SST warming phase and the second half corresponds to SST cooling. Also note that the Reynolds measurements fail to capture the large diurnal variation in SST (blue dashed line) and thus does not capture the variations in surface evaporation or longwave emission. The longwave and evaporative cooling contributions calculated in the NCEP model are lower than they otherwise should be because of this oversight. |
Evap |
Evaporation Excellent agreement with NCEP in general because winds are good. This is true despite the large scale discrepency of the measurements. Lower caption is a comparison of NCEP with a satellite derivation method over a large scale again with excellent agreement. |
Wind |
Wind Fluxes compare well because wind compares well. |
Radiation |
Radiation
NCEP does a generally poor job based on this comparison, however a technique developed by Shinoda and Hendon (1997) from OLR regressions does substantially better. In the comparison above, both NCEP and OLR shortwave radiation are shown and compared. |
Sui and Lau (1997, J. Climate) found near equal roles for
evaporation and solar flux anomalies in cooling the ocean during
intraseasonal variations. The representativeness of their results are uncertain however. With the techniques for diagnosing surface fluxes discussed above, attention can be broadened to other temporal and spatial domains.
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A CLIMATOLOGY OF SURFACE FLUX ANOMALIES | |
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COMPOSITE RESULTSidentified warm events over the small domain. identified warm events over the medium (Pacific) domain. Indian Medium Domain identified warm events over the large (Pacific) domain. Indian Large Domain | |
Net Shortwave Flux Latent Flux Net Shortwave (Med Scale) Latent Flux (Med Scale) Net Shortwave (Large Scale) Latent Flux (Large Scale) | Integrated Flux Anomalies
Integrated Flux Anomalies (Med Scale) Integrated Flux Anomalies (Large Scale) |
THE PUNCH LINE
Warming of SST is primarily associated with below normal evaporation and, to a slightly lesser extent, enhanced surface solar radiation. Fighting against the warming are longwave and sensible fluxes which act to cool the mixed layer during this stage. There is no evidence of a "super greenhouse" in the NCEP product...ie: The enhanced downwelling flux from the atmosphere (at the surface) is less than the increased upwelling flux associated with a warming surface. Cooling of SST is largely associated with both stronger than normal evaporation and reduced solar flux anomalies. Over the Pacific, these flux anomalies are often nearly equal. Over the Indian Ocean warm pool, the flux anomalies are often dominated by solar flux anomalies. In both cases, there exist important couplings to the large-scale atmosphere and the variability of SST appears to be largely DRIVEN BY THE ATMOSPHERE rather than the reverse. Thus in the variability observed during in-situ experiments, there is little evidence of SST regulation via an internal thermodynamic "threshold". In press, Journal of Climate. |