Air-Sea Fluxes: A New Approach for Validation and Estimation - PowerPoint PPT Presentation

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Air-Sea Fluxes: A New Approach for Validation and Estimation

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Air-Sea Fluxes: A New Approach for Validation and Estimation Huai-Min Zhang NOAA/NESDIS/NCDC/ScSD John M. Toole and Michael J. Caruso Woods Hole Oceanographic Institution – PowerPoint PPT presentation

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Title: Air-Sea Fluxes: A New Approach for Validation and Estimation


1
Air-Sea Fluxes A New Approach for Validation
and Estimation
  • Huai-Min Zhang
  • NOAA/NESDIS/NCDC/ScSD

John M. Toole and Michael J. Caruso Woods Hole
Oceanographic Institution
OBJECTIVE Use the conservation law of the
internal energy (heat) of an ocean volume to
validate and constrain the air-sea flux estimates
based on surface and atmospheric measurements.
2
  • PROBLEM
  • Accurate air-sea fluxes are very important for
    weather and climate study predictions.
  • There large uncertainties on currently available
    air-sea fluxes. Validation against measurements
    is rare and of limited use. Cross checks of
    different products (NOAA/NESDIS/NCEP and
    NODC/COADS, European ECMWF, British SOC, etc.)
    reveal large differences, but cannot tell which
    one is better.
  • The correct air-sea fluxes must also be
    consistent with the ocean dynamics and
    energetics. In this work we use the conservation
    of oceanic internal energy to validate and
    constrain the air-sea fluxes.

3
  • METHOD
  • Internal energy (heat) equation
  • ?Cp DT/Dt Fh
    (1)
  • where ?density Cpheat capacity
    Tpotential temperature D/Dttime differential
    Fhdivergences of the turbulent and radiative
    heat fluxes including the air-sea exchange terms.
  • Integrating the above equation over a volume, V,
    defined by the instantaneous position of an
    isotherm ( T x), applying Liebnitz's rule on the
    left and Gauss' Theorem on the right and shifting
    terms yields
  • D/Dt ??? ?Cp dV ?Cp T x dV/dt ?? ?Cp(SST- T
    x)(P-E)dA ?? Fh dA . (2)
  • ?? Fh dA ?? FsdAs - ??FpdAb - ??FddAb.
    (3)
  • FsAir-Sea flux FpShortwave penetration flux
    Fd Diffusive flux at bottom.
  • In words, this equation relates the rate of
    change in the volumetric
  • heat content of a warm pool to the time rate of
    change in its volume,

4
RESULTS
  • Climatological Seasonal Cycle
  • Data Sets
  • Ocean Data NOAA/NODC WOA98
  • Air-Sea Fluxes NOAA/NODC
  • COADS94 NOAA/NCEP
  • ECMWF SOC.
  • Isotherms 28oC for the Pacific
  • and Indian Oceans
  • 27oC for the Atlantic Ocean.

Climatological SST September
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8
CONCLUSIONS
  • Validation Issues
  • The un-constrained COADS (which has a large
    globally averaged heat imbalance of 40 W/m2!!)
    and the SOC fluxes are consistent with ocean
    dynamics (down-gradient turbulent flux and
    positive diffusivity), while others (constrained
    COADS with globally balanced heat, NCEP and
    ECMWF) are not - with up-gradient turbulent flux
    and negative diffusivity in the ocean in some
    time periods, esp. the ECMWF fluxes.
  • Scientific Issues
  • The warm pool volume and heat content curves tend
    to parallel each other.
  • The warm pools expand and contract meridionally
    with the seasonal heating and cooling cycles in
    the respective hemispheres.
  • Hemispheric assymetries in the western Atlantic
    and Pacific boundaries and the corresponding
    continental influence on regional air-sea
    exchange give rise to the annual-period signals
    in total pool volume and heat content that are
    phased with the northern hemisphere (warmest
    waters in mid-to-late boreal summer). In
    contrast, the limited geographic extent of the
    North Indian Ocean results in the southern
    hemisphere driving the seasonal cycle.

9
  • The first two terms in eq.2 are by far the
    largest (an order larger) but they have the same
    sign and appear on opposite sides of the
    equation. Thus it is the residual of the two
    heat content terms (net heat storage) that
    relates (in the same order) to the turbulent flux
    divergences.
  • Although the net storage is smaller (and changes
    signs) than other terms of eq.2, in time sequence
    it is parallel to the air-sea flux change. In
    other words, the net heat storage captures the
    time change signals of the air-sea flux the
    large non-zero time-mean air-sea flux is balanced
    by the relatively constant diffusive flux through
    the bottom of the warm pool.
  • The diffusivity is relatively constant,
    physically reasonable and largely consistent with
    measurements.
  • Work in Progress
  • Interannual and ENSO variability
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