Ogcm sensitivity experiments on the annual cycle of western hemisphere warm pool whwp
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OGCM sensitivity experiments on the annual cycle of Western Hemisphere Warm Pool (WHWP). By S.-K. Lee (CIMAS/UM), D. Enfield (AOML/NOAA), C. Wang (AOML/NOAA), and G. Halliwell Jr. (RSMAS/UM) Objectives:

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Ogcm sensitivity experiments on the annual cycle of western hemisphere warm pool whwp
OGCM sensitivity experiments on the annual cycle of Western Hemisphere Warm Pool (WHWP)

  • By S.-K. Lee (CIMAS/UM), D. Enfield (AOML/NOAA), C. Wang (AOML/NOAA), and G. Halliwell Jr. (RSMAS/UM)

  • Objectives:

  • To assess the appropriateness of commonly used surface heat flux data sets in driving HYCOM simulation of WHWP

  • To assure that the model will optimally simulate the warm pool behavior (i.e, fine-tuning).

  • To understand the annual cycle of the WHWP heat budget (i.e, forcing and damping mechanisms)


Outline
Outline Hemisphere Warm Pool (WHWP)

  • What is WHWP and why do we want to study it?

  • HYCOM configuration

  • 16 Numerical experiments

  • Two important numerical issues

    (a) Heat budget analysis using HYCOM?

    (b) Synoptic variability of surface turbulent heat flux

  • Model-Data and Model-Model comparisons

  • Conclusion and Discussion


What is whwp and why do we want to study it
What is WHWP and why do we want to study it? Hemisphere Warm Pool (WHWP)

  • Importance of WHWP:

  • A heating source for the summer Atlantic Hadley circulation (Wang and Enfield, 2003)

  • A source of moisture for North America, affecting summer rainfall in the US, Caribbean, Central & South America (Bosilovich et al. 2002; Mestas-Nunez et al., 2005)

  • Important for Atlantic Hurricanes (Wang et al., 2005)

  • Interannual variability large, a part of ENSO-Atlantic bridge signal


Ogcm simulation
OGCM simulation Hemisphere Warm Pool (WHWP)

Model: Hybrid Coordinate Ocean Model (version 2.1.27)

Model domain: contains both Pacific and Atlantic (100oE- 20oE, 35oS-65oN)

Resolution: uniform 1o in zonally and variable in meridional direction; 0.5o at the equator increasing linearly to 1o at 40o latitude and 1o poleward of 40o; 22 hybrid layers

Miscs.: T-S are advected; Levitus climatology used for initial condition, also for relaxation at the southern boundary.


Numerical experiments
Numerical experiments Hemisphere Warm Pool (WHWP)


Two important numerical issues
Two important numerical issues Hemisphere Warm Pool (WHWP)

  • Issue-1: Heat budget analysis using HYCOM?

  • Hybrid grid generator acts like an upstream vertical advection operator (Bleck, 2002), contributing in ocean heat budget

  • A practical solution is to enforce the surface layers (~50m) to be in purely z-coordinate (the problem still exist in hybrid layers, but not in purely z-coordinate or isopycnal layers).

  • Use unrealistically small target density for the upper layers to enforce the surface layers (~50m) to be in purely z-coordinate (Bleck, 2002)

  • PCM scheme do nothing in z-coordinate layers, but PLM (HYCOM default) still operate in purely z-coordinate layers.

  • Heat budget computation routine is implemented in the codes. Using archive data is not a good idea.


Issue 1 heat budget analysis using hycom
Issue-1: Heat budget analysis using HYCOM? Hemisphere Warm Pool (WHWP)



Issue 2 synoptic variability of surface turbulent heat flux1
Issue-2: Synoptic variability of surface turbulent heat flux Hemisphere Warm Pool (WHWP)

  • Synoptic variability in the surface turbulent heat fluxes (anisotropic heat flux term) is significant over the WHWP (~50W/m2).

  • Anisotropic heat flux is independent from the mean fluxes, thus can not be parameterized (Gulev, 1979).

  • When monthly heat flux data sets are used to force HYCOM, anisotropic heat flux must be treated separately.

  • A practical way is to estimate the anisotropic heat flux for each surface heat flux climatology, then use it as an additional heat flux in HYCOM.






Shc kpp obh kpp
SHC-KPP & OBH-KPP Hemisphere Warm Pool (WHWP)


Shu kpp dsu kpp
SHU-KPP & DSU-KPP Hemisphere Warm Pool (WHWP)


Ncep1 kpp ncep2 kpp
NCEP1-KPP & NCEP2-KPP Hemisphere Warm Pool (WHWP)


Era15 kpp era40 kpp
ERA15-KPP & ERA40-KPP Hemisphere Warm Pool (WHWP)


Model sst bias versus surface net heat flux q net
Model SST bias versus surface net heat flux ( Hemisphere Warm Pool (WHWP)QNET)


Q net dsu era15
Hemisphere Warm Pool (WHWP)QNET (DSU – ERA15)



Summary
Summary Hemisphere Warm Pool (WHWP)

  • The simulated SST is closest to the observations when Southampton constrained (SHC) data is used to force HYCOM; unrealistically high when forced with unconstrained in-situ data products (SHU, DSU) and unrealistically low when forced with model-based reanalysis products (NCEP1, NCEP2, ERA15 and ERA40).

  • The model SST bias is minimized when monthly KPAR climatology is used. For reasonable variations of the critical Richarson number (0.25 ~ 1.00) in two different mixing models (KPP and GISS), there is no significant impact on the results. Heat flux bias is too high to make any conclusion.

  • Yes, HYCOM can be used for mixed layer heat budget studies, but the numerical diffusion due to hybrid-grid generator must be properly dealt with, and the heat budget computation routine must be implemented in the codes.

  • When monthly heat flux data sets are used to force HYCOM, synoptic variability of the QLAT and QSENmust beincorporatedin HYCOM.


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