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
  • 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?
  • 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

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.

two important numerical issues
Two important numerical issues
  • 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 2 synoptic variability of surface turbulent heat flux1
Issue-2: Synoptic variability of surface turbulent heat flux
  • 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.
summary
Summary
  • 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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