Ocean sensitivity experiments#

Ocean-only sensitivity experiments with FESOM and ICON-O that differ in their vertical mixing scheme and its parameter settings. In most of the experiments, the same mixing scheme and the same settings were used in FESOM and ICON-O. The purpose of the runs was to find the best settings for the coupled nextGEMS configurations. The two-year simulations were chosen to include the year 2015, because a strong Near-Inertial Wave (NIW) mixing event occurred in that year and was observed during a RV Meteor cruise in the tropical North Atlantic, allowing for a comparison between model and observations. The results of these analyses are available in Bastin et al. (2024).

Common settings and experiment descriptions#

  • Common vertical axis with 128 vertical levels for both models, with thicknesses ranging from 2m near the surface to about 200m near the seafloor

  • Same vertical mixing schemes (TKE and KPP) as well as the same parameter settings (details on mixing schemes below)

  • Use versions of TKE and KPP schemes provided by the CVMix (Community ocean Vertical Mixing) project

  • All model runs forced with hourly ERA5 reanalysis (Hersbach et al. 2020)

  • Run from the end of the 5-year spinup with adjusted mixing parameter settings for two years (2014 and 2015).

  • The model runs were started on 1 January 2014, and run until 31 December 2015

  • Additional ICON-O run with increased minimum background vertical mixing via increased minimum background TKE

  • Additional ICON-O run with increased minimum background vertical mixing via minimum background value for vertical diffusivity and viscosity

  • Additional ICON-O run using the standard FESOM bulk formulae (details on bulk formulae below).

List of sensitivity experiments#

Sensitivity experiments#

Name

Model

Mixing scheme

Parameter settings

F_TKE_01

FESOM

TKE

\(c_k=0.1\)

F_TKE_02

FESOM

TKE

\(c_k=0.2\)

F_TKE_03

FESOM

TKE

\(c_k=0.3\)

F_KPP_030

FESOM

KPP

\(Ri_{crit}=0.3\)

F_KPP_027

FESOM

KPP

\(Ri_{crit}=0.27\)

I_TKE_01

ICON-O

TKE

\(c_k=0.1\)

I_TKE_02

ICON-O

TKE

\(c_k=0.2\)

I_TKE_03

ICON-O

TKE

\(c_k=0.3\)

I_KPP_030

ICON-O

KPP

\(Ri_{crit}=0.3\)

I_KPP_027

ICON-O

KPP

\(Ri_{crit}=0.27\)

I_TKE_02_Langmuir

ICON-O

TKE

\(c_k=0.2\), additional Langmuir parameterisation

I_TKE_02_minTKE

ICON-O

TKE

\(c_k=0.2\), minimum background TKE = $10^{-5}$ J/kg (default: $10^{-6}$ J/kg)

I_TKE_02_minKv

ICON-O

TKE

\(c_k=0.2\), minimum background diffusivity = $10^{-5}$ m$^2$/s (viscosity = $10^{-4}$ m$^2$/s)

I_TKE_02_ncarbf

ICON-O

TKE

\(c_k=0.2\), FESOM default bulk formulae

Output data#

  • All output variables are available at 3-hourly intervals

  • Output variables include:

    • temperature

    • salinity

    • density

    • velocities

    • shear

    • buoyancy frequency

    • Richardson Number

    • turbulent kinetic energy

    • vertical diffusivity and viscosity (3D for the upper 200m)

    • surface fluxes (heat, momentum, freshwater)

    • mixed layer depth

  • For easier analysis and comparability between the two models, data from both models has been regridded to a \(0.1^{\circ} \times 0.1^{\circ}\) longitude-latitude grid for the tropical Atlantic domain \((10^{\circ}S - 30^{\circ}N, 60^{\circ}W - 15^{\circ}E)\).

Model data access#

Ocean-only sensitivity runs are published under nextGEMS data project in the World Data Center for Climate (WDCC) of the German Climate Computing Center (DKRZ) and can be found in Bastin et al. 2023a . In addition, there are time series of the model output at locations where observational data is available during the modelled time period that can be found in Bastin et al. 2023b .

Observational data#

Horizontal grid mesh#

  • FESOM: FESOM2 mesh that has 50 km resolution over most of the globe, except for the equatorial Atlantic, where it is set to 13 km resolution. FESOM uses a \(z^*\) vertical coordinate, where the total change in SSH is distributed equally over all layers, except the layer that touches the bottom.

  • ICON-O: ICON grid with globally approximately uniform horizontal resolution of about 10km is used. Horizontal resolution comparable in the tropical Atlantic. In the vertical, a \(z^*\) coordinate is used where model levels follow the free surface.

Details on mixing schemes#

In the TKE (turbulent kinetic energy) scheme (Gaspar et al. 1990), the eddy diffusivity is determined as a function of the turbulent kinetic energy, for which an additional prognostic equation is included in the model. For TKE, the value of \(c_k\) parameter (cf. Gaspar et al. (1990), Equation 10) is varied. In the KPP (K-profile parameterisation) scheme (Large et al. 1994), the eddy diffusivity is instead given as a specified vertical profile in the surface ocean boundary layer. The boundary layer depth is determined as the depth below which the bulk Richardson Number becomes larger than a critical value \(Ri_{crit}\). \(Ri_{crit}\) is usually set to 0.3 but should ideally approach the critical gradient Richardson Number of 0.25 with increasingly fine vertical resolution. For KPP, the value of \(Ri_{crit}\) is varied in the sensitivity runs. Additionally number of runs only with ICON-O changing different aspects of the TKE scheme: One of these has an extension of the TKE scheme: the parameterisation of Langmuir turbulence (Axell 2002). Langmuir turbulence, which is generated through the interaction of wind-driven surface currents and wind-generated surface waves, is responsible for additional turbulent energy input into the upper ocean, and it has been shown to be important over much of the global ocean area (e.g. Belcher et al. (2012)). Since ICON-O does not simulate surface waves, the effect of the Langmuir turbulence is missing if it is not parameterised. Two runs were done with increased minimum background vertical mixing, one by increasing the minimum background TKE from \(10^{-6}\) J/kg to \(10^{-5}\) J/kg (in this case the diffusivity and viscosity are then still dependent on the stability of the water column), and one by setting a minimum background value directly for the vertical diffusivity \((10^{-5} m^2/s)\) and viscosity \((10^{-4} m^2/s)\) as it is also done in the KPP scheme. The last run was done to check the effect of the different sets of default forcing bulk formulae that are used in ICON-O and FESOM with ERA5 forcing. Here, ICON-O was run with the same settings as in the ICON_TKE_02 run, except that the default FESOM bulk formulae were used.

Bulk formula#

Bulk formulae that are by default used in FESOM and ICON-O together with ERA5 forcing are different.

  • ICON-O: Default bulk formulae for the ERA5 forcing are those of Kara et al. (2002) over ocean and sea ice, with water vapor pressure and 2_m specific humidity calculated using the (modified) equations from Buck (1981) and longwave radiation calculated using Berliand (1952).

  • FESOM: Bulk formulae calculated according to Large and Yeager (2009) over the ocean and with constant bulk exchange coefficients over sea ice, as described in Tsujino et al. (2018). These are also implemented in ICON-O, but usually only used together with JRA55-do forcing (Tsujino et al. (2018)).

References#

  • Axell, L. B., Wind-driven internal waves and Langmuir circulations in a numerical ocean model of the southern Baltic Sea. J. Geophys. Res., 107(C11), 3204, doi:10.1029/2001JC000922, 2002.

  • Bastin, S. et al. (2023a) High frequency tropical Atlantic data from vertical mixing sensitivity runs with FESOM and ICON-O (nextGEMS WP6). World Data Center for Climate (WDCC) at DKRZ. doi: 10.26050/WDCC/nextGEMS_WP6oc

  • Bastin, S. et al. (2023b) nextGEMS: Output of the WP6 ocean vertical mixing sensitivity runs (timeseries). Zenodo. doi: 10.5281/zenodo.8225706.

  • Bastin, Swantje, et al. Sensitivity of the tropical Atlantic to vertical mixing in two ocean models (ICON-O v2. 6.6 and FESOM v2. 5). EGUsphere 2024 (2024): 1-44.

  • Belcher, S. E., et al. (2012), A global perspective on Langmuir turbulence in the ocean surface boundary layer. Geophys. Res. Lett., 39, L18605, doi:10.1029/2012GL052932.

  • BERLIAND M. E. Determining the net long-wave radiation of the earth with consideration of the effect of cloudiness. Izv.Akad.Nauk.SSSR Ser.Geofiz, 1952, 1, 64-78, https://cir.nii.ac.jp/crid/1570572699293459712,

  • Buck, Arden L. New Equations for Computing Vapor Pressure and Enhancement Factor. Journal of Applied Meteorology (1962-1982), vol. 20, no. 12, 1981, pp. 1527–32. JSTOR, http://www.jstor.org/stable/26180379

  • Gaspar, P., Y. Grégoris, and J.-M. Lefevre (1990), A simple eddy kinetic energy model for simulations of the oceanic vertical mixing: Tests at station Papa and long-term upper ocean study site. J. Geophys. Res., 95(C9), 16179–16193, doi:10.1029/JC095iC09p16179.

  • Hersbach H, Bell B, Berrisford P, et al. The ERA5 global reanalysis. Q J R Meteorol Soc. 2020; 146: 1999–2049. https://doi.org/10.1002/qj.3803

  • Kara, A.B., Rochford, P.A. & Hurlburt, H.E. Air–Sea Flux Estimates And The 1997–1998 Enso Event. Boundary-Layer Meteorology 103, 439–458 (2002). https://doi.org/10.1023/A:1014945408605

  • Large, W. G., J. C. McWilliams, and S. C. Doney (1994), Oceanic vertical mixing: A review and a model with a nonlocal boundary layer parameterization. Rev. Geophys., 32(4), 363–403, doi:10.1029/94RG01872.

  • Large, W.G., Yeager, S.G. The global climatology of an interannually varying air–sea flux data set. Clim Dyn 33, 341–364 (2009). https://doi.org/10.1007/s00382-008-0441-3

  • Mrozowska, M. A., et al. Using NIW observations to assess mixed layer parameterizations: A case study in the tropical Atlantic. Journal of Geophysical Research: Oceans 129.5 (2024): e2024JC020985.

  • Tsujino, Hiroyuki, et al. JRA-55 based surface dataset for driving ocean–sea-ice models (JRA55-do). Ocean Modelling 130 (2018): 79-139.