SODA4 Documentation

Structure of SODA4.15.2 Fields

SODA4 is a global eddy-resolving mesoscale ocean/sea ice reanalysis. The core experiment configuration is SODA4.15.2, which incorporates data assimilation constraints using a localized optimal interpolation framework. The companion SODA4.15.2.0 simulation (no data assimilation) is included as well for users who wish to execute strict closed closed-budget diagnostic evaluations.

Atmospheric forcing datasets frequently exhibit systematic discrepancies compared directly with targeted oceanic observations. To mitigate these artifacts, an iterative bias correction algorithm is applied to bring the net surface heat and freshwater fluxes into alignment with the ocean uptake. In effect, ocean observations are used to correct atmospheric fluxes.

File Naming Conventions

Files follow the standardized layout structure: soda4.X.x_Z_V_G_YYYY.nc

• 4.X (Meteorological Forcing Identifier):
  • 4.15 indicates ERA5 high-resolution atmospheric forcing profiles.
• x (Experiment & Assimilation Mode):
  • 2 indicates sequential multi-variate data assimilation implemented using the COARE3.5 bulk formulation.
• Z (Temporal Averaging/Sampling Interval):
  • mn indicates calendar monthly average context fields.
  • 5dy indicates 5-day average frames.
  • daily indicates daily mean resolution fields (available for specialized surface and transport segments).
• V (Variable Set & Vertical Resolution Scheme):
  • ocean indicates 3D high-resolution fields of temperature, salinity, and velocity (75 vertical levels), alongside standard 2D diagnostics.
  • isopycn indicates fields remapped onto constant potential density coordinates.
  • ice specifies sea ice-related surface fields generated via the SIS1 model component.
  • transp contains 3D vector volume transport metrics across key hydrographic pathways.
• G (Horizontal Coordinate Projection Mapping):
  • reg indicates reanalysis outputs uniformly gridded and interpolated onto a high-resolution 1/10°x1/10° geographical coordinate footprint.
  • or indicates data provided directly on the native, isotropic tripolar mesoscale grid array.
• YYYY (Date & Time Stamp Formats):
  • year (4 digits) for unified monthly archive packages.
  • year_mo_dy (8 digits) for high-frequency daily or pentad files.

Native and Remapped Grids

SODA4 represents a major advancement in spatial configuration, transitioning to a global eddy-resolving mesoscale structure. The underlying numerical model utilizes the GFDL MOM5.1 ocean / SIS1 sea ice numerics. The native horizontal layout leverages a tripolar isotropic coordinate matrix consisting of a 3600x2700 grid point structure with a nominal 0.1°x0.1° (1/10°) horizontal resolution.

A comprehensive grid topography baseline layout embedding precise partial bottom cell configurations is maintained across our repositories, facilitating highly exact calculations of boundary flow structures and topographic steering currents.

The vertical grid structure features an upgraded allocation of 75 vertical layers, using a z* vertical coordinate scheme (increasing downward). Levels are densely distributed near the surface (~1.1 meter separation) to accurately capture summer stratified heating, river plume discharges, and ice-melt boundaries, telescoping out to a maximum of 200m thickness layers within the deep abyssal plains.

Thermodynamic variables are explicitly evaluated at the 75 layer midpoint centers. The complete vertical level midpoint map (meters) can be found in the accompanying technical metadata manifests.

Regridded 1/10° Ocean Data Structure

Remapping from native configurations onto the uniform 1/10°x1/10°x75lev Mercator coordinate structure is executed via the GFDL Flexible Modeling System (FMS) engine. Conservative remapping is used for thermodynamic variables (Temperature and Salinity) while nonconservative mapping is used for dynamic variables (Velocity).

3D Ocean Tracer & Flow Dynamics

• temp: Potential Temperature [°C].
• salt: Practical Salinity [psu] constrained via the World Ocean Database (WOD23) profile indices.
• u, v, w: Horizontal and vertical components of mesoscale flow velocity [m/s].
• prho: Potential Density values evaluated via TEOS-10 equations [kg/m³].

2D Ocean Surface Parameters

• ssh: dynamic Sea surface height [m]. Evaluated via complete mass conservation, tracking steric height alterations and sea level variations.
• mlt, mlp, mls: Mixed layer depth metrics computed dynamically.
• taux, tauy: Wind stress components tracking integrated relative ocean motion feedbacks [N/m²].

2D Sea Ice Diagnostics (SIS1 Engine)

• hi: Sea ice vertical thickness [m ice], constrained using satellite and in-situ thickness estimations.
• mi: Sea ice integrated mass density [kg/m²].
• hs: Snow accumulation thickness layers [m snow].
• aice: Total aggregate fractional ice concentration [0 to 1].

3D Ocean Mass Transport Systems

• mx, my: High-resolution volumetric horizontal vector mass transport across discrete grid cell boundaries [10^9 kg/s], critical for cross-sectional passage transport checks.

Citations & Support Reference

When deploying this SODA4 mesoscale reanalysis configuration within formal research contexts or manuscripts, please cite the definitive preprint validation reference detailing the architecture layout:

Chepurin, G. A., Carton, J. A., Sun, L., and Penny, S. G. (2025): SODA4: a mesoscale ocean/sea ice reanalysis 1980–2024, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-3810