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Eulerian and Lagrangian near-surface velocity and sea surface height from one year of the global HYbrid Coordinate Ocean Model (HYCOM)

This is a description of the dataset contained in the publicly available AWS S3 bucket "hycom-global-drifters".

This bucket contains two distinct and related datasets:

  1. The dataset in hycom-global-drifters/eulerian/ consists of Eulerian (fixed grid) field variables which were generated from a 1-year simulation of the HYCOM model as described in Arbic et al. 2022. The S3 bucket described here provides these data as cloud-optimized zarr directory stores.

  2. The dataset in hycom-global-drifters/lagrangian/ consists of Lagrangian particle trajectories advected in the velocity fields of the model at two depth (0 m and 15 m) and are not available elsewhere.

A manuscript providing further details on how the Lagrangian dataset was produced with the Ocean Parcels software is in preparation. Once submitted this manuscript will be referenced here. In the meantime, for further information not available in this README file, please email Dr. Shane Elipot at selipot@miami.edu.

1. hycom-global-drifters/eulerian/

The data correspond to field variables at 8759 hourly time steps from 2014-01-01T01:00:00 to 2014-12-31T23:00:00. These data are split in 12 zarr stores containing 720 hourly steps or 60 days of data, for stores number 1 to 11, and a 12th store with 839 steps to complete the year. The data are further split between velocity data and sea surface height data for a total of 24 stores. The date range and steps of each of the two sets of 12 stores are:

Stores First Step Datetime Last Step Datetime Number of Steps Indices (1-based)
1 2014-01-01T01:00:00 2014-01-31T01:00:00 720 1 to 720
2 2014-01-31T01:00:00 2014-03-02T01:00:00 720 721 to 1440
3 2014-03-02T01:00:00 2014-04-01T01:00:00 720 1441 to 2160
4 2014-04-01T01:00:00 2014-05-01T01:00:00 720 2161 to 2880
5 2014-05-01T01:00:00 2014-05-31T01:00:00 720 2881 to 3600
6 2014-05-31T01:00:00 2014-06-30T01:00:00 720 3601 to 4320
7 2014-06-30T01:00:00 2014-07-30T01:00:00 720 4321 to 5040
8 2014-07-30T01:00:00 2014-08-29T01:00:00 720 5041 to 5760
9 2014-08-29T01:00:00 2014-09-28T01:00:00 720 5761 to 6480
10 2014-09-28T01:00:00 2014-10-28T01:00:00 720 6481 to 7200
11 2014-10-28T01:00:00 2014-11-27T01:00:00 720 7201 to 7920
12 2014-11-27T01:00:00 2014-12-31T23:00:00 839 7921 to 8759

Velocity data are contained in zarr stores hycom12-x-rechunked-corr.zarr with x from 1 to 12 and organized as follows (in the xarray Dataset sense).

  • The dimensions and their lengths are:

    • Depth: 2,
    • Y: 7055,
    • X: 9000,
    • time: 720 or 839.

X and Y are the non-regular tri-pole grid nodes on the spherical Earth used by HYCOM (see Arbic et al. 2022).

  • The coordinates with dimensions of the same names and their units are:
    • Depth (Depth) : depth 0.0 or 15.0, m,
    • X (X), 1 to 9000, no unit,
    • Y (Y), 1 to 7055, no unit,
    • time (time): 0 to 719 (or 838), hours since first step.

Please note that the units of time are hours since the first step of the set/store.

  • The Data variables and their dimensions are:
    • Latitude (Y, X) : degree north,
    • Longitude (Y, X) : degree east,
    • u (time, Depth, Y, X): eastward_sea_water_velocity, m s-1,
    • v (time, Depth, Y, X): northward_sea_water_velocity, m s-1.

The zarr chunks for the core variables u and v are (720, 1, 1, 9000) such as the data are optimized for analysis along the time dimension (and X dimension).

Note about model velocity depth: We reproduce here an excerpt from Arbic et al. 2022 pertaining to the model depths :

The global HYCOM simulation employs nominal 1/25° horizontal grid spacing and 41 hybrid layers (Bleck, 2002) covering the vertical direction. HYCOM employs terrain-following coordinates in shallow waters and isopycnal coordinates in the subsurface open ocean. In the near-surface open ocean, the uppermost 14 layers are in z-mode, with seven z-levels, having spacing ranging from 1.00 to 6.87 m, in the uppermost 30 m (Figure 1). The HYCOM “0 m” (surface) results actually represent the midpoint of the uppermost layer. The HYCOM “15 m” results represent interpolations to 15 m. The interpolation largely reflects results from the HYCOM level at 13.185 m, which lies in between adjacent levels at 8.38 and 18.55 m. The latter three depth values represent middepth points of the respective vertical layers in which they lie.

Sea surface height data are contained in stores hycom12-ssh-x-rechunked-corr.zarr with x from 1 to 12. These stores have the same dimensions and coordinates as the velocity data stores except for the coordinate and dimension Depth (Depth). Their Data variables are:

  • ssh (time, Y, X): sea surface height, m,
  • steric_ssh (time, Y, X): steric change in sea surface height, m.

The ssh is the sum of the steric ssh and the non steric ssh which can be calculated as nonsteric_ssh = ssh - steric_ssh.

The bathymetry of the model is also provided in the file hycom12-bathy.zarr. The variables bathymetry, Latitude, and Longitude all have dimensions Y: 7055 and X: 9000.

Please note that these Eulerian model data are alternatively available as NetCDF files through the OSiRIS/Globus infrastructure at OSiRIS (2024), Non-assim HYCOM 1/25 SSH and Surface velocities for 2014.

2. hycom-global-drifters/lagrangian/

The Lagrangian dataset is constituted of 22 zarr stores. There are 11 stores for numerical particles advected at the surface of the model (0 m) in stores global_hycom_0m_step_x.zarr with x from 1 to 11, and 11 stores for numerical particles advected at 15 m depth in stores global_hycom_15m_step_x.zarr with x from 1 to 11. Each zarr store contains either 593,292 (0 m) or 587,225 (15 m) particle trajectories at 1440 hourly time steps or 60 days.

The first (step 1), middle (step 720), and end (step 1440) datetimes for the two 11 sets of trajectories are:

Set/Store First Step Middle Step End Step
1 2014-01-01 01:00:00 2014-01-31 01:00:00 2014-03-02 00:00:00
2 2014-01-31 01:00:00 2014-03-02 01:00:00 2014-04-01 00:00:00
3 2014-03-02 01:00:00 2014-04-01 01:00:00 2014-05-01 00:00:00
4 2014-04-01 01:00:00 2014-05-01 01:00:00 2014-05-31 00:00:00
5 2014-05-01 01:00:00 2014-05-31 01:00:00 2014-06-30 00:00:00
6 2014-05-31 01:00:00 2014-06-30 01:00:00 2014-07-30 00:00:00
7 2014-06-30 01:00:00 2014-07-30 01:00:00 2014-08-29 00:00:00
8 2014-07-30 01:00:00 2014-08-29 01:00:00 2014-09-28 00:00:00
9 2014-08-29 01:00:00 2014-09-28 01:00:00 2014-10-28 00:00:00
10 2014-09-28 01:00:00 2014-10-28 01:00:00 2014-11-27 00:00:00
11 2014-10-28 01:00:00 2014-11-27 01:00:00 2014-12-27 00:00:00

For all Lagrangian sets/stores:

  • The Dimensions are:

    • traj (593292 or 587225),
    • obs (1440).

  • The Coordinates and their dimensions are:

    • id (traj),
    • obs (obs), 0 to 1439.

  • The Data Variables and their dimensions and units are:

    • depth (traj,obs): model depth, m,
    • grounding (traj,obs): a boolean variable which is True if the particle is estimated grounded,
    • lat (traj,obs): latitude, degree north
    • lon (traj,obs): longitude, degree east
    • ssh (traj,obs): sea surface height, m,
    • steric_ssh (traj,obs): steric change in sea surface height, m,
    • time (traj,obs): hours since first step of each set,
    • ve (traj,obs): eastward particle velocity, m s-1
    • vn (traj,obs): northward particle velocity, m s-1.

Note that grounding of particles may occur both at the beginnings and at the ends of trajectories because these trajectories were generated by advecting particle forward in time and backward in time from the middle time step.

The zarr chunks for the data variables are (23301, 1440) except for grounding (93206, 1440) and time (9176, 45). As such, the data are generally optimized for analysis along the obs dimension, that is along trajectories (or equivalently time).

The coordinate variable id is a tagging number assigned to a particle by the Ocean Parcels software. The same id numbers are all found in each of the 11 sets at 0 m, and the same is true for the 11 sets at 15 m. There are 587,225 common ids between a set at 0 m and a set at 15 m. Across all sets, a unique id is associated with a particle located at a given geographical position at middle time step 720. As an example, all particles with id 105763 are located at (lon,lat) = (-134.5, -64.75) at time step 720 of their respective 1440-step trajectories. This has been achieved by assembling trajectories of particle advected forward in time and backward in time from a common time, starting from a common location on a regular 1/4 degree longitude-latitude grid.