Work Description

Date: 2nd August, 2023

Dataset Title: Surface and sub-subsurface internal gravity wave kinetic energy spectra in global ocean models and observations

Dataset Creators: Joseph.K. Ansong, Brian.K.Arbic

Dataset Contact: Joseph K. Ansong, jkansong@umich.edu (jkansong@ug.edu.gh), Brian K. Arbic, arbic@umich.edu

Funding: National Science Foundation (NSF, OCE-0968783), Office of Naval Research (ONR, N00014-11-1-0487 and N00014-15-1-2288),
ONR (N00014-19-1-2635), ONR (0602435N & N00014-11-1-2288), NASA (NNX13AD95G, NNX16AH79G, NNX17AH55G, & 80NSSC20K1135),
University of Michigan Associate Professor Support Fund, supported by the Margaret and Herman Sokol Faculty Awards

Key Points:

-Vertical wavenumber spectra of internal gravity wave kinetic energy in two high-resolution global models are compared to observed spectra.
-Both models under-estimate motions at high vertical wavenumbers (small vertical scales), flagging this as a target for model improvement.
-The ratio of high- vs low-frequency surface kinetic energy at small horizontal scales is dependent on the model and grid spacings employed.

Research Overview:

Recently, a small but growing number of global ocean models have begun to employ simultaneous tidal and atmospheric forcing.
At the same time, increasing supercomputer power has allowed for simulations of oceanic motions with increasing accuracy,
increasing feature (spatial) resolution, and more frequent time slices. Global ocean models with fine grid spacing,
and simultaneous tidal and atmospheric forcing, host a vigorous spectrum of high-frequency waves that control mixing over
most of the ocean water column, and are important for many operational oceanography challenges. As an example of the latter,
high-resolution global internal wave models have been used to study the relative balance of high-frequency vs. low-frequency motions
at the small horizontal scales that will be measured by the upcoming Surface Water Ocean Topography (SWOT) mission.
The balance described above depends on the model employed and the grid spacing employed within that model, meaning that
conclusions about the balance are dependent on the model used to estimate it. Comparisons between the models and vertically
profiling instruments indicate that resolving fine scale motions in the vertical direction, where ocean mixing takes place,
is not yet handled well by the models. Modeling of fine-vertical scale motions is therefore an important future research direction.

Methodology:

The data are model output from two global simulations of the HYbrid Coordinate Ocean Model (HYCOM) and
three global simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). The data are not global
in nature, but consists of seven cut-out oceanic regions from the global simulations. The regions are the North Pacific (NP),
Kuroshio (KU), Gulf Stream (GS), Equatorial Pacific (EQPAC), South Pacific (SP), South East Pacific (SEP), and Indian Ocean (IN).
The HYCOM simulations have horizontal resolutions of 1/12.5 degree (HYCOM12; path: HYCOM/exp_6.1/one12th) and
1/25 degree (HYCOM25; path: HYCOM/expt_10.2/one25th), and MITgcm has horizontal
resolutions of 1/12 degree (MITgcm12, path: MITgcm/one12th), 1/24 degree (MITgcm24, path: MITgcm/one24th) and
1/48 degree (MITgcm48, path: MITgcm/one48th).

For MITgcm/one24th, there is no data for the Indian Ocean.

Instrument and/or Software specifications: MATLAB 9.0 R2016a and later versions. Some earlier versions might work too.

Files contained here:

1. The folders show divisions based on the model simulation (HYCOM vs MITGCM), as well as the particular region
(NP, KU, GS, EQPAC, SP, SEP, IN).

-For each simulation and each region, the files/data contain an abbreviation of the simulation (e.g., MITgcm48, with path: MITgcm/one48th/),
the region (e.g., "NP" for North Pacific), the length of time series (e.g., "90days" for 90 days of hourly data), and the variable for which
we computed the horizontal kinetic energy (HKE) spectra (e.g., u for zonal velocity, and v for meridional velocity).

-For instance, the folder path MITgcm/one48th/NP contains the HKE spectra for the MITgcm48 simulation for the North Pacific (NP) region.

-For example, the MATLAB files of HKE from MITgcm48 (three-dimensional) for the North Pacific region constain the files:

u_spectra_klomegaMITgcm_48th_NP_90days.mat
v_spectra_klomegaMITgcm_48th_NP_90days.mat

-The 2D versions of the u and v files above (for the same simulation and region) are named as:

u_spectra_kisotropicomegaMITgcm_48th_NP_90days.mat
v_spectra_kisotropicomegaMITgcm_48th_NP_90days.mat

-The same folder MITgcm/one48th/NP contains the NetCDF file "NP_walls.nc" containing temperature and salinity profiles at the Southern
and Northern ends of the region.

- The same folder (MITGCM/one48th/NP) contains the computed normal modes at the Southern and Northern boundaries and named as
normalanalysis_high_and_lowlats.mat

-Similar files with this naming convention are found in the other folders for the other regions in MITgcm48:

MITgcm/one48th/KU
MITgcm/one48th/GS
MITgcm/one48th/IN
MITgcm/one48th/SEP
MITgcm/one48th/SP
MITgcm/one48th/EQPAC

-The folder structure for the regions in HYCOM is similar, but there are two intermediate subfolders. The paths are

HYCOM/exp_6.1/one12th/ for files/folders for HYCOM12
HYCOM/exp_10.2/one25th/ for files/folders for HYCOM25

2. The folder "KEspectra_integrated_freq_bands" contains computed KE spectra data that has been integrated
over frequency or over wavenumber from the model simulation. The mat file is a structure containing the simulations,
regions, and the field over which the integration is done.

For example:
data.HKEspectra_integrated
ans =
MITgcm12th: [1x1 struct]
MITgcm24th: [1x1 struct]
MITgcm48th: [1x1 struct]
HYCOM12th: [1x1 struct]
HYCOM25th: [1x1 struct]
data.HKEspectra_integrated.MITgcm12th
ans =
KU: [1x1 struct]
NP: [1x1 struct]
GS: [1x1 struct]
EQPAC: [1x1 struct]
SP: [1x1 struct]
SEP: [1x1 struct]
IND: [1x1 struct]

3. Matlab codes for plotting HKE data, and computing vertical normal modes can be found in
"matlab_codes_KE_DATA". It contains the folder:

- matlab: this folder has common matlab functions, e.g., paramaters for each simulation and region

- matlabstuff: this folder contains the folders:
(a) "plotting_KE_spectra_and_regions_on_globe" with scripts for plotting
spectra data.
(b) "VerticalModes_Joe" with scripts for calculationg vertical normal modes.
The file "calculate_vertical_modes.m" is an example script which uses the test data
"TestData.mat" to calculate three vertical modes. The other files are functions that
are called for this calculation.

Related publication(s):
Ansong, J.K., et al., "forthcoming", Surface and sub-subsurface internal gravity wave kinetic energy spectra in global ocean models and observations

Use and Access:
This data set is made available under a Creative Commons Public Domain license (CC0 1.0).

To Cite Data:
Ansong, J.K., Arbic, B.K., Nelson, A.D., Alford, M.H., Kunze, E., Menemenlis, D., Savage, A.C., Shriver, J.F., Wallcraft, A.J.,
Buijsman, M.C. "forthcoming", Surface and sub-subsurface internal gravity wave kinetic energy spectra in global ocean models and observations