WOA18 Decav Sound Velocity Datasets (Annual, Seasonal and Monthly) 1/4 degree resolution

This dataset contains derived climatological seawater sound-speed fields based on the World Ocean Atlas 2018 (WOA18), NOAA NCEI’s collection of objectively analysed oceanographic climatologies derived from quality-controlled World Ocean Database 2018 observations. The decav reference represents 1955–2017, combining six climatological periods: 1955–1964, 1965–1974, 1975–1984, 1985–1994, 1995–2004 and 2005–2017. It is an average of the period climatologies, not a pooled average of all observations. Resolution code 04 denotes a 0.25° latitude × 0.25° longitude grid, equivalent to 15 angular minutes, not one-quarter of an angular minute.

Temporal identifiers distinguish annual climatology (00); monthly climatologies (01 January, 02 February, 03  March, 04 April, 05 May, 06 June, 07 July, 08 August, 09 September, 10 October, 11 November and 12 December); and seasonal climatologies (13 January–March, 14 April–June, 15 July–September and 16 October–December). The seasonal names winter, spring, summer and autumn refer to the Northern Hemisphere. Annual and seasonal source temperature and salinity products provide 102 standard depth levels spanning 0–5500 m, whereas monthly products provide 57 levels spanning 0–1500 m. The depth limits describe the nominal vertical grid, not complete valid profiles at every location; source masks distinguish land and the ocean bottom.

Sound-speed fields for the WOA18  decav , resolution  04  collection were derived using the Python CalculateSvp class from the objectively analysed temperature ( t_an ) and salinity ( s_an ) fields at the selected source grid cells and standard depths. Two separate estimates are produced:  v_an_chen , calculated using the implemented Chen–Millero/UNESCO-type temperature–salinity–pressure polynomial, and  v_an_leroy , calculated using the Leroy, Robinson and Goldsmith (2008) temperature–salinity–depth–latitude equation; both yield sound speed in metres per second. For the former, pressure is estimated from depth and latitude using Leroy and Parthiot (1998), with an optional configured regional correction, and converted from MPa to bar before evaluation. The class applies these calculations directly to the climatological inputs without additional spatial or vertical interpolation, extrapolation or missing-value filling. The results are derived climatological sound-speed estimates rather than direct acoustic measurements.

The files in this dataset are used with these MB-System tools: mbsvpbest, mbsvproute, mbsvplocation, and mbsvpnearest. They are available here:

https://github.com/dwcaress/MB-System

References:

https://www.ncei.noaa.gov/sites/default/files/2022-06/woa18documentation.pdf

https://www.ncei.noaa.gov/access/world-ocean-atlas-2018/

https://www.ncei.noaa.gov/access/world-ocean-atlas-2023/