This document describes the HydrOffice Sound Speed Library and Manager (2019.2). For the source code, go here.

2.5. Appendix A - Oceanographic Atlases

2.5.1. World Ocean Atlas

The World Ocean Atlas is a 3-dimensional grid of mean temperature and salinity for the world’s oceans that is based upon a large set of archived oceanographic measurements in the World Ocean Database.

More information about the World Ocean Atlas 2009 (WOA09) can be found online

The WOA09 netCDF temperature and salinity grids used by the package can be accessed from

The files required are:







Basin and land/sea masks can be downloaded from:

2.5.2. Global Real-Time Ocean Forecast System

The Global Real-Time Ocean Forecast System (RTOFS Global) is a 1/12°, 3-D oceanographic forecast model. More information can be found online at:

Daily forecast/nowcast grids can be downloaded via the URL listed above, but the file sizes for the daily forecast are prohibitive for use at sea. Instead, the package relies on the OpenDAP portal to download only small segments of the nowcast grids for surrounding a specified query location. The downloaded subset is a 5x5 grid centered on the query location.

2.5.3. Regional Operational Forecast System

The NOAA Operational Forecast System (RegOFS) is a network of operational nowcast/forecast hydrodynamic models covering a number of coastal regions in the USA. Models are run 4 times a day and generate current and short-term (0 to 48 hour) forecast predictions of pertinent parameters such as water levels, currents, temperature, and salinity.

More information about RegOFS models can be found at:

Daily forecast/nowcast grids can be downloaded via the URL:

Similar to the RTOFS Global atlas, the package relies on the OpenDAP portal to download only small segments of the nowcast grids surrounding a specified query location.

Currently the package supports a limited number of the RegOFS models. The full list of models with related support is provided in the table below.

Regional Operational Forecast System Model



Chesapeake Bay



Deleware Bay



Gulf of Maine



New York and New Jersey


St. John’s River


Northern Gulf of Mexico



Tampa Bay



Lake Erie



Lake Huron


Lake Michigan


Lake Ontario


Lake Superior


Columbia River Estuary



Sanfrancisco Bay



2.5.4. Synthetic cast values derived from atlases

The cast extrapolation algorithm vertically extends temperature and salinity profiles as deep as possible using the estimates immediately local to the area of the cast in either WOA or OFS. WOA09-based profiles

The World Ocean Atlas 2009 (WOA09) extension algorithm uses a nearest neighbor lookup in each of the 33 depth levels in the grids within a 3x3 grid node search box centered on the cast’s geographic position. This is roughly equivalent to a search radius of 1.5° or 90 nmi at the equator. Note that this grid node search box becomes rapidly narrower in the east-west direction with latitude. The nearest-neighbor geodetic distance is, however, correctly computed and the nearest neighbor will indeed be the geographically most proximal grid node; the only shortcoming is that the lookup will ignore potentially closer data in the east-west direction at high latitudes.

Future updates to the WOA09 extraction algorithms will remedy this shortcoming. The search radius is set this large to enable the extension to at least estimate deeper temperature and salinity values in the case where the true depth at the requested location is significantly larger than the coarse depth reported in the WOA09 grid for that location (the WOA09 grid depth will generally always be smaller than the true depth).

The search algorithm will not respect topographic boundaries and may extrapolate profiles using data from a neighboring oceanographic basin. Future versions of the algorithm will address this shortcoming as well, likely with the use of the basin mask file provided with the WOA09 data set. WOA13-based profiles

WOA13 represents the ocean state variables of temperature and salinity with more detail and less uncertainty than WOA09 due to large increases in data holdings and better temporal and spatial coverage coupled with refined analysis and quality control techniques:

  • Increased vertical resolution (3x in the upper ocean, 2x below 1500 m.)

  • Increased spatial resolution (16x)

  • Release of the decadal climatologies which were used to calculate the final 1955-2012 long-term climatological mean fields.

In the specific, the package uses the WOA13v2 release that was prepared to address both methodology concerns and, to a lesser extent, quality control concerns which have surfaced since the initial release of WOA13. RTOFS-based profiles

The RTOFS extension algorithm differs in the size of the search area (5x5), roughly equivalent to a search radius of 0.2° or 12.5 nmi at the equator. All of the shortcomings of the WOA09 lookup described above also apply to the RTOFS lookup. RegOFS-based profiles

The Regional Operational Forecast System Models follow an approach similar to the RTOFS-based profiles. However, the resulting search radius is function of the regular grid resolution at which each model output is published. Profile Finalization

The final extrapolation to a depth of 12,000 m is done using the values measured by (Taira et al., 2005) in Challenger Deep. This could be improved by searching for the nearest neighbor grid node at the deepest level observed in the basin using the basin mask file.