RTOFS

From the Navy's HYCOM to NOAA's daily ocean

NOAA's National Centers for Environmental Prediction (NCEP), which run the GFS weather model, started forecasting the ocean in December 2005 with an Atlantic RTOFS, covering the North Atlantic basin and US coastal waters, the Gulf Stream and the Gulf of Mexico. Its global successor, RTOFS Global, went operational on 25 October 2011: NOAA's first global eddy-resolving ocean forecast system, built in close collaboration with the US Navy. In 2016 NCEP proposed to retire the Atlantic system, by then superseded by the global one.

Its ocean model, HYCOM (HYbrid Coordinate Ocean Model), grew out of isopycnic models such as the Miami Isopycnic Coordinate Ocean Model (MICOM), on foundations laid by Rainer Bleck, and was developed by the University of Miami with the Naval Research Laboratory. The Navy used it in its own Global Ocean Forecasting System (GOFS 3.1 — 41-layer HYCOM with NCODA analyses at 1/12°). RTOFS version 1.0 restarted every day from an analysis made by the Naval Oceanographic Office with NCODA, and NCEP converted the model to its own atmospheric forcing, from GDAS and GFS.

On 17 October 2015 version 1.1 raised the layers from 32 to 41, nine of them added near the surface, coupled HYCOM with the Los Alamos CICE sea ice model, improved the bathymetry down to a minimum depth of 5 m and moved to a 17-term equation of state. Version 2.0, operational in December 2020, brought the daily analysis home to NCEP's Environmental Modeling Center with its own system, RTOFS-DA. Then came bias corrections near the equator and new satellites (v2.3, August 2022), stability fixes (v2.4, September 2024) and, on 29 July 2025, v2.5, which constrains the assimilation of sea level and assimilates current measurements from drifting buoys and HF radars.

The next step was announced in March 2026: RTOFS v3.0 would replace HYCOM with MOM6 (Modular Ocean Model) and CICE 4 with CICE 6, the components of NOAA's Unified Forecast System. As of October 2026 the operational feed is still v2.5, the HYCOM one described here.

HYCOM: an ocean of three kinds of layer

HYCOM solves the primitive equations of a hydrostatic ocean, like other circulation models; what sets it apart is its vertical coordinate. In the open, stratified ocean its layers follow surfaces of equal density (isopycnic layers), which keeps water masses from blurring over time; in the surface mixed layer they turn into levels at fixed depth; and over shallow shelves they follow the sea floor (terrain-following, or sigma, layers). Each layer has a reference density and the model keeps moving its interfaces back towards it, unless that would crowd them together.

Global RTOFS has 41 such hybrid layers, the fixed-depth ones mostly in the top 200 m, on a 1/12° tripolar grid of 4500 × 3298 points: a Mercator grid south of 47°N and, north of it, an Arctic patch with two poles, so that no point lies on the geographic pole. At about 9 km the model is called eddy-resolving: the ocean's eddies and the meanders of the Gulf Stream or the Kuroshio are computed, not averaged away.

The ocean is coupled every hour with CICE 4, the Los Alamos sea ice model, and driven by NCEP's atmosphere: GDAS analyses for the past, then the GFS forecast's wind stress, radiation and precipitation for the 8 days ahead. The operational system has no tide, even though the v2.0 code added a tidal forcing for future use, and it is not coupled with a wave model, so its current carries no Stokes drift, the net forward motion of water under passing waves.

RTOFS-DA and the daily cycle

Each day RTOFS-DA gathers sea surface temperature from satellites (METOP-B, VIIRS) and from ships and buoys, sea surface salinity from the SMAP and SMOS satellites and from buoys, temperature and salinity profiles (Argo and Alamo floats, gliders, CTD casts, expendable probes, moored buoys and even sensors carried by marine animals), the sea surface height of the altimetry satellites (AltiKa, CryoSat, Jason-3, Sentinel-3A, 3B and 6A) and sea ice concentration from SSMIS and AMSR2. Since v2.5 it also takes the currents measured by drifting buoys and coastal HF radars. Every observation goes through automatic quality control and thinning first.

RTOFS-DA is a multivariate, multi-scale 3DVar, built on the Navy's NCODA (Navy Coupled Ocean Data Assimilation). A 3DVar looks for the ocean state that best fits both the previous day's 24-hour forecast, used as a first guess, and the observations, each weighted by its expected error, at a single analysis time rather than over a window as a 4D-Var would. Away from the equator, the density corrections are given currents in geostrophic balance, the balance between pressure and the Earth's rotation; since 2022 a bias correction tames the spurious waves this initialisation used to start at the equator.

The cycle is daily, from 00 UTC: the corrections are blended into the model gradually (incremental analysis update) rather than all at once, then the forecast runs to 8 days, its first four days and its last four in two steps of the day. The outputs, hourly surface fields and 6-hourly three-dimensional ones, in GRIB2 and NetCDF, are published on NOAA's NOMADS servers and on NOAAPort, and mirrored in NOAA's open data on the cloud.

Like everything the National Weather Service publishes, RTOFS is in the US public domain: free, reusable for any purpose, on the sole condition of not presenting it as one's own nor implying NOAA's endorsement. Data that have been processed, regridded for instance, must not be passed off as original NOAA data.

At sea

RTOFS was born around the Gulf Stream: the Atlantic system already gave guidance on it, and NCEP's evaluation of the global system checked the stream's position and the flow measured by the Florida cable across the Florida Straits. In the open ocean it draws the big currents and their eddies wherever altimetry and floats pin them down; these features move over days rather than hours, which is what makes an 8-day forecast of them worth reading.

Its blind spot for a sailor is the tide. On continental shelves and in straits where tidal streams dominate, the English Channel, the Gulf of Maine or the Bay of Fundy, RTOFS only gives the non-tidal current, so the real stream can run against it for half of each tide. There it is best read for the residual drift, with tidal atlases, or SMOC, which adds the tide, beside it.

Without Stokes drift, it also leaves out the downwind push of the waves that SMOC adds in a blow. Next to SMOC, RTOFS is a second opinion from an independent system, another model, another assimilation, other winds: in the open ocean, where the two agree on a current, it is worth trusting; where they place an eddy differently, the current there is uncertain. Near the coast, most of their difference is simply the tide.

RTOFS in Saily WR

RTOFS is one of the two currents a sailor can choose in Saily WR, beside SMOC or no current at all; the chosen current is added to the boat's speed at each point and hour, on the route of every wind model, so that they stay comparable side by side. The app takes RTOFS's surface current, without tide nor Stokes drift, from its one run a day at 00 UTC: hourly to H+72, then every 3 hours to H+192, 8 days ahead. NOAA publishes each run in two parts, and the app takes each as soon as it is out: the first 72 hours reach the server about 09:30 UTC, the rest about 15:30 UTC; in between, past H+72, a route goes on in the run of the day before, which the server keeps, up to its horizon: from one run to the other, the current is interpolated in time as between any two steps.

Its tripolar grid, of about 1/12°, is mapped onto a regular 1/12° grid: each point of that grid takes the value of the nearest RTOFS point within 15 km. A server downloads each new run as soon as it is published, and the Data page shows live which run RTOFS is on, its horizon and its coverage.

Computing the same offshore trip with RTOFS, then with SMOC, shows how much a route hangs on the ocean analysis: if both currents send it the same way, the choice is sound; if the routes split around an eddy, that eddy deserves a look on the water. Near the coast, a gap between the two is first of all the tide, which SMOC adds and RTOFS leaves out.