Notes on Ocean Currents (starpath.com/currents)
...always a work in progress brought to you by Starpath School of Navigation.
Here is the main eye-opener to modern knowledge of currents. It is a fantastic video from NASA called Perpetual Ocean. We reference here the primary source link. It has been placed on YouTube by many folks, but the resolution is compromised in many and most omit the full credits. Click the picture below to go to the NASA source. You can also download a hi-res copy of the video from that link. Watch the dates change in the video to get a rough estimate of time scale. These are model output, but still a good look at what we must deal with in the ocean.
The key word here is mesoscale eddy. They are all over the ocean. They can pass by you in half a day, or they can last for a month. Peak currents in these eddies can reach 5 kts. Two to three knots is not uncommon. Read about mesoscale eddies at the excellent and extensive AVISO web site. (You will have a hard time finding it on their web site, but AVISO stands for Archiving, Validation and Interpretation of Satellite Oceanographic data.)
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The live current presention shown below is not in place any longer, but we leave it here to show what can be done. It is a state of the art presentation of live ocean current predictions and vessel positions that is possible with modern techniques. When functional, you would click the pic for latest live data. This is the fine work of Angeline Pendergrass, then at UW Atmospheric Sciences Dept, now at UCAR in Bolder, CO. This was a Python project. We were routing a row boat across the tropical Atlantic. Here is the view as they approached Cape Verde Islands having departed from Dakar, Senegal on the way to Miami. Each pic showed the vessel position at present time and at the 24h forecast time. Unfortunately, we found that the RTOFS model at that time did not do a very good job in forecasting the details of the currents we needed. We also had very accurate current measurements as we monitored the GPS position, along with COG and SOG every hour.
There are better ways now, but back then we got the data from NCEP Central Operations and then we analyzed it with a remarkable free program called Panoply (there is a learning curve, but tutorials are online). There are other public programs that will read netcdf data, which is needed for this exercise. The data come as N-S (called U) and E-W (called V) components of the current velocities, which must be added as vectors to make the current vectors. This program is valuable for essentially all model and measurment data display. ( We have sense added a couple videos and articles on how to display currents using Panoply and how to convert the output to KML files to view in Google earth. See below. )
During that period we also compared several ocean models available at the time (2013). The results at that time were not very encouraging. This is described here: A comparison of ocean current models. RTOFS is based on GOFS and that was just udated (mid 2017) to v3.1, which shows notable improvements, so all of this eary work is outdated. We did do a recent study comparing RTOFS and NCOM predictions with HF Radar measurements, which had mixed results.
PS. We also routed this same rowboat across the North Atlantic in 2006 from NYC to Falmouth, England. For that work we used the Navy maps (below) augmented by SST data from Johns Hopkins. The models were not much use in those days, and we had no grib versions to work with. This venture deserves a special write up as we were successful in keeping the boat in favorable water and learned a couple tricks. They won the first (and last) Great North Atlantic Rowing Race, setting a new record in the Guinness Book for the first row across the Atlantic that actually arrived at the intended destination. Earlier records just claimed victory crossing a specific longitude, even if they were in the wrong country.
* * * VARIOUS OCEAN AND COASTAL CURRENT REFERENCES, IN NO PARTICULAR ORDER * * *
Keeping Current with Ocean Currents (an overview and update, which ends up coming back to this page!)
For discussion of global currents see Chapter 36 of the latest (2017) Bowditch... or maybe better still find an older version to supplement this one. The coverage seems to be going down. This It has an excellent chart of major current flow patterns and short discussions of each system. PS, as of some point in 2018, NGA discontinued chapter downloads so you need to get the full 109 MB book.
A very informative link on all ocean currents at RSMAS Miami (Please sign their Guest Book if you like their work).
Six-day view of tropical Atlantic in low resolution (Scrolling the pics shows how the mesoscale eddies evolve and move around.)
HF Radar currents
See also this SUPER NEAT PRESENTATION of Global Hycom Current forecasts from SOEST HI. Scroll the left panel to see all the many options there. You can compare their ROMS model predictions with HF-Radar measurements. See this video on how to get to the HYCOM current forecasts.
• RTOFS currents in GRIB format are avilable from Saildocs and Zygrib, and several other sources that offer direct download. (A new version of Zygrib called XyGrib is imrpoved and well maintained, but it does not as of Jan 10, 2019, have direct links to ocean currents. Get RTOFS in GRIB format direct from NOMADS. Get RTOFS in netcdf (.nc) format to open in Panoply and make exports to Google Earth. The data are issued daily starting with h12 and then every 24h to h108. That is 5 forecasts, 24h apart starting on 12z.
• NCOM is a regional Navy model and considered superior to RTOFS where it is available. NCOM is offered by LuckGrib and there is a direct link to each of the NCOM regions from within Expedition under MyGribs section. You can also get NCOM data by full region direct from NOMADS. These grb2 files can be viewed in OpenCPN. They are about 29 MB.
• OSCAR currents is GRIB format are available from Saildocs. OSCAR are not forecasts, but averages of observed data related to current flow over the past 10 days, issued every 5 days. These valuable current summaries are developed here in Seattle by Kathleen Dohan. Samples are in our comparison link above. We have found that these OSCAR averages are the best way to plan a route across the Equitorial currents and countercurrents. Here is a video note on how to download and view global OSCAR currents for any date. We use Panoply and Google Earth, but these files can be obtained from saildocs. Our text Modern Marine Weather has an extended section on how to obtain various ocean weather relatated products from saildocs as well as from FTPmail.
OSCAR currents in netcdf (.nc) format. This data will take Panoply to read.
• Global HYCOM currents in GRIB format are available via Ocens WeatherNet
US Navy NCOM (note special definition of W Lon = 360 - Lon), sample below
OpenDAC software For requesting and viewing ocean data (have not been able to make any of these work yet!).
Ocean Model Current Areas
The NCEP Global Real-Time Ocean Forecast System (RTOFS-Global) data is based on the U.S. Navy implementation of Global HYCOM. Regional NCOM data is provided by the Naval Oceanographic Office (NAVO).
(Mouseover and click the desired area to view the valid Global Ocean Model Currents 0-72 hour forecast data)
For NCOM Regional Data via ftp: http://ftp.opc.ncep.noaa.gov/grids/operational/NCOM/regional/
For archived NCOM Data: http://www.ncddc.noaa.gov/ocean-nomads/global-ncom/
For Data via Nomads GDS server: http://nomads.ncep.noaa.gov:9090/dods/ncom
For East Coast images: http://ftp.opc.ncep.noaa.gov/grids/operational/NCOM/images/
For RTOFS Global Data via ftp: http://ftp.opc.ncep.noaa.gov/grids/operational/GLOBALHYCOM/
RTOFS GLOBAL MODEL INFORMATION
The Global Real-Time Ocean Forecast System is based on NAVO's configuration of the 1/12 deg. eddy resolving global HYCOM model and is initialized daily with NAVO generated initial conditions using the NCODA system. RTOFS Global is forced with NCEP's operational GFS winds. For more details see:NOAA/NCEP/MMAB RTOFS Global Web Page
REGIONAL NCOM MODEL INFORMATION
The Regional NCOM models have a resolution of 1/36 deg. (3 km). The U.S. Navy Operational Global Ocean Model (NCOM), developed by the Naval Research Laboratory (Barron et al, 1&2) and maintained by the Naval Oceanographic Office, provides boundary conditions for the regional models. The regional NCOM models found here include the U.S. East Coast (ncom_useast), The Southern California Coast (ncom_socal), Hawaii Coasts (ncom_relo_hawaii) and the Gulf of Mexico and Caribbean Seas (ncom_relo_amseas). The Navy Coupled Ocean Data Assimilation System (NCODA) provides the data assimilation for NCOM including SSH, SST, and in situ observations. For more information about NCOM see:
New Section on Gulf Stream, in progress...
See super nice presentation by Frank Bohlen at the bermudarace.com site. Includes actual analysis of past races across the current.
NAVO color images. THE STARTING POINT (in our approach) FOR GULF STREAM ANALYSIS: North part color graphic (BW digital) and the South part and GOM color graphic (BW digital ) Save these links they can be hard to find; for a while they were at the OPC, then as of Jul. 2018 they were gone. Compare these with the OPC data: Inshore limit of the southern Gulf Stream
* * * COMPARE RTOFS WITH NCOM (both of these are available in grib format)
Our note on Gulf Stream Sources off the Coast of Florida
Video on importing the Navy Gulf Stream boundaries into echart program as a gpx file.
Sites on the Web for Sea Surface Temperature (SST) and Gulf Stream information.