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Tidal Streams

StormGeo Adds Tidal Streams into HYCOM Ocean Current Data

StormGeo Shipping Insights, April 2014
By Stephen Santilena, Data Manager & Weather Analyst

One of the many tools that StormGeo uses to evaluate and optimize ship performance is ocean current data provided by the National Oceanic and Atmospheric Administration (NOAA) Real Time Ocean Forecast System (RTOFS). This ocean forecast system uses the Hybrid Coordinate Ocean Model (HYCOM) to calculate the ocean circulation.

Since its inception, HYCOM has not incorporated the effect of tides on its ocean circulation output. Over the open ocean, the tidal influence is minimal. However, near many straits, coastal channels and ports, the tides can be the dominant force on the ocean circulation. Neglecting the tidal influence in these areas would not give a complete representation of conditions that may affect a ship's performance.

Recognizing this fact, StormGeo incorporates tidal streams into the HYCOM ocean current data. StormGeo Shipping's full tidal model consists of 22 separate domains of varying resolution that can be used to calculate the tidal current and tidal height at any water point in the world. We incorporate output from all of these domains to produce a global 1/12 x 1/12 degree resolution data set. We then add this global tidal data set to the modeled HYCOM ocean circulation data. The resulting output is a global ocean current data set that shows the large-scale circulation as well as the tidal influences in smaller coastal regions. Having this tidal data helps in predicting strong, short-lived currents that could affect a ship's performance when transiting in and around areas where the tidal influences are strong.

The figures in this article show how the inclusion of the tides can significantly affect the current data in certain areas.

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Figure 1: Plot showing the unedited HYCOM data.

Figure 1 above shows the unedited HYCOM data in and around the Bay of Fundy, valid December 25 at 12:00 UTC. As you can see, there is no significant current present except for a few small eddies that have spun off the Gulf Stream.

However, Figure 2 below shows the HYCOM data with the tides included for the same time. It can be seen that around 45N 65.5W, the new data shows currents between 2.0 and 2.5 knots; currents of this magnitude do not exist for the same area in Figure 1. These strong currents are a direct result of the tidal influence and they have a direct impact on ship performance.

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Figure 2: Plot showing the HYCOM current data with the tidal data added.

Ocean current data that include these tidal influences are used in all StormGeo products that currently use the RTOFS, including all of StormGeo's ocean voyage simulations that predict vessel speed and consumption.

We expect that future versions of HYCOM will eventually include the tidal influence in the model output. Until then, StormGeo will continue to supplement the HYCOM model output with our calculated tidal streams in order to provide our clients with the most comprehensive ocean circulation information available.