And why should sailors care about it?
It’s generally understood that ocean wind waves form due to a transfer of energy from the wind to the water surface boundary.
What’s less well-known is that when wind blows over a water surface with a current flowing in the opposite direction, the waves generated will be taller and steeper than they would otherwise, with commensurate additional chance of becoming breaking waves.
This phenomenon is particularly demonstrated in areas of regular ocean currents, such as when a north or north-east wind blows over the Gulf Stream current:
“When ocean wind or swell waves encounter a current moving in the opposite direction, the response will be for wave speed and length to decrease, wave period will not change, but wave heights will increase, resulting in taller, steeper waves. In some cases, this can even lead to waves breaking, resulting in greater energy against hulls.”
“Crossing the Gulf Stream where ocean currents can exceed 2 knots takes some skill and if you try in under certain weather conditions, could be dangerous for any size vessel.”Fred Pickhardt, High Wind and Wave Events Crossing the Gulf Stream, Explained
Another prime example occurs at the Agulhas Current off South Africa:
“Abnormal waves of up to 20 meters in height, preceded by deep troughs may be encountered in the area between the edge of the continental shelf and twenty miles to seaward thereof.”Tony Herrick, Southern African Cruising Notes
One possible source of so-called “rogue waves” is suspected to be wind opposing current:
“As wind blows over the ocean, energy is transferred to the sea surface. When strong winds from a storm happen to blow in the opposing direction of the ocean current, the forces might be strong enough to randomly generate rogue waves.”Rogue wave (Wikipedia)
As a result of their steep and potentially breaking nature, the waves formed by wind opposing current can be uncomfortable or even dangerous for small vessels.
What Can Sailors Do to Help Avoid Encountering Wind Opposing Current?
- Study and understand local weather phenomena, in particular around strong regular ocean currents like the Gulf Stream.
- Pay special attention to the timing and wind when traversing channels, inlets, and bars. Avoid traversing when the tidal current will be opposing wind or swell.
- Check weather conditions before going out and while underway. It’s tedious and time consuming, but you can manually check forecast GRiBs (you need to look at both wind and current) and/or weather fax to try to discern by eyeball where wind may oppose current.
- Check the Weathermuffin app, which saves you the trouble of menially scanning GRiBs by monitoring the forecasts continually and warning when the forecasts indicate you might encounter wind opposing current in the next 72 hours (premium subscription required).
References and Resources
- Why does wind blowing against a strong current create way bigger waves?
- What visual wave behaviour help to tell if a tide is going in or out?
- Why is wind against current a problem?
- The wind and the waves
- Wind against tide by Alan Lapworth
- The Wave Energy Concentration at the Agulhas Current off South Africa by I. V. Lavrenov