Wind against tide
David Mackay’s letter ‘Wind against tide’ (YM December 2023) caught my eye, not because of his semantic protest but because it reminded me I have never had a scientifically satisfying explanation of the phenomenon.
Having negotiated the infamous Agulhas current from Durban to Cape Town in January 2019, where the northeast to southwest current runs at up to five knots, I am well aware that wind over current can create dangerous waters. What has never made sense to me, however, is why wind over current/tide isn’t simply the sum of the vectors. That is, if the wind is 20 knots from the southwest and the current five knots from the northeast, why isn’t the resulting sea state just that of a 25-knot wind?
I have seen many explanations but most involve tidal streams flowing over shallower water or through narrow constrictions, which doesn’t apply in the Agulhas current. I would love to see this addressed scientifically in the pages of YM.
But back to the semantics. David Mackay overlooks the nuanced use of ‘over’, often signifying antagonism rather than physical elevation: hence mind over matter, or good over evil, adding another layer to the discourse on wind and tide interactions. Andrew Furley
Ken Endean replies:
The term ‘wind against tide’ is potentially misleading because the wind’s only role is in creating waves and sometimes those waves arrive as swell, not created by the local wind, so we should really use the term ‘waves against tide’.
Disproportionately rough water occurs where waves encounter an adverse current. Two things happen simultaneously: the current directly reduces the waves’ speed over the ground and as each wave in turn is slowed, the following wave moves closer, foreshortening the wavelength. As shorter waves travel slower, this foreshortening reduces their speed through the water, with a corresponding further reduction in their speed over the ground.
Shortening the wavelength compresses the wave energy into a smaller area, so that the waves grow higher. Being both shorter and higher than before, the waves are therefore much steeper.
The change in sea state is also influenced by the properties of wave motion, in that the energy in a wave train only moves at half the speed of the waves. When all these factors are combined, some fiddly maths indicates that a contrary current will completely stop a train of waves that are moving four times as fast. For example, if waves meet a contrary five-knot current, they will all become higher and steeper; those that were moving at less than 20 knots will progress no further and will be eight times as steep as before. Longer, faster waves will continue slowly upstream, but will be almost as steep.
Waves and currents interact in other ways. Tongues of current, like the Algulhas, concentrate wave energy through wave refraction, so that the current becomes continuously rougher while the slack water on each side becomes calmer. Interaction between those trapped waves can also produce a further increase in height. Even waves that are moving with currents may create chaos under certain conditions. However, further explanations would take several pages! The subject is explored in more depth in Chapters 6 and 7 of my book Coastal Turmoil – Winds, Waves and Tidal Races (Adlard Coles, £18).