Today we got exactly what we came here for…and then some. We’re sat in the middle of a storm with winds of storm force 11 gusting well up into hurricane force 12 – over 60 knots (30 m/s)*. The wind picked up quickly yesterday afternoon, soon after we deployed the fully repaired and working spar buoy. Last night was what is colloquially known as ‘a bit lumpy’, with waves of about 5-6 m. Sleeping isn’t easy in those conditions – you get rolled back and forth in your bunk, there’s a lot of noise from waves hitting the ship, engines straining, and what seems like every single object in the cabin creaking, groaning, or squeaking as it all moves around. Usually simple activities can become a major challenge – taking a shower without being thrown out of it by a sudden roll of the ship for instance.
This morning the wind speed increased, and the waves built rapidly to about 10 m or so. The view from the bridge is very impressive and we’ve all spent at least an hour or two up there watching the show.
Most ships and sensible people would go to great lengths to stay out of conditions like this. But it’s exactly what we want. We’re out here as part of a study of air-sea interaction, and particularly gas exchange, under high winds. The High Wind Gas exchange Study (HiWinGS) is a joint US-UK project to use direct measurements of the rate of exchange of various gases between the ocean and atmosphere.
One of the major uncertainties in the global budget of carbon dioxide is the rate of exchange of the as between the ocean and atmosphere. The oceans are known to absorb around 30%-50% of the CO2 emitted by burning fossil fuels. The rate of exchange depends on the difference in concentration (technically the partial pressure) of CO2 between the ocean and atmosphere at any given location, and the strength of turbulent mixing in both the ocean and atmosphere – this increases strongly with wind speed. We need to determine exactly how the rate of gas exchange varies with the environmental conditions in order to be able to include it properly in models of global climate.
Over the last 20 years or so a number of studies have determined exchange rates; these generally agree reasonably well with each other at low wind speeds, but diverge increasingly with increasing wind speed. It is thought that the reason for this is the influence of waves, wave breaking, and the bubbles generated by them. The higher the wind speed, the larger waves can grow, but the longer it takes for them to reach their maximum size. So at low winds different studies measure under roughly similar conditions, but at high winds they may measure very different conditions depending on the size the waves have reached. Bigger waves generate more bubbles when they break. This is important because the presence of bubbles enhances the gas exchange – gas is exchanged not only between air and water across the sea surface, but between all the bubbles and the surrounding water.
There are theoretical models of how bubbles might enhance the gas transfer rate, but very few measurements. We are out here to try and get the measurements necessary to validate the simple models of the bubble contribution to gas exchange. And to do that, we need high winds and waves. More on exactly what we are measuring, and how, in a later post.
*For the meteorologically inclined the wind peaked today with a 30-minute average U10 of just over 27 m/s. For everyone else, that’s very very windy.