The four forecasts are obtained for different assumptions about winds and biology:
Notice how variable the forecasts are, depending on the assumptions about winds and growth/loss. Notice also that each forecast is based on a number of ‘ensemble members’ that we average together (the thick purple line), accounting for considerable uncertainty in wind, currents and temperatures, for any particular year – a bit like the weather forecast.
Upper left: drift with current only
Upper right: drift with current plus winds
Lower left: drift with current only, subject to growth and loss
Lower right: drift with current plus winds, subject to growth and loss
We estimate growth and loss (mortality) along each particle trajectory, according to surface temperature. Sargassum grows fastest at 26-27oC, but increasingly dies off at temperatures above 29oC. Through recent laboratory and mesocosm experiments, we now know more about the dependence of growth on temperature, in particular that the ‘new’ sargassum grows well at higher temperatures than previously thought, so these results may need updating.
Along with the forecasts, the green line shows the actual sargassum recorded by satellite around Jamaica. This ‘truth’ is by no means accurate, as the satellite’s view is affected by clouds to a greater or lesser extent over the forecast period. Nevertheless, some of our forecasts seem to do a reasonable job of predicting the summer peak in sargassum, here for 2018.
Notice the dominant influence of winds in the case of sargassum forecast along the coast of Ghana. In this part of the world, in summer, the winds act to rapidly blow the sargassum away from the region, substantially reducing quantities forecast near the coast, here in 2020. Unlike for Jamaica, we have very little satellite data in the region of Ghana, so for now we do not know which forecast is most realistic.