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I'm thinking about a device (in a story I'm writing) that could control ocean currents, and I'm wondering how it could be weaponized. For example, if an ocean current was suddenly stopped, what might happen to the climate around it? Would stopping one ocean have a significant effect, or would it take stopping many/all?
Furthermore, what if an ocean current was reversed or shifted? Could it be used to cause a localized drought or freeze? Or otherwise severe, dangerous weather?
Oceanic currents have a large effect in redistributing the heat absorbed by the ocean water.
Stopping or altering them would, as a consequence, have a large impact on the climate of the regions bordering the current, like we see with the Gulf stream.
The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe. Although there has been recent debate, there is consensus that the climate of Western Europe and Northern Europe is warmer than it would otherwise be due to the North Atlantic drift, one of the branches from the tail of the Gulf Stream.
More in detail, if the circulation should somehow stop,, there would be sever impact on weather
Hansen et al. 2015 found, that the shutdown or substantial slowdown of the Atlantic meridional overturning circulation, besides possibly contributing to extreme end-Eemian events, will cause a more general increase of severe weather. Additional surface cooling from ice melt increases surface and lower tropospheric temperature gradients, and causes in model simulations a large increase of mid-latitude eddy energy throughout the midlatitude troposphere. This in turn leads to an increase of baroclinicity produced by stronger temperature gradients, which provides energy for more severe weather events.
Many of the most memorable and devastating storms in eastern North America and western Europe, popularly known as superstorms, have been winter cyclonic storms, though sometimes occurring in late fall or early spring, that generate near-hurricane-force winds and often large amounts of snowfall. Continued warming of low latitude oceans in coming decades will provide more water vapor to strengthen such storms. If this tropical warming is combined with a cooler North Atlantic Ocean from AMOC slowdown and an increase in midlatitude eddy energy, we can anticipate more severe baroclinic storms.
Answering your question, yes, altering the oceanic currents would have a large impact on the weather.
However keep in mind that it would take some years to see it. If that is a suitable time scale for your world, let it be. But if you rely on it to kill the bear chasing you while you harvest blueberries in the wild you will be history by the time the weapon strikes.
