What happens when one of the most valuable climate heartbeats on earth is involved in the thin water passage between Florida and the Bahamas?
During one of her night research shifts with the Straits of Florida, oceanographer Ryan Smith saw a heavy cage of sensors sink into the Atlantic about 1,000 pounds of tubes and electronics to measure temperature, salinity and density of the ocean. At two thirds the distance, the data stream became dark. The phone could not be replaced with any spare parts available, so the crew had to carry the package back up and re-troubleshoot in the moving blackness and test it again, since the water rushing past the ship was the very system the instruments were calibrated to measure.
It aims at the Florida Current, a rapid current that sustains the Gulf Stream and contributes to powering a bigger motor in the Atlantic called the Atlantic Meridional Overturning Circulation or AMOC. It is frequently said that the circulation is a conveyor: the warm surface water is swept northwards, cooled and becoming heavier, sinks and moves southwards at depth. The point is not poetic. The AMOC circulates nutrients, heat, and salt in a manner that determines the distribution of rainfall and temperatures on the Atlantic basin and farther.
The effect of the AMOC in the climate: It transfers massive amounts of heat between the equator and the poles, the workhorse of the climate, explained Denis Volkov, one of the co-principal investigators of the Western Boundary Time Series initiative of NOAA.
The concern is simple: a warmer world can pump in fresh water along the North Atlantic thinning out salinity and density and making more difficult surface waters to sink- one of the physical processes that maintain the overturning circulation going. One of the most significant disturbances has been associated in the literature with alteration in the reliability of rainfall, and with changes in sea level at coastal East Coast. The sheer mass of the Gulf Stream can cause a decrease in the coastal sea level by about 3 to 5 feet compared to the value it should have been without the current, such that changes in the behavior of the current can be converted into change that people experience at tide gauges and in the streets of low lying cities.
Scientists are divided over whether the AMOC is already starting to turn negative and this is partly because natural variability of the ocean can obscure the long-term signals. Others rely on proxies, including sea surface temperature patterns, to detect change, including a Nature Communications analysis of sea surface temperature data that contained an argument that the system could be closer to the tipping point than many models imply it to be. According to other scientists, direct, continuous observations of the AMOC are relatively new and this restricts the faith in far-flung assertions.
It is why the Florida Current has taken so high a value. Over the last 40 years it was traced by scientists with the help of a very unlikely tool: a now-decommissioned telecommunications cable buried on the bottom of the water between the states of Florida and Grand Bahama. Seawater flowing causes minute voltages in that cable; these voltages have been calibrated with ship measurements, and may be converted into transport estimates. In 2024, Volkov et al. reprocessed the Florida Current record adjusted by variations in the geomagnetic field of the Earth, and concluded that the current itself appeared to be widely stabilized over the multi-decade record and that some of the earlier AMOC estimates at that latitude must have been overshooting.
However, there is also a weak point of the monitoring system: the cable on which the record was held broke in 2023. Since the free data stream is disrupted, scientists have turned to seafloor devices, underwater acoustic barometers that are maintained by divers, to continue the time series. Now we vote to service them more often, said Volkov, because, just now, this is all we know of our Florida Current transport estimates.
As a matter of fact, the work itself resembles less a debate on global climatic conditions and more a maintenance exercise: scuba diving to collect instruments that record the data at intervals of five minutes; CTD rosettes dropped down to hundreds of meters to record the layered structure of the water; and, on one occasion, a sensor failure at 2 a.m. that causes the crew to repair the equipment before the ship goes to the next station.
With the increase in intensity of storms in a warming world, due in part to increased sea surface temperatures, the physical state of the Atlantic is becoming more difficult to treat like the background landscape. The Florida Current record is a record among the few that have a long history of near-continuous observation of how the ocean is behaving, not only at the surface but also in the moving machinery beneath.
The understatement is made when the spreadsheets made by the team then include each station with a simple, Profile looks good; use these data. The largest signals in the science of ocean climates are usually those based on the smallest and most recalcitrant details remaining measurable over decades.
