AMOC Strength Estimator

The Atlantic Meridional Overturning Circulation is the system of currents that carries warm, salty surface water northward in the Atlantic and returns cold, dense water southward at depth. Its strength, measured in sverdrups, sets how much heat the ocean delivers toward the North Atlantic and northwest Europe. At roughly 17 Sv it transports on the order of 1.2 petawatts of heat, comparable to a substantial fraction of the total poleward energy transport at that latitude.

One sverdrup (Sv) is one million cubic metres of water per second, or 10 to the sixth power cubic metres per second. It is the standard unit for ocean volume transport because ocean currents move volumes far too large to express conveniently in cubic metres per second. For scale, the Amazon River discharges roughly 0.2 Sv, so a 17 Sv AMOC moves about 85 times more water.

Transport mode treats the overturning upper limb as a rectangular box: volume transport equals basin width times upper limb depth times mean northward velocity, divided by one million to convert to sverdrups. With a 5000 km wide basin, an 1100 m deep upper limb and a 0.3 cm per second mean velocity, that gives 16.5 Sv, close to the RAPID array long-term mean.

Heat transport equals seawater density times specific heat capacity times volume transport times the temperature contrast between the northward upper limb and the southward deep return. Using 1025 kg per cubic metre and 3990 joules per kilogram per kelvin, a 16.5 Sv overturning with a 17 degree Celsius contrast carries about 1.147 petawatts.

RAPID is a mooring array across the Atlantic at 26.5 degrees north that has continuously measured the overturning since 2004. Its long-term mean is roughly 17 Sv, with very large month-to-month swings driven mainly by wind-forced Ekman transport. Individual months have ranged from under 5 Sv to over 25 Sv, which is why short records cannot resolve a slow trend.

Proxy reconstructions suggest the AMOC is weaker now than it was in the mid 20th century and probably weaker than at any point in the last millennium, but the direct RAPID record is still too short and too noisy to isolate a statistically robust trend on its own. The IPCC AR6 assessed that the AMOC will very likely weaken over the 21st century, while assigning only medium confidence to a decline already having occurred.

A 20 percent reduction in overturning cuts the associated ocean heat transport by roughly the same proportion, on the order of 0.2 petawatts at a typical limb contrast. In practice this shows up as relative cooling of the subpolar North Atlantic and northwest Europe compared with the global warming trend, a southward shift in tropical rainfall belts, and enhanced sea level rise along the northeast coast of North America.

A full collapse, meaning a shutdown of the deep overturning rather than a gradual weakening, is considered unlikely before 2100 in the IPCC AR6 assessment, but it cannot be ruled out and is treated as a low-likelihood, high-impact outcome. Some simplified models show a tipping threshold beyond which the circulation cannot recover, though the freshwater forcing level required is poorly constrained in the full coupled models.

Trend mode applies a fixed percentage weakening per decade, which compounds and produces an exponential decay curve rather than a straight line. This keeps the projected strength positive at every horizon, which a linear percent-of-baseline decline does not do once the elapsed time exceeds the full decline period.

The velocity entered here is the basin-wide mean of the net northward flow across the whole upper limb cross section, not the speed of any individual current. Fast western boundary currents such as the Gulf Stream reach well over 100 cm per second, but they occupy a narrow strip and are largely balanced by southward flow across the rest of the basin, so the net mean is small.

No. This is a bulk geometric estimate of the overturning cell, so it lumps together everything crossing the section. The RAPID decomposition splits the total into a Florida Current component, an Ekman component and an upper mid-ocean geostrophic component, and the Ekman part is what drives most of the month-to-month variability.

For the subtropical North Atlantic near 26.5 degrees north, a contrast of roughly 15 to 19 degrees Celsius between the warm upper limb and the cold deep return flow is typical, so 17 degrees Celsius is a reasonable central value. The contrast is smaller at higher latitudes where the upper limb has already lost much of its heat to the atmosphere.