Wind-driven and buoyancy-driven circulation in the subtropical North Atlantic Ocean.

Area of Science:

• Oceanography
• Climate Science
• Geophysics

Background:

• The Atlantic Meridional Overturning Circulation (AMOC) is crucial for global heat distribution.
• Understanding AMOC's strength and variability is key to predicting climate change impacts.
• Previous studies often considered deep water flow as purely buoyancy-driven.

Purpose of the Study:

• To quantify the strength and variability of the AMOC at 26°N in the subtropical Atlantic.
• To investigate the drivers (wind vs. buoyancy) of different AMOC components.
• To analyze long-term trends and significant variability events in the AMOC.

Main Methods:

• Continuous oceanographic observations at 26°N since April 2004.
• Analysis of northward western boundary current transport (Gulf Stream, Antilles Current).
• Quantification of surface Ekman transport and thermocline recirculation.
• Monitoring of southward deep water flow below 1100m depth.

Main Results:

• The Gulf Stream is identified as partially wind-driven and partially buoyancy-driven.
• Seasonal to interannual variations in circulation are primarily wind-driven.
• A significant reduction (30%) in southward Lower North Atlantic Deep Water transport occurred in 2009.
• Over 14 years (2004-2018), the AMOC strength decreased by 2.4 Sv (from 18.3 to 15.9 Sv).

Conclusions:

• The AMOC exhibits significant variability, with both wind and buoyancy forces playing roles.
• Deep water circulation is not solely buoyancy-driven, with implications for climate models.
• The observed decline in AMOC strength warrants further investigation into its long-term implications for climate.