The powerful current of the Gulf Stream is like a highway, carrying warm tropical waters from the Caribbean to Europe. The current is known to meander and to shed rotating rings of water on both sides. However, its interaction with the surrounding water tends to be limited to the outer edges of the current, especially along the well defined northern wall of the Gulf Stream.


But satellite images presented by Xiaofeng Li and his colleagues in Geophysical Research Letters show a parcel of cold water from a region known as the Middle Atlantic Bight breaching the northern boundary of the Gulf Stream, and traversing the full width of the current (Geophys. Res. Lett. 29, 10.1029/2002GL015378; 2002).

The left panel of the picture, taken at 7:08 a.m. on 3 October 2001, shows the penetration of cold water as a green tongue that extends into the main current just east of Cape Hatteras, where the Gulf Stream leaves the American East Coast and veers off into the North Atlantic Ocean. The right panel shows the fate of the intrusion about 24 hours later: the cold tongue has been swept along with the current while extending southeastwards.

In early October 2001, strong and persistent winds from northerly directions blowing along the shore north of Cape Hatteras piled up cold water from the Middle Atlantic Bight in the corner formed by the coastline and the Gulf Stream's north wall. Under less extraordinary wind conditions, long streaks of the relatively cold shelf water are slowly mixed into the Gulf Stream along its northwestern edge. But after three days of wind speeds exceeding 12.8 m s−1 — conditions unique for early autumn in the 11-year period from 1991 to 2001 — the cold coastal water broke into the main Gulf Stream and eventually crossed it.

Li and colleagues say they are not aware of any other reports of such a breaching event. After all, crossing a busy highway is rarely attempted and is even less often successful.