Origin and Evolution of the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is the dominant variability mode in the North Atlantic region and plays important roles in weather and climate. When did the NAO first emerge in the past, and how did it evolve over geological timescales? To answer these questions, we examined a set of time-slice paleoclimate simulations with varying continental configurations from ~160 million years ago (Ma). We show that the present-day-like NAO mode emerges stably at around 80 Ma when the North Atlantic Ocean is wide enough to form a high-pressure system that separates the North Atlantic jet from the Euro-Asia jet. A set of idealized simulations confirms that a robust NAO mode will emerge when the width of the ocean basin is greater than 40°. This study depicts the evolutionary history of NAO over geological time and reveals the essential nature of NAO and its relationship with topography.