Dynamic controls on surface ocean circulation

If you look at a map of the ocean, one of the most striking features is how fast currents tend to hug the western edges of ocean basins. This is not breaking news. Even Benjamin Franklin, in his role as postmaster general, noticed strong currents while trying to speed up transatlantic mail routes, what we now call the Gulf Stream. The Kuroshio is another familiar example. The tl;dr version is that most ocean basins host narrow, energetic flows along their western flanks that transport heat from the tropics toward higher latitudes and influence both regional and global climate.

The dynamics of these currents have been explored for decades, yet they remain an active area of research. Their structure reflects the combined effects of winds, Earth’s rotation, and stratification. I am especially interested, however, in something that is easy to overlook: the shape of the ocean basin itself. How wide is it? How far north and south does it extend? And how might those geometric constraints influence the circulation that can exist within it?

My work explores these questions using a mix of theory and numerical models, with the aim of isolating geometry as a control on circulation. A useful way to frame this problem is through a basin’s aspect ratio, defined as the ratio of its north–south extent to its east–west width. While the east–west width is set by continental margins, the north–south extent is determined by the separation between wind-stress maxima. In that sense, basin geometry reflects both tectonic constraints and atmospheric forcing. What emerges is that the aspect ratio appears to matter a great deal. As basin geometry changes, the strength of the western boundary current (WBC) and the amount of water that recirculates through it do not necessarily vary smoothly or monotonically. Instead, there are regimes in which circulation strengthens, weakens, or reaches a maximum.

One of the more intriguing outcomes of this work is that certain basin geometries can maximize the WBC’s poleward transport. When a basin becomes either too wide or too narrow, recirculation tends to weaken. This offers a useful way of thinking about why different ocean basins may respond differently to similar wind forcing. For example, the Gulf Stream in the relatively narrow Atlantic and the Kuroshio in the wider Pacific are not simply scaled versions of the same system. Their geometry may place them in different dynamical regimes.

This geometric perspective is also helpful when thinking about past climates. During the Cretaceous, tectonic changes gradually reshaped the Pacific Ocean. In climate model simulations, these changes in basin geometry are associated with weaker subtropical gyres and increased temperature contrasts between the equator and the poles. Encouragingly, these modeled patterns are consistent with independent geological proxy records.

Looking ahead, I am interested in how this large-scale geometric control connects to smaller-scale ocean variability. If recirculation is particularly sensitive to basin shape, then mesoscale eddies and sea surface height variability may inherit some of that sensitivity as well. Changes in circulation strength and structure can also influence the tilt of density surfaces, the depth of the thermocline, and sea level along western boundaries. I plan to explore these potential links using a combination of simplified models, comprehensive climate simulations, and satellite observations of sea surface height.