Improved quantification of SST dynamics from a new inverse method

We present a novel inverse method that operates on satellite SST measurements and quantifies the

physical mechanisms governing SST dynamics at unprecedented resolution. The method uses a

Fluctuation­Dissipation inverse model under the assumption that SST anomalies behave as the

integrated response to a stochastic weather forcing. The novelty is that instead of using the

common EOF method to reduce dimensionality, we assume that the linear operator is a discretized

partial differential equation, meaning that it is sparse and local. We deduce a set of best fit

discretization coefficients at each grid point and decompose the coefficients into advection,

diffusion, and decay rate fields. Applying this method to a NOAA North Atlantic satellite SST

dataset produces high resolution (1/4 degree) spatial maps of the velocity (0-­7 cm/s), subgrid

diffusivity (500­-2000 m2/s), and decay rate fields (10­-100 days) that govern SST anomaly

transport. The estimates compare favorably with previous studies while providing much increased

detail and low errors. The added detail shows that: First, SST anomalies propagate with the surface

ocean current in some regions and not in others. Second, SST anomalies diffuse most rapidly in the

subtropical gyre interior, not the western boundary currents. And, third, SST anomalies decay

slowly (rapidly) on the northwest (southeast) flank of the Gulf Stream. We suggest physical

mechanisms to explain these findings. Finally, the approach estimates for the first time the SST

impulse response function that gives the distribution of travel­ times between any two points on the ocean

surface.