Air-Sea Interaction and Large-Scale Sea Surface Temperature Variability

Sarah Larson 

Abstract:

Sea surface temperature (SST) variability is driven by an aggregate of intrinsic processes and in recent decades, anthropogenic forcing. Identifying drivers of intrinsic climate variability is of key interest because different mechanisms (or the interplay between them) can set the timescales of the SST variability, the spatial pattern, amplitude, and impact the predictability. Quantifying the relative contribution of intrinsic air-sea processes in driving large-scale patterns of SST remains an elusive task, because of sparse ocean observations and complicated dynamics. This issue is exacerbated by a wide gap in the climate model hierarchy, between fully coupled models with a complete dynamical representation of both the ocean and atmosphere and models that represent the ocean as a “slab”, a motionless layer that is only thermally coupled to the atmosphere. This gap in the model hierarchy leads to a gap in our mechanistic understanding of how buoyancy fluxes and winds together and separately drive the ocean circulation and climate variations. This presentation will introduce a “mechanically decoupled” (MD) model configuration that is uniquely designed to isolate climate variations that are solely driven by buoyancy from those that are wind-driven. Different from a slab ocean model, the MD model includes a dynamic ocean component and hence simulates a realistic mean ocean circulation. Results reveal a different role for wind-driven ocean temperature advection, often referred to as Ekman advection, in the midlatitudes versus the subtropics in driving large-scale SST variability. This latitudinal difference in the role of Ekman advection plays a key role in shaping the canonical pattern of the Pacific Decadal Oscillation. The results reveal that SST variability in the subtropical oceans is less understood compared to the midlatitudes and suggest that the relative role of Ekman advection in driving SST variability on the large scale is likely to change in a warming climate.