[SLS] Zhengyu Liu (Ohio State University)

“Strong oceanic forcing on decadal SST variability over global ocean”

Sea surface temperature (SST) variability on decadal timescales has been associated with global and regional climate variability and impacts. The mechanisms that drive decadal SST variability, however, remain highly uncertain. Many previous studies have examined the role of atmospheric variability in driving decadal SST variations. The relative importance of atmospheric vs oceanic forcing on decadal SST variability, however, has remained highly controversial. Here we assess the strength of oceanic forcing in driving decadal SST variability in observations and state-of-the-art climate models by analyzing the relationship between surface heat flux and SST. First, we develop a stochastic climate theory and show how the sign of heat flux-SST correlation is affected by atmospheric and oceanic forcing with time scale. In particular, a red noise ocean forcing is necessary for the sign reversal of heat flux-SST correlation. Furthermore, this sign reversal can be detected equivalently in three approaches: the low-pass correlation at lag zero, the unfiltered correlation at long (heat flux) lead and the real part of the heat flux-SST coherence. Second, we develop a new scheme in combination with the theory to assess the magnitude of oceanic forcing on decadal SST variability over the globe. We find a largely similar pattern of decadal oceanic forcing across all ocean basins, characterized by oceanic forcing about twice the strength of the atmospheric forcing in the mid- and high latitude regions, but comparable or weaker than the atmospheric forcing in the subtropics. The decadal oceanic forcing is hypothesized to be associated with the wind-driven oceanic circulation, which is common across all ocean basins.

References

Liu, Z., P. Gu and T. Delworth, 2023: Strong red noise ocean forcing on Atlantic Multidecadal Variability assessed from surface heat flux: theory and application. J. Clim. 36, 53-80, 10.1175/JCLI-D-22-0063.1.
Gu, P, Z. Liu, T. Delworth, 2023: Strong oceanic forcing on decadal surface temperature variability over global ocean. Geophys. Res. Lett. 51, e2023GL107401, https://doi.org/10.1029/2023GL107401

Sack Lunch Seminar Series —

Informal seminar series within PAOC (Program in Atmospheres, Oceans, and Climate) that focuses on more specialized topics than the PAOC Colloquium. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest.
Contact: sacklunch-committee@mit.edu