ESRL/PSD Seminar Series

Abstract

---

The dominant mode of coupled ocean-atmosphere variability in the Tropical Atlantic, the Atlantic Meridional Mode (AMM), responds strongly to seasonally dependent remote forcing. While many studies have found that the AMM is easily excited by ENSO and the NAO during boreal spring, little is known about AMM variability during boreal fall, when it strongly regulates Atlantic hurricane activity. Recent findings using a linear inverse model (LIM) of near-global SST anomalies suggest that the boreal fall AMM is forced by extratropical Atlantic SST anomalies and shows remarkable predictability with lead times approaching one year.

To investigate the validity and mechanism of the LIM, we run a set of ensemble simulations using an AGCM coupled to a slab-ocean. By initializing the GCM with LIM-derived SST anomalies, we have reproduced the LIM result in an alternate physical framework. Excitation of the AMM is found to involve at least two processes: one is the thermodynamic, wind-evaporation-SST feedback, and the other is a low-cloud/SST feedback. Further tests confirm that the former is much more important than the latter in exciting the AMM. Additionally, we find that the GCM response depends strongly on the season of forcing (consistent with the LIM), and exhibits non-linearity with respect to the polarity of the forcing. Another important conclusion is that oceanic dynamics are not needed to explain the LIM result. We discuss our results in context of previous investigations of extratropical-tropical forcing, and highlight areas for future research.

---