Strong Damping of Decadal Variability in the West Pacific Warm Pool By Atmospheric Heat Transport. (Invited Presentation)

In recent decades, sea surface temperatures (SSTs) in the eastern tropical Pacific have been observed to be cooling, while the western Indo-Pacific warm pool has been experiencing intensified warming. However, models in the Coupled Model Intercomparison Project phase 6 (CMIP6) have been unable to recreate this warming pattern. Understanding the cause for this discrepancy is made difficult by the fact that we currently do not have a good understanding of the mechanisms that control decadal scale variability in the tropical Pacific.

Here we focus on understanding the unforced component of decadal-scale variability in the tropical Pacific. We first show that CMIP6 climate models robustly agree that interdecadal surface temperature variability is very muted in tropical areas of deep convection, i.e. the Warm Pool, ITCZ and SPCZ. Across CMIP6 models, we find that the strength of ascent is positively correlated with the signal to noise ratio of forced response to unforced variability. As a result, observed decadal variability in regions of deep convection is strongly dominated by externally-forced variations.

We then use a simple Hasselmann model of coupled atmosphere-ocean variability to show that the GCM behaviour is underpinned by a robust physical mechanism. Specifically, we use the Hasselmann model to partition decadal SST variance into contributions from forcing and feedback terms for both oceanic and atmospheric heat fluxes. We find that in regions of deep convection decadal SST anomalies are very efficiently damped by atmospheric heat flux divergence.