How will the tropical Pacific respond to global warming? The importance of timescale when considering apparent paleo-paradoxes

Several oceanic and atmospheric mechanisms have been put forward to describe the response of the tropical Pacific to global warming. Still uncertainties persist in their interaction and relative importance, with projections varying substantially across climate models. When we turn to the last time in Earth’s history that atmospheric CO2 estimates exceed 400 ppm, the Pliocene, several apparent paradoxes appear, clouding our view of the tropical Pacific during the Pliocene and its utility as a potential analogue for future warming. However, when proper consideration is given to the timescales associated with the oceanic and atmospheric mechanisms that support tropical Pacific climate variability, several of these apparent paradoxes can be resolved. The first apparent paradox is between the reduced east-west SST gradient (weaker Walker Circulation) mean state reconstructions for the Pliocene and the strengthening of the east-west SST gradient during 20th century warming. This paradox can be resolved by considering the transient versus equilibrium processes involved in the response to global warming. The second apparent paradox is evidence for ENSO during the Pliocene even while the mean state resembled more El Nino-like (i.e., El Padre) conditions. This apparent paradox is resolved by the relative importance of coupled feedbacks, other than the thermocline feedback, generating inter-annual variability in the Pliocene. Thirdly, an apparent paradox between the reduced Pliocene east-west SST gradient, indicative of reduced wind-driven upwelling, and evidence of enhanced biological productivity in the east Pacific where productivity is typically driven by upwelling. This apparent paradox can be reconciled by evidence for older, more acidic, and nutrient-rich water reaching the equatorial Pacific by way of a Pacific meridional overturning circulation during the Pliocene, providing a mechanism for enhanced productivity existing alongside a reduced east-west SST gradient.