Kris is visiting as a Columbia Climate Center Visiting Lecturer, and as a former postdoctoral Fellow in OCP, his seminar is also part of the Lamont75 OCP seminar series.
Title:
How fast is the mean upwelling in the equatorial Pacific? New observational insights and circulation biases of relevance to the simulated pattern of tropical warming
Abstract:
The tropical Pacific zonal SST gradient plays an outsized role in the global climate system, yet observations and climate models exhibit notable discrepancies over the response of the tropical Pacific to historical anthropogenic radiative forcing. If the observed trends are forced (i.e., not just internal variability or a result of sampling issues), this discrepancy suggests that climate models are not representing key physical processes constraining the response of the tropical Pacific to anthropogenic forcing, with implications for confidence in the future projections of myriad climate impacts. Focusing on the equatorial ocean circulation, this talk examines three climate model biases with potential contributions to this discrepancy: (1) a weak Equatorial Undercurrent (EUC) despite improvements from CMIP3 to CMIP5 and CMIP6, (2) insufficient vertical transmission of friction from the easterly trades to the EUC, and (3) equatorial upwelling that is an order of magnitude too slow. The latter of the three biases represents the newest avenue of this work for me, and is the most alarming because of how closely CMIP6 model solutions hover around what I will argue is a severely biased observational benchmark. By synthesizing over a dozen historical observational studies attempting to estimate the mean upwelling velocity going back to Wyrtki (1981), I will show that the sampling strategies of these campaigns almost perfectly explain their spread. A reinterpretation of historical estimates, along with a fresh look at a massive collection of surface velocity observations from the NOAA drifter program, reveals that the highest of the range of historical estimates of equatorial upwelling (~10 m per day) are consistent with the mean. Given the crucial role of upwelling velocity as the crux of the ocean dynamical thermostat—a mechanism whereby vertical advection damps the warming of the eastern equatorial Pacific under climate change—we should be rather skeptical of climate models in terms of their balance of processes governing the pattern of warming in the tropical Pacific and consequently all of the projected impacts that are sensitive to remote forcing from the eastern tropical Pacific.
