Inconsistent Atmosphere-Ocean Dynamics and Multidecadal Zonal SST Gradient Trends Across the Equatorial Pacific Ocean in Reanalysis Products

Ocean reanalysis products are routinely employed as reality checks in model evaluations and for process studies. This is especially so for critical regions such as the equatorial cold tongue (ECT) in the eastern equatorial Pacific where models suffer a chronic cold bias. ECT is a major player in the Pacific equatorial zonal sea surface temperature (SST) gradient (Delta EWSST) that has a significant impact on oceanic heat uptake and thus global climate. Hence, we investigate the reliability of three ocean reanalysis products for surface flux and ocean dynamic controls on Delta EWSST and Nino3.4 SST trends. We infer that while Nino3.4 SST trends are positive in all products, the signs of Delta EWSST trends do not agree with each other because initial conditions likely play a big role in their evolution. However, for Delta EWSST trends, the effect of initial conditions gets canceled out to some extent. Mixed layer heat budget and trends in ocean dynamic features such as tropical and subtropical cells, equatorial undercurrent, and subsurface temperatures are also diagnosed. We show that two reanalysis products that show a strengthening of Delta EWSST have contradicting trends in their surface heat flux and ocean dynamic contributions. This suggests that without accurate surface heat and momentum fluxes, data assimilation techniques may produce an east-west trend that is inconsistent among each other. Reanalysis products must address these issues considering the importance of this gradient. Plain Language Summary The east to west sea surface temperature (SST) gradient (.EWSST) of the equatorial Pacific Ocean (EQPO) is characterized by a highly variable cold SST over the eastern EQPO and relatively less variable warm, fresh pool in the west. This gradient reinforces the equatorial easterlies because of a positive feedback with wind-driven upwelling in the east. Anomalous weakening (strengthening) of the.EWSST and the resulting weaker (stronger) easterlies lead to an El Nino (La Nina). On climate change timescales, this feedback modulates oceanic heat uptake and atmospheric warming as seen by the La Nina-like state during 1998 to 2013 and the so-called global warming pause. The reliability of future climate projections depends on simulating the coupled dynamics of the region faithfully due to its global reach. Therefore, climate models have to be thoroughly evaluated. As observations are insufficient, ocean reanalysis products, which are comprehensive records produced by merging observations and models, serve as surrogates for observations in this context. However, the surface heat fluxes and ocean dynamics are inconsistent among three reanalysis products analyzed. The inconsistencies in surface fluxes in two products still produce a similar.EWSST trend but with different ocean dynamic pathways.