The effect of changes in surface winds and ocean stratification on coastal upwelling and sea surface temperatures in the Pliocene

Abstract

Sea surface temperature (SST) in subtropical eastern boundary upwelling zones is shown to be affected by three main factors: large‐scale ocean stratification, upwelling‐favorable sea surface wind stress, and the surface concentration (baroclinicity) of the alongshore pressure gradient driving the incoming geostrophic flow which balances the Ekman surface outflow. Pliocene‐aged SST proxies suggest that some combination of differences in upwelling forcing enable the sea surface temperatures in these zones to increase by up to 11°C. We find that large warming in SST in response to the three factors, of up to about 10°C in addition to a mean Pliocene ocean warming of 2–3°C, is concentrated in the direct upwelling zone. In the location of proxy sea surface temperatures, about 120 km away from the coast, and outside the coastal upwelling zone, the SST response to changes in wind and stratification is weaker, only accounting for up to 3.4°C above the mean Pliocene warming. Increased baroclinicity of the alongshore pressure gradient has a smaller effect, accounting for less than 2°C increases at both the coast and proxy site. The SST seaward (westward) of the upwelling zone is primarily determined by ocean‐atmosphere heat exchange and basin‐scale ocean forcing, rather than by changes in upwelling. The spatial pattern of SST change with each of the three forcing factors is similar, and therefore, all could contribute to the Pliocene‐modern difference in coastal SST.

Reductions in Strong Upwelling‐Favorable Wind Events in the Pliocene

“…An important caveat is that we have focused on upwelling‐favorable wind events here, yet the use of an atmosphere‐only model in this study does not allow us to study the effects of a different thermocline depth. The same wind will lead to a different SST and proxy upwelling signal, and this was explored by Miller and Tziperman (). We emphasize that the mechanism explored here involves the basin‐scale SST gradient altering synoptic‐scale wind patterns.…”

“…It is thus possible to consider how the large-scale differences in Pliocene SST would impact the coastal winds, while neglecting the relatively small feedback from the local SST. This approach, of asking how the large-scale SST affects the wind, complements studies that use a dynamical ocean model with prescribed winds to study the local ocean upwelling response (Miller and Tziperman, 2017). Eventually, of course, the problem would need to be tackled by a fully coupled ocean-atmosphere model.…”

“…They found reductions of 10% to 50% in both coastal upwelling driven by AWS and offshore upwelling driven by wind stress curl. However, such reductions alone appear to be insufficient to explain the anomalies suggested by temperature proxies (Miller & Tziperman, ).…”

The effect of mountain uplift on eastern boundary currents and upwelling systems

“…5.1). Proxy records of the major EBUSs indicate a SST cooling of around 3-10 • C (Marlow et al, 2000;Dekens et al, 2007;Rommerskirchen et al, 2011;Miller and Tziperman, 2017), Figure 17. Responses in low-cloud coverage (shading) and temperature (contours, intervals: 0.5 • C) to African (CTRL -AF) (a), North American (CTRL -NA) (b), and South American (CTRL -AND) (c) mountain uplift.…”

Mid-Pliocene Atlantic Meridional Overturning Circulation simulated in PlioMIP2

“…Despite the uncertainties in reconstructions, the simulated warming in the mid-latitude upwelling regions in PlioMIP2 can be found in the low end of the proxy-estimated range. Realistic simulations in upwelling regions require good model-abilities in simulating large-scale ocean stratification and sea surface wind stress (Miller and Tziperman, 2017;Li et al, 2019), which are partly model-resolution dependent (Small et al, 2015).…”