Subtropical-tropical pathways of spiciness anomalies and their impact on equatorial Pacific temperature

Abstract

Understanding mechanisms of tropical Pacific decadal variability (TPDV) is of high importance for differentiating between natural climate variability and human induced climate change as this region sustains strong global teleconnections. Here, we use an ocean general circulation model along with a Lagrangian tracer simulator to investigate the advection of density compensated temperature anomalies ("spiciness mechanism") as a potential contributor to TPDV during the 1980-2016 period. Consistent with observations, we find the primary regions of spiciness generation in the eastern subtropics of each hemisphere. Our results indicate that 75% of the equatorial subsurface water originates in the subtropics, of which two thirds come from the Southern hemisphere. We further show two prominent cases where remotely generated spiciness anomalies are advected to the equatorial Pacific, impacting subsurface temperature. The relative contribution of Northern versus Southern Hemisphere prominence and/or interior versus western boundary pathways depends on the specific event. The anomalously warm case largely results from advection via the Southern hemisphere interior (65% ), while the anomalously cold case largely results from advection via the Northern hemisphere western boundary (48% ). The relatively slow travel times from the subtropics to the equator (> 4 years) suggests that these spiciness anomalies underpin a potentially predictable contribution to TPDV. However, not all decadal peaks in equatorial spiciness can be explained by remotely generated spiciness anomalies. In those cases, we propose that spiciness anomalies are generated in the equatorial zone through changes in the proportion of Northern/Southern hemisphere source waters due to their different mean spiciness distribution.

A stronger versus weaker Walker: understanding model differences in fast and slow tropical Pacific responses to global warming

“…A related important point is that changes in oceanic and atmospheric circulation patterns cannot be simply classi ed as weakening or strengthening, but they may also change spatially. For example, in both models the Walker circulation exhibits a characteristic westward shift, and the STCs can exhibit an equatorward narrowing, in particular in the f19 model, which may in turn change the make-up of the source waters upwelled at equator (Zeller et al, 2020). Thus, both the magnitude of changes and changes in the spatial structure should be considered when analyzing the tropical-extra-tropical connection with regard to the tropical Paci c response to warming.…”

Decadal climate variability in the tropical Pacific: Characteristics, causes, predictability, and prospects

“…Other modeling studies suggest that temperature anomalies subducted in the subtropics can reach the equatorial thermocline and influence equatorial SSTs (35), especially when they are densitycompensated by associated salinity anomalies (so-called "spiciness" anomalies) and can then propagate toward the equator along mean density surfaces with minimal dissipation (36). While the ability of spiciness anomalies to reach the equator from their source regions has been recently demonstrated in a modeling context (36), their impact on equatorial SSTs in nature remains to be determined.…”

Subsurface Cooling in the Tropical Pacific Under a Warming Climate

“…Instead of the general advection through the meridional overturning circulation, the advection of the spiciness anomalies at a certain isopycnal surface is another process that can demonstrate the effect of the subtropical variability on tropical climate (Zeller et al, 2021). Observations and model studies substantiate the existence of spiciness anomalies in the midlatitudes in both the North and South Pacific, which act as passive tracers to propagate toward the tropics along the geostrophic streamlines at isopycnal surfaces (Giese et al, 2002;Schneider, 2000;Yeager & Large, 2007;Zeller et al, 2021). But most works are of concern The maximum spiciness trends also appear in the subtropics on a longer timescale and are located on a denser isopycnal surface in the South Pacific than in the North Pacific (Figure 4a).…”