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PO Research / Mechanisms of tropical Pacific decadal variability

Mechanisms of tropical Pacific decadal variability

Capotondi, A., McGregor, S., McPhaden, M., Cravatte, S., Holbrook, N., Imada, Y., Sanchez, S., Sprintall, J., Stuecker, M., Ummenhofer, C., Zeller, M., Farneti, R., Graffino, G., Hu, S., Karnauskas, K., Kosaka, Y., Kucharski, F., Mayer, M., Qiu, B., Santoso, A., Taschetto, A. S., Wang, F., Zhang, X., Holmes, R. M., Luo, J.-J., Maher, N., Martinez-Villalobos, C., Meehl, G. A., Naha, R., Schneider, N., Stevenson, S., Sullivan, A., van Rensch, P., & Xu, T. (2023). Mechanisms of tropical Pacific decadal variability. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-023-00486-x

Mechanisms of tropical Pacific decadal variability

Subtropical cells influence on tropical Pacific decadal variability and two different mechanisms are illustrated schematically: (a) equatorward advection of temperature anomalies within the pycnocline (the vbarT’ hypothesis) is schematically illustrated in the upper panel as the advection of spiciness anomalies (pink shading) by the mean circulation (black arrows) on the 25.0kg mm−3 isopycnal surface. The lower panel depicts zonally averaged isopycnal depths (from 23 kg m−3 to 25.5 kg m−3 with a spacing of 0.5 kg m−3; solid lines: 23, 24 and 25 kg m−3; dashed lines: 23.5, 24.5 and 25.5 kg m−3), and the flow of equatorward spiciness anomalies along isopycnal surfaces. (b) As in (a), but for anomalous upwelling that can also drive TPDV via changes in the transport of the subtropical cells (the v’Tbar hypothesis) is schematically illustrated with mean (black arrows) and anomalous (red arrows) flows, which reveal how flow along isopycnal surfaces connects the subtropics to the tropics. Both mechanisms are proposed as potential contributors to TPDV. EUC, equatorial undercurrent; LLWBC, low-latitude western boundary current. [Adapted from Capotondi et al., 2023] (Figure provided by Caroline Ummenhofer)

Naturally occurring tropical Pacific variations at timescales of 7–70 years — tropical Pacific decadal variability (TPDV) — describe basin-scale sea surface temperature (SST), sea-level pressure and heat content anomalies. Several mechanisms are proposed to explain TPDV, which can originate through oceanic processes, atmospheric processes, or as an El Niño-Southern Oscillation (ENSO) residual. In this Review, we synthesize knowledge of these mechanisms, their characteristics and contribution to TPDV. Oceanic processes include off-equatorial Rossby waves, which mediate oceanic adjustment and contribute to variations in equatorial thermocline depth and SST; variations in the strength of the shallow upper-ocean overturning circulation, which exhibit a large anti-correlation with equatorial Pacific SST at interannual and decadal timescales; and the propagation of salinity-compensated temperature (spiciness) anomalies from the subtropics to the equatorial thermocline. Atmospheric processes include midlatitude internal variability leading to tropical and subtropical wind anomalies, which result in equatorial SST anomalies and feedbacks that enhance persistence; and atmospheric teleconnections from Atlantic and Indian Ocean SST variability, which induce winds conducive to decadal anomalies of the opposite sign in the Pacific. Although uncertain, the tropical adjustment through Rossby wave activity is likely a dominant mechanism. A deeper understanding of the origin and spectral characteristics of TPDV-related winds is a key priority.