Study takes in-depth look at decade-long variations in the tropical Pacific that influence global climate
Decade-long variations in the tropical Pacific (referred to as TPDV) have a significant influence on the global climate, which can impact extreme weather events, precipitation, and agricultural production. Major international efforts are underway to provide decadal climate predictions, but scientists lack a mechanistic understanding of TPDV, and its degree of predictability.
A new study published in Science, co-authored by CIRES/PSL, researchers, provides an in-depth look at the current knowledge on this topic, drawing from both observational analyses and modeling studies. Low-frequency variability in the tropical Pacific occurs on a broad range of timescales, so this study adopts a definition of ”decadal” that includes all timescales longer than approximately 8 years, to exclude variations associated with the interannual El Niño Southern Oscillation (ENSO) phenomenon. The researchers consider decadal variations arising from processes internal to the climate system, as well as variations that are driven by either natural (e.g., volcanoes) or anthropogenic (e.g., greenhouse gases, aerosols) causes.
TPDV is evident in instrumental records, paleoclimate records over past millennia, and climate models. This study emphasizes that a large fraction of internally-generated TPDV may arise as a residual of the event-to-event differences in ENSO patterns and evolution. The authors also highlight ocean processes that may play an important role in generating low-frequency variations in this region, as they could contribute some degree of predictability. The exact nature of these mechanisms, and their relative role in producing decadal variations is still poorly understood, and in need of further investigation. The study also identifies aspects of forced variations that appear robust. For example, warming induced by greenhouse gas forcing in the western tropical Pacific has reached levels that are unprecedented in the historical record. Some future projections of climate models, like the expansion of the component of atmospheric circulation known as “Hadley Cell” in the Southern Hemisphere, appear to be robust across models. However, projected changes in eastern equatorial Pacific surface temperature, and in the strength of the tropical Pacific trade winds, remain very uncertain.
Some skill is found in decadal predictions of temperature in the western Pacific, although it appears to be largely associated with anthropogenic warming. There are also indications of multi-year skill in predicting some biogeochemical quantities important for fisheries and the global carbon budget.
This study assesses the degree of uncertainty in the characterization of TPDV by examining how it is represented in both instrumental and multi-century-long paleo records as well as state-of-the-art climate models. It also identifies aspects of this topic that are relatively well understood versus those that need further clarification. One important issue in need of greater understanding is the role played by the upper ocean circulation in controlling tropical Pacific sea surface temperatures at decadal timescales. Insights from this study will guide research needed to improve understanding of decadal variability and its predictability in the context of our changing climate.
Power, Scott, Matthieu Lengaigne, Antonietta Capotondi (CIRES/PSL), Myriam Khodri, Jérôme Vialard, Beyrem Jebri, Eric Guilyardi, Shayne McGregor, Jong-Seon Kug, Matthew Newman (CIRES/PSL), et al. (October 2021): Decadal climate variability in the tropical Pacific: characteristics, causes, predictability and prospects. Science, https://doi.org/10.1126/science.aay9165
Posted: October 8, 2021