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- Title
The Tropical Pacific Annual Cycle and ENSO in PMIP4 Simulations of the Mid‐Holocene.
- Authors
Zhang, Xiaolin; Atwood, Alyssa R.; Nag, Bappaditya; Cobb, Kim M.
- Abstract
We investigate the tropical Pacific annual cycle and the El Niño/Southern Oscillation (ENSO) in four mid‐Holocene simulations. Our results show that both ENSO variability and the amplitude of the annual cycle of the tropical Pacific cold tongue are reduced under mid‐Holocene forcing, along with a modified annual cycle in ENSO variance. The weakened annual cycle of the cold tongue is attributed to an ocean dynamical response to westerly wind anomalies in the western equatorial Pacific in boreal spring in addition to a thermodynamic response to local insolation changes in the eastern Pacific. The anomalous westerly winds in boreal spring excite an annual downwelling Kelvin wave that deepens the thermocline and propagates eastward along the equator, reaching the central and eastern equatorial Pacific during the development season of ENSO in boreal summer. Upon reaching the eastern Pacific, the downwelling Kelvin wave deepens the near‐surface thermocline, warming the surface ocean and weakening the local ocean‐atmosphere coupling critical to the growth of ENSO events. The westerly wind anomaly is associated with a shift in convection in the western Pacific driven by greater cooling of the Maritime Continent than western Pacific Ocean during the first half of the year (January to June) under tropical insolation forcing. By elucidating a common set of mechanisms responsible for a reduced cold tongue annual cycle and ENSO variability in a diverse range of mid‐Holocene simulations, this work yields important insight into the linkages between the tropical Pacific annual cycle and ENSO that are critical for understanding tropical Pacific climate variability. Plain Language Summary: Paleoclimate records and climate models alike indicate that the tropical Pacific underwent substantial changes during the mid‐Holocene, including weakened variability associated with the El Niño/Southern Oscillation (ENSO). In this study, we analyzed four global climate models included in the latest Paleoclimate Model Intercomparison Project and show that seasonal changes in the amount of sunlight received by the tropics caused a weakened annual cycle of sea surface temperatures in the eastern Pacific Ocean due in part to changes in wind and rainfall patterns in the western Pacific. These changes in the annual cycle cause ENSO variability to be reduced because the coupling between the ocean and atmosphere is weakened during the part of the year that is important for the growth of ENSO events. These findings support previous results that highlight an important linkage between changes in the tropical Pacific annual cycle and ENSO during the mid‐Holocene. Key Points: The annual cycle of the tropical Pacific cold tongue and El Niño/Southern Oscillation (ENSO) variability is reduced during the mid‐Holocene in PMIP4 modelsWesterly wind anomalies in the western Pacific in boreal spring drive an annual downwelling Kelvin wave that deepens the eastern equatorial Pacific thermocline in boreal summerThe westerly wind anomaly is associated with a shift in convection driven by differential cooling of the Maritime Continent and western Pacific in response to insolation forcing
- Subjects
WESTERLIES; EL Nino; SOUTHERN oscillation; OCEAN waves; ATMOSPHERIC models; OCEAN temperature; TROPICAL climate
- Publication
Journal of Geophysical Research. Oceans, 2022, Vol 127, Issue 8, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2021JC017587