The Tropical Pacific Annual Cycle and ENSO in PMIP4 Simulations of the Mid-Holocene

We investigate the tropical Pacific annual cycle and the El Nino/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.