Session: Cross-cutting Group 2 (Paleovar, Past to future, Data assimilation)
Author: Zhengyao Lu / firstname.lastname@example.org / Lund University, Sweden
Co-author: Zhengyu Liu, University of Wisconsin Madison, Peking University;
Modern El Niño-Southern Oscillation (ENSO) events are characterized by their phase locking of variability to the seasonal cycle and tend to peak at the end of calendar year. However, in an idealized CCSM3 simulation of the climate of the last 300 thousand years, ENSO seasonal phase locking is shifted periodically following the precessional forcing: ENSO tends to peak in boreal winter when perihelion is near vernal equinox, but peak in boreal summer when perihelion ranges between autumnal equinox and winter solstice.
The mechanism for the change of ENSO’s phase locking is proposed to be caused by the change of seasonality of the growth rate, or the intensity of ocean-atmosphere feedbacks, of ENSO. It is found that the December peak of ‘winter ENSO’ is caused by the continuous growth of ENSO anomaly from June to November, while the June peak of ‘summer ENSO’ appears to be caused jointly by the seasonal shift of positive growth rate in spring and relatively stronger stochastic noise in the first half of the year. Furthermore, the change of the seasonal cycle of feedbacks is contributed predominantly by that of the Ekman upwelling feedback (EK) with the thermodynamic damping playing a secondary role. The summer peak of ENSO is proposed to be caused by the following mechanism. A perihelion in the late fall to early winter forces spring cooling of SST in the eastern equatorial Pacific (EEP) due to reduced insolation, which, reinforced by an oceanic process, leads to weakened thermodynamic damping. The EEP thus becomes more sensitive and favors the growth of the eastern Pacific-like ENSO (opposing to the central Pacific-like ENSO). This generates more uniform basin-wide wind response (quantified as a component of EK), in turn, increased instability in spring, ultimately benefiting the subsequent summer ENSO peak.