Modulations of solar activity on El Niño Modoki and possible mechanisms

Abstract

This paper uses the sunspot number (SSN) index and the El Niño modoki index (EMI) to examine the possible modulation of El Niño Modoki events by variations in solar activity. A significant positive correlation was found between SSN and EMI with a lag of two years, and both SSN and EMI have an obvious period of about 11–12 years. The evolution of El Niño Modoki events was investigated using composite analysis. There was a clear evolution of El Niño Modoki events in the three years after the solar peak year. An ocean mixed layer heat budget diagnostic method is used to investigate the contributor to the anomalous patterns in the three years after the solar peak. The atmosphere radiation fluxes are confirmed as the major contributor to the warming response in the central tropical Pacific. Two possible mechanisms are proposed, one is the direct mechanism that the solar radiation warms up the tropical pacific with a geographical difference, due to the cloud distribution. The warming response in the central Pacific is amplified by the coupled positive feedback between the ocean and atmosphere with 1–2 years lag. Another possible way can be described as follows: the solar heating effect propagating from the upper atmosphere modulates the strength and variation of atmospheric anomaly at high and mid-latitudes in the northern hemisphere winter, which results in an anomalous subtropical cyclone over the northeastern Pacific in the winter seasons following the solar peak years. The anomalous cyclone reduces the cloud cover over the northeastern Pacific and enhances the local input of solar radiation. As a result, a positive sea surface temperature (SST) anomaly occurs over the northeastern Pacific and extends towards the central tropical Pacific along the path of anomalous southwesterly winds, which may trigger an El Niño Modoki event in the following years.