Weakened feedback of the Indian Ocean on El Niño since the early 1990s

Abstract

The present paper investigates changes in the feedback of Indian Ocean warming on the western Pacific by using observational data and model experiments. It is found that the relationship between easterly wind anomalies over the equatorial western Pacific and Indian Ocean warming during the mature and decaying phases of El Nino has changed since 1994. Before 1994, when El Nino reached its peak in the winter, significant warming of the entire Indian Ocean caused tropospheric temperature anomalies and emanated Kelvin waves into the western Pacific. Thus, Indian Ocean warming produced easterly wind anomalies over the western Pacific. However, significant warming of the Indian Ocean has been confined to the west of the Indian Ocean and Kelvin waves have been weakened since 1994. Therefore, the Indian Ocean has contributed less to easterly wind anomalies in the western Pacific after 1994. Changes in the intensity and pattern of the Indian Ocean dipole (IOD) mode may be responsible for these results. After 1994, the intensity of the IOD, especially the intensity of the cold component of the IOD mode, has become stronger, resulting in Indian Ocean warming being confined to the west of the Indian Ocean. These results are confirmed by numerical model experiments. The paper also discusses the enhancement in the feedback of the north tropical Atlantic on El Nino after 1994.