Does vertical temperature gradient of the atmosphere matter for El Niño development?

Abstract

In this work, we examine the connection of vertical temperature gradient of the tropospheric atmosphere along the equator with El Nino–Southern Oscillation (ENSO) and the possible impact of the long-term change of the gradient. It is suggested that when the temperature anomalies in the lower troposphere are relatively warmer (cooler) than in the upper troposphere, the atmosphere is less (more) stable and favors an El Nino (a La Nina) event to develop. ENSO evolutions in 1997–1998 and 2014–2015 events are good examples of this relationship. They started from similar ocean anomaly states in the springs of 1997 and 2014, but developed into an extreme El Nino in 1997–1998 and a borderline El Nino in 2014–2015. That may be partially due to differences in the evolutions of the vertical temperature anomaly gradient in troposphere. Thus, in addition to the significant atmospheric response to ENSO, the preconditioning of vertical gradient of the tropospheric temperature due to internal atmospheric processes to some extent may play an active role in affecting ENSO evolution. The long-term trend with more pronounced warming in the upper troposphere than in the lower troposphere causes a reduction in the vertical temperature gradient in the troposphere. Moreover, unlike almost homogenous warm anomalies in the upper troposphere, the lower troposphere shows remarkable regional features along the equator during 1979–2014, with cold anomaly trends over the central and eastern Pacific Ocean associated with the so-called hiatus and some warm anomalies on its two sides in the east and west. This vertical and zonal distribution of the air temperature trends in the troposphere over the Pacific Ocean is consistent with the convection suppression over the central Pacific since 2000, implying a weakening of atmosphere and ocean coupling.