Mechanism underlying enhanced Predictions of Atlantic Nino in the Equatorial Atlantic

Abstract

The Atlantic Niño is the dominant mode of interannual sea surface temperature (SST) anomalies in the equatorial Atlantic, exerting a profound influence on modulating global and regional weather patterns, including West African rainfall and tropical Atlantic hurricane activity. The dynamics of the Atlantic Niño share similarities with the El Niño-Southern Oscillation (ENSO) in the tropical Pacific, both involving atmosphere-ocean coupled positive feedback processes referred to as the Bjerknes positive feedback. Despite similarities with ENSO-particularly in its coupled ocean-atmosphere interactions, the Atlantic Niño has historically shown lower predictability and weaker ocean-atmosphere positive feedback strength. This thesis investigates the underlying physical mechanisms, classification, and evolving predictability of Atlantic Niño events using a combination of observational datasets, reanalysis products, and state-of-the-art seasonal forecast and global climate model outputs. The first component of the present study is to revisit the classical Bjerknes positive feedback loop that governs the dynamics of equatorial Atlantic SST anomalies. Using regression analyses of SST, zonal winds, zonal wind stress, upper heat content, sea surface height, and precipitation, the presence of a complete coupled positive feedback loop in the equatorial Atlantic was investigated. The results indicate that the SST-wind, SST-thermocline, and SST-heat content couplings are all active, with explained variances ranging from 21% to 46%. Additionally, the seasonal migration of the intertropical convergence zone (ITCZ) and associated changes in deep atmospheric convection are found to modulate the strength and spatial structure of the feedback processes. These findings underscore the central role of atmospheric diabatic heating in initiating and sustaining Atlantic Niño events. The second component of the present study examines the evolving nature of atmosphere-to-ocean coupling over the equatorial Atlantic in recent decades. By comparing the periods 1979-1999 and 2000-2021 using both observations and a multi-model ensemble from the CMIP6 Atmospheric Model Intercomparison Project (AMIP), the significant post-2000 weakening in key subprocesses of the Bjerknes feedback was identified and investigated. Specifically, the study observes a diminished sensitivity of deep convection to zonal SST gradients and a weaker relationship between precipitation and sea level pressure gradients. While the wind stress response to pressure gradients remains essentially unchanged, the overall weakening is most pronounced in the western and central equatorial Atlantic regions associated with peak convection and SST variability. These findings suggest a decline in the efficiency of coupled ocean-atmosphere feedback, with implications for the intensity, evolution, and forecast skill of Atlantic Niño events in a changing climate. The third component of this research is to investigate the prediction skill of the Atlantic Niño events by distinguishing between canonical and non-canonical Atlantic Niño event types. Canonical Atlantic Niño events exhibit strong coupled dynamics analogous to Pacific ENSO events and are often preceded by La Niña-like SST variability in the tropical Pacific. These events can be predicted up to six months in advance using a 51-member ensemble operational seasonal forecast system from the European Centre for Medium-Range Weather Forecasts (ECMWF). In contrast, the non-canonical Atlantic Niño events lack strong atmosphere-ocean coupling and are scarcely predictable, but they lead to the emergence of a Pacific La Niña-like variability. This classification provides a new framework to disentangle the two-way connection between the equatorial Atlantic and the Pacific Oceans and improve seasonal climate prediction across the tropics. The study results provide new insights into the coupled dynamics, classification, and evolving predictability of Atlantic Niño events.