PhD Defense : Maria Gabriela Escobar Franco “Interactions between Kelvin and tropical instability waves in the equatorial pacific ocean”
YouTube Link :https://www.youtube.com/watch?v=w5DzpEpPJIU
The tropical mean climate is the result of a tight interplay between the ocean and overlaying atmosphere. Any perturbations of this subtle balance can trigger intense oceanic and atmospheric responses, resulting in a modulation of climate variability over a wide range of time scales. In particular, the Eastern Equatorial Pacific (EEP) is home to the El Niño/Southern Oscillation (ENSO), the strongest year-to-year climate fluctuation with widespread effects on global climate, weather patterns and societies. Despite tremendous progress in ENSO research over the last two decades, considerable challenges remain to understand its newly recognized pattern diversity and temporal complexity. Nonlinear processes and associated time scales interactions have been pointed out as a source of irregularity for ENSO and the climate variability in general. Among others, high frequency oceanic processes such as the intraseasonal tropical variability have been suggested to play a significant role in ENSO onset, evolution, termination, spatial diversity and temporal complexity.
In particular, intraseasonal Kelvin waves (IEKWs) and Tropical Instability Waves (TIWs) are essential components of the tropical Pacific coupled climate variability. While downwelling IKWs are precursors of ENSO, TIWs can contribute to its asymmetry by mixing more/less warm off-equatorial and cold tongue waters during La Niña/El Niño. While both Kelvin waves and TIWs have long been recognized to be part of a complex interplay of time scales interactions in the equatorial Pacific, far fewer studies have highlighted the potential for interactions between them. Yet the few studies that have addressed this issue have suggested that their interaction can lead to strong impacts in the eastern equatorial Pacific.
The purpose of this thesis is to enhance our understanding of the nonlinear interactions occurring at intraseasonal time scales between the transient TIWs mesoscale activity and equatorial Kelvin Waves. By documenting for the first time their interaction from satellite observations over a period spanning almost 30 years, we evidence that 42% of the variance of TIWs-induced intraseasonal sea level anomalies are associated with equatorial Kelvin waves activity. Then, the analysis of two types of simulations' ensembles of the tropical Pacific basin, one with the full mesoscale activity associated with the presence of TIWs, the other without non-linear terms in the momentum equation to damp the TIWs variability, reveals a marked asymmetric response of TIWs to the passage of IEKWs between their two different phases. In particular, we observe that the passage of upwelling IEKWs induces a much larger increase in the barotropic instabilities in the upper-ocean and baroclinic instabilities in the subsurface layer than of downwelling IEKWs. In addition, we show that the interaction between TIWs and IEKWs can promote substantial changes in non-linear heat advection in the eastern equatorial Pacific, which may have strong implications for the oceanic teleconnection between the western and eastern Pacific and the low-frequency variability in this key region for the ENSO.
- Jérôme Vialard – LOCEAN – Referee
- Xavier Carton – UBO – Referee
- Véronique Garçon – LEGOS – Examiner
- Daria Gushchina – Université de Moscou – Examiner
- Karina von Schuckmann – MERCATOR – Invited Member
- Julien Boucharel – LEGO – Advisor
- Boris Dewitte – CERFACS/CECI – Co-Advisor