The Indian Ocean Dipole, ENSO, and their interactions
About the Event:
Both the Indian Ocean Dipole (IOD) and ENSO are modes of interannual climate variability arising from tropical ocean-atmosphere interaction. Although the climatological setting of the Indian and Pacific oceans from which these modes arise are significantly different, both phenomena can be understood in the frame work of equatorial dynamics --- in particular, Rossby and Kelvin wave dynamics. In the first part of the talk, I will introduce and compare key elements of each mode of variation, and discuss the main driving mechanisms.
ENSO complexity is a recent topic of much interest, framed in terms of the structural differences between observed El Ninos. Current research posit that two main forms of El Ninos exist: in one, strong SST variations occur in the far-eastern Pacific; the other flavour of El Nino -- sometimes referred to as a Modoki (or fake in Japanese) El Nino – features strong SST variations in the central Pacific. Based on recent researches, I present a new framework to understand the structural differences between El Ninos. A key concept in this framework is that of self-limitation in ENSO dynamics. Self-limitation refers to a feedback mechanism that dampens the growth of SST anomalies in the far-eastern Pacific during El Ninos. In the talk, I will explain how self-limitation can be understood in terms of the atmospheric Kelvin wave response that arises during El Nino events.
Self-limitation implies that El Ninos events should not have strong SST variations in the far-eastern Pacific, a concept that challenges the conventional picture of El Nino behaviour. I will provide observational analyses to suggest that the conventional picture of El Nino as an eastern Pacific phenomenon arises due to the presence of a very small number of eastern-ocean intensified El Ninos. We call these rare events Super El Ninos, in light of their strong SST anomalies in the far-eastern Pacific. Super El Nino events also feature strong rainfall variations in the eastern Pacific and have strong impacts on weather systems and ecosystems in the tropical Pacific and elsewhere. Using simple oceanic and atmospheric modelling, I proceed to demonstrate that super El Ninos arise from the interaction between a positive IOD and El Nino. A particular focus will be on the atmospheric Kelvin waves forced by IOD associated Indian Ocean convection, and their role in generating El Nino-like SST anomalies over the tropical Pacific. Our simple modeling studies suggest, that by persistently modulating western Pacific wind anomalies, IOD teleconnections are a key process involved in the formation of super El Ninos. Finally, we discuss the strong 2015 El Nino and show that decadal SST anomalies account for a significant proportion of its well-noted extreme SST amplitude.
About the Speaker:
Dr. Hameed studies interactions between the ocean and the atmosphere. These interactions---major building blocks of Earth's climate---explain why penguins inhabit the equatorial latitudes of Galápagos, make earth warmer than radiative equilibrium dictates, and, once in many years, generate El Niños that wreak havoc on ecosystems and people across the planet. Dr. Hameed’s research expands the frontiers of this field, with a geographical focus on the Asia-Pacific region. Hosting 60% of the of the world's population, this region is the planet's most disaster-prone region. Further, a rich and varied spectrum of oceanic and atmospheric phenomena that abound in the region provide a natural laboratory for studying instabilities, chaos, pattern formation, and complexity within the realm of geophysical fluid dynamics. Finally, it is the tropical oceans in the region --- the Indian and Pacific oceans--- that generate the major climate modes affecting Earth's climate the most. The dynamics and impacts of the Indian Ocean Dipole (IOD) have taken up a large part of his past research. Recently, Dr. Hameed is trying to understand how interactions between the tropical oceans affect the temporal and spatial structure of inherent tropical modes such as IOD, ENSO, and the Atlantic Niño.