Abstract:
The term ``teleconnections'' is used to describe relationships between weather phenomena or climate patterns occurring at widely separated locations on Earth.
The understanding of such connections enables the formulation of predictions with a lead time of several weeks to months which allows a better preparation for extreme events.
A specific focus of this thesis is set on the investigation of teleconnection structures of extreme rainfall events (EREs) within the Indo-Pacific Monsoon domain.
EREs are a major source of natural disasters in this region and are often associated with severe flooding, landslides, and loss of life and property.
A better understanding of the teleconnection structures of EREs is crucial for the development of early warning systems in the region.
However, the complexity of the climate system and the multitude of processes involved in the formation of teleconnections make it challenging to identify teleconnection structures and understand their underlying mechanisms.
This work uses so-called climate networks, derived from complex network science, to capture teleconnection structures. Climate networks are constructed by representing the climate system as a network of nodes representing grid points on Earth which are connected by links derived from statistical relationships between climate variables. The topology of the network allows conclusions about the underlying physical processes forming teleconnection patterns.
In this work, two different network measures are introduced to quantify the strength and spatial organization of teleconnection patterns.
The first measure to be drawn, the curvature approach, is based on the concept of detecting the structure of the most relevant bundles of links in the network.
The second measure, the community detection approach, is based on the concept of detecting groups of densely connected nodes in the network.
These two measures are applied in different studies:
i) The first study presents the curvature approach, which is an intuitive way to visualize differences between regional links and long-range teleconnection structures within climate networks.
This study employs this to investigate differences in teleconnection patterns of the El Nino Southern Oscillation (ENSO).
The analysis suggests that El Nino events with positive sea surface temperatures (SSTs) anomalies in the Eastern Pacific (EP) reveal teleconnection patterns primarily confined to the Tropics, whereas the teleconnection patterns of El Nino events with positive SST anomalies in the Central Pacific (CP) show teleconnections to the Subtropics and Midlatitudes as well.
ii) The second study investigates the teleconnection structures of EREs within the Indo-Pacific Monsoon domain. The community detection approach reveals that the spatial organization of EREs is dominated by the Boreal Summer Intraseasonal Oscillation (BSISO), a quasi-periodic north-eastward movement of convective precipitation from the Indian Ocean to the Western Pacific. This study reveals different local propagation pathways of EREs within the Indo-Pacific Monsoon domain and uncovers the El Nino Southern Oscillation (ENSO) as the key driver in modulating these pathways.
iii) The third study analyzes the synchronization between two of the most pronounced monsoon systems on Earth, the Indian and the West African Monsoon. This work identifies a teleconnection structure between these two monsoon systems that exists on intraseasonal timescale. The teleconnection is modulated by a variety of different atmospheric processes.
Taken together, the three studies included in this thesis present two different ways to intuitively visualize the most dominant teleconnections
for the area under investigation. Using these insights our studies contribute to the understanding of teleconnection structures from local to global scales and provide a new perspective on the mechanisms that govern the yearly variability in the occurrences of EREs.
