UC Berkeley

The monsoon is the most important mode of seasonal climate variation in the tropics, responsible for bringing life-giving rainfall to the heavily-populated regions of East Asia, South Asia, and West Africa. Previous paleoproxy and modeling studies have shown that the North Atlantic can exert considerable influence on the strength of the East Asian, South Asian and West African monsoons: colder temperatures over the extratropical North Atlantic lead to their weakening. This dissertation focuses on understanding the mechanisms of the teleconnection from extratropical North Atlantic sea surface temperature (SST) to the East Asian, South Asian and West African monsoons, and the spatiotemporal characteristics of the teleconnection in present and future climate.

We investigate the atmospheric teleconnection mechanism between extratropical North Atlantic cooling and the West African monsoon weakening in an atmospheric general circulation model (GCM), focusing on the role of atmospheric temperatures as the mediator. We find that the simulated atmospheric cooling readily reaches North Africa and the Sahel, and that the cooling in turn weakens the West African monsoon. We explicitly show the latter connection by imposing atmospheric cooling anomalies on the northern boundary of a regional climate model of the Sahel; it responds with a rainfall reduction over the Sahel that is similar to the GCM. In the GCM simulation, extratropical North Atlantic cooling is augmented by a positive low cloud feedback and advected downstream, cooling Europe and North Africa, where the cooling is further amplified by a reduced greenhouse effect from decreased atmospheric specific humidity. The results suggest a thermodynamic pathway for the extratropics-tropics teleconnection found in paleoclimate studies of the last glacial period.

We then investigate whether this teleconnection is relevant in monsoonal changes in present-day and future climates. We focus on the Sahel drought that started during the late 1960's and persisted until the late 1990's. By analyzing 20th century observations and reanalysis, we find a late 1960's climate shift over Eurasia and Africa, characterized by a reduction in summer monsoon rainfall, and surface cooling and pressure increases over the Eurasian and North African continental interiors. The spatial patterns resemble the simulated impact of extratropical North Atlantic cooling in paleoclimate simulations. This late 1960's climate shift is reproduced in GCM simulations with imposed 20th century observed SST and climate forcing. Our analysis of these simulations suggests that the shift originates from an abrupt cooling of extratropical North Atlantic SST over the late 1960's. Idealized GCM simulations with prescribed extratropical North Atlantic cooling also simulate the climate shift. The result herein shows that the observed climate signature of the 1960's abrupt shift in Eurasian and North African climate is consistent with the influence of the abrupt extratropical North Atlantic cooling that occurred in the late 1960's.

Lastly, we investigate if the extratropical North Atlantic is relevant for the future monsoon climate evolution, through analyzing climate projections of the 21st century from the Coupled Model Intercomparison project phase 5 (CMIP5). The dominant pattern of climate change in future projections is global warming; there is also an important pattern that is related to the change of extratropical North Atlantic temperature relative to the global mean. This pattern features coherent changes in extratropical North Atlantic temperature, and Eurasian and North African temperature and precipitation, similar to the pattern found for the late 1960's shift or paleoclimate. We show this in two ways: first, by applying a principle component analysis (PCA) to the spatial patterns of temperature and rainfall changes at the end of the 21st century across CMIP5 models; and second, by applying the PCA to the correlation pattern of extratropical North Atlantic temperature with temperature and precipitation for each ensemble member. The results suggest that the extratropical North Atlantic teleconnection to the Eurasian and North African monsoons is also relevant in future climate projections. As such, it suggests for better understanding of the processes controlling extratropical North Atlantic climate in order to constrain estimates of future monsoon activity.