This project seeks to understand the processes through which the cooling of the extratropical North Atlantic results in a weakening of the North African and Asian monsoons. The weakening of the monsoons in response to North Atlantic cooling is suggested in paleoclimate evidence from the last glacial period, and is also relevant to 20th century climate change including the decades-long Sahel drought. Work under this award will examine the teleconnection between North Atlantic cooling and the monsoon systems through simulations with a simplified climate model, in which the ocean component model is replaced with a "slab ocean" which represents ocean thermodynamics but not dynamics. Preliminary work shows that the model is capable of reproducing the teleconnection, and the research will consist of diagnostic analysis of model output, combined with a suite of model experiments, to identify the primary mechanisms through which the influence of North Atlantic cooling is communicated to the monsoon regions.
In addition to its scientific interest, the question addressed here has societal relevance due to the large number of people who live in the monsoon regions of Asia and North Africa. In addition, the project will support and train a graduate student, thereby providing for the next generation of scientists engaged in this research area.
Embedded during the glacial period in earth's climate history were rapid climate changes occurring on millennial timescales, where cooling (warming) over the high latitude North Atlantic co-incided with a cooling (warming) over the entire Northern Hemisphere, a weakening (strengthening) of the northern hemisphere summer monsoons and a general southward (northward) shift of the tropical rainband. We know now that these events were caused by a slowdown (speedup) of the Atlantic Meridional Overturning circulation (AMOC), and that climate models forced to alter its AMOC strength simulate these global climate changes (figure 1). There is a nice correspondence between models and data. We actually know very little of the mechanisms underlying how these global climate changes are set up once AMOC changes, in particular how the communication (aka 'teleconnection') between the North Atlantic and the tropical rainfall comes about. Moreover, the concept of tropical rainfall influenced by high latitude forcings has not really yet been fully explored to understand tropical rainfall changes in today's climate. The paleoclimate analog suggests that the hemispheric contrast in temperatures - the interhemispheric thermal gradient - plays a crucial role as the tropical rainband 'likes' the warmer hemisphere. Also, thermal forcing from the extratropics is effective at setting up hemispheric temperature changes. The goals of the project was to further our mechanistic understanding of the high-to-low latitude connections, and with an eye towards informing us of possible tropical rainfall changes in today and future climates. To this end, we pursued two general approaches - one where we tried to gain as much knowledge as possible about mechanisms from diagnosing climate model simulations that reproduce this teleconnection. The other approach was to understand what determined the behavior of the interhemispheric thermal gradient in the recent past and future projections, given the strong influence that it has on tropical rainfall. Our project produced four significant results. First, with regards to the teleconnection, we found that tropspheric air temperature is key to meditating the extratropical North Atlantic influence to the North African monsoon. Once the air is cooled over the Sahara, the rainband over the Sahel weakens both because of the reduction to the magnitude of rainfall under colder conditions, but also because of reduced north-south pressure gradients that drove the monsoonal winds. We also found that low-cloud and water vapor raditative feedbacks acted positively to cool northern hemisphere temperatures once the extratropical North Atlantic is cooled. Second, with regards to the interhemispheric thermal gradient, we found that models reproduced the lack in the trend of the intehermipsheric thermal gradient (North minus South) over the 20th centiry, but that this trend becomes positive and increases into the 21st century indicating that the North warms faster than the South. We attributed this latter behavior to both the effects of increased greenhouse gas concentrations that warm the North more than the South; as well as a projected reduction to Northern Hemisphere aerosols. The implication of this is that tropical rainbands may shift northwards in the future, since it 'likes' the warmer hemisphere. Third, we found that an abrupt shift in the interhemispheric gradient in the late 1960's was not anthropogenically forced, but rather likely a result of internal variations. Based on past literature and our own data analysis of subsurface ocean quantities, we concluded that this shift was a result of an abrupt cooling in the high-latitude North Atlantic, possibly induced by the so-called 'Great Salinity Anomaly' where unusually fresh waters invaded the high-latitude North Atlantic Ocean. Lastly, assisting a collaborator at the University of Washington, our work helped show that the increase in vegetation in the Northern extratropics during the mid-Holocene could have induced the North African monsoon to be stronger; this would partly explain the 'Green Sahara' effect. The mechanisms operating there are similar to that for the extratropical thermal influence on tropical rainfall. In summary, our research has advanced our understanding of the mechanisms by which extratropical thermal forcing affects tropcial monsoon rainfall; and also shown how the concept (via the interhemispheric thermal gradient) could potentially be applied to understanding tropical rainfall changes in the past and future. Much more work remains to be done, however, in both understanding the details of the teleconnection, as well as its applicability to understanding tropical rainfall changes in the past and future. The project has produced three peer-reviewed articles in leading climate dynamics journals (a fourth is still being prepared); as well as a book chapter reviewing the ideas and applications of the interhemispheric thermal graident in interpreting past and future tropical rainfall climate changes. Two Ph.D. students were supported on this project, and this research formed the basis for their respective Ph.D. theses. Both students have recently graduated (Spring 2014), and are intending to further pursue postdoctoral research in the field of climate dynamics.
