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.

The surface temperature difference between the northern and southern hemispheres is the simplest climate change indicator following global mean temperature, and reveals unique information about the state of the global climate, in particular regarding tropical atmospheric circulation and rainfall. This dissertation examines the historical behavior, future projections, and tropical hydrologic impacts of this interhemispheric (north-south) temperature difference.

Historically, most of the variability in the interhemispheric temperature difference is from the northern hemisphere. Investigation of specific-forcing simulations shows that globally uniform radiative forcing from well-mixed greenhouse gases causes asymmetric northern warming due to the hemispheric land-ocean contrast and Arctic amplification. However, sulfate aerosols, which were disproportionately emitted in the northern hemisphere, caused cooling that masked the northern warming until the mid 1970s. Air pollution regulations in North America and Europe combined with sustained emissions of greenhouse gases have resulted in a positive trend in the interhemispheric temperature difference in the past few decades.

Future simulations of phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5) project that this recent asymmetric northern hemispheric warming will continue in the 21st century. The projected increase is well outside the range of historical variability in both moderate and business-as-usual scenarios. There is also a projected multimodel mean northward shift in the Hadley circulation and tropical rainfall, though the multimodel spread is much larger than for temperature.

A prominent feature in the interhemispheric temperature record is an abrupt decrease around 1970, which was most pronounced in sea surface temperature (SST). Examination of the shift using surface and subsurface ocean datasets reveals that there were pronounced cooling and freshening at depth in the subpolar North Atlantic north of 50°N, which coincided with opposing warming and salting in the western mid-latitude North Atlantic between 35° and 45°N. These combined features cannot be completely accounted for by atmospheric forcing alone. Rather, they point to a discrete subpolar North Atlantic freshening known as the Great Salinity Anomaly and the corresponding weakening of the North Atlantic thermohaline circulation as causes of the 1970 interhemispheric shift.

Spatially, the strongest surface temperature correlations with the interhemispheric SST difference are found in the extratropical North Atlantic and southern hemisphere oceans, which are key regions for the shift around 1970. Additionally, the correlations with low-latitude rainfall and the flow of major rivers are examined. Positive rainfall correlations are found in tropical North Africa, South and Southeast Asia; and negative correlations are found in Australia, southern and eastern South America, and northern Mexico. Among the rivers examined, the Niger, which flows through the Sahel, is most strongly correlated with the interhemispheric SST difference. The Indus is also significantly positively correlated, while the Mississippi and Parana are significantly negatively correlated. Based on this assessment, these regions are likely to experience impacts in response to future variations in the interhemispheric temperature difference from the extratropical Atlantic and southern hemisphere oceans, such as from the meridional overturning circulation or aerosol forcing.

In the first part of the thesis, the role of the Central Asian mountains on North Pacific storminess is examined using an atmospheric general circulation model by varying the height and area of the mountains. A series of model integrations shows that the presence of the Central Asian mountains suppresses North Pacific storminess by 20-30% during boreal winter. Their impact on storminess is found to be small during other seasons. Two main causes of the reduced storminess are diagnosed. First, the decrease in storminess appears to be associated with a weakening of downstream eddy development. The mountains disorganize the zonal coherency of wave packets and refract them more equatorward. As the zonal traveling distance of wave packets gets substantially shorter, downstream eddy development gets weaker, leading to the weakening of transient eddy kinetic energy and storminess. Second, the Central Asian mountains suppress the global baroclinic energy conversion. The decreased baroclinic energy conversion, particularly over the Eastern Eurasian continent decreases the number of eddy disturbances entering into the Western North Pacific. The `barotropic governor' does not appear to be a dominant factor in explaining the results.

In the second part of the thesis, the impact of the Tibetan Plateau on the onset of the South Asian summer monsoon is examined using an atmospheric general circulation model. A series of model integrations shows that rainfall increases downstream of the mountains, whereas upstream of the mountains experiences anomalous subsidence. In particular, the Tibetan Plateau triggers low-level cross-equatorial flow and substantial rainfall over the Bay of Bengal during the pre-monsoon season (April-May). While the low-level cross-equatorial flow begins in May, the monsoon's onset over the Arabian Sea is suppressed until early June. The delayed onset over the Arabian Sea is probably because of the anomalous subsidence induced by the earlier moist convection over the Bay of Bengal. It is suggested that the earlier rainfall over the Bay of Bengal could be the response of stationary waves to the orographic forcing, such as low-level cyclonic motions downstream of the Tibetan Plateau. Because low-level westerlies exist over the Southern Tibetan Plateau during pre-monsoon season, the stationary wave response to the mountains is likely to occur.

This dissertation explores the dynamical linkage between the orographic forcing of the Ti- betan Plateau on the westerlies, and the seasonal migration of the East Asian summer monsoonal rainfall.

Chapter 2 focuses on evolution of the East Asian summer monsoon (EASM) from early to late Holocene. The Holocene EASM was previously characterized as a trend toward weaker monsoon intensity paced by orbital insolation. It is demonstrated here that this evolution is more accurately characterized as changes in the transition timing and duration of the EASM seasonal stages (spring, pre-mei-yu, mei-yu, midsummer), and tied to the north–south dis- placement of the westerlies relative to Tibet. To this end, time-slice simulations across the Holocene are employed using an atmospheric general circulation model. Self-organizing maps are used to objectively identify the transition timing and duration of the EASM seasonal stages. Compared to the late Holocene, an earlier onset of mei-yu and an earlier transition from mei-yu to midsummer in the early to mid-Holocene are found, resulting in a short- ened mei-yu and prolonged midsummer stage. These changes are accompanied by an earlier northward positioning of the westerlies relative to Tibet. Invoking changes to seasonal tran- sitions also provides a more satisfactory explanation for two key observations of Holocene East Asian climate: the “asynchronous Holocene optimum” and changes to dust emissions. A mechanism is proposed to explain the altered EASM seasonality in the simulated early to mid-Holocene. The insolation increase over the boreal summer reduces the pole–equator temperature gradient, leading to northward-shifted and weakened westerlies. The meridional position of the westerlies relative to the Tibetan Plateau determines the onset of mei-yu and possibly the onset of the midsummer stage. The northward shift in the westerlies triggers ear- lier seasonal rainfall transitions and, in particular, a shorter mei-yu and longer midsummer stage.

Chapter 3 explores how the termination of the mei-yu is dynamically linked to the west- erlies impinging on the Tibetan Plateau. It is found that the mei-yu stage terminates when the maximum upper-tropospheric westerlies shift beyond the northern edge of the plateau, around 40◦ N. This termination is accompanied by the disappearance of tropospheric norther- lies over northeastern China. The link between the transit of the jet axis across the northern edge of the plateau, the disappearance of northerlies, and termination of the mei-yu holds on a range of time scales from interannual through seasonal and pentad. Diagnostic analysis indicates that the weakening of the meridional moisture contrast and meridional wind con- vergence, mainly resulting from the disappearance of northerlies, causes the demise of the mei-yu front. We propose that the westerlies migrating north of the plateau and consequent weakening of the extratropical northerlies triggers the mei-yu termination. Model simula- tions are employed to test the causality between the jet and the orographic downstream northerlies by repositioning the northern edge of the plateau. As the plateau edge extends northward, orographic forcing on the westerlies strengthens, leading to persistent strong downstream northerlies and a prolonged mei-yu. Idealized simulations with a dry dynamical core further demonstrate the dynamical link between the weakening of orographically forced downstream northerlies with the positioning of the jet from south to north of the plateau. Changes in the magnitude of orographically forced stationary waves are proposed to explain why the downstream northerlies disappear when the jet axis migrates beyond the northern edge of the plateau.

Chapter 4 proposes an interpretation on the teleconnection between El Niño and East Asian early summer rainband. An intensification of East Asian rainfall usually occurs sum- mers following El Niño. We propose that this teleconnection is mediated via the westerlies impinging on the Tibetan Plateau, through El Niño’s control on tropical tropospheric tem- perature. This is distinct from previous studies that attribute the El Niño’s influence to changes in the Western Pacific subtropical anticyclone. The warming in the eastern equato- rial Pacific leads to uniform warming of the entire tropical troposphere, which sharpens the temperature gradient between the tropics and the subtropics and shifts the westerlies south- ward. The westerlies impinge over the Tibetan Plateau for longer and, through interaction with the topography, induce intense and persistent extratropical northerlies downstream of the plateau that in turn intensifies the East Asian rainband. The rainband has previously been shown to intensify in a similar manner in a warming climate, suggesting that the El Niño response provides an analog for future changes.