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During the 1990s, pronounced changes occurred in major climatological fields worldwide. In the Northern Hemisphere, regarding the major shifts in the dry and wet patterns, the mid-1990s decadal change occurred more rapidly in winter than in summer. What accompanied by this decadal change were the strengthened lower-tropospheric anticyclones over the subtropical Pacific Ocean and a northward shift of the midlatitude westerly jet stream across the Afro–Asia–Pacific region. Increased trade winds resulted in considerable sea surface cooling over the central and eastern Pacific regions. These circulation changes induced the poleward expansion of two boreal winter systems: convection over the Maritime Continent and dry conditions in North Africa and West Asia. Moreover, the weakening of the Arctic upper-tropospheric circulation further modulated the seasonal shift of the midlatitude jet stream. In boreal summer, the more gradual decadal change was associated with the westward shift of Asian monsoon and northward penetration of African monsoon. What used to be a substantial vertical coupling of the westerly flows above these two monsoons was weakened during the 1990s. Judging from the effects of these circulation changes, it appears that the different rate of change in the jet streams may have exerted a dynamic effect on the pace of the decadal change in different seasons. [ABSTRACT FROM AUTHOR]

The late Pliocene is a pre-Quaternary warming period compared to the preindustrial period. Here, based on the updated PRISM4 (Pliocene Research, Interpretation and Synoptic Mapping version 4) boundary conditions from the Pliocene Model Intercomparison Project phase 2 (PlioMIP2), seven experiments are conducted to simulate the global monsoon in the late Pliocene interglacial period and further investigate the impacts from changes in the individual boundary conditions by using the Community Earth System Model (CESM 1.0.4). The simulated late Pliocene interglacial climate is generally warmer and wetter than the preindustrial period climate, with an expanded monsoon domain, particularly in North Africa, Asia and Australia. The differences in topography, vegetation and lakes outside of Greenland and Antarctica account for the greatest contribution to monsoon changes. Moreover, orbital parameter variations further significantly affect monsoon domains and lead to significant fluctuations in monsoon precipitation and winds on the orbital scale, and the changed magnitude is even larger than that between the late Pliocene and preindustrial period when orbital forcing is not considered. [ABSTRACT FROM AUTHOR]

We investigate, for the first time, the response of the North African summer monsoon to separate precession and obliquity forcings using a high-resolution state-of-the-art coupled general circulation model, EC-Earth. Our aim is to better understand the mechanisms underlying the astronomical forcing of this low-latitude climate system in detail. The North African monsoon is strengthened when northern hemisphere summer insolation is higher, as is the case in the minimum precession and maximum obliquity experiments. In these experiments, the low surface pressure areas over the Sahara are intensified and located farther north, and the meridional pressure gradient is further enhanced by a stronger South Atlantic high pressure area. As a result, the southwesterly monsoon winds are stronger and bring more moisture into the monsoon region from both the northern and southern tropical Atlantic. The monsoon winds, precipitation and convection also extend farther north into North Africa. The precession-induced changes are much larger than those induced by obliquity, but the latter are remarkable because obliquity-induced changes in summer insolation over the tropics are nearly zero. Our results provide a different explanation than previously proposed for mechanisms underlying the precession- and, especially, obliquity-related signals in paleoclimate proxy records of the North African monsoon. The EC-Earth experiments reveal that, instead of higher latitude mechanisms, increased moisture transport from both the northern and southern tropical Atlantic is responsible for the precession and obliquity signals in the North African monsoon. This increased moisture transport results from both increased insolation and an increased tropical insolation gradient. [ABSTRACT FROM AUTHOR]