Last Glacial Maximum lacustrine and fluviatile Formations in the Tibesti and other Saharan mountains, and large-scale climatic teleconnections linked to the activity of the Subtropical Jet Stream
In the mountains of the central Sahara (lat ca. 20° to 22°N, long 16° to 19°E) and particularly in the Tibesti mountains, important lacustrine formations developed during the late Pleistocene, primarily during the Last Glacial Maximum (LGM). Two main phases, separated by a brief regression, intervened between ca. 20,000 and 15,500 BP, and between 15,000 and 12,500 BP. Pollen analyses were carried out on four samples of this formation. The high lacustrine levels were associated to both important precipitations and a reduced evaporation linked to lower temperatures. Similar lacustrine deposits were found in the Djebel Marra in the south of the Sahara. In the mountains of the central and eastern Sahara, during the same period and until the middle Holocene, the "Middle Terrace" Formation was deposited in the river valleys of the Tibesti, Hoggar, Air and the Red Sea Hills. Since the southern headwaters of the Nile were dry from ca. 20,000 to 12,500 BP, the fluviatile sediments deposited in the Nile valley in Nubia may have resulted almost entirely from the numerous wadis flowing from the Red Sea Hills. The rainfalls which fed these lacustrine and fluviatile formations were related to the Tropical Depressions which are formed in the southern part of the westerlies and are linked to the activity of the Subtropical Jet Stream (STJ), whose path remained over the central Sahara from 20,000 BP to the early Holocene. In the Rocky Mountains of the western US, the palaeolakes Lahontan and Bonneville were very large during the LGM and the main fluctuations exhibit similar chronology to that of the Saharan mountains. Broecker [Broecker, W.S., 1994. Massive iceberg discharges as triggers for global climate change. Nature 372, 421-424] estimates that these two large U.S. wet events between ca. 20,000-15,500 BP and ca. 15,000-12,500 BP may have been an indirect result of two large ice surges in the North Atlantic, related to Heinrich layers 1 and 2. We can assume, however, that the similar climatic variation of the Rocky Mountains and the central and eastern Saharan mountains was also a result of the activity of the STJ all along its path, which marks the boundary between the polar and tropical circulations. STJ activity can apparently produce long-distance climatic teleconnections. During the LGM similar teleconnections also existed in the Southern Hemisphere between South Africa and Australia. The Tropical Depressions result from the interaction of polar troughs and the influx of humid equatorial air forming transversal cloud bands. The large increase in the intensity of atmospheric circulation during the LGM was responsible for a large increase in Tropical Depressions in both hemispheres.