Contraction of the Western Pacific Intertropical Convergence Zone During the Little Ice Age

Abstract:

Both palaeoclimate reconstructions and climate modeling have demonstrated that
precipitation in low latitudes is primarily controlled by north- south migration of the global
Intertropical Convergence Zone on millennial to orbital timescales. These migrations are
associated with the occurrence of opposite rainfall variations between the two
hemispheres. However, the pattern of rainfalls around the marine-continental tropical
western Pacific region over the last millennium remains unclear. Several recent studies
suggest a southward migration of the Intertropical Convergence Zone during the Little Ice
Age (~AD 1400-1850). Concomitantly, dry Little Ice Age conditions should have occurred in
the northern extent of the Intertropical Convergence Zone and wet conditions around the
southern limit. However, our synthesis of proxy hydrology records from the Asian-
Australian monsoon area documents a distinctly different rainfall pattern that violates this
expectation, suggesting instead the occurrence of synchronous retreat of the East Asian
Summer Monsoon and the Australian Summer Monsoon during the Little Ice Age. Thus we
propose an alternative dynamic scenario: rather than strict north-south migration, the
apparent mode of multi-decadal to centennial change for the western Pacific Intertropical
Convergence Zone is contraction/expansion in response to external forcings such as solar
irradiance variation and large volcanic eruptions.