The full programme book (PDF) - Royal Geographical Society

T3
Spatial and temporal variability of North American Monsoon during the Holocene
S.J. Davies 1 *, J. Barron 2 , S. E. Metcalfe 3
1 Department of Geography and Earth Sciences, Aberystwyth University, Penglais, Aberystwyth, SY23
3DB
2 United States Geological Survey, 345 Middlefield Road, Menlo Park, Colorado 94025-3561, USA
3 School of Geography, University of Nottingham, University Park, Nottingham, NG7 2RD
The North American Monsoon (NAM) is a vital moisture source for central and northern
Mexico and the south-west USA. With climate change projections suggesting both
increased temperatures and reduced precipitation in the region, a more complete
understanding of long term variability and forcing mechanisms is required. Whilst a direct
relationship between insolation and monsoon strength is to be expected through the
Holocene, as exists for other monsoon regions, considerable variability is observed across
the NAM region. The present day summer monsoon regime is influenced by precipitation
from the Gulf of Mexico / Atlantic and the Gulf of California / Pacific, although there is
regional variation in the relative importance of the two moisture sources. This results in a
spatially variable response to forcings such as the Atlantic Multidecadal Oscillation (AMO),
the Pacific Decadal Oscillation (PDO) and the El Niño Southern Oscillation (ENSO).
Seasonality of precipitation is also key factor, with winter rainfall being more important in
the northern NAM region.
Here, we bring together palaeoclimate data from both marine and continental proxy
records to reconstruct spatial patterns of change in the NAM through the Holocene. The
evidence indicates a muted response to enhanced orbital forcing during the early
Holocene, which we attribute to the continuing influence of the residual Laurentide Ice
Sheet. The monsoon appears to have been centred over a more westerly location at this
time. By 7.5 cal ka BP, conditions were wetter than present across the region. A more
complex picture emerges during the mid-Holocene, with more regional variability around 6
cal ka BP. The interval between c. 3 and 4 cal. ka BP represents a period of major
reorganisation in the NAM system. After this, conditions are generally drier and oscillating,
particularly in the south, with a more southerly ITCZ and ENSO playing an important role
in driving regional differences.
Keywords: North American Monsoon; Holocene; ENSO; ITCZ