HR: 17:45h
AN: PP34B-08    [Abstracts]
TI: Was the North Pacific wintertime climate less stormy during the mid-Holocene?
AU: * Chiang, J C
EM: jchiang@atmos.berkeley.edu
AF: Dept of Geography and Center for Atmospheric Sciences, University of California, Berkeley, CA 94720-4740, United States
AU: Fang, Y
EM: yfang@atmos.berkeley.edu
AF: Dept of Geography and Center for Atmospheric Sciences, University of California, Berkeley, CA 94720-4740, United States
AU: Chang, P
EM: ping@ocean.tamu.edu
AF: Dept of Oceanography, Texas A&M, College Station, TX 77843, United States
AB: We present intriguing model evidence that North Pacific wintertime activity in the storm track region was significantly weaker during the mid-Holocene (~6,000 bp), though a mechanism that has an analog in today's climate. This decreased activity is shown across several PMIP2 coupled model simulations of the mid-Holocene climate, and in our coupled ocean atmosphere model. The reduced activity is co-incident with basinwide climate changes over the North and tropical Pacific, including a deepening of the Aleutian low, colder SSTs in the western and central North Pacific, a strengthening and southward shift of the subtropical jet, and a strengthened South Pacific Convergence Zone (SPCZ). Northward heat transport by transient eddies is reduced, but is more than compensated for by an increase in the stationary eddy transport. The climate changes show striking parallels to similar changes observed in the modern climate during times of reduced storm track activity associated with the ‘mid-winter suppression' first documented by H. Nakamura. The linkage suggests that dynamically the mid-Holocene and present-day share common origins. We postulate that the change in the North Pacific storminess originates from a strengthened SPCZ and weakened north branch of the ITCZ induced as a simple consequence of the mid-Holocene insolation changes. The changed convection alters the Hadley circulation and increases the northward angular momentum transport that strengthens the north Pacific subtropical jet, creating the ‘midwinter suppression' conditions that reduce activity over the storm track region. A similar reduction in the storm track activity also occurs over the North Atlantic during the mid-Holocene as well, with similar changes in the large-scale North and tropical Atlantic climates (deeper Icelandic low; colder midlatitude north Atlantic SST; strengthened north Atlantic subtropical jet; and southward shifted Atlantic ITCZ). If the reduced mid-Holocene atmospheric activity turns out to be correct, then it offers potentially a straightforward explanation for why mid-Holocene ENSO activity was significantly reduced as well (as noted in several paleoproxies), as the atmosphere provides less stochastic forcing to the ENSO system.
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3344 Paleoclimatology (0473, 4900)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting