Phase relationship of moisture distribution between the Midwest and Southwest of USA during the last termination

Our dune records from Midwest suggest inversed moisture distribution between the Midwest and Southwest on phase level changes within the H1 interval. The comparison shows that during H1b phase from 17.7 to 16.5 ka, Midwest aridity was correlated to Southwest humidity, while a minor ice drafting occurred at the Iberian margin in North Atlantic, which corresponded to one of the coldest Greenland climate. During the H1a phase from 16.5 to 15 ka, the climate reversal from dry to wet in Midwest and wet to dry in the Southwest, responded to a combination that Greenland stayed cold in the first but became warmer in the second half phase and Antarctica experienced an accelerating trend of deglacial warming, while a major ice drafting occurred at the Iberian Margin. During the transition (t-B/A) phase from 15 to 14.7 ka, the return of aridity in the Midwest and humidity in the Southwest corresponded to some cooling in Greenland and perhaps in Antarctica, while no ice drafting occurred in North Atlantic. The moisture distribution over the North America during the H1 stadial seems to respond to the hemispheric thermal changes more rapidly than the teleconnection of air-ocean circulation over North Atlantic. It could result from the different degree of ITCZ displacement in response to hemispheric thermal changes (Chiang and Bitz, 2005). Because North Atlantic circulation was zonal and the atmosphere over the North Atlantic Ocean flowed across variable landmass and ice sheets during H1, their teleconnection could respond to hemispheric thermal changes slowly especially in the east Atlantic Ocean. On the other hand, open water between Arctic and Antarctica in east Pacific Ocean allowed unobstructed shifts of ocean circulation associated with ITCZ. The modern ITCZ is positioned further north over West Pacific and North America compared with North Atlantic basin. If paleo-ITCZ displacement resembled the present analogue during the last termination, the accelerated Southern Hemispheric (SH) warming during H1a phase, perhaps resulting from the bipolar seesaw oscillation, should have impacted the ITCZ and Polar Jet Stream (PJS) northward shifting more effectively over North America than that over the North Atlantic. This interpretation consistent with modeling results that the Pacific-Atlantic seesaw took place during the H1 interval: when Atlantic meridional overturning circulation was shut down and the Pacific meridional overturning circulation became active (Hu et al., 2012). We propose that the SH warming during H1a phase briefly shifted the ITCZ and pushed PJS northward to 41° N in the Mississippi Valley, allowing Pacific and Gulf of Mexico moisture across the southern U.S.A. causing a wetter condition in Midwest and drier condition in Southwest. During H1b and t-B/A phases, Northern Hemisphere cooling shifted the ITCZ southward and pushed the PJS down to 34° N in the Mississippi Valley, tracking rain storms near its front causing a wetter condition in Southwest and drier condition in Midwest. Reference Chiang, J.C.H., Bitz, C.M., 2005. Influence of high latitude ice cover on the marine Intertropical Convergence Zone. Climate Dynamics 25, 477-496. Hu, A., Meehl, G.A., Han, W., Abe-Ouchi, A., Morrill, C., Okazaki, Y., and Chikamoto, M.O., 2012. The Pacific-Atlantic seesaw and the Bering Strait. Geophysical Research letters 39, L03702.