Search

Paleo-proxy reconstructions reveal a moistening of the American Southwest during the Last Glacial Maximum (LGM; 21 ka). However, the primary mechanisms driving the moistening trend are still debated, with relatively few studies focused on hypotheses related to synoptic changes in precipitation. Analysis of the Paleoclimate Intercomparison Model Project (PMIP3) simulations shows enhancement of precipitation in the southwest and south-central United States during the winter and summer. Here, we suggest that summertime eastward phase shifting of stationary waves at the LGMenhanced precipitation in the south-central United States and dried the southeastern United States. Mechanism denial experiments performed with version 3 of the Hadley Centre Coupled Model (HadCM3) indicate that the thermodynamic effect of the Laurentide Ice Sheet forced eastward phase shifting of stationary waves. By comparing a synthesis of LGM proxies to the PMIP3 ensemble, we find models that compare more favorably to the reconstructions simulate a weaker Laurentide ice thermodynamic effect, smaller eastward phase shifting of stationary waves, and weaker jet stream anomalies. [ABSTRACT FROM AUTHOR]

In the past 40 years, the global annual mean surface temperature has experienced a nonuniform warming, differing from the spatially uniform warming simulated by the forced responses of large multimodel ensembles to anthropogenic forcing. Rather, it exhibits significant asymmetry between the Arctic and Antarctic, with intermittent and spatially varying warming trends along the Northern Hemisphere (NH) midlatitudes and a slight cooling in the tropical eastern Pacific. In particular, this "wavy" pattern of temperature changes over the NH midlatitudes features strong cooling over Eurasia in boreal winter. Here, we show that these nonuniform features of surface temperature changes are likely tied together by tropical eastern Pacific sea surface temperatures (SSTs), via a global atmospheric teleconnection. Using six reanalyses, we find that this teleconnection can be consistently obtained as a leading circulation mode in the past century. This tropically driven teleconnection is associated with a Pacific SST pattern resembling the interdecadal Pacific oscillation (IPO), and hereafter referred to as the IPO-related bipolar teleconnection (IPO-BT). Further, two paleo-reanalysis reconstruction datasets show that the IPO-BT is a robust recurrent mode over the past 400 and 2000 years. The IPO-BT mode may thus serve as an important internal mode that regulates high-latitude climate variability on multidecadal time scales, favoring a warming (cooling) episode in the Arctic accompanied by cooling (warming) over Eurasia and the Southern Ocean (SO). Thus, the spatial nonuniformity of recent surface temperature trends may be partially explained by the enhanced appearance of the IPO-BT mode by a transition of the IPO toward a cooling phase in the eastern Pacific in the past decades. [ABSTRACT FROM AUTHOR]