|Title||Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Tokinaga H., Xie SP, Mukougawa H.|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Type of Article||Article|
|Keywords||air-temperature; atlantic; circulation; climate; climate-change; early 20th-century Arctic warming; Multidecadal variability; north pacific; ocean-atmosphere interaction; oscillation; Pacific decadal variability; pressure; sea-ice; simulations; variability; version 2; wind speeds|
With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20th-century Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean-atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-to-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region.
We have shown that a concurrent phase shift of Pacific decadal variability (PDV) and Atlantic multidecadal variability (AMV) modes is a major mechanism for the unusually intense early 20th-century Arctic warming, and that the atmospheric circulation change is important. Our atmospheric general circulation model (AGCM) experiments indicate constructive contributions of the tropical and extratropical sea surface temperature (SST) forcings. The tropical Pacific warming excites a Pacific/North America (PNA)-like circulation change while the extratropical SST warming strengthens meridional sea level pressure (SLP) gradient over northern Eurasia. The North Atlantic plays a key role in changing atmospheric circulation over the Eurasian Arctic. The Pacific/Atlantic SST warming in the early 20th century was underrepresented in previous reconstructed SST datasets. Our AGCM successfully reproduces the magnitude and spatial distribution of the early Arctic warming when the phase shift of PDV/AMV modes is properly represented. Long coupled model simulations confirm that concurrent PDV–AMV phase shifts affect Arctic temperature trends (Fig. 5), highlighting the importance of regional patterns of SST change. The sensitivity to SST also highlights the need for the reliable reconstruction of the historical evolution, especially before 1950.