Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability

Abstract

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. T he Arctic has warmed faster than the global average by a factor of 2 or more since the mid-20th century, a phenomenon known as the Arctic amplification. The recent temperature warming over the Arctic is strongly linked to a drastic reduction in sea ice extent since the 1970s, contributing to the Arctic amplification through positive ice-albedo feedbacks (1-3). A similar rapid warming occurred in the Arctic during the early 20th century (4-8). Compared with the recent warming, the early 20th-century Arctic warming (hereafter referred to as the early Arctic warming) is mysterious as greenhouse gas (GHG) radiative forcing was three to four times weaker than at present (9) and changes in sea ice extent were small (10). The comparison of these two warming epochs suggests that mechanisms other than GHG forcing are important for the early Arctic warming.Several hypotheses have been proposed for the early Arctic warming, including intensified natural forcing due to decreased volcanic aerosols and increased solar radiation (11, 12); increased cloud long-wave emissivity due to sulfate aerosols transported from Central Europe (6, 13); uncertain but possible reduction in the Arctic sea ice extent (4,5,14); variability of the North Atlantic ocean-ice-atmosphere system (15); and atmospheric internal variability (16). Neither coupled ocean-atmosphere models nor atmospheric models driven by historical radiative forcing and observed sea surface tempe...

Regime shifts of Mediterranean forest carbon uptake and reduced resilience driven by multidecadal ocean surface temperatures

“…Therefore, amplified AMO, changes in ENSO dynamics and global warming could jointly affect fire frequency in the next decades and impact forest carbon sinks ( Figure S21c). For example, recent works indicate that Arctic warming has been enhanced by the current phase (PDO−, AMO+), leading to a reduction in the poleward temperature gradient and to reduced westerlies (Screen & Francis, 2016;Su, Zhang, & Wang, 2017;Tokinaga, Xie, & Mukougawa, 2017). For example, recent works indicate that Arctic warming has been enhanced by the current phase (PDO−, AMO+), leading to a reduction in the poleward temperature gradient and to reduced westerlies (Screen & Francis, 2016;Su, Zhang, & Wang, 2017;Tokinaga, Xie, & Mukougawa, 2017).…”

“…In addition, Arctic climate amplification processes significantly interact with ocean multidecadal phases. For example, recent works indicate that Arctic warming has been enhanced by the current phase (PDO−, AMO+), leading to a reduction in the poleward temperature gradient and to reduced westerlies (Screen & Francis, 2016;Su, Zhang, & Wang, 2017;Tokinaga, Xie, & Mukougawa, 2017). Additional modelling evidence has forecasted strong interactions between AMOC, AMO and global warming in the next decades (Hansen et al, 2016;Sgubin et al, 2017).…”

On the Increasing Importance of Air-Sea Exchanges in a Thawing Arctic: A Review

“…Seasonality of ocean temperatures is also influenced by the Pacific Decadal Oscillation (PDO) and the co-location of Arctic storm tracks and sea ice cover [39,178,233,234]. On decadal time scales, Pacific and Atlantic inter-decadal variability was linked to rapid early 20th-century Arctic warming though interactions with SST anomalies and the atmospheric circulation [235].…”

Pacific Ocean Variability Influences the Time of Emergence of a Seasonally Ice‐Free Arctic Ocean

“…Negative year-0 IPO is linked to subsequent decreasing mean sea level pressure trends in the North Pacific (Figure 2e), reflecting a strengthening of the AL. A strengthening AL is expected to increase lower atmospheric heat and moisture transport into the Pacific sector of the Arctic (Svendson et al, 2018;Tokinaga et al, 2017). This can be understood in terms of a transitioning IPO, with a shift toward the positive IPO phase coinciding with a strengthening of the AL.…”