Impacts of the Atlantic Multidecadal Variability on North American Summer Climate and Heat Waves

Abstract

The impacts of the Atlantic multidecadal variability (AMV) on summertime North American climate are investigated using three coupled global climate models (CGCMs) in which North Atlantic sea surface temperatures (SSTs) are restored to observed AMV anomalies. Large ensemble simulations are performed to estimate how AMV can modulate the occurrence of extreme weather such as heat waves. It is shown that, in response to an AMV warming, all models simulate a precipitation deficit and a warming over northern Mexico and the southern United States that lead to an increased number of heat wave days by about 30% compared to an AMV cooling. The physical mechanisms associated with these impacts are discussed. The positive tropical Atlantic SST anomalies associated with the warm AMV drive a Matsuno–Gill-like atmospheric response that favors subsidence over northern Mexico and the southern United States. This leads to a warming of the whole tropospheric column, and to a decrease in relative humidity, cloud cover, and precipitation. Soil moisture response to AMV also plays a role in the modulation of heat wave occurrence. An AMV warming favors dry soil conditions over northern Mexico and the southern United States by driving a year-round precipitation deficit through atmospheric teleconnections coming both directly from the North Atlantic SST forcing and indirectly from the Pacific. The indirect AMV teleconnections highlight the importance of using CGCMs to fully assess the AMV impacts on North America. Given the potential predictability of the AMV, the teleconnections discussed here suggest a source of predictability for the North American climate variability and in particular for the occurrence of heat waves at multiyear time scales.

Current and Emerging Developments in Subseasonal to Decadal Prediction

“…A primary general source of S2D atmospheric predictability is remote influences from a variety of teleconnections (e.g., Yuan et al 2018;Ruprich-Robert et al 2018;Beverley et al 2019). Teleconnections associated with anomalous atmospheric circulation patterns arise from changes to the Walker circulation usually driven by anomalous zonal SST gradients (Cai et al 2019), and changes to the Hadley circulation usually driven by anomalous meridional SST gradients, especially interhemispheric differences (Kang et al 2018).…”

“…The causes of AMV are not fully understood, especially the relative roles of internal variability and external forcing from aerosols. However, AMV is modulated to some extent by the oceanic Atlantic meridional overturning circulation (Yeager and Robson 2017), which together with the North Atlantic subpolar gyre is influenced by deep ocean density anomalies particularly in the Labrador Sea (Robson et al 2016); these influences contribute to the especially high multiyear predictability in the North Atlantic (Buckley et al 2019). AMV couples to the Hadley circulation, affecting hurricanes and Sahel rainfall as illustrated in Fig.…”

Half‐a‐Degree Matters for Reducing and Delaying Global Land Exposure to Combined Daytime‐Nighttime Hot Extremes

“…The agreement among the majority of ensemble members/models ensures a robust detection of such events (Masson-Delmotte et al, 2018). The requirement of occurrences with a probability of once in 2 years at least aims to eliminate coincident outlier events randomly modulated by internal variability modes (e.g., Atlantic Multidecadal Oscillation; Ruprich-Robert et al, 2018). Extending the period to the entire twentieth century for selecting the strongest historical case does not affect the result in a measurable manner, because most regions saw their record hot after 1985.…”

Atlantic Ocean influence on Middle East summer surface air temperature

“…Links between summer ME-SAT and Atlantic SST have received even less scientific attention, 36 despite the potential for synergistic effects that might favor highimpact extreme events, particularly record-breaking summer temperatures and extreme heat waves. [37][38][39] In this study, we examine the impact of North Atlantic SST variation, one of the main drivers for Northern Hemisphere climate, on summer ME-SAT. We show that the leading mode of summer ME-SAT variability is strongly correlated with the internal multidecadal mode of variability in the North Atlantic, and that the strong recent warming trend over the ME could persist as long as the North Atlantic Ocean remains anomalously warm.…”