Tropical Rainfall Linked to Stronger Future ENSO‐NAO Teleconnection in CMIP5 Models

Abstract

The El Niño-Southern Oscillation (ENSO) has previously been shown to influence the winter North Atlantic Oscillation (NAO). Here we investigate the ENSO-NAO teleconnection in historical and RCP8.5 scenario CMIP5 simulations and show a future strengthening of the teleconnection under RCP8.5. The teleconnection strength is associated with increased East Pacific tropical rainfall variability. Stratospheric and tropospheric teleconnection pathways are examined, with both pathways having stronger links in the future. The stratospheric pathway involves the Aleutian Low and the stratospheric polar vortex with a downward influence on the NAO. This pathway is clearest in the high-top models that better resolve the stratosphere. The tropospheric pathway is driven by the Pacific subtropical jet strengthening and extending further into the Atlantic in the future, generating increased baroclinicity in the Caribbean and influencing the Atlantic storm track. These results suggest increasing influence of tropical rainfall on extratropical circulation in the future. Plain Language Summary Winter weather over Europe is driven by the North Atlantic Oscillation (NAO) an atmospheric circulation pattern linked to the fluctuating strength of westerly winds across the North Atlantic. Previous work has shown that the NAO is influenced by rainfall in the tropical East Pacific linked to the El Niño-Southern Oscillation (ENSO) an irregular warming and cooling of the tropical Pacific Ocean. We show that the ENSO-NAO link strengthens in a future warmer climate according to climate models. Models with stronger links simulate bigger year-to-year changes in tropical east Pacific rainfall. We investigate two pathways in which the tropical rainfall influences the NAO. In the first pathway, the rainfall is linked to a persistent low-pressure system over the North Pacific; this then weakens the winter stratospheric polar vortex, which in turn affects the NAO. The models that best simulate the stratosphere show this pathway is stronger in the future. In the second pathway, the jet stream over the Pacific extends further into the Atlantic in the future. This affects the Atlantic storm track, which is itself closely linked to the NAO. These results suggest that tropical rainfall has an increasing influence on the winter weather over Europe in the future, which may lead to better future predictability.

Strengthened Causal Connections Between the MJO and the North Atlantic With Climate Warming

“…While a systematic study of the relative importance of each of these mechanisms is outside the scope of this study, Zhou et al (2020) suggest that the extension of the North Pacific jet with warming can explain shifts in MJO teleconnections over the western United States in CMIP5 and CMIP6 simulations. On seasonal timescales, Fereday et al (2020) also concluded that an extension of the Pacific and Atlantic jet-streams under RCP8.5 lead to a strengthening of the tropospheric pathway between the El Nino-Southern Oscillation and the NAO. We also find a robust extension of the North Pacific and North Atlantic jet-streams within CESM2-WACCM (contours in Figure 2c).…”

“…On seasonal timescales, Fereday et al. (2020) also concluded that an extension of the Pacific and Atlantic jet‐streams under RCP8.5 lead to a strengthening of the tropospheric pathway between the El Nino‐Southern Oscillation and the NAO. We also find a robust extension of the North Pacific and North Atlantic jet‐streams within CESM2‐WACCM (contours in Figure 2c).…”

Future Changes in the Indian Ocean Walker Circulation and Links to Kenyan Rainfall

“…Fereday et al. (2020) have recently argued that tropical rainfall will also have an increasing dynamic impact on extratropical climate, meaning that an understanding of tropical rainfall dynamics is essential for both near‐term predictability and longer‐term climate projections on increasingly large spatial scales. We therefore consider these process‐based assessments of rainfall projections to be essential for other parts of the tropics, and they must be used for the new CMIP6 model generation.…”

“…The general pattern of omega decrease to the north of the domain and increase to the south is suggestive of a northward shift of the zone of maximum tropical convergence (Creese, Washington, & Jones 2019;Creese, Washington, & Munday, 2019). Fereday et al (2020) have recently argued that tropical rainfall will also have an increasing dynamic impact on extratropical climate, meaning that an understanding of tropical rainfall dynamics is essential for both near-term predictability and longer-term climate projections on increasingly large spatial scales. We therefore consider these process-based assessments of rainfall projections to be essential for other parts of the tropics, and they must be used for the new CMIP6 model generation.…”

“…The uncertainty in future East African rainfall has been argued to be due to the unresolved nature of the relationship between dynamic and thermodynamic drivers of precipitation change; specifically, the extent to which humidity increases (following the Clausius‐Clapeyron relation) are canceled out by changes to the atmospheric circulation as a result of warming (Chadwick et al., 2013; Fereday et al., 2020). The nature of tropical precipitation changes are argued to be determined by shifts in spatial patterns of zones of convergence and divergence as a result of these circulation changes (Cook et al., 2020; Pfahl et al., 2017), and the primary contributor to uncertainty in model projections of East African rainfall in both rainy seasons is varying simulations of regional circulation dynamics (Rowell & Chadwick, 2018).…”