A Modeling- and Process-Oriented Study to Investigate the Projected Change of ENSO-Forced Wintertime Teleconnectivity in a Warmer World

Abstract

Considerable uncertainties remain about the expected changes of ENSO and associated teleconnectivity as the climate is warming. Two ensembles of pacemaker experiments using the CNRM-CM5 coupled model are designed in a perfect model framework to contrast ENSO-forced teleconnectivity between the preindustrial period versus a warmer background state (obtained from a long stabilized simulation under late-twenty-first-century RCP8.5 constant forcing). The most notable sensitivity to the mean background state is found over the North Atlantic, where the ENSO–NAO teleconnection is considerably reinforced in a warmer world. We attribute this change to (i) a stronger and eastward-extended mean upper-level jet over the North Pacific, (ii) an eastward-shifted ENSO teleconnection over the North Pacific, and (iii) an equatorward-shifted and reinforced mean jet over the North Atlantic. These altogether act as a more efficient waveguide, leading to a better penetration of synoptic storms coming from the Pacific into the Atlantic. This downstream penetration into the North Atlantic basin forces more systematically the NAO through wave breaking. The reinforcement in the teleconnection is asymmetrical with respect to the ENSO phase and is mainly sensitive to La Niña events. Even though the Pacific jet tends to retract westward and move northward during cold events, mean changes are such that both Pacific and Atlantic jets remain connected in a warmer climate by contrast to the preindustrial period, thus ensuring preferred anticyclonic wave breaking downstream over the North Atlantic leading ultimately to NAO+ events.

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

“…The strengthened teleconnection is associated with increased future variability in tropical East Pacific rainfall; the large majority of models show increases in the mean and interannual variability of rainfall in this region, with the strongest teleconnection seen in models with the strongest rainfall variability. This finding contrasts with Drouard and Cassou (2019), who show a stronger NAO‐ENSO link in future simulations for which tropical Pacific SST has a warmer mean state, but the same variability as their control simulations.…”

“…Figure 3c suggests an eastward shift in the Aleutian Low, with a stronger and more zonally oriented low‐pressure region extending across North America into the NAE sector, linked to a significantly intensified negative NAO pattern. Consistently, previous work shows that the PNA teleconnection pattern forced by ENSO moves east and strengthens under global warming (Drouard & Cassou, 2019; Zhou et al, 2014). We now examine two ENSO‐NAO teleconnection mechanisms, one stratospheric and one tropospheric.…”

“…Figure 5e shows that these changes are significant. For 21C, a coherent jet structure therefore extends from the Pacific to the Atlantic at upper levels, consistent with Drouard and Cassou (2019).…”

“…Figure 5 suggests that the stronger Pacific jet in the future could further enhance baroclinicity and eddy growth in this Atlantic region during El Niño, creating a stronger connection to the negative NAO. The stronger connection could also be due to the stronger subtropical Pacific jet acting as a waveguide that enhances propagation of synoptic wave packets from the Pacific into the Atlantic (Drouard & Cassou, 2019).…”

“…Given the projected increased rainfall variability, one might expect a stronger and more predictable ENSO‐NAO teleconnection. Studies based on single models suggest stronger ENSO‐NAE region links (Herceg Bulić et al, 2012) or stronger ENSO‐NAO links (Drouard & Cassou, 2019; Müller & Roeckner, 2006, 2008) in the future. Drouard and Cassou (2019) find that the synoptic wave packet mechanism between the Pacific and Atlantic basins strengthens in a warmer climate.…”