The role of local sea surface temperature pattern changes in shaping climate change in the North Atlantic sector

Abstract

Beside its global effects, climate change is manifested in many regionally pronounced features mainly resulting from changes in the oceanic and atmospheric circulation. Here we investigate the influence of the North Atlantic SST on shaping the wintertime response to global warming. Our results are based on a long-term climate projection with the Max Planck Institute Earth System Model (MPI-ESM) to investigate the influence of North Atlantic sea surface temperature pattern changes on shaping the atmospheric climate change signal. In sensitivity experiments with the model's atmospheric component we decompose the response into components controlled by the local SST structure and components controlled by global/remote changes. MPI-ESM simulates a global warming response in SST similar to other climate models: there is a warming minimum-or "warming hole"-in the subpolar North Atlantic, and the sharp SST gradients associated with the Gulf Stream and the North Atlantic Current shift northward by a few a degrees. Over the warming hole, global warming causes a relatively weak increase in rainfall. Beyond this, our experiments show more localized effects, likely resulting from future SST gradient changes in the North Atlantic. This includes a significant precipitation decrease to the south of the Gulf Stream despite increased underlying SSTs. Since this region is characterised by a strong band of precipitation in the current climate, this is contrary to the usual case that wet regions become wetter and dry regions become drier in a warmer climate. A moisture budget analysis identifies a complex interplay of various processes in the region of modified SST gradients: reduced surface winds cause a decrease in evaporation; and thermodynamic, modified atmospheric eddy transports, and coastal processes cause a change in the moisture convergence. The changes in the the North Atlantic storm track are mainly controlled by the non-regional changes in the forcing. The impact of the local SST pattern changes on regions outside the North Atlantic is small in our setup.

Broad Consistency Between Observed and Simulated Trends in Sea Surface Temperature Patterns

“…We now investigate three ocean regions that are prone to large model trend biases and large observational uncertainty, namely, the equatorial Pacific (Figure 4a; Bordbar et al, 2017; Coats & Karnauskas, 2017; Seager et al, 2019), the Southern Ocean (Figure 4b; Latif et al, 2013; Hobbs et al, 2016; Kostov et al, 2018; Zhang et al, 2019), and the North Atlantic (Figure 4c; Ting et al, 2009; Ruiz‐Barradas et al, 2018; Hand et al, 2019). The changes in the east‐to‐west SST gradient in the equatorial Pacific (3°S–3°N, 120–180°E minus 3°S–3°N, 85–155°W) range from strongly negative to strongly positive for all periods and each single‐model ensemble, while the SST gradient slightly strengthens with time in the observations.…”

Regional Climate Impacts of Future Changes in the Mid–Latitude Atmospheric Circulation: a Storyline View

“…Supporting this pathway, the strength of the North Atlantic storm track increases in experiments inducing a "collapse" of the AMOC [128,129] and it covaries with the AMV on multi-decadal timescales [61,130]. However, climate experiments directly modelling the influence of the projected North Atlantic SST warming patterns provide a less consistent picture: while some studies back up a direct influence on circulation from North Atlantic SSTs [131,132], others only identify a small response [133,134]. The latter results would suggest that the weakening of the AMOC is largely communicated via a modulation of remote climate responses, such as the ratio between the Arctic and tropical warming [134].…”

Observed High-Latitude Precipitation Amount and Pattern and CMIP5 Model Projections

“…This occurs to some extent over NH land and ocean, but the counterclockwise shift in their future points indicates changes to the pattern of precipitation. This can be inferred from inspection of Figure 4: firstly, the NH ocean south of Greenland shows heavy present day precipitation, but near-zero future changes, for which a number of causes have been proposed [53]. Meanwhile the Bering Sea shows moderate present day precipitation and a substantial future percentage increase.…”

“…Their future correlation coefficients are not distinguishably different from those of the non-selected models, suggesting that they show similar future patterns of precipitation but with greater mean increase. future changes, for which a number of causes have been proposed [53]. Meanwhile the Bering Sea shows moderate present day precipitation and a substantial future percentage increase.…”