Water mass transformation (WMT) in the North Atlantic plays a key role in driving the Atlantic Meridional Overturning Circulation (AMOC) and its variability. Here, we analyze subpolar North Atlantic WMT in high- and low-resolution versions of the Community Earth System Model version 1 (CESM1) and investigate whether differences in resolution and climatological WMT impact low-frequency AMOC variability and the atmospheric response to this variability. We find that high-resolution simulations reproduce the WMT found in a reanalysis-forced high-resolution ocean simulation more accurately than low-resolution simulations. We also find that the low-resolution simulations, including one forced with the same atmospheric reanalysis data, have larger biases in surface heat fluxes, sea-surface temperatures, and salinities compared to the high-resolution simulations. Despite these major climatological differences, the mechanisms of low-frequency AMOC variability are similar in the high- and low-resolution versions of CESM1. The Labrador Sea WMT plays a major role in driving AMOC variability, and a similar North Atlantic Oscillation-like sea-level pressure pattern leads AMOC changes. However, the high-resolution simulation shows a pronounced atmospheric response to the AMOC variability not found in the low-resolution version. The consistent role of Labrador Sea WMT in low-frequency AMOC variability across high- and low-resolution coupled simulations, including a simulation which accurately reproduces the WMT found in an atmospheric-reanalysis-forced high-resolution ocean simulation, suggests that the mechanisms may be similar in nature.

State-of-the-art climate models simulate a large spread in the projected decline of Arctic sea-ice area (SIA) over the 21st century. Here we diagnose causes of this intermodel spread using a simple model that approximates future SIA based on present SIA and the sensitivity of SIA to Arctic temperatures. This model accounts for 70-95% of the intermodel variance, with the majority of the spread arising from present-day biases. The remaining spread arises from intermodel differences in Arctic warming, with some contribution from differences in the local sea-ice sensitivity. Using observations to constrain the projections moves the probability of an ice-free Arctic forward by 10-35 years when compared to unconstrained projections. Under a high-emissions scenario, an ice-free Arctic will likely (>66% probability) occur between 2036-2056 in September and 2050-2068 from July-October. Under a medium-emissions scenario, the ‘likely’ date occurs between 2040-2062 in September and much later in the 21st century from July-October.

North Atlantic sea-surface temperatures (SSTs) exhibit variability on seasonal to decadal timescales, providing a potential source of predictability for the atmospheric circulation and regional climate on these timescales. Recent work has shown that initialized climate models have skill in predicting the decadal evolution of North Atlantic SSTs [1], but this will only help to predict regional climate in the surrounding continents if models can correctly simulate the atmospheric response to these SST anomalies. There is growing evidence that models systematically underestimate the atmospheric response to extratropical SST anomalies [2], and that this may be rectified by increasing the atmospheric resolution to resolve mesoscale processes over ocean frontal zones [3]. Here, we investigate the large-scale atmospheric circulation response to idealized Gulf Stream SST anomalies in two configurations of the Community Atmospheric Model (CAM6), one with 1-degree resolution globally and one with regional grid refinement of 1/8-degree over the North Atlantic. The variable resolution configuration, which resolves mesoscale atmospheric processes, shows a large negative response of the wintertime North Atlantic Oscillation (NAO) to a strengthening of the SST gradient across the Gulf Stream (a 2-standard-deviation NAO anomaly for SST anomalies that vary between ±2°C). The response is substantially weaker and has a different spatial structure in the lower resolution simulations. The large-scale atmospheric circulation response in the variable resolution simulations results from mesoscale processes that enhance convection over the Gulf Stream and lead to latent-heating and divergence anomalies in the upper troposphere. These results suggest that the atmospheric circulation response to extratropical SST anomalies may be fundamentally different at higher resolution. Regional refinement in key regions offers a potential pathway towards improving simulation of the atmospheric response to extratropical SST anomalies and thus improving multi-year regional climate predictions. [1] Yeager, S.G., et al., 2018, https://doi.org/10.1175/BAMS-D-17-0098.1. [2] Simpson, I.R., et al., 2018, https://doi.org/10.1175/JCLI-D-18-0168.1. [3] Czaja, A., et al., 2019, https://doi.org/10.1007/s40641-019-00148-5.

North Pacific atmospheric and oceanic circulations are key missing pieces in our understanding of the reorganisation of the global climate system since the Last Glacial Maximum (LGM). Here, using a basin-wide compilation of planktic foraminiferal δ18O, we show that the North Pacific subpolar gyre extended ~3 degrees further south during the LGM, consistent with sea surface temperature and productivity proxy data. Analysis of an ensemble of climate models indicates that the expansion of the subpolar gyre was associated with a substantial gyre strengthening. These gyre circulation changes were driven by a southward shift in the mid-latitude westerlies and increased wind-stress from the polar easterlies. Using single-forcing model runs, we show these atmospheric circulation changes are a non-linear response to the combined topographic and albedo effects of the Laurentide Ice Sheet. Our reconstruction suggests the gyre boundary (and thus westerly winds) began to migrate northward at ~17-16 ka, during Heinrich Stadial 1.

Observed surface temperature trends over recent decades are characterized by (i) intensified warming in the Indo-Pacific Warm Pool and slight cooling in the eastern equatorial Pacific, consistent with strengthening of the Walker circulation, and (ii) cooling in the Southern Ocean. In contrast, state-of-the-art coupled climate models generally project Walker circulation weakening, enhanced warming in the eastern equatorial Pacific, and warming in the Southern Ocean. Here we investigate the ability of 16 climate model large ensembles to reproduce observed sea-surface temperature and sea-level pressure trends over 1979-2020 through a combination of externally forced climate change and internal variability. We find large-scale differences between observed and modeled trends that are very unlikely (<5% probability) to occur due to internal variability as represented in models. Disparate trends are found even in regions with weak multi-decadal variability, suggesting that model biases in the transient response to anthropogenic forcing constitute part of the discrepancy.