Observed sea level pressure (SLP) trends for 1901-2010, 1951-2010, and 1981-2010 are assessed using two observed data sources (HadSLP2_lowvar and 20CRv3) compared to a CMIP5 multi-model ensemble. The CMIP5 simulations include runs with: i) no external forcing (Control runs), ii) natural external forcing only (Natural-Forcing), or iii) natural plus anthropogenic forcings combined (All-Forcings). We assess: i) whether the CMIP5 All-Forcing ensemble is consistent with observations; and ii) whether there is model-based evidence for detectable anthropogenic influence for the observed SLP trends.

For the 1901-2010 and 1951-2010 trends, a robustly detectable anthropogenic signal in both observational datasets is a zonal band of SLP increase extending over much of the southern hemisphere extratropics (30°S-50°S). In contrast, the HadSLP2_lowvar and 20CRv3 observed datasets disagree on the sign of the century-scale trends in SLP over much of the low-latitude region 25°N-25°S. These differences will limit confident detection/attribution/consistency conclusions for lower-latitude regions at least until the observational data product discrepancies are better reconciled. The northern hemisphere extratropics remains a difficult region for identifying any detectable anthropogenic influence for annual- or seasonal-mean SLP trends. Overall, our results highlight the difficulty in detecting and attributing anthropogenic signals in SLP for relatively short time scales. The observed 1981-2010 regional trends typically have a different pattern and magnitude from the simulated externally forced trends. Consequently, our results suggest that internal variability is likely the dominant driver of most observed 1981-2010 regional trend features, including the pronounced increase in SLP over the central and eastern equatorial Pacific.

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