Low-frequency internal climate variability (ICV) plays an important role in modulating global surface temperature, regional climate, and climate extremes. However, it has not been completely characterized in the instrumental record and in the Coupled Model Intercomparison Project phase 5 (CMIP5) model ensemble. In this study, the surface temperature ICV of the North Pacific (NP), North Atlantic (NA), and Northern Hemisphere (NH) in the instrumental record and historical CMIP5 all-forcing simulations is isolated using a semiempirical method wherein the CMIP5 ensemble mean is applied as the external forcing signal and removed from each time series. Comparison of ICV signals derived from this semiempirical method as well as from analysis of ICV in CMIP5 preindustrial control runs reveals disagreement in the spatial pattern and amplitude between models and instrumental data on multidecadal time scales (.20 yr). Analysis of the amplitude of total variability and the ICV in the models and instrumental data indicates that the models underestimate ICV amplitude on low-frequency time scales (.20 yr in the NA; .40 yr in the NP), while agreement is found in the NH variability. A multiple linear regression analysis of ICV in the instrumental record shows that variability in the NP drives decadal-to-interdecadal variability in the NH, whereas the NA drives multidecadal variability in the NH. Analysis of the CMIP5 historical simulations does not reveal such a relationship, indicating model limitations in simulating ICV. These findings demonstrate the need to better characterize low-frequency ICV, which may help improve attribution and decadal prediction.

Coral Sr/Ca provides a well-established proxy for sea surface temperature (SST) (e.g.,

The temporary slowdown in large‐scale surface warming during the early 2000s has been attributed to both external and internal sources of climate variability. Using semiempirical estimates of the internal low‐frequency variability component in Northern Hemisphere, Atlantic, and Pacific surface temperatures in concert with statistical hindcast experiments, we investigate whether the slowdown and its recent recovery were predictable. We conclude that the internal variability of the North Pacific, which played a critical role in the slowdown, does not appear to have been predictable using statistical forecast methods. An additional minor contribution from the North Atlantic, by contrast, appears to exhibit some predictability. While our analyses focus on combining semiempirical estimates of internal climatic variability with statistical hindcast experiments, possible implications for initialized model predictions are also discussed.

Variability of North Pacific sea surface temperatures (SSTs) on multidecadal timescales plays an important role in modulating global climate and regional hydroclimate. The principle modern phenomenon associated with this variability is the Pacific Decadal Oscillation (PDO); however, our understanding of the PDO and multidecadal SST variability more broadly is limited to SST observations from the historical era and land-based proxy reconstructions. Here, we reconstruct multidecadal Pacific Ocean SST variability with an 1,800-year, continuous, high-resolution alkenone-derived SST reconstruction from Baja California, a region sensitive to changes in the PDO, and compare the record with a reconstruction of Southwestern North American hydroclimate. Our SST reconstruction displays persistent multicentennial and discontinuous intervals of multidecadal variability with periodicities similar to instrumental PDO observations. The most severe droughts in Southwestern North America during the last two millennia are coeval with strong multidecadal variability, suggesting that multidecadal SST variability plays an important role in regional megadroughts.Plain Language Summary The distribution of heat in the ocean's surface plays a profound role in global and regional climate on timescales of seasons to millennia. The observational record of ocean temperatures only spans the last 150 years, requiring us to use paleoclimate archives to understand the mechanisms of climate variations on multidecadal and longer timescales. Here, for the first time, we reconstruct changes in ocean temperature off Baja California in the Eastern Subtropical Pacific Ocean during the last two millennia. We find that so-called "megadroughts" that occurred in Southwestern North America during the last millennium coincide with time intervals that have the strongest variations in ocean temperatures. The combined alterations to atmospheric circulation and Pacific temperature caused by the Pacific Decadal Oscillation together likely played an important role in the development of Southwestern North American megadroughts.