The Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere-ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of processes often more independent of the tropics than is observed. Finally, it is suggested that the assessment of PDO-related regional climate impacts, reconstruction of PDO-related variability into the past with proxy records, and diagnosis of Pacific variability within coupled GCMs should all account for the effects of these different processes, which only partly represent the direct forcing of the atmosphere by North Pacific Ocean SSTs.

The Gulf Stream transports large amounts of heat from the tropics to mid-and high-latitudes, and thereby affects cyclogenesis 1,2 and low cloud formation 3 . Its influence on climate on monthly and longer timescales remains poorly understood, however. In particular, it is unclear to what depth the warm current affects the free atmosphere above the marine atmospheric boundary layer. Here we consider the Gulf Stream's influence on the troposphere using a combination of operational weather analyses, satellite observations, and an atmospheric general circulation model 4 . Our results reveal that the Gulf Stream affects the entire troposphere. In the marine boundary layer, atmospheric pressure adjustments to sharp sea surface temperature gradients lead to surface wind convergence that anchors a narrow precipitation band along the Gulf Stream. In this rain band, upward motions and cloud formation extend into the upper troposphere as corroborated by the frequent occurrence of very cold cloud-top temperatures. These mechanisms offer a direct pathway by which the Gulf Stream can affect the atmosphere both locally and possibly in remote regions via planetary wave propagation 5,6 . This pathway may have implications for our understanding of climate change because the Gulf Stream is the upper limb of the Atlantic meridional overturning circulation, which displays large variations in records of past climate change 7 and is predicted to slow down in response to global warming 8 .

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Internal variability in the climate system confounds assessment of human-induced climate change and imposes irreducible limits on the accuracy of climate change projections, especially at regional and decadal scales. A new collection of initial-condition large ensembles (LEs) generated with seven Earth system models under historical and future radiative forcing scenarios provides new insights into uncertainties due to internal variability versus model differences. These data enhance the assessment of climate change risks, including extreme events, and offer a powerful testbed for new methodologies aimed at separating forced signals from internal variability in the observational record. Opportunities and challenges confronting the design and dissemination of future LEs, including increased spatial resolution and model complexity alongside emerging Earth system applications, are discussed.