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Changes in the amplitude of decadal climate variability over the twentieth century have been noted, with most evidence derived from tropical Pacific sea surface temperature records. However, the length, spatial coverage, and stability of most instrumental records are insufficient to robustly identify such nonstationarity, or resolve its global spatial structure. Here, it is found that the long-term, stable, observing platform provided by tide gauges reveals a dramatic increase in the amplitude and spatial coherence of decadal (11–14-yr period) coastal sea level (ζ) variability between 1960 and 2000. During this epoch, western North American ζ was approximately out of phase with ζ in Sydney, Australia, and led northeastern U.S. ζ by approximately 1–2 years. The amplitude and timing of changes in decadal ζ variability in these regions are consistent with changes in atmospheric variability. Specifically, central equatorial Pacific wind stress and Labrador Sea heat flux are highly coherent and exhibit contemporaneous, order-of-magnitude increases in decadal power. These statistical relationships have a mechanistic underpinning: Along the western North American coastline, equatorial winds are known to drive rapidly propagating ζ signals along equatorial and coastal waveguides, while a 1–2-yr lag between Labrador Sea heat fluxes and northeastern United States ζ is consistent with a remotely forced, buoyancy-driven, mechanism. Tide gauges thus provide strong independent support for an increase in interbasin coherence on decadal time scales over the second half of the twentieth century, with implications for both the interpretation and prediction of climate and sea level variability. Significance Statement: Decadal climate variability influences the frequency and severity of many natural hazards (e.g., drought), with considerable human and ecological impacts. Understanding observed changes and predicting future impacts relies upon an understanding of the physical processes and any changes in their variability and relationship over time. However, identifying such changes requires very long observational records. This paper leverages a large set of tide gauge records to show that decadal time scale coastal sea level variability increased dramatically in the second half of the twentieth century, in widely separated geographic locations. The increase was driven by a shift in the amplitude, spatial pattern, and interbasin coherence of atmospheric pressure, wind, and sea surface temperature variability. [ABSTRACT FROM AUTHOR]

Tide gauges provide a rich, long‐term, record of the amplitude and spatiotemporal structure of interannual to multidecadal coastal sea‐level variability, including that related to North American east coast sea level "hotspots." Here, using wavelet analyses, we find evidence for multidecadal epochs of enhanced decadal (10–15 year period) sea‐level variability at almost all long (>70 years) east coast tide gauge records. Within this frequency band, large‐scale spatial covariance is time‐dependent; notably, coastal sectors north and south of Cape Hatteras exhibit multidecadal epochs of coherence (∼1960–1990) and incoherence (∼1990‐present). Results suggest that previous interpretations of along coast covariance, and its underlying physical drivers, are clouded by time‐dependence and frequency‐dependence. Although further work is required to clarify the mechanisms driving sea‐level variability in this frequency band, we highlight potential associations with the North Atlantic sea surface temperature tripole and Atlantic Multidecadal Variability. Plain Language Summary: The prediction of future sea‐level change along the densely populated North American east coast is of considerable societal value. However, robust predictions require additional efforts to (a) characterize the nature of observed sea‐level variability; and (b) identify key underlying physical processes. Such efforts are also required to understand whether, and how, the tide gauge record can be used to reconstruct ocean circulation and climate variability. While many studies have investigated North American east coast sea‐level variability, there remains a lack of clarity in its spatial structure. Of particular importance to reconstructions and tide gauge indices is the degree to which sea‐level variability is common ('"coherent") across Cape Hatteras. Here, we identify and characterize sea‐level variability across a large set of tide gauge records, highlighting changes occurring at roughly decadal periods. Coastal sea level exhibits damped and enhanced 30‐40‐year epochs of decadal variability, and coherence (∼1960–1990), and incoherence (∼1990‐present) across Cape Hatteras. These findings (a) reveal limitations in assumptions embedded in previous coastal sea‐level indices and sea‐level reconstructions and (b) inform interpretations of geographic shifts in sea level "hotspots" observed over the past few decades. Key Points: The large‐scale spatial structure of sea‐level variability along the North American East Coast is time‐dependent and frequency‐dependentMultidecadal epochs of enhanced (up to ∼10 cm) decadal sea‐level variability are evident at most east coast tide gaugesFrom approximately 1960 to 1990, decadal sea‐level variability was coherent across Cape Hatteras [ABSTRACT FROM AUTHOR]

The difference between North Atlantic subpolar gyre sea surface temperatures (SPG SSTs) and hemispheric‐ or global‐scale surface temperatures has been utilized as an index of centennial‐timescale changes in Atlantic Meridional Overturning Circulation (AMOC) strength. Here, using Community Earth System Model ensembles, we show that surface temperature‐based indices (STIs) proposed to date largely reflect global‐scale temperature trends and thus do not reflect dynamical relationships with AMOC. More broadly, we find that relationships between STIs, SPG SSTs, and AMOC strength differ greatly in significance and magnitude over different time periods because they are dependent upon the nature of external forcing. In the twentieth century, characterized by offsetting greenhouse gas and aerosol forcing, the relationship between SSTs and AMOC strength varies widely and changes sign across a 20‐member ensemble. We conclude that STIs and SPG SSTs are poor predictors of centennial‐timescale AMOC strength variations. Plain Language Summary: The short observational record of the Atlantic Meridional Overturning Circulation (AMOC) limits our ability to assess changes in its strength over the instrumental and preinstrumental periods. Indirect proxies of ocean circulation are thus required to make inferences about past trends, for example, those over the past century. Several previous analyses have used surface temperature indices to interpret twentieth century AMOC trends. However, the robustness of this indirect AMOC proxy, including its sensitivity to time period, timescale, and/or climate state, has not been assessed. We use two state‐of‐the art climate model ensembles to assess AMOC/surface temperature relationships over century timescales, finding a strong dependence upon time period and climate forcing. Our results clarify the origins of discrepancies in AMOC/surface temperature relationships and suggest that interpretations of twentieth century climate and ocean circulation change based on surface temperature indices are limited. Key Points: Previously proposed surface temperature indices (STIs) of AMOC strength are dominated by global temperature trendsSTIs are poor predictors of AMOC strength outside their calibration period, calling into question previous interpretations of twentieth century trendsOver centennial timescales, AMOC/STI relationships are sensitive to the nature of external forcing and unforced variability [ABSTRACT FROM AUTHOR]

Climate model projections have previously been used to compute ice shelf basal melt rates in ice sheet models, but the strategies employed – e.g., ocean input, parameterization, calibration technique, and corrections – have varied widely and are often ad hoc. Here, a methodology is proposed for the calculation of circum-Antarctic basal melt rates for floating ice, based on climate models, that is suitable for ISMIP6, the Ice Sheet Model Intercomparison Project for CMIP6 (6th Coupled Model Intercomparison Project). The past and future evolution of ocean temperature and salinity is derived from a climate model by estimating anomalies with respect to the modern day, which are added to a present-day climatology constructed from existing observational datasets. Temperature and salinity are extrapolated to any position potentially occupied by a simulated ice shelf. A simple formulation is proposed for a basal melt parameterization in ISMIP6, constrained by the observed temperature climatology, with a quadratic dependency on either the nonlocal or local thermal forcing. Two calibration methods are proposed: (1) based on the mean Antarctic melt rate (MeanAnt) and (2) based on melt rates near Pine Island's deep grounding line (PIGL). Future Antarctic mean melt rates are an order of magnitude greater in PIGL than in MeanAnt. The PIGL calibration and the local parameterization result in more realistic melt rates near grounding lines. PIGL is also more consistent with observations of interannual melt rate variability underneath Pine Island and Dotson ice shelves. This work stresses the need for more physics and less calibration in the parameterizations and for more observations of hydrographic properties and melt rates at interannual and decadal timescales. [ABSTRACT FROM AUTHOR]

The ice sheet model intercomparison project for CMIP6 (ISMIP6) effort brings together the ice sheet and climate modeling communities to gain understanding of the ice sheet contribution to sea level rise. ISMIP6 conducts stand-alone ice sheet experiments that use space- and time-varying forcing derived from atmosphere–ocean coupled global climate models (AOGCMs) to reflect plausible trajectories for climate projections. The goal of this study is to recommend a subset of CMIP5 AOGCMs (three core and three targeted) to produce forcing for ISMIP6 stand-alone ice sheet simulations, based on (i) their representation of current climate near Antarctica and Greenland relative to observations and (ii) their ability to sample a diversity of projected atmosphere and ocean changes over the 21st century. The selection is performed separately for Greenland and Antarctica. Model evaluation over the historical period focuses on variables used to generate ice sheet forcing. For stage (i), we combine metrics of atmosphere and surface ocean state (annual- and seasonal-mean variables over large spatial domains) with metrics of time-mean subsurface ocean temperature biases averaged over sectors of the continental shelf. For stage (ii), we maximize the diversity of climate projections among the best-performing models. Model selection is also constrained by technical limitations, such as availability of required data from RCP2.6 and RCP8.5 projections. The selected top three CMIP5 climate models are CCSM4, MIROC-ESM-CHEM, and NorESM1-M for Antarctica and HadGEM2-ES, MIROC5, and NorESM1-M for Greenland. This model selection was designed specifically for ISMIP6 but can be adapted for other applications. [ABSTRACT FROM AUTHOR]