There is debate about slowing of the Atlantic Meridional Overturning Circulation (AMOC), a key component of the global climate system. Some focus is on the sea surface temperature (SST) slightly cooling in parts of the subpolar North Atlantic despite widespread ocean warming. Atlantic SST is influenced by the AMOC, especially on decadal timescales and beyond. The local cooling could thus reflect AMOC slowing and diminishing heat transport, consistent with climate model responses to rising atmospheric greenhouse gas concentrations. Here we show from Atlantic SST the prevalence of natural AMOC variability since 1900. This is consistent with historical climate model simulations for 1900–2014 predicting on average AMOC slowing of about 1 Sv at 30° N after 1980, which is within the range of internal multidecadal variability derived from the models’ preindustrial control runs. These results highlight the importance of systematic and sustained in-situ monitoring systems that can detect and attribute with high confidence an anthropogenic AMOC signal.
Sea-level rise 1 is one of the most pressing aspects of anthropogenic global warming with far-reaching consequences for coastal societies. However, sea-level rise did 2-7 and will strongly vary from coast to coast 8-10 . Here we investigate the long-term internal variability e ects on centennial projections of dynamic sea level (DSL), the local departure from the globally averaged sea level. A large ensemble of global warming integrations has been conducted with a climate model, where each realization was forced by identical CO 2 increase but started from di erent atmospheric and oceanic initial conditions. In large parts of the mid-and high latitudes, the ensemble spread of the projected centennial DSL trends is of the same order of magnitude as the globally averaged steric sea-level rise, suggesting that internal variability cannot be ignored when assessing twenty-first-century DSL trends. The ensemble spread is considerably reduced in the mid-to high latitudes when only the atmospheric initial conditions di er while keeping the oceanic initial state identical; indicating that centennial DSL projections are strongly dependent on ocean initial conditions.Globally averaged sea level has risen by about 20 cm since 1900 and at a rate of about 3 mm yr −1 during the past two decades, but with strong regional variation 1-7 . For example, the western tropical Pacific featured a much stronger rise than the global average during the recent decades, whereas even falling sea levels were observed in the eastern tropical Pacific and along the west coast of the Americas 7 . Dynamic sea surface topography, the departure from the Earth's geoid, is influenced by ocean currents, local mass balance and density changes of the water column 11-14 . The DSL, which is the focus of this study, has been introduced to describe the collective effect of the local steric (thermosteric and halosteric) and dynamical ocean adjustment contribution 9,11,14 . As the observed sealevel changes include the effects of both external forcing (natural, for example, solar; and anthropogenic, for example, CO 2 ) and internal variability, we need to understand both drivers to assess twentieth and twenty-first-century sea-level changes.Climate modes, patterns with identifiable characteristics and specific regional effects, are prominent examples of internal variability. The El Niño/Southern Oscillation 15 , a quasi-periodic fluctuation of the equatorial Pacific sea surface temperature with a period of about 4 years, is the leading mode of tropical interannual variability. El Niño/Southern Oscillation is associated with zonal redistributions of heat, causing large sea-level anomalies across the equatorial Pacific and along the west coasts of the Americas 15 . The Pacific Decadal Oscillation, a decadal climate mode, also strongly affects sea level in the Pacific 2,16,17 and tropical South Indian Ocean 18 . Other regions of strong internal decadal sea-level variations are the North 19,20 and South Atlantic 19 . Here we address the influence of the longer centen...
While the Earth's surface has considerably warmed over the past two decades, the tropical Pacific has featured a cooling of sea surface temperatures in its eastern and central parts, which went along with an unprecedented strengthening of the equatorial trade winds, the surface component of the Pacific Walker Circulation (PWC). Previous studies show that this decadal trend in the trade winds is generally beyond the range of decadal trends simulated by climate models when forced by historical radiative forcing. There is still a debate on the origin of and the potential role that internal variability may have played in the recent decadal surface wind trend. Using a number of long control (unforced) integrations of global climate models and several observational data sets, we address the question as to whether the recent decadal to multidecadal trends are robustly classified as an unusual event or the persistent response to external forcing. The observed trends in the tropical Pacific surface climate are still within the range of the long‐term internal variability spanned by the models but represent an extreme realization of this variability. Thus, the recent observed decadal trends in the tropical Pacific, though highly unusual, could be of natural origin. We note that the long‐term trends in the selected PWC indices exhibit a large observational uncertainty, even hindering definitive statements about the sign of the trends.
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