Effects of long-term variability on projections of twenty-first century dynamic sea level

Abstract

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...

Evaluating Model Simulations of Twentieth-Century Sea-Level Rise. Part II: Regional Sea-Level Changes

“…It is substantial at interannual to multidecadal time scales ( Fig. S4; see also Lyu et al 2016;Bordbar et al 2015) and it masks part of the forced signal during the twentieth century (Carson et al 2015;Richter and Marzeion 2014;Lyu et al 2014;Jordà 2014). Here the unforced variability explains a significant part of the spread among climate models' twentieth-century dynamic sea level change patterns (cf.…”

The Sea Level Response to External Forcings in Historical Simulations of CMIP5 Climate Models*

“…While the internal climate variability has a relatively small effect on GMTSL, it can be larger regionally (Hu and Deser 2013), mainly on shorter (up to multidecadal) time scales, but also on centennial time scales (Carson et al 2015;Bordbar et al 2015;Monselesan et al 2015). We have compared DSL trends for the five different forcing experiments over various periods and find that, while the overall response to the external forcings is consistent, the patterns show more variability over shorter (decadal to multidecadal) periods because of the larger influence of internal climate variability.…”

Recent Progress in Understanding and Projecting Regional and Global Mean Sea Level Change

“…Hu and Deser [161] showed that even by the middle of this century, unforced variability leads to an uncertainty of up to a factor of two in coastal projections, the largest spread (in proportional terms) being on the north Pacific and north Atlantic coasts, despite the very small spread in projections of global mean thermal expansion. Bordbar et al [162] found that even after 100 years, the spread in local projections due to unforced variability may be comparable to global thermal expansion.…”

“…Several recent studies have evaluated the time of emergence of sea level change, with various definitions, in the CMIP5 dataset. For ocean dynamical sea level change (which excludes GMSLR), the local signal is not detectable in the majority of the ocean even after 100 years of anthropogenic forcing [162,164] (Figs. 4 and 5).…”