Coastal sea levels are contributing to coastal flooding and erosion as global sea levels rise, partly due to the thermal expansion of seawater, which accelerates with increasing temperatures. Climate model simulations with increasing greenhouse gas emissions show that future sea-level variability will increase in many regions around the world.
In a new Communications Earth & Environment article, authors Matthew Widlansky, Xiaoyu Long, and Fabian Schloesser present an analysis of CMIP5 climate model projections of future sea level to show that there is a tendency for a near-global increase in sea level variability with continued warming that is robust across models, regardless of whether ocean temperature variability increases.
Observed sea levels vary seasonally and interannually everywhere, although there are pronounced gradients between regions of larger and smaller variability in sea surface height. Specifically, for an upper-ocean warming by 2 °C, which is likely to be reached by the end of this century, sea-level variability increases by 4 to 10% globally on seasonal-to-interannual timescales because of the nonlinear thermal expansion of seawater.
Some of the largest annual ranges of sea level (10 cm to greater than 30 cm from the minimum to maximum of monthly averages) occur near the continental margins of the northwestern Atlantic and Pacific Oceans, as well as in the northern Indian and the tropical Pacific Oceans (Fig. 1a).
Figure 1. a, b The observed annual cycle range and interannual standard deviation (cm; shading), respectively from ORA-S5 (Methods). Contours enclose annual ranges and interannual standard deviations greater than 10 cm and 5 cm, respectively. c, d Multi-model mean (29 CMIP5 models) future projection for RCP8.5 with respect to the historical experiments for the annual cycle range and interannual standard deviation (% change). Stippling indicates grid points where less than 19 out of 29 models agree on the future change sign for annual cycle and interannual changes.
For example, in the Florida Strait near the City of Miami, the sea level annual range averages 13 cm (Fig. 2a; 22 cm at the local tide gauge). The annual maximum of sea level, which typically occurs during September–November in Miami, often determines the season of greatest risk for coastal flooding. Whereas the sea level annual cycle is incorporated into most tidal predictions, interannual variability (Figs. 1b and 2a) that either amplifies or dampens the annual cycle, is not. Should interannual high sea level anomalies (Fig. 1b; e.g., standard deviations greater than 5 cm) occur during the season of highest sea levels, then coastal flooding is more likely to occur; especially if accompanied by large astronomical tides or exacerbating meteorological events (e.g., storm surges or runoffs from heavy rainfall).
Figure 2: a, c The sea level and T100 annual cycles, respectively, for ORA-S5 (black) averaged over the 1° ocean grid box nearest the Virginia Key tide gauge (purple; 25.7°N, 279.8°E). The seasonally-dependent interannual variability of the tide gauge and ORA-S5 are also shown in (a) (bars; ±0.5 standard deviation). b, d The sea level and T100 annual cycles for the historical (blue) and RCP8.5 (orange) experiments averaged over the nearest 1° ocean grid to the tide gauge for each model. Sea level annual cycles are normalized to have a mean of zero (a, b). The vertical lines indicate the magnitude of the annual ranges during the historical and future periods (b, d).
With unabated greenhouse warming, the majority of models that were assessed project increasing sea level variability this century: 28 out of 29 models show increased annual cycle ranges in the tropics as well as the mid-latitudes; and, the number of models showing increased interannual standard deviations are 22 and 26 for the tropics and mid-latitudes, respectively.
There are stark regional differences in the projected increases of the annual range and interannual standard deviation (Fig. 1c, d), with reduced variability also being projected in some areas. Considered regionally, the inter-model uncertainty of future sea level variability changes is substantial. As the oceans continue to warm, future ocean temperature oscillations will cause increasingly larger buoyancy-related sea level fluctuations that may alter coastal risks. With many communities regularly experiencing flooding on calm weather days, these small seasonal variations become significant.
This study was partially funded by the MAPP program.
The Modeling, Analysis, Predictions, and Projections (MAPP) Program is a competitive research program in NOAA Research’s Climate Program Office. MAPP’s mission is to enhance the Nation’s and NOAA’s capability to understand, predict, and project variability and long-term changes in Earth’s system and mitigate human and economic impacts. To achieve its mission, MAPP supports foundational research, transition of research to applications, and engagement across other parts of NOAA, among partner agencies, and with the external research community. MAPP plays a crucial role in enabling national preparedness for extreme events like drought and longer-term climate changes. For more information, please visit www.cpo.noaa.gov/MAPP.