No abstract

[1] Decadal sea level variations from tide gauge records along the western European coast and in the Mediterranean Sea commencing in the late 19th and early 20th centuries are examined relative to large-scale atmospheric forcing. Recent studies have provided evidence for a link between sea level in the eastern North Atlantic and atmospheric forcing, however the nature of this relationship is still unclear. Here the outputs of a regional barotropic model and a nearly global baroclinic model are used in conjunction with wind stress and heat flux data to explore the physical mechanisms responsible for the observed sea level variability. All tide gauge records show significant decadal variability (up to 15 cm) and are highly correlated with the NAO and among themselves at decadal periods. There is a coherent sea level signal that affects the eastern boundary of the North Atlantic northward of 25 N and is limited to a narrow band of the order of a few hundred kilometers along the coast. This band tends to become narrower towards higher latitudes. We find that longshore wind and wave propagation along the boundary are the major contributors to coastal sea level variability but no significant contribution from mass redistribution linked to changes in the strength of the subtropical gyre is observed. The mass component dominates sea level in the Mediterranean and is mainly driven by mass exchanges with the Atlantic, which explains the correlation between both regions. Southward of 25 N, sea level changes are mainly driven by heat advection through Ekman fluxes.

There is observational evidence that global sea level is rising and there is concern that the rate of rise will increase, significantly threatening coastal communities. However, considerable debate remains as to whether the rate of sea level rise is currently increasing and, if so, by how much. Here we provide new insights into sea level accelerations by applying the main methods that have been used previously to search for accelerations in historical data, to identify the timings (with uncertainties) at which accelerations might first be recognized in a statistically significant manner (if not apparent already) in sea level records that we have artificially extended to 2100. We find that the most important approach to earliest possible detection of a significant sea level acceleration lies in improved understanding (and subsequent removal) of interannual to multidecadal variability in sea level records.

Mean sea level (MSL) variations across a range of time scales are examined for the North Sea under the consideration of different forcing factors since the late 19th century. We use multiple linear regression models, which are validated for the second half of the 20th century against the output of a tidesurge model, to determine the barotropic response of the ocean to fluctuations in atmospheric forcing. We find that local atmospheric forcing mainly initiates MSL variability on time scales up to a few years, with the inverted barometric effect dominating the variability along the UK and Norwegian coastlines and wind controlling the MSL variability in the south from Belgium up to Denmark. On decadal time scales, MSL variability mainly reflects steric changes, which are largely forced remotely. A spatial correlation analysis of altimetry observations and gridded steric heights suggests evidence for a coherent signal extending from the Norwegian shelf down to the Canary Islands. This fits with the theory of longshore wind forcing along the eastern boundary of the North Atlantic causing coastally trapped waves to propagate over thousands of kilometers along the continental slope. Implications of these findings are assessed with statistical Monte-Carlo experiments. It is demonstrated that the removal of known variability increases the signal to noise ratio with the result that: (i) linear trends can be estimated more accurately; (ii) possible accelerations (as expected, e.g., due to anthropogenic climate change) can be detected much earlier. Such information is of crucial importance for anticipatory coastal management, engineering, and planning.

Decadal to multidecadal variations in sea level extremes unrelated to mean sea level changes have been investigated using long tide gauge records distributed worldwide. A state space approach has been applied that provides robust solutions and uncertainties of the time evolving characteristics of extremes, allowing for data gaps and uneven sampling, both common features of historical sea level time series. Two different models have been formulated for the intensity and for the occurrence of extreme sea level events and have been applied independently to each tide gauge record. Our results reveal two key findings: first, the intensity and the frequency of occurrence of extreme sea levels unrelated to mean sea level vary coherently on decadal scales in most of the sites examined (63 out of 77) and, second, extreme sea level changes are regionally consistent, thus pointing toward a common large‐scale forcing. This variability of extremes associated with climate drivers should be considered in the framework of climate change studies.

[1] Sea level observations suggest that the rate of sea level rise has accelerated during the last 20 years. However, the presence of considerable decadal-scale variability, especially on a regional scale, makes it difficult to assess whether the observed changes are due to natural or anthropogenic causes. Here we use a regression model with atmospheric pressure, wind, and climate indices as independent variables to quantify the contribution of internal climate variability to the sea level at nine tide gauges from around the world for the period 1920-2011. Removing this contribution reveals a statistically significant acceleration (0.022 ± 0.015 mm/yr 2 ) between 1952 and 2011, which is unique over the whole period. Furthermore, we have found that the acceleration is increasing over time. This acceleration appears to be the result of increasing greenhouse gas concentrations, along with changes in volcanic forcing and tropospheric aerosol loading. Citation: Calafat, F. M., and D. P. Chambers (2013), Quantifying recent acceleration in sea level unrelated to internal climate variability, Geophys. Res. Lett., 40,[3661][3662][3663][3664][3665][3666]

We investigate how well methods based on empirical orthogonal functions (EOFs) can reconstruct global mean sea level (GMSL). We first explore the analytical solution of the method and then perform a series of numerical experiments using modeled data. In addition, we present a new GMSL reconstruction for the period 1900-2011 computed both with and without a spatially uniform EOF (EOF0). The method without the EOF0 uses global information, which leads to a better reconstruction of the variability, though with some underestimation. The trend, however, is not captured, which motivates the use of the EOF0. When the EOF0 is used the method reduces to the generalized weighted mean with regularization of altimetry records at tide-gauge locations, and thus it uses no global information. This results in a poor reconstruction of the variability. Although the trend is better captured (biases smaller than 625%) with the EOF0, using the covariance matrix of deseasonalized monthly time series as the basis for determining the contribution of each tide gauge to the trend is dubious because it assumes that the interannual variability and the trend are driven by the same mechanisms. A significant fraction of the interannual to decadal variability (4 mm peak-to-peak and 2 mm standard error) in the new GMSL reconstruction without the EOF0 is consistent with land hydrology changes associated with the El Niño-Southern Oscillation (ENSO). When the EOF0 is used, we find no correlation with either the ENSO or land hydrology changes, and decadal fluctuations are 5 times greater.

We examine the issue of sustained measurements of sea level in the coastal zone, first by summarizing the long-term observations from tide gauges, then showing how those are now complemented by improved satellite altimetry products in the coastal ocean. We present some of the progresses in coastal altimetry, both from dedicated reprocessing of the radar waveforms and from the development of improved corrections for the atmospheric effects. This trend towards better altimetric data at the coast comes also from technological innovations such as Ka-band altimetry and SAR altimetry, and we discuss the advantages deriving from the AltiKa Ka-band altimeter and the SIRAL altimeter on CryoSat-2 that can be operated in SAR mode. A case study along the UK coast demonstrates the good agreement between coastal altimetry and tide gauge observations, with root mean square differences as low as 4 cm at many stations, allowing the characterization of the annual cycle of sea level along the UK coasts. Finally, we examine the evolution of the sea level trend from the open to the coastal ocean along the western coast of Africa, comparing standard and coastally improved products. Different products give different sea level trend profiles, so the recommendation is that additional efforts are needed to study sea level trends in the coastal zone from past and present satellite altimeters. Further improvements are expected from more refined processing and screening of data, but in particular from the constant improvements in the geophysical corrections.