An extreme event of sea-level rise along the Northeast coast of North America in 2009–2010

The coastal sea levels along the Northeast Coast of North America show significant year-to-year fluctuations in a general upward trend. The analysis of long-term tide gauge records identified an extreme sea-level rise (SLR) event during 2009–10. Within this 2-year period, the coastal sea level north of New York City jumped by 128 mm. This magnitude of interannual SLR is unprecedented (a 1-in-850 year event) during the entire history of the tide gauge records. Here we show that this extreme SLR event is a combined effect of two factors: an observed 30% downturn of the Atlantic meridional overturning circulation during 2009–10, and a significant negative North Atlantic Oscillation index. The extreme nature of the 2009–10 SLR event suggests that such a significant downturn of the Atlantic overturning circulation is very unusual. During the twenty-first century, climate models project an increase in magnitude and frequency of extreme interannual SLR events along this densely populated coast. Extreme sea level rises are a threat to coastal communities, but their cause, in terms of seasonal or interannual time scales, has received little attention. Here, the authors combine observational and model data to show that one such rise in 2009–10 was caused by a 30% downturn in the Atlantic overturning circulation.

T he Intergovernmental Panel on Climate Change Fifth Assessment Report 1,2 lists extreme sea levels among the top impacts of climate change. Hourly to daily extreme sea levels are typically associated with transient storms and eddies, tides and tsunamis. Once they occur and superimpose, these events pose a threat to coastal communities. On seasonal to interannual time scales, extreme sea-level events are usually linked to large-scale ocean dynamics and climate extremes, but have received little attention thus far.
In the North Atlantic, especially along the Northeast (NE) Coast of North America, sea levels are critically influenced by the Atlantic meridional overturning circulation (AMOC) [3][4][5][6][7] . Since 2004, the AMOC has been systematically monitored at 26.5°N in the North Atlantic 8,9 . The available data reveal a strength of 18.5 ± 1.0 Sv (mean ± 1s) and small interannual variability of the AMOC from 1 April 2004 to 31 March 2009. From April 2009 through March 2010, by contrast, the AMOC shows a significant 30% downturn to 12.8 Sv, followed by a second minimum during the winter of 2010-11 (refs 9-11). Additional observations at other latitudes in the Atlantic indicate that this downturn is a basin-wide phenomenon due to the spatial coherence of the AMOC 12,13 .
Whether this downturn of the AMOC is a sign of its long-term trend 14 or a part of its natural variability 11,15 , or both 16 , will need further research with longer observational data. Regardless, this event provides a valuable opportunity to study the climate impact of the AMOC, quantify the AMOC-sea-level rise (SLR) relationship and test model simulation results. Here we analyse long-term tide gauge (TG) data and report an extreme and unprecedented SLR event in 2009-10 along the NE Coast of North America. It should be noted that we use the term SLR here to indicate interannual sea-level changes, while it is usually referred to the long-term and gradual trend of sea level in literature. With various observation and model data, we show that the 2009-10 SLR event was caused by the 30% downturn of the AMOC and the wind stress anomalies associated with the significant negative North Atlantic Oscillation (NAO) index 17 .

TG records. Sea level along the East Coast of the United States and
Canada exhibits interannual fluctuations superimposed on multidecadal variations and a long-term upward trend 5,7,[18][19][20] . Modelling and long-term TG data indicate that the behaviour of sea level is similar and highly correlated north or south of Cape Hatteras 4,5,7,18,20 . By taking the long-term rate and especially the 2009 SLR rate into account ( Supplementary Fig. 1 (Fig. 2b). Between 2009 and 2010, the sea level in this region jumped by nearly 100 mm on average (Figs 1b and 3a), contributing significantly to the identified SLR acceleration during the recent decades 5,7,18 . To calculate the return period, we fit a Gaussian distribution to the yearly SLR rates from 1920 to 2011. The result shows that the 2009 SLR rate is a 1-in-850 year event (Fig. 2c).   To test the robustness of the result and its sensitivity to different methods, we repeat the yearly SLR calculation based on the linear fit to the monthly data on interannual time scales ( Supplementary Fig. 2). We also identify record-breaking years of sea level and compare the sea-level increase over the previous records ( Supplementary Fig. 3). The 2009-10 event stands out in all three methods ( Table 1).
The 2009-10 extreme SLR event is also evident in individual TG data ( Supplementary Fig. 4) Supplementary Fig. 4). The spatial coherency and extreme SLR rate suggest that large-scale ocean climate dynamics (for example, the AMOC 4,21 , Gulf Stream 6,22 and wind effect 20,23 ) rather than local mechanisms (for example, land subsidence 24 ) is the main cause of the 2009 SLR event. Land subsidence is at least one order of magnitude smaller than the yearly SLR rates.
The signal of the 2009-10 extreme SLR event attenuates towards the south. Compared with the NE region, the amplitude of the yearly sea-level fluctuation is larger (s ¼ 19.9 mm per year) in the Mid-Atlantic region, while the 2009 SLR rate is less extreme (Fig. 2d,e and Supplementary Fig. 4). The composite SLR rate is 31.2 mm per year (41s) in 2009, with a range of 22.0-41.5 mm per year at individual stations. In the Mid-Atlantic region, especially the Chesapeake Bay, land subsidence induced by glacial isostatic adjustment contributes to the long-term SLR 24 . The pronounced SLR rate during 1982-83 (42.5s and with a return period of 150 years) is likely related to the strong El Niño in the Pacific and the resulting more coastal storms in the Mid-Atlantic region 23 (Fig. 2d,f).
South of Cape Hatteras, the 2009 SLR rate further reduces to 3.0 mm per year and falls within ± 1s (Fig. 2g,h). The coastal sea level in this region is influenced by the North Atlantic subtropical gyre 25 . An extreme sea-level fall (B À 3s) occurred in 1949, following a rapid and continuous SLR during much of the 1940s. This decade was characterized by faster global SLR 26 . The detailed investigation about this extreme event is beyond the scope of this study.
The satellite altimetry data indicate that the most significant interannual variability of the dynamic sea level (DSL) occurs in the ocean interior, especially along the Gulf Stream and its extension (Fig. 3a). Along the East Coast of North America, the altimetry data is generally consistent with the TG data regarding the 2009-10 SLR event, but the exact magnitude differs.
Role of the AMOC. The 2009-10 sea-level spike in the NE region coincides with two significant ocean and climate events: a 30%   (Fig. 4b).
Especially, the two sea-level spikes during the winters of 2009-10 and 2010-11 coincide with the two AMOC minima, respectively. Regression based on all monthly data from 2004 to 2012 further reveals a 13.2-mm SLR along the NE coast in response to 1 Sv AMOC slowdown. After excluding the 30% downturn period of the AMOC, the AMOC-SLR ratio increases to 16.6 mm Sv À 1 , reflecting the sea-level response to the gradual decline of the AMOC during 2004-2012. In the altimetry data, the AMOC-DSL correlation extends far offshore, especially from the NE coast of North America (Fig. 3c). It changes sign across the Gulf Stream, implying that a decrease in the AMOC transport reduces the cross-current sea-level gradient and thus lifts sea level along the NE coast 5,6 . Interestingly, the 30% downturn of the AMOC in 2009-2010 followed a brief return to deep convection in the Labrador Sea during the winter of 2007-2008 (ref. 28). A data assimilation product by GFDL 29,30 (see the Methods section) shows that strong deep downwelling in 2007-2008 was mainly induced by a reduction in the freshwater input into the Labrador Sea ( Supplementary Fig. 5). However, a record low of oceanic  heat and buoyancy loss, dominated by the sensible heat flux ( Supplementary Fig. 6), occurred in the subsequent years 2008-2010. Associated with these changes is a significant positive steric sea-level anomaly in 2009 (4100 mm), southeastward of the shelf break (Fig. 3b). The gradient across the shelf break can drive more water mass towards the shelf, thereby causing SLR along the NE coast of North America 4,31 .
To better understand extreme SLR events in this region, we use two sets of state-of-the-art climate models: an ensemble of ten models developed at GFDL, including high-resolution models with eddying oceans 32 (Table 2), and the other set including 14 CMIP5 models with the AMOC and sea-level data available at the CMIP5 archive 33 (Table 3; also see the Methods section). The GFDL ensemble represents different model generations with progressive improvement and a systematic model development effort at one modelling centre. On the other hand, the CMIP5 ensemble represents similar generation models from different institutes, which bear significant difference in many aspects. Therefore, these models cover a wide range in the model formulation, parameterization and uncertainty space.
In the long-term control runs of these models without changing external forcing, the DSL along the NE coast of North America shows an instantaneous correlation with the AMOC on the interannual time scale (Fig. 3d and Supplementary Figs 7 and  8). This correlation suggests that a 30% weakening of the AMOC in 2009-2010 may have contributed to and increased the chance of extreme coastal SLR events. The two models with eddying oceans (GFDL CM2.6 and CM2.5) show weaker correlation, probably due to the relatively weak AMOC in these two models 32 (Supplementary Fig. 9).     Fig. 5). Second, the NAO-induced wind stress anomalies can pile up waters directly against the NE coast or generate onshore Ekman transport.
The control runs with both the GFDL and other CMIP5 models indicate that extreme SLR events along the NE coast typically occur when the nearby wind stress shows an onshore (easterly) or alongshore (northeasterly) anomaly pattern ( Fig. 3f and Supplementary Figs 10 and 11). Indeed, the lower sea-level pressure east of North America in 2009 results in northeasterly wind stress anomalies near the NE coastal regions (Fig. 3e). The anomalous Ekman transport contributes to the 2009-2010 SLR extreme. The negative NAO can also influence storminess, which contributes to higher sea levels due to more frequent storm surges 34 .
The NE sea-level composite shows correlation (R ¼ 0.6) with the NAO index during 2004-2012 ( Supplementary Fig. 12). However, the correlation reduces significantly for the entire period of 1920-2012. In addition to 2009-2010, extreme negative NAO index also occurred in 1969 and other years ( Supplementary Fig. 13). The lack of extreme SLR signal on the NE coast during these years indicates that the NAO is not the sole mechanism of the 2009-2010 SLR event. Finally, the lower sea-level pressure (by B1.6 hPa) in 2009 could account for B15% of the 2009-10 SLR event through the inverse barometer effect (Fig. 3e).
Future projections of extreme SLR events. Similar to extreme temperature and precipitation events, extreme SLR events on the interannual time scale may be also linked to human-induced climate change 35 . Increased greenhouse gas concentrations are likely to shift the probability density function towards more extremes. To study future changes of extreme SLR events, we consider the ten GFDL climate models and their long-term control runs, and idealized 1% per year CO 2 increase experiments for 100 years. Along the NE coast of North America, most of these models (CM2.6, CM2.5, CM2.5 FLORa6, CM2.5 FLOR, ESM2M and ESM2preG) suggest an increase in the magnitude and frequency of the extreme SLR events in response to the CO 2 increase (Fig. 5 and Supplementary Fig. 14). In five of these models (CM2.6, CM2.5 FLORa6, CM2.5 FLOR, ESM2M and ESM2preG), the increase in the yearly SLR extrema is unproportionately larger than the increase in mean sea level ( Supplementary Fig. 14).
There are several reasons for this increase in extremes: first, the global mean SLR from thermal expansion and land ice melt (note that the latter is not included in Fig. 5); second, the overall weakening of the AMOC in the CO 2 experiment leads to record lows of the circulation in some years ( Supplementary Fig. 9), thereby facilitating SLR extremes along the NE coast of North America; and third, the NAO variability remains strong in the CO 2 experiments 36 . Recently, projections of the melting of glaciers 37 , the Greenland 38 and Antarctic 39 ice sheets, have been made for the twenty-first century under greenhouse gas emission scenarios. Adding these contributions would further increase the probability of extreme SLR events along the East Coast of North America, especially its northeastern sector.

Discussion
In the present study, we focus on the extreme SLR event on interannual time scales. With long-term TG data, we calculate the sea-level composite and yearly SLR rates for three SLR regimes along the East Coast of North America. The resulting time series contain rich information about both climate variability and individual events. The extreme and unprecedented SLR event in 2009-2010 is particularly notable along the NE coast of North America. Our analysis suggests that this event was mainly caused by a 30% downturn of the AMOC and the wind stress anomalies associated with the negative NAO, although the two factors are inherently linked 17 .
There is no direct observation of the AMOC before 2004. Some recent model hindcast suggests that similar downturns of the AMOC may have punctuated in the twentieth century 11 . Our analysis based on the long-term TG data and the AMOC-SLR relationship indicates that the 2009-2010 event is very unusual. In addition to internal variability, anthropogenic forcing could be another impact factor, as most climate models project a weakening of the AMOC during the twenty-first century in response to the increase in the atmospheric greenhouse gas concentrations 40 . Continuing observations of the AMOC is essential to confirm these modelling results.
In addition to the absolute SLR rate, the 2009-2010 event is very unusual also in the sense that it occurred during a short period of global sea-level fall 41 . Unlike storm surge, this event caused persistent and widespread coastal flooding 22 even without apparent weather processes. In terms of beach erosion, the impact of the 2009-2010 SLR event is almost as significant as some hurricane events 42 . For the twenty-first century, modelling results suggest that the increase in the greenhouse gas concentrations is likely to cause more extreme SLR events on the interannual time scale along this densely populated coast. Once coastal storms compound high sea levels, more damages will result.   Fig. 2).

Methods
Return period. Based on the millennial time-scale control simulations of the ten GFDL models (not shown), we find the yearly SLR rates are well fitted with a Gaussian distribution. For a return level x t , the probability of exceedance is where Pr is the cumulative density function associated with the Gaussian distribution. The return period (T) is calculated as T ¼ 1/Pr.
GFDL ocean data assimilation data. The GFDL ocean data assimilation data are taken from the oceanic component of climate reanalysis by coupled data assimilation 30 . We use longwave (LW), shortwave (SW), sensible heat (SH), latent heat (LH), precipitation minus evaporation (P À E), and river runoff (R) fluxes from the GFDL climate reanalysis product 29 , where Q HF (W m À 2 ) and Q WF (m s À 1 ) are the net atmosphere-ocean heat (positive values indicate flux into ocean) and freshwater flux, respectively. Ocean temperature, salinity and steric sea-level data. We use the data from the National Oceanographic Data Center (http://www.nodc.noaa.gov/OC5/ 3M_HEAT_CONTENT/). The steric sea-level anomaly data for the upper 2,000 m span from 2005-2013.
GFDL model suite. We consider ten coupled climate models built over the past decade at GFDL (Table 2). CM2.0 and CM2.1 were used as part of the CMIP3 projects, whereas CM3 and ESM2M/ESM2G were used as part of the CMIP5 project. ESM2preG is an early version of ESM2G. Each of these models uses a nominally 1°ocean, with various changes made to the atmosphere and ocean components, leading to the different configurations. Notably, the ESM2G and ESM2preG are based on an isopycnal ocean, whereas the other models use a level coordinate MOM ocean component. CM2.6, CM2.5, CM2.5 FLORa6 and CM2.5 FLOR all use the same 50-km finite volume atmospheric core, with CM2.6 using a 1/10°ocean, CM2.5 a 1/4°ocean and the FLOR simulations using a 1°ocean. The two FLOR simulations differ in their choice for ocean subgrid scale parameterizations, with FLORa6 using a larger lateral viscosity than FLOR. CM2.5 FLOR is used for studies of tropical cyclones.
CMIP5 model suite. The data are downloaded from the CMIP5 archive 33 (Table 3). Detailed model description and experimental design can be found at http://cmip-pcmdi.llnl.gov/cmip5/.