Recently, a significant sea level rise acceleration has been detected along the U.S. east coast north of Cape Hatteras (NCH) using tide gauge data [1
]. The region is also believed to be a “hot-spot” for accelerating tidal flooding [4
]. Some studies have revealed that the climate-related changes in the Atlantic Meridional Overturning Circulation (AMOC) and Gulf Stream, as well as the subsistence of the coastal zones are potential drivers behind this [3
Rising sea levels amplify the threat and magnitude of the storm surge-induced urban flooding in coastal areas. Therefore, it is important to combine tide gauge measurements, satellite altimetry and sea level data on longer time series (e.g., the sea level reconstruction) together to better understand the forcing mechanism of the regional sea level variability and help to implement the sea level rise adaptation and mitigation strategy. Yearly averaged tide gauge data have been found to be highly and positively correlated with satellite altimetry data in NCH near shore regions [6
]. But are there teleconnections between low-frequency sea level variability in the U.S. east coast and deep-water regions of the North Atlantic (e.g., [7
])? If so, how do they link to each other? Do the correlations exist on long time scales? To answer these questions, we investigated the relationships of tide-gauge measured long-term sea level variability to satellite altimetry observations and sea level reconstruction data (1900–2012, [8
]) in the North Atlantic.
The paper is organized as follows. The data and methods used in this work are described in Section 2
. Section 3
presents the variations of correlations between long-term sea level variability along the U.S. east coast observed by tide gauges and that in the North Atlantic observed by satellite altimeters. Analysis of the factors contributing to the variations are given in Section 4
. The results from the sea level reconstruction are shown in Section 5
. The summary is presented in Section 6
3. Spatio-Temporal Correlations between Sea Level Variability in the U.S. East Coast and the North Atlantic Ocean
In order to show the teleconnections of long-term sea level variability in the U.S. east coast with that in the North Atlantic Ocean, the spatio-temporal correlation coefficients between tide gauge records in SCH (Figure 2
a–c)/NCH (Figure 2
d–f) and altimeter data at each grid are shown in Figure 2
. All the time series were low-pass filtered with a filter half amplitude period of 1 year. To exhibit the changes of the teleconnection patterns on a decadal time scale, the calculations were made over 1993–2012 (Figure 2
a,d), 1993–2002 (Figure 2
b,e) and 2003–2012 (Figure 2
c,f), respectively. Only the correlations significant at 95% confidence level are shown in Figure 2
In Figure 2
a, the SCH averaged sea level variability observed by tide gauges are highly correlated with the altimeter observations in the U.S. near shore areas. Higher correlations are found over these areas in 2003–2012 (Figure 2
c) than a decade ago (Figure 2
b, 1993–2002). In addition, there is an obvious negative correlation in the sub-polar gyre (e.g., the Labrador Sea and Irminger Sea) in 2003–2012 (Figure 2
c), which is not observed in the first decade. This might be due to the possible change in the forcing factors of the regional sea level variability.
Compared with the region SCH, the patterns of the spatio-temporal correlations between altimetry data and tide gauge measurements in NCH (Figure 2
d–f) are distinct, indicating the forcing mechanisms are possibly different in these two regions. High correlations are shown from the coasts to the outer shelf and sub-tropical regions (10° N–20° N) in Figure 2
d. This has not been investigated in previous works [6
]. Compared with the first decade of 1993–2002 (Figure 2
e), there is a correlation reversal in the northern North Atlantic and coastal regions of Greenland in 2003–2012 (Figure 2
f) and an enhanced correlation in NCH near shore and sub-tropical regions. The change of the high correlations implies a shift from subpolar-related to tropical-related regional sea level variability along the NCH coast in the past two decades. In the following sections, considering that the change of correlation in NCH is more significant than that in SCH and it is a “hot-spot” for accelerating flooding, we focus on the sea level variability in this area.
As shown in Figure 2
e, over 1993–2002, the tide gauge data in NCH are negatively correlated with altimeter data near 36° N and 73° W, the southeast of the Gulf Stream’s mean position. This was also pointed out in Reference [6
]. However, the correlations disappear in the following decade and a wider spatial coverage of higher correlations exhibits on the NCH shelves, which may be related to the eastward shift of the Gulf Stream path [5
5. Long-Term Sea Level Variability from Sea Level Reconstruction
In the above sections, we investigated the variations of correlations between tide gauge measurements, altimetry data and climate indices over the period 1993–2012. Does the correlation and teleconnection exist on a longer time scale? Here we try to answer the question based on the reconstructed sea level data over the 20th century (1901–2012).
Before further analysis of the low-frequency sea level variability in the North Atlantic Ocean, we first evaluated the reconstructed sea level data against satellite altimetry data in the four representative regions in Figure 1
. The time series of sea level anomaly from the reconstruction dataset and altimeters are shown in Figure 6
a–d with trends and seasonal signals removed. All regions exhibit high correlations larger than 0.7. In Figure 6
a, the sea level peak in 1998 in Region 1 is captured by the sea level reconstruction dataset. Strong sea level variability in the Gulf Stream is well reproduced (Figure 6
b). In the subpolar (Figure 6
c) and subtropical (Figure 6
d) regions, the correlations exceed 0.8.
Similar to Figure 3
, Figure 7
a–c show the correlation coefficient between the filtered sea level reconstruction data and AMOC/NAO winter/AMO indices over 1901–2012 (1935–2012 for AMOC). Compared with the satellite altimetry era (Figure 3
a), coherent spatial patterns are observed in the 20th century (Figure 7
a) but with a weaker correlation. Comparing Figure 7
b with Figure 3
d, the reconstructed sea level shows stronger correlation in NCH near shore and subpolar regions. As for the AMO, very similar spatial patterns are presented in altimetry data and the sea level reconstruction dataset (Figure 3
g and Figure 7
shows the time evolution of the reconstructed sea level anomaly in the past 112 years and AMOC/NAO/AMO indices in the four selected regions. In Figure 8
a, a higher correlation could be obtained if the calculation started from 1960. The correlations between the sea level anomalies in the four regions are listed in Table 1
. Negative correlations are found between sea level in the Gulf Stream front and the other three regions, with a correlation coefficient (CC) of −0.45, −0.53 and −0.73, respectively. Sea level variability in the coastal regions are tele-connected with that in the subpolar and subtropical regions with a CC of 0.38 and 0.51. Similar to the altimetry era, the sea level in the subpolar and subtropical regions are highly correlated with each other with an absolute value of CC over 0.7. Combining Figure 8
a with Figure 5
, it is obvious that the remarkable high sea level variations in NCH coastal regions in 1998–1999 and 2010–2011 are related to AMOC variations and low NAO/high AMO indices.
summarizes the correlations between regional sea level reconstructions and the three climate indices. NAO is correlated with the reconstructed sea level in the four regions, but AMO is only correlated with that in the subpolar and subtropical regions. Compared with the other regions, a higher correlation between AMOC and sea level is shown in the NCH near shore area.
In this study, we found a high teleconnection between sea level variability in NCH observed by tide gauges and that captured by satellite altimetry in the subpolar and tropical North Atlantic. The correlation varies on a decadal time scale. In the Labrador Sea, the positive correlations during 1993 to 2002 reverses to negative in the following decade (2003–2012). Compared with the subpolar North Atlantic, satellite data in the tropical region exhibit a stronger connection with tide gauge data in NCH in 2003–2012, which implies a shift of local sea level variability from subpolar-related to tropical-related in the past two decades. Moreover, the negative correlations between tide gauge measurements and altimeter data near 36° N and 73° W disappear in 2003–2012, which could be related to the shift in the Gulf Stream path and has not been reported in previous studies.
At basin scales, surface heat flux changes contribute a lot to the low-frequency sea level variations [16
]. Hence, we further investigated the variability of ocean heat content (OHC) to explain the remote connections between tide gauge and satellite altimeter data in the last 20 years. Figure 9
a,b show the distribution of the standard deviation of the detrended OHC over 1993–2002 and 2003–2012, respectively. The AMOC transports the heat from the South Atlantic and tropical North Atlantic to high latitudes. In Figure 9
, note the decrease in OHC variability along the Gulf Stream main path, which is possibly related to the slowdown of the AMOC accompanied with less transportation of warm water to the subpolar Atlantic Ocean.
The climatic warming and the melting of the Greenland ice sheet have a great impact on the deep (dense) water formation in the northern North Atlantic and Labrador Sea. The reversed correlations in the Labrador Sea in the last two decades (Figure 2
e,f) are consistent with the cooling regions caused by the slowdown of AMOC due to the freshwater anomaly added to the northern North Atlantic [17
]. Therefore, the decrease in deep-water formation in the Labrador Sea and the associated reduction of AMOC may partly lead to the correlation variations. In the tropical regions, the areas with positive correlations agree well with the warm westward limb of the AMOC at 10°–20° N. The weakening of AMOC decreases the westward transportation of the warm water. That explains the variability of the sea level here, particularly in 1998, 2005 and 2010 (see Figure 5
The variations of the correlations between tide gauge observations in NCH and altimeter data are accompanied with lower or higher negative correlations between NAO winter and annual sea level data in the subpolar and tropical Atlantic Ocean (Figure 3
d–f), suggesting that the NAO may be an important factor remotely influencing the local sea level variability.
Another factor linked to the teleconnection of NHC–North Atlantic sea level variability is AMO (Figure 3
g–i). A positive AMO phase can increase sea level by thermal expansion, and the associated enhanced AMOC tends to weaken the sea level rise rate in the northeast coastal areas of the U.S. and Subpolar Gyre regions (e.g., [18
]). The two effects combine to yield the spatial patterns of sea level change associated with the AMO (Figure 4
b and Figure 5
c). The sea level variability in the NCH near shore area may be linked with that in the subtropical North Atlantic through the observed warming of the tropical Atlantic Ocean (Figure 2
h and Figure 4
b), and remotely connected with that in the northern North Atlantic through NAO (Figure 4
a) and its relationship with AMOC variability.
The climate indices AMOC, AMO and NAO are related to each other. Multidecadal variation of the NAO can induce multidecadal variation of the AMOC and poleward ocean heat transport in the Atlantic (e.g., AMO). The NAO variation could change the AMOC. The sea level reconstruction (1900–2012) demonstrates remarkable decadal sea level variability in the North Atlantic Ocean. Coherent with altimetry data, the reconstructed dataset in the 20th century exhibits similar spatial correlation patterns with the AMOC, NAO and AMO indices (Figure 7
and Table 2
). It confirms the teleconnection between sea level variability in NCH and the subpolar/subtropical North Atlantic, which may be attributed to variations in AMOC and NAO/AMO.