Recent studies reported weakening in the Atlantic Meridional Overturning Circulation (AMOC) and in the Gulf Stream (GS), using records of about a decade (RAPID project) or two (altimeter data). ...Coastal sea level records are much longer, so the possibility of detecting climatic changes in ocean circulation from sea level data is intriguing and thus been examined here. First, it is shown that variations in the AMOC transport from the RAPID project since 2004 are consistent with the flow between Bermuda and the U. S. coast derived from the Oleander measurements and from sea level difference (SLDIF). Despite apparent disagreement between recent studies on the ability of data to detect weakening in the GS flow, estimated transport changes from 3 different independent data sources agree quite well with each other on the extreme decline in transport in 2009–2010. Due to eddies and meandering, the flow representing the GS part of the Oleander line is not correlated with AMOC or with the Florida Current, only the flow across the entire Oleander line from the U.S. coast to Bermuda is correlated with climatic transport changes. Second, Empirical Mode Decomposition (EMD) analysis shows that SLDIF can detect (with lag) the portion of the variations in the AMOC transport that are associated with the Florida Current and the wind-driven Ekman transport (SLDIF-transport correlations of ~0.7–0.9). The SLDIF has thus been used to estimate variations in transport since 1935 and compared with AMOC obtained from reanalysis data. The significant weakening in AMOC after ~2000 (~4.5Svperdecade) is comparable to weakening seen in the 1960s to early 1970s. Both periods of weakening AMOC, in the 1960s and 2000s, are characterized by faster than normal sea level rise along the northeastern U.S. coast, so monitoring changes in AMOC has practical implications for coastal protection.
•Three different data sets agree on the 2009/10 decline in AMOC and the Gulf Stream.•Sea level difference across Gulf Stream detects variations in AMOC.•Sea level is used to construct a proxy for AMOC variability for 1935–2012.•The weakening in AMOC after 2004 is compared to a similar decline in 1960s–1970s.
The classical Ekman theoretical solution for steady-state wind-driven currents of homogeneous ocean with constant eddy viscosity was obtained more than a century ago. However, it is not clear how ...applicable this solution is for realistic stratified ocean with depth-dependent turbulent mixing coefficient (KM). In this study, the Ekman analytical solution is compared with currents obtained by one-dimensional Mellor–Yamada turbulent ocean model (1D-MY) to assess the accuracy of the Ekman solution under various oceanic conditions. For experiments with constant density but depth-dependent KM, the Ekman solution is close to the 1D model calculation if the analytical solution uses the mean KM obtained by the 1D model for each wind speed. Inclusion in the 1D-MY model, the Craig–Banner (C-B) turbulence induced by surface breaking waves makes the surface velocity in the model more like the Ekman surface velocity; however, C-B mixing only affects current direction and speed of the upper ~ 5 m and only for strong winds. Model experiments with different mixed layer (ML) depths show abrupt decline in turbulence and vanishing currents below the ML, so model currents below the ML are weaker than the Ekman solution for an unstratified ocean. The best comparison between the model and the Ekman solutions was found when the Ekman equations use mean KM calculated from the model over the ML depth plus 10 m of the thermocline below. Sensitivity model experiments with different winds and different stratifications resulted in an empirical formula that estimates the mean KM from observed wind and ML depth, and this relation can complement the classical Ekman formula in cases where KM is unknown.
Fast sea level rise (SLR) is causing a growing risk of flooding to coastal communities around the Chesapeake Bay (hereafter, CB or “the Bay”), but there are also significant differences in sea level ...variability and sea level rise rates within the bay that have not been fully investigated in the past. Therefore, monthly sea level records for 1975–2021 from eight tide gauge stations, from the upper bay at Baltimore, MD, to the lower bay at Norfolk, VA, are analyzed and compared. The results show significant spatial variations within the Bay over a wide range of time scales. The largest contribution to the seasonal variations of mean sea level in the Bay is from the annual (S
A
) and semiannual (S
SA
) tides, while the contribution from thermosteric changes is relatively smaller. The lower Bay has a ~ 5 cm smaller mean annual sea level range than the upper Bay and has a secondary minimum in mid-year due to a larger semiannual tide than the upper Bay which is dominated by the annual tide. Variations in sea level anomaly (after removing the mean seasonal cycle) show anticorrelation between the upper and lower bay. Empirical mode decomposition (EMD) analysis reveals that variations with opposite phases at the two edges of the Bay appear mostly on decadal time scales that are linked with the North Atlantic Oscillation (NAO). Sea level trends vary along the Bay—linear SLR rates (4.5–6.1 mm y
−1
) increase from north to south, while sea level acceleration rates (all positive in the range 0.012–0.16 mm y
−2
) increase from south to north. The linear SLR pattern is driven by land subsidence rates, while the acceleration pattern suggests potential impacts from climate change signals that enter the mouth of the Bay in the southeast and amplified farther north by local dynamics. Monthly sea level projections until 2100, based on past trends and the seasonal cycle of each station, are compared with different SLR scenarios based on climate models. The results suggest that accounting for local sea level acceleration in projections can result in large differences in local future sea level rise.
Sea level rise (SLR) is causing acceleration in the frequency and duration of minor tidal flooding (often called “sunny-day” or “nuisance” flooding) along the U.S. East Coast. Those floods have a ...seasonal pattern that often follows the monthly mean sea level anomaly which peaks in September–October for stations between New York and south Florida. However, there are large differences between coasts: for example, over 75% of the minor floods occur during the fall in Florida, but during the spring and winter in Boston. Various data and forcing, such as tide gauge records, surface temperatures, winds, long-term tidal cycles, and the Gulf Stream flow, were analyzed to examine potential drivers and mechanisms that can contribute to the seasonal pattern of floods. The seasonal water temperature cycle, with maximum temperatures in August, could not by itself explain the seasonal sea level pattern, but two mechanisms that significantly correlate with the seasonal sea level and flooding patterns are the annual and semi-annual tidal cycles (correlation of ~ 0.97) and changes in the Gulf Stream (GS) flow (correlation of − 0.6; the GS shows a maximum decline in September–October during the period of peak flooding). The combination of the seasonal pattern of tropical storms and high coastal sea level can explain the high frequency of fall flooding along the Southeastern U.S. coasts, while winter storms have more influence on the northeastern coasts. In recent decades however, the seasonal pattern seemed to have shifted so that increased flooding is seen on the northeastern coasts during spring and summer, while in the Mid-Atlantic and southeastern coasts, a dramatic increase in flooding is seen almost exclusively during the fall. A long-term change in the mean zonal wind pattern along the coast can contribute to the recent shift in the seasonal flooding pattern. The study can help regional adaptation and resilience planning for flood-prone coastal cities and communities.
Recent studies appear to show that a “hot spot” for accelerated sea level rise (SLR) shifted around 2010 from the Mid‐Atlantic Bight (MAB) to the South Atlantic Bight (SAB) and south Florida. The ...role of the Gulf Stream (GS) in this shift was thus investigated. The findings show that in the ~15–20 years before, SLR was accelerating in the MAB due to weakening and southward shifting of the GS. After 2010, however, SLR started slowing down in the MAB due to strengthening and northward shifting of the GS. Thermosteric effects seen in altimeter data indicate a warming trend south of 35°N that started around 2010 and contributed to increased SLR south of Cape Hatteras. However, in the MAB, after the GS separated from the coast, the warming of the Subtropical Gyre and cooling of nearshore waters resulted in an opposite SLR response and strengthening of the GS front. Oscillations with periods of 2–5 years dominated the GS flow and coastal sea level variability, but the GS in the MAB is often out of phase with the GS in the SAB due to eddies and recirculation gyres. These oscillations can create temporal changes in SLR rates that are ~10 times larger than the long‐term trend, so recent changes in the local “hot spot” may not be interpreted as a sign of a shift in the long‐term trend, but more likely a temporal shift associated with interannual and decadal variations in the North Atlantic.
Plain Language Summary
Regional changes in sea level rise (SLR) rates along the U.S. East Coast and the role of the Gulf Stream (GS) in those changes are investigated, explaining why the largest SLR rates were found north of Cape Hatteras before 2010 and south of Cape Hatteras after 2010. Before 2010 SLR was accelerating in the MAB due to weakening and southward shifting of the GS, but after 2010 SLR started slowing down in the MAB due to strengthening and northward shifting of the GS. Because of the GS dynamics and its distance to shore, a warming trend south of 35°N that started around 2010 had different sea level response in the north and south. Oscillations with periods of 2–5 years were also investigated.
Key Points
A recent shift in the location of a hot spot for accelerated sea level rise along the U.S. East Coast was investigated
Changes in the Gulf Stream strength and position around 2010 seemed to explain the regional changes in coastal sea level
Interannual and decadal variations in the Gulf Stream can contribute to temporal and spatial changes in sea level rise rates
Recent studies found that on long time scales there are often unexplained opposite trends in sea level variability between the upper and lower Chesapeake Bay (CB). Therefore, daily sea level and ...temperature records were analyzed in two locations, Norfolk in the southern CB and Baltimore in the northern CB; surface currents from Coastal Ocean Dynamics Application Radar (CODAR) near the mouth of CB were also analyzed to examine connections between the CB and the Atlantic Ocean. The observations in the bay were compared with daily Atlantic Meridional Overturning Circulation (AMOC) observations during 2005–2021. Empirical Mode Decomposition (EMD) analysis was used to show that variations of sea level and temperature in the upper and lower CB are positively correlated with each other for short time scales of months to few years, but anticorrelated on low frequency modes representing decadal variability and long-term nonlinear trends. The long-term CB modes seem to be linked with AMOC variability through variations in the Gulf Stream and the wind-driven Ekman transports over the North Atlantic Ocean. AMOC variability correlates more strongly with variability in the southern CB near the mouth of the bay, where surface currents indicate potential links with AMOC variability. For example, when AMOC and the Gulf Stream were especially weak during 2009–2010, sea level in the southern bay was abnormally high, temperatures were colder than normal and outflow through the mouth of CB was especially high. Sea level in the upper bay responded to this change only 1–2 years later, which partly explains phase differences within the bay. A persistent trend of 0.22 cm/s per year of increased outflow from the CB, may be a sign of a climate-related trend associated with combination of weakening AMOC and increased precipitation and river discharge into the CB.
Global sea level reconstruction (RecSL) for 1900–2015 was used to estimate the variations in oceanic kinetic energy (OKE) and compare OKE with changes in wind patterns and wind kinetic energy (WKE); ...the comparison was done for each latitude and for 5 western boundary currents (WBCs). Two contributors to variability in sea level were analyzed: gravitational, rotational and deformational effects (GRD) related to changes in water masses (barystatic sea level change), and changes in the sterodynamic sea level (SDSL), associated with changes in wind, steric sea level and ocean circulation. GRD changes were responsible for latitudinal multidecadal variations with time scale of ~ 60 to 80 years, while SDSL changes were responsible for interannual and decadal variability, and together with the Greenland ice melt, to sea level acceleration since the 1960s. Regional changes near WBCs show a coherent upward trend in OKE (+ 24% ± 3 increase per century), while trends in WKE over the same regions changed widely from -11% (decrease) over the Gulf Stream region to + 28% (increase) over the Brazil Current region. Low frequency oscillations of wind and oceanic kinetic energy are correlated in some WBCs (e.g., R = 0.5 in the Kuroshio region) but not in others (e.g., R = -0.05 in the Gulf Stream region). The study suggests that several forcing mechanisms contribute to the increased OKE, they include an increased wind-stress curl over subtropical gyres, local changes in wind patterns that impact some WBCs, and large uneven warming near WBCs that increased sea level gradients and thus intensified OKE.
Sea level data from the Chesapeake Bay are used to test a novel new analysis method for studies of sea level rise (SLR). The method, based on Empirical Mode Decomposition and Hilbert‐Huang ...Transformation, separates the sea level trend from other oscillating modes and reveals how the mean sea level changes over time. Bootstrap calculations test the robustness of the method and provide confidence levels. The analysis shows that rates of SLR have increased from ∼1–3 mm y−1 in the 1930s to ∼4–10 mm y−1 in 2011, an acceleration of ∼0.05–0.10 mm y−2 that is larger than most previous studies, but comparable to recent findings by Sallenger and collaborators. While land subsidence increases SLR rates in the bay relative to global SLR, the acceleration results support Sallenger et al.'s proposition that an additional contribution to SLR from climatic changes in ocean circulation is affecting the region.
Key Points
Sea level rise in Chesapeake Bay is accelerating
New sea level analysis method tested
Ocean dynamics contribute to sea level rise
Recent studies indicate that the rates of sea level rise (SLR) along the U.S. mid‐Atlantic coast have accelerated in recent decades, possibly due to a slowdown of the Atlantic Meridional Overturning ...Circulation (AMOC) and its upper branch, the Gulf Stream (GS). We analyzed the GS elevation gradient obtained from altimeter data, the Florida Current transport obtained from cable measurements, the North Atlantic Oscillation (NAO) index, and coastal sea level obtained from 10 tide gauge stations in the Chesapeake Bay and the mid‐Atlantic coast. An Empirical Mode Decomposition/Hilbert‐Huang Transformation (EMD/HHT) method was used to separate long‐term trends from oscillating modes. The coastal sea level variations were found to be strongly influenced by variations in the GS on timescales ranging from a few months to decades. It appears that the GS has shifted from a 6–8 year oscillation cycle to a continuous weakening trend since about 2004 and that this trend may be responsible for recent acceleration in local SLR. The correlation between long‐term changes in the coastal sea level and changes in the GS strength was extremely high (R = −0.85 with more than 99.99% confidence that the correlation is not zero). The impact of the GS on SLR rates over the past decade seems to be larger in the southern portion of the mid‐Atlantic Bight near Cape Hatteras and is reduced northward along the coast. The study suggests that regional coastal sea level rise projections due to climate change must take into account the impact of spatial changes in ocean dynamics.
Key Points
coastal sea level is driven by Gulf Stream
Gulf Stream weakening in recent years
climate change may slow down the Gulf Stream
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