Acceleration of sea-level rise in the southeastern United States from 2011 to 2015; unexplained by AMOC

Variations in AMOC unable to explain short-lived accelerations of sea level rise.

Arnoldo Valle-Levinson, Andrea Dutton, Jonathan B. Martin

University of Florida

1

Abstract

Sea-level rise (SLR) has accelerated in the eastern United States north of Cape Hatteras (CH)

over the past several decades, including an abrupt, short-lived rise in 2009-2010. These

accelerations are paired with slowdowns in Atlantic Meridional Overturning Circulation

(AMOC), although this causal link has yet to be firmly established. We document an abrupt

increase in SLR south of CH from 2011-2015 accompanied by regional sea-level fall north of

CH and continued decline in AMOC strength. Local rates of SLR south of CH over this 5-yr

window (18-20 mm/yr) are anomalous, but not unprecedented in this region. The potential for

such a pattern is not captured in model scenarios of AMOC decline, requiring a re-analysis of the

mechanisms driving this regional variability.

2

Introduction

Coastal counties of the United States are home to ~123 million people, who represent close to

39% of the total population (NOAA, 2013). These coastal communities are threatened by

transient phenomena such as storms, storm surge, flooding, and increased erosion that capture

most of the attention (NRC, 2007). Less consideration is given to the recently persistent threat of

rising sea level and its risk to coastal water resources and infrastructure. Rates of sea-level rise

vary through time as shown by sea-level reconstructions that span timescales of 10 to105 years

(e.g. Lambeck et al., 2014; Rohling et al., 2009) and direct observations based on tide gauges and

satellite altimetry (Church et al., 2013).

A recent acceleration in global (or eustatic) rates of sea-level rise has been observed in tide

gauge and satellite altimetry data, which show an increase from a global mean rate of ~1.2-1.9

mm/yr between 1901 and 1990 to ~2.8-3.7 mm/yr for 1993 to 2010 (Haigh et al., 2014). But

these rates vary spatially over hundreds of kilometers (Sallenger et al., 2012). For instance, tide

gauge data show a rise of ~5 mm/yr along the Mid-Atlantic Bight (MAB) of the United States

(north of Cape Hatteras) over the past two decades (Boon, 2012; Knight et al., 2005; Goddard et

al., 2015), clearly larger than the global mean. These high rates have been attributed to changes

in wind forcing, deceleration of the Gulf Stream, and decline in the Atlantic Meridional

Overturning Circulation (AMOC, e.g., Ezer et al., 2013; Woodworth et al., 2014; Thompson and

Mitchum, 2014). The increased rates have also been related to the Atlantic Multi-decadal

Oscillation and slowing of the Florida current (Frankcombe and Dijkstra, 2009; Park and Dusek,

2013).

3

Several of the above studies have pointed to a particular period of accelerated rise in the MAB,

between Massachusetts and North Carolina, from 2009 to 2010 that was 3 to 4 times higher than

the global mean rate (e.g., Sallenger et al., 2012; Goddard et al., 2015). Those studies indicated

negligible increase rates south of Cape Hatteras (CH). However, cursory inspection at the

National Oceanic and Atmospheric Administration (NOAA)’s website of interannual variation of

mean sea level on the east Florida shelf (e.g., tidesandcurrents.noaa.gov/sltrends/residual. htm?

stnid=8720030) and a recent analysis in southernmost Florida (Park and Sweet, 2015) revealed a

marked upward trend in the period 2010-2015. The rapid rise in the MAB appears to have now

shifted to the south of CH, with tide gauges showing rates of ~18 to 20 mm/yr since 2010. The

purpose of this study is to describe the magnitude and the geographic extent of the recent sea-

level rise acceleration south of CH and to examine whether it may be linked to the AMOC.

Methods

Sea level data were compiled for the east coast of the US from Florida to Maine (Fig. 1a) from

two sea level data repositories: Hawaii Sea Level Center (uhslc.soest.hawaii.edu/) and NOAA’s

tide stations (tidesandcurrents.noaa.gov). The Hawaii Sea Level Center provided hourly data

from January 1, 1920 to December 31, 2012 (“long period”) at stations in black letters. NOAA

provided data from January 1, 1996 to May 1, 2015 (“short period”) at stations in red numbers..

The short period represents the last 19 years, or one full nodal tidal cycle, following the

convention adopted by the National Ocean Service to represent the time segment over which tide

observations are taken to obtain mean values. The short period also spanned a shorter distance,

4

from Florida to New York, relative to the long period, which extended to the Canada - United

States border. A few stations report data beginning at later dates than those mentioned above. In

addition, wind mean speeds and gust speeds were compiled from 1987 to 2015 at NOAA buoy

41009 from the National Data Buoy Center station. The station is 20 nautical miles east of Cape

Canaveral, in Central Florida. Data were compiled to analyze other atmospheric forcings that

could produce sea-level rise change.

In order to identify recent changes in sea-level rise, data from all stations, and for both the long

and short periods, were detrended (linear trend) and filtered with a cosine Lanczos filter (e.g.

Thomson and Emery, 2014) centered at 365 days. This filter smoothed out monthly, seasonal

and semiannual variations in sea level (e.g. Fig. 1b for the short period at 3 stations in south

Florida). Thus, reported values represented accelerations/decelerations rather than absolute

elevations. One-year low-pass filtering resulted in loss of one half year at the beginning and end

of the time series.

One-year low-pass filtered data of water level were arranged in Hovmöller (or phase) diagrams

for most of the eastern seaboard of the United States at uniformly gridded values at intervals of

30 days and 50 km. Phase diagrams display the propagation of signals in space and time, in this

case of sea level increase or decrease. These diagrams were constructed with the long and the

short periods to identify spatial structures and magnitudes of the interannual variability. One-

year filtered hourly data from recording stations were interpolated to this uniform space-time

grid with Delaunay triangulations (e.g. Fang and Piegl, 1992; 1993).

5

One-year filtered time series of the long period, as portrayed in the phase diagram, were then

decomposed into Empirical Orthogonal Functions (EOFs). The EOF analysis was performed

only on the long period phase diagrams to ensure statistical reliability. These functions depict the

spatial structure of sea level variability throughout the eastern United States and the temporal

variations, from 1921 to 2011, of those spatial structures.

In addition, linear trends in sea-level rise were determined for 5-year periods throughout the span

of the long and short period observations. Five-year rates were determined from the monthly

values gridded on the phase diagram. Most recent trends were compared to trends starting 1920,

in the context of probability density functions.

Results

One-year filtered water levels in southeast Florida at 3 stations from Miami to Trident (located at

Cape Canaveral in the middle of the peninsula) display a trend of rising sea levels from 1996 to

2010 of 4-6 mm/yr, which is about 33 to 100% greater than the rate of global mean sea-level rise

of ~3 mm/yr over this period (Fig. X). A marked change is observed from 2011 to 2015, with

sea-level rise becoming 25 5 mm/yr (Fig. 1b). This increase is unprecedented for 5-year

intervals during at least the last 19 years. Considering the change since 2010, the rate is between

18 and 20 mm/yr. A comparable rise has been identified for 2 other sites in the Florida Keys in

southernmost Florida for the same period (Park and Sweet, 2015). Including the 3 previously

identified locations with accelerating sea-level rise makes at least five stations in Florida that

have shown values of sea-level rise that are 6 to 9 times the global mean value of 2 to 3 mm/yr.

6

Phase or Hovmöller diagrams of the long and short period time series were generated to

determine the spatial extent of this rapid change in the rate of sea-level rise and to assess whether

it is anomalous in the context of past measurements of variability in sea level in this region. The

Hovmöller diagram of the long period time series shows intervals of depressed and heightened

water levels throughout the last 90+ years, relative to the linear trend (Fig. 2a). Intra-decadal

variations may be attributed to wind and other indices (Woodworth et al., 2014). The highest

water levels have occurred in the MAB in 2009-2010, south of CH in 1947-1948, and centered

around CH in 1973. The long period time series ended in 2012 and fell short in documenting the

recent sea-level rise south of CH captured in the short period time series (Fig. 2b).

The Hovmöller diagram of the short period time series (Fig. 2b) accentuates ephemeral

differences in sea level between the MAB and south of CH. The diagram shows elevated sea

levels during the period 2010-2015, although the nature of the regional response differs to the

north and south of CH. A dramatic rise in sea level occurs in the MAB in 2009-2010 (Goddard,

2015; Ezer, 2015) but slowly tapers off over the next several years. In contrast, there is a

sustained increase in sea level south of CH after 2011 (Fig. 2b) that translates to a rate of sea-

level rise of ~20 mm/yr over the period 2010-2015. Such extreme values of sea-level rise south

of CH have not been observed at other times in the period starting in 1996 (Fig. 2b), although the

long term period suggests a similar rise around 1947 (Fig. 2a). An EOF analysis places these

rates in the context of the long period time series.

7

The spatial structure of EOF mode 1 explains variations in the same direction (filtered sea level

increases or decreases) throughout the eastern seaboard of the United States between 1920 and

2012 (Fig. 3a). Unidirectional variations throughout the eastern seaboard have occurred 64% of

the time in 9+ decades. The greatest increases, as illustrated by temporal variations of mode 1

(Fig. 3b), occurred in 1949, 1973 and 2009, while the largest decreases occurred in 1977, 1981,

and 1988-1990. The spatial structure of EOF mode 2 (Fig. 3a) describes opposite changes of sea

level in the MAB relative to south of CH; positive values in Figure 3c represent increases in the

north relative to the south, while negative values denote the opposite. The greatest increases in

the MAB with negligible, or even negative, change south of CH (maxima in positive values of

EOF mode 2) occurred in 1970-1972 and in 2010. On the other hand, the greatest increase south

of CH with negligible, or even negative, change in the MAB has occurred in 1947-1949 and

1995 (greatest negative values). The type of variability depicted by Mode 2 has occurred 20% of

the time in the span 1920-2012. The 2009-2010 increase in the MAB appears in this EOF mode

2. Finally, the spatial structure of EOF Mode 3 defines a change in the middle of the eastern

seaboard, around CH, which was in a different direction from changes in the MAB and south of

CH (Fig. 3a). The greatest influence of Mode 3 (Fig. 3d) has occurred in 1955 (positive) and

1939 (negative), representing 6% of the variability of one-year filtered water levels between

1920 and 2012.

Prevailing variabilities of the time series of the EOFs, which in turn represent temporal

variability of one-year filtered sea level variations, were characterized by their spectra (Fig. 3e).

Modes 1 and 2 exhibit intra-decadal and quasi-decadal variability. Intra-decadal variations are

dominated by oscillations between 4 and 5 years, and between 6 and 7 years. Quasi-decadal

8

oscillations are around 12 years for Mode 1 and 14 years for Mode 2. In addition, Mode 2

exhibits broad-banded multi-decadal variability centered at ~40 years. Mode 3 variations have

been mostly associated with multi-decadal oscillations. The relevance of the overall temporal

variability is now explored with analysis of linear 5-year trends in sea level. These trends were

obtained from the difference, over 5 years, of values portrayed in Figure 4.

Linear 5-yr trends for the long period time series show positive and negative variations with

magnitudes that exceed the global mean rate of 2-3 mm/yr by severalfold (Fig. 4a). Maximum

rates of sea-level rise occurred south of CH in the mid-1940s. In the middle of the eastern

seaboard of the United States, maximum rates occurred in the early 1970s. The MAB has

repeatedly had extended periods (~5-10 yrs) of sea-level rise throughout this time interval,

separated by shorter periods (<5 yrs) of sea-level fall. However, none of these increases in the

MAB have been as large as the maximum that appeared south of CH in the mid-1940s. The

Probability Density Function for the rates of sea-level rise in the span 1920-2012 showed that the

most common rates were highest (around 3 mm/yr) between New York (Montauk) and CH

(dotted line on Fig. 4a). Outside the MAB, the most common rates are near zero or 1 mm/yr.

The last (2008-2012) 5-yr trend for the long period dataset (denoted by white dots) displayed

values of sea-level rise up to 10 mm/yr in the MAB, even leaking to North Carolina (Wilmington

record). This increase was mainly linked to the extremely high sea levels recorded in the MAB

during 2009-2010 (Goddard et al., 2015; Ezer et a., 2015). In contrast, during 2008-2012, the

trend was 0-1 mm/yr in most of the south of CH.

9

Linear trends from the short period data also showed values that surpass rates of global mean

sea-level rise by severalfold (Fig. 4b). Since 1996, trends of sea-level rise have shown spatially

variable structures. In the MAB there have been two periods of rapid rise centered in late 2002,

and 2009 through 2010. The rapid rise in the latest period in the MAB leaked into North

Carolina but not farther south. South of CH there has been only one span of rapid increasing

trend, where most dramatic increases have been seen in Florida, Georgia, and South Carolina (up

to 1200 km in Fig. 4b). Examination of the Probability Density Function of the 5-yr sea-level

rise rates indicated that, since 1996, the most frequently observed sea-level rise trend in the

MAB has been near 10 mm/yr, in agreement with results from the long period data. In contrast,

south of CH has shown most frequent values around 2 mm/yr, except in South Carolina where

the most frequent trend has been slightly negative. The most recent 5-yr interval (denoted by

white dots on the probability figure) indicated a slightly negative tendency in the MAB, after the

large increase in 2009-2010. South of CH, in particular from Charleston to Miami, the trends

were near 20 mm/yr. The only previously observed period of a similarly high rate of rise in sea

level occurred in the mid-1940s (Fig. 4a). Altimeter data from the Archiving, Validation and

Interpretation of Satellite Oceanographic data (AVISO) products display the same overall

increase of sea-level rise offshore of Florida since 2010 as tide gauge data (data not shown).

Hence, both tide gauge and altimetry data have indicated a definitive acceleration in sea-level

rise after 2011.

Discussion

10

The main findings of this study are that sea-level rise rates are heterogeneously distributed in

space and time along the east coast of the US and that. south of CH the rates have increased from

1-2 mm/yr to 18-20 mm/yr. These findings extend previous studies that documented sea-level

rise accelerations in the MAB over the past several decades, with no corresponding acceleration

south of CH in records through 2012 (e.g. Sallenger et al., 2012; Ezer et al., 2013). An

anomalous high rate of sea-level rise was also recorded in the MAB during 2009-2010 with no

corresponding increase observed south of CH (Ezer 2015; Goodard 2015). The recent increase of

sea level south of CH seems to be rare, with a similar rate of rise only observed from 1947 to

1948, according to the Probability Density Function of the rates observed in the last 90+ years

(Fig. 4).

Possible reasons for the recent acceleration in sea-level rise in the MAB have been attributed to

different phenomena, including wind stress and ocean circulation (e.g. Ezer et al., 2013;

Woodworth et al., 2014; Thompson and Mitchum, 2014). Some of these causes can be from

natural variability in atmospheric and ocean processes over the Atlantic Ocean (North Atlantic

Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO)), and other processes (not

mutually exclusive) associated to global changes that could be influenced by humans.

The NAO has a periodicity of 2-15 years and can produce sea level variations through the

inverse barometer effect (e.g. Olafsdottir et al., 2013; Piecuch and Ponte, 2015). Spectral

amplitudes of EOF Modes (Fig. 3) indicated periodicities that fall within those expected from

these inter-decadal and multi-decadal oscillations. In fact, there are significant coherences, albeit

only with values of ~0.5 and at only 90% confidence, between Mode 3 and the NAO (Fig. 3f).

11

Significant coherences occurred only at periods of ~10 years with lags (not shown) close to 180º,

which indicated opposite changes between Mode 3 water level and the NAO index, likely from

the inverse barometer effect. There was no significant coherence (at 90% confidence) with EOF

Mode 1 and the NAO, although there was weak coherence with Mode 2 at periods of 2 and 3

years. Therefore, linkages between observed variations of sea level in the eastern United States

and the NAO is tenuous.

The AMO has periods between 60 and 80 years (Schlesinger and Ramankutty, 1994) and shows

anomalously warm sea surface waters (positive phase) in the North Atlantic between 1925 and

1965, and after 2000. There was significant broad-band coherence (95% confidence with values

>0.6) between the AMO index and Modes 2 and 3 at periods >20 years (Fig. 3f). The lag for

Mode 2 was ~90º (not shown) indicating that sea level response to the AMO index was greatest

at the transition between negative and positive phases. The lag for Mode 3 was around 50º (not

shown). There was no coherence between the AMO index and EOF Mode 1. Relationships

between AMO and sea level oscillations in the eastern United States seem related to spatially

heterogeneous responses, separated by CH, as portrayed by EOF Modes 2 and 3.

A frequently mentioned attribution to the recent acceleration of sea-level rise in the MAB is the

slowdown of the Atlantic meridional overturning circulation (AMOC) (e.g., Sallenger et al.,

2012; Ezer and Corlett, 2012; Rhamstorf et al., 2015). The overturning circulation is related to

the strength of North Atlantic Deep Water formation. Such overturning circulation is linked to

the North Atlantic Subtropical Gyre through the Gulf Stream. Therefore, any perturbations to

the AMOC would affect the subtropical gyre and the Gulf Stream. Introduction of freshwater to

12

the surface ocean at high latitudes would increase the static stability of North Atlantic waters and

slow down the overturning circulation. There are several studies pointing out the decline of the

overturning circulation (e.g. Smeed et al., 2014; Boulton et al, 2014). There is also evidence for

reduced Gulf Stream transport (Ezer et al., 2013, Ezer, 2015) and Florida Current transport (Park

and Sweet, 2015) by at least 1.5 Sv after 2005. However, the decline of the Florida Current

transport seems to be independent of the decline in the AMOC (Yin and Goddard, 2013).

Further evidence of drastic changes in the North Atlantic subtropical gyre is the anomalously

large amounts of sargassum running aground on the beaches of the entire Caribbean Sea from

late 2014 to the summer of 2015. It is possible that these strandings are related, at least partially,

to the relaxation of the Subtropical Gyre because of declines in the Florida Current, though this

linkage is speculative and requires further scrutiny.

Wind speed shows an equivocal tendency in the last 5-10 years (Fig. 5a). However, both east

and north wind velocity components seem to have become weaker since 2011 (Fig. 5b). This

apparent wind relaxation could contribute to waning of the Florida Current as wind is a major

driving force of the subtropical gyre. Reduced wind velocities have also been consistent with

decreased significant wave heights (Fig. 5c), most evidently since 2005. Atmospheric pressure

has not shown a clear trend in the period of record (Fig. 5d) nor their oscillations seem to be tied

to the variations in sea level of Figure 2b. But there is a clear increasing trend in air temperature

and water temperature, consistent with global tendencies. It is possible that the increasing sea

level can have a contribution from regionalocean warming, but this would need a thorough

exploration beyond the scope of this study.

13

Despite many possible explanations for the regional sea level patterns identified here, there are

no clear candidates that explain the magnitude and pattern of sea-level change witnessed along

the eastern seaboard of the United States over the past 5 years, or actually over the last 90 years.

The increase in the rate of sea-level rise south of CH may not be linked to a decline in the

AMOC given that models predict an effect on coastal sea level in the MAB, but not south of CH

(e.g., Yin and Goddard, 2013?). Although the high rates of sea-level rise currently observed

south of CH are not without precedent, the only previous such event on record that occurred in

the mid 1940s did not occur at a similar phase of the NAO but at a warm pahse of the AMO.

These differences suggest the potential influence of AMO in the variability observed.

Importantly, the high magnitude of sea-level rise south of CH contributes to a higher base sea

level upon which extreme events such as storm surges and king tides are superimposed. For

example, the tide gauges on the east coast of Florida display >100 mm of sea-level rise over a

mere 5 years (old fig 2b new figure). These rates exacerbate efforts to defend coastlines using

smooth sea level projections that do not consider the possibility for such variability in the rate of

sea-level rise, even if it turns out to be short-lived. Additionally, because seasonal variability

determines the amplitude of king tides that regularly cause incursions of seawater into streets of

many coastal communities, including—but not limited to—south Florida, it is essential to

understand how this observed rate of sea-level rise is distributed throughout the year. Though

not shown here, most of the increase in the rate of sea-level rise south of CH since 2010 has been

driven by higher sea levels during what is normally the seasonal low (the spring). If the seasonal

high in the tidal cycle south of CH (September and October) were somehow similarly affected in

the future, the king tides and late season hurricanes will have a much more devastating effect.

14

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Figure 1. a) Eastern United States showing the station locations for the long period (in white letters and black stars) and the short period (in red numbers and yellow stars). Numbers and letters correspond to station names given in other figures. b) one-year filtered sea level at 3 stations in Southeastern Florida).

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Figure 2. Hovmöller or phase diagram for the one-year filtered water levels from the long period (a) and short period (b) time series. The horizontal dotted line represents the location of Cape Hatteras (CH).

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Figure 3. a) Spatial structure of the 3 dominant EOFs from the long period time series. b), c) and d) Temporal variation of EOF modes 1 (blue), 2 (red) and 3 (green), showing the portion of the variability explained by each mode. In c), the North Atlantic Oscillation (NAO) index, divided by 10, is shown in black. In d), the Atlantic Multi-decadal Oscillation (AMO), divided by 2, is shown in black. e) Exhibits the spectral amplitudes of the temporal variations of the 3 EOF modes. f) and g) display coherence between each one of the 3 EOF modes (color coding for each mode is consistent with other panels) and NAO (f) and AMO (g). The horizontal dashed lines indicate significance levels at 90, 95 and 99%.

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Figure 4. Five-year trends of sea-level rise for the a) long period and b) short period time series. The horizontal dotted line represents the location of Cape Hatteras. Probability density functions (PDF) are shown to the right of each panel, indicating (with white dots) the last values of sea-level rise obtained from each time series.

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Figure 5. Meteorological variables, together with significant wave height and water temperature obtained from the National Data Buoy Center, station 41009 (28.522º N 80.188º W) from 1988 to early 2015. A linear trend was also plotted on top of the north and east components of wind velocity, air temperature and water temperature. The corresponding slopes were -0.005 and 0.006 m/s/y for the north and east components of wind, and 0.01 and 0.02 ºC/y for air and water temperature.

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