OSOcean ScienceOSOcean Sci.1812-0792Copernicus GmbHGöttingen, Germany10.5194/os-11-607-2015Accelerated sea level rise and Florida Current transportParkJ.joseph_park@nps.govhttps://orcid.org/0000-0001-5411-1409SweetW.National Park Service, Everglades National Park, 950 N Krome Ave, Homestead, FL, USANational Oceanic and Atmospheric Administration, 1305 East West Hwy, Silver Spring, MD, USAJ. Park (joseph_park@nps.gov)30July201511460761524March201530April201528June201516July2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://os.copernicus.org/articles/11/607/2015/os-11-607-2015.htmlThe full text article is available as a PDF file from https://os.copernicus.org/articles/11/607/2015/os-11-607-2015.pdf
The Florida Current is the headwater of the Gulf Stream and is
a component of the North Atlantic western boundary current from
which a geostrophic balance between sea surface height and mass
transport directly influence coastal sea levels along the Florida Straits. A linear regression of daily Florida Current transport
estimates does not find a significant change in transport over the
last decade; however, a nonlinear trend extracted from empirical mode decomposition (EMD) suggests a 3 Sv decline in mean
transport. This decline is consistent with observed tide gauge
records in Florida Bay and the straits exhibiting an
acceleration of mean sea level (MSL) rise over the decade. It is not
known whether this recent change represents natural variability or
the onset of the anticipated secular decline in Atlantic meridional
overturning circulation (AMOC); nonetheless, such changes have direct
impacts on the sensitive ecological systems of the Everglades as
well as the climate of western Europe and eastern North America.
Introduction
The Florida Current is a progenitor of the Gulf Stream, a component of
the North Atlantic subtropical gyre western boundary current, and
a surface component of the Atlantic meridional overturning circulation (AMOC).
This current is climatically important with
model simulations predicting a decrease in AMOC in response to
increasing greenhouse gases . Models also suggest
the potential for significant mean sea level (MSL) changes across
the western North Atlantic in response to AMOC dynamics
. In terms of mass transport found
that a weakening AMOC would be manifested as a reduction in southward
deep water transport balanced by a decline in the northward upper
ocean western boundary current. A decrease in AMOC since 2004 has
been reported by and , finding a weakening
of the southward flowing lower North Atlantic deep water. It is then
reasonable to expect a concurrent decrease in western boundary
transport.
The Florida Current is not only important in terms of meridional heat
transport but also tightly confined in the Straits of Florida, and a
geostrophic balance between transport and western boundary sea surface
height exert a significant influence on coastal sea levels as noted by
. More recent investigations corroborate the
transport–MSL link, e.g., , and quantify
transport variations . It should be noted that there
are a multitude of dynamical processes acting in concert with the
transport–MSL mechanism, such as regional wind stress, barometric pressure,
and bottom friction on the shelf , although in South Florida
the shelf is narrow and frictional effects are minimal. These coastal
sea level variations have important implications on ecological and
anthropogenic responses along the Florida Straits and eastern seaboard
of North America, including the fresh water resources of the environmentally
critical ecosystems of the Everglades.
Map of Florida Straits with the cable, tide gauge, and ship
measurement locations. Bathymetry contours are in meters.
Gulf Stream dynamics in the Middle Atlantic Bight of North
America (between Cape Hatteras and Cape Cod) is known to express
similar influences on MSL . For example,
related AMOC and wind stress to MSL finding that
wind stress dominates at interannual and shorter timescales.
analyzed the MSL elevation gradient across the western boundary of the
Gulf Stream, Florida Current transport, and coastal sea level at 10 tide gauges in
the Chesapeake Bay and middle Atlantic coast concluding that the Gulf
Stream 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 a recent acceleration of middle Atlantic coast sea
level rise . Subsequently, reported
2 decades of ship-based velocity measurements across the Gulf Stream
finding no significant decrease based on a linear regression.
We note that based their results on empirical mode decomposition (EMD), a component of the Hilbert–Huang transform
capable of extracting nonlinear trends, in an area
limited to the US east coast, while analyzed
stream-wide transects from the east coast to Bermuda. More recently,
re-examined this apparent tension finding that the contrasting
results are actually consistent when one considers the nonlinear nature
and spatial dependence of the data.
The question of which statistical estimator is appropriate for a given
data set is important. When dealing with multidimensional geophysical
signals which have significant variance not only due to random process noise
but also include nonlinear, nonstationary components across a range of
timescales, linear regression over a long period is perhaps not appropriate
. In the case of Florida Current and MSL
variability, we suggest that a single
coefficient over multiple decades is not appropriate to capture
time-varying trends such as the recent decline in AMOC and acceleration
of mean sea level. Instead, we employ EMD to extract time-dependent
trends in Florida Current transport and mean sea level.
EMD is a heuristic, basis-free,
data-adaptive modal decomposition which extracts modes in order of
increasing oscillatory scales (lower instantaneous frequencies) into
intrinsic mode functions (IMFs). It is essentially a modern
implementation of Chrystal's graphical method of residuation
, with an excellent introduction and review provided
by . In contrast to Fourier decomposition employing
time-invariant sinusoidal bases, wavelet transforms using
scale-invariant wavelets, or empirical orthogonal functions based on
eigenmodes, EMD is a recursive residuation extracted from envelopes of
the data itself. It performs well on non-stationary and nonlinear
signals, and is suited to the extraction of time-adaptive nonlinear
trends . However, as with any analysis technique it has
limitations which can include mode mixing where different IMFs may
contain portions of a signal with the same temporal scale.
The mode-mixing problem can be resolved with ensemble
empirical mode decomposition, the reader is referred to
for a complete discussion, although in our analysis we focus on the
EMD residuals and decadal-scale IMFs where there is no evidence of
mode mixing.
The question of whether or not recent changes in Florida Current and AMOC
transport are secular has not yet
been answered. examined tide gauge records along the
middle Atlantic coast together with the Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) indices, finding that all
are currently within the bounds of 20th century
variability and that another decade of observations are required to
determine whether the recent middle Atlantic coast sea level
acceleration represents the onset of trend, or an extension of
historical variability. Specific to sea level rise acceleration,
indicate that interannual to multidecadal variability
dominates the records such that in most locations several additional
decades are needed to reliably quantify any tide gauge accelerations.
Transport estimates
Daily transport estimates derived from electromotive induction
voltages in a submarine cable spanning the Florida Straits near
27∘ N have been made since 1982 (Fig. ), with
independent ship-based calibration measurements obtained on
a recurring basis. A review of Florida Current transport measurements
including details on the cable observational program and data can be
found in along with an assessment of the
temporal variability of Florida Current transport. Specifically, two-thirds of the variance is at sub-annual timescales, less than
10 % annually, 13 % interannually (13–42 months), and less
than 10 % at periods longer than 42 months. also found no evidence for
a long-term linear trend in Florida Current transport based on data
for 1982–2007, and variability on decadal timescales of roughly
±1 Sv during 1982–2007.
recently assessed accuracy of the cable transport
estimates, finding that annual transport averages are accurate to
within 0.3 Sv and that the cable is capable of observing small
but important climate-induced changes in Florida Current transport.
The focus of this paper is to examine recent data linking Florida
Current transport and coastal MSL anomalies from a nonlinear,
time-varying trend perspective.
Florida Current trend
Daily estimates of Florida Current transport from the cable, as well
as ship-based dropsonde or lowered acoustic Doppler profiler
measurements are available from the NOAA Climate Observation Division
as part of the Western Boundary Time Series project .
Figure a shows daily data from 18 March 1982 to
19 November 2014 along with data reconstruction (gray vertical bands)
over the period 23 October 1998 through 18 June 2000 when data were not
collected, and five other periods with gaps greater than 30 days
(2 July to 16 August 1995; 27 May to 8 August 1996; 21 November 2001
to 2 February 2002; 2 September to 8 October 2001; 3 September to
29 October 2004).
IMF minimum and maximum instantaneous frequency expressed as periods in years.
Florida Current NAO MEI IMFminmaxIMFminmaxIMFminmax(yr)(yr)(yr)(yr)(yr)(yr)110.413.5773.2510.6740.753.92120.624.9385.9214.0851.174.92130.994.5297.7516.0861.926.25141.826.4573.586.83152.977.22165.8812.51179.9315.26
(a) Florida Current daily transport estimates from
induced cable voltages at 27∘ N (green). The gray
background bounds missing measurements and data reconstructed from
daily distributions across all available years. Also shown are the
sum of the EMD residual and IMFs 11–17 (black), and the EMD
residual itself (blue). The total number of IMFs is
NIMF=17. (b) Florida Current transport
estimates from ship-based measurements (green) and EMD residual
(blue). (c) Comparison of EMD residuals to linear models
of the transport data, and to a linear model of the cable transport EMD
residual.
The reconstructed data are uniformly sampled from distributions
constructed from all available data for a missing yearday. For example,
if 1 January 2000 is missing, a Gaussian kernel is fit to all available
data for 1 January. A uniform random sample is then drawn from this
distribution and used as the reconstructed value. This preserves the
overall distribution of the data for a yearday capturing the seasonal
trends, while realistically allowing for variance away from the mean on
the daily timescale. All other data gaps (46 one
day gaps, 33 between 2 and 5 days, 21 between 6 and 30 days) are
filled with linear interpolation. The ship data have no
reconstruction applied.
A linear regression of the ship data with time results in a decline
of 0.77±0.55Sv from 1982 to 2014 with a p value of 0.17,
suggesting an 83 % probability that a linear model decline is not
a random artifact. A linear model of cable transport from 1982
to September 2014 finds a decrease of 1.08±0.11Sv
(p value less than 0.001), while from October 2004 to
October 2014 a linear regression decrease of 0.57±0.19Sv (p value 0.002) is obtained. Since decadal transport
variability of the Florida Current has been estimated as
±1 Sv, linear regressions of
the data, which are less than 1 Svdecade-1, can lead one
to conclude that there has been no significant change in mean transport.
However, a linear model can be biased by incomplete modal
oscillations, is not data adaptive within the record (there is only
one fit coefficient), and may provide an incomplete picture for the
complex dynamics of coupled nonlinear processes, providing motivation
to employ EMD in an effort to extract
time-varying trends in the data. The black line in Fig. a
shows the sum of the cable data EMD
residual with IMFs 11 through 17 which are the seven longest temporal scale, or
lowest instantaneous frequency modes, while
the blue line plots the EMD residual representing the trend. IMFs are not
restricted to a constant frequency of oscillation, and Table
lists the range of oscillation periods (inverse of instantaneous
frequency) for the IMFs discussed in this paper. The trend in
cable data from 1982 through 2004 exhibits a ±1 Sv
variation, while the most recent decade indicates a 3 Sv
decline. Figure b shows the ship data with its EMD
residual, which
in contrast to the cable data exhibits a mildly declining trend.
Linear models of Florida Current transport rates and change
over the period of record (March 1982–November 2014) from ship and cable data.
A comparison of transport data EMD residuals with linear models
is shown in Fig. c. The dashed blue line shows
a linear regression of the cable data
EMD residual, although in practice it makes little sense to linearize
a nonlinear trend, while the dashed red line is a linear regression of
the ship data, and the solid black line a linear regression of the
cable data. The ship data EMD residual and linear
regressions are nearly degenerate, whereas the cable EMD residual
is time dependent and significantly different from the corresponding
linear trend, although both suggest a general decline in
transport over the period of record. Table presents a
comparison of the linear models suggesting that there is not a great
disparity between the linear model decline of the cable data and a linear
model of the cable EMD residual. However, Fig. c
does illustrate the considerable difference in trends for the cable data
when considering a linear vs. a time-varying nonlinear model.
The remarkable divergence between the ship-based and cable transport
estimates over the last decade has not been attributed and needs
further study. One factor is that the cable estimates are based on an
integrated measurement of electromotive flux across the entire cable,
whereas ship measurements are sampled with spatially limited vertical
profiles.
Since the mean transport decline of the cable data over the last
decade as expressed in the EMD residual is consistent with the change
of Gulf Stream MSL gradient in
the Middle Atlantic Bight as found by , with a reduction
in AMOC since 2004 , and also with coastal MSL
anomalies in South Florida (discussed below), there is observational evidence to support
a recent decline in mean transport as detected in the cable data and
we will focus on it in the remainder of the analysis.
Comparison of Florida Current IMFs with the NAO, AMO, and MEI
indices. (a) Florida Current and NAO index EMD
residuals. (b) Decadal-scale IMFs of Florida Current
(NIMF=17) and NAO index
(NIMF=10). (c) Florida Current and AMO
index EMD residuals. (d) Decadal-scale IMFs of Florida
Current and AMO index (NIMF=9). (e)
Sum of Florida Current IMFs 12–17 (blue) and sum of MEI IMFs 4–7
(green; NIMF=9). (f) MEI IMFs
4–7. IMFs are vertically offset for clarity. The dashed vertical
lines in (e and f) bracket strong El Niño
events in June 1982–October 1983, and March 1997–August 1998.
Climate indices
The El Niño–Southern Oscillation (ENSO), NAO, and AMO express
global teleconnections influencing atmospheric and oceanic
circulation, as well as coastal sea levels. We examine each of these
in relation to the Florida Current transport in the following
sections. NAO data are available at
www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml, AMO
data at www.esrl.noaa.gov/psd/data/timeseries/AMO/, and the
multivariate ENSO index (MEI) at
www.esrl.noaa.gov/psd/enso/mei/.
North Atlantic Oscillation
noted that decadal changes in transport were
inversely correlated to the NAO index over the
period 1982–1998, motivating to propose that wind-stress curl forcing of fast propagating Rossby waves was a causative
mechanism. Concurrently, suggested that the correlation
is phase dependent such that a strong positive NAO initiates
a transport peak in May, while a strong negative NAO delays the
transport peak until July, and concluded that transport variability is
influenced by variability of internal ocean dynamics forced by NAO,
rather than by NAO directly. More recently,
examined a longer data record (through
2007) than available to , concluding that the
NAO-transport correlation is not evident outside the 1982–1998 time
frame, and the mechanism proposed by may be only one
of several mechanisms contributing to interannual and longer
variability in the Florida Current.
A comparison of EMD residual trends, as well as decadal and
interannual IMFs of Florida Current, NAO, AMO, and MEI climate
indices are shown in Fig. . There does not appear to be
relationship between EMD residuals of Florida Current and NAO; however
in (b) we identify decadal-scale IMFs of Florida Current and NAO which
are approximately anti-phased from 1985–2006, while beyond 2006 the
NAO mode appears to be increasing in period and decreasing in
amplitude. We note that made their
determination of NAO and transport synchronization by comparison of 3
and 5 year centered means (their Fig. 7). Examination of
their 5 year mean timeseries suggests a stable
anti-correlation from 1986 to 2002. Since an arithmetic mean cannot
discriminate contributions to the mean from longer period cycles, it
may be that decadal-scale IMFs provide a more complete representation
of decadal cycles, and is perhaps why we find the anti-correlation to
hold beyond 2002. Nonetheless, the finding by
that a synchronous relation between
transport and NAO on decadal timescales is not persistent is supported by our EMD analysis.
Atlantic Multidecadal Oscillation
The AMO is coherently related to North Atlantic coastal MSL anomalies
and it has been suggested that it is
a manifestation of the AMOC . Since the
Florida Current is a component of the AMOC, we examine EMD residuals
and decadal IMFs of Florida Current and AMO index in
Fig. c and d. While there is no clear association
between the Florida Current and AMO EMD residuals, there appears to be
a link between decadal modes with Florida Current transport leading
the AMO index by an average of 1.7 years. This is physically
reasonable since the Florida Current transports warm water into the
North Atlantic.
El Niño–Southern Oscillation
Regarding interannual scales, the ENSO globally influences oceanic
processes and is readily apparent in geostrophic transport time series
spectra of the Florida Current . We note a positive
Florida Current anomaly coincident with the strong 1997–1998
El Niño in Figs. and e; however,
there is no evidence of a Florida Current anomaly during the
1982–1983 El Niño suggesting that ENSO influence on Florida
Current transport is indirect and only one component of a complex
system.
It is also interesting to note that inspection of MEI IMFs during the
1982–1983 and 1997–1998 El Niño events
indicate that strong El Niño events correspond to
a synchronization of MEI IMFs 4–7 as shown between the vertical
dashed lines in Fig. f. While this observation does
not appear to be directly relevant to the MSL–transport link, it may
nonetheless provide a useful starting point for investigations of
physical forcings associated with the respective IMFs and their
episodic synchronization into El Niño events.
Sea level
Rising sea levels are a major concern from both ecological and
sociological perspectives, and understanding impacts from Florida
Current variability can provide valuable information for modeling
efforts and decision makers. Figure plots monthly mean
sea levels from March 1982 to October 2014 from the NOAA tide
gauge at Vaca Key, Florida, from January 1994 to January 2015 at
the Virginia Key, Florida, NOAA gauge (data and station information
available at tidesandcurrents.noaa.gov), and from October 1993 to
October 2014 at a National Park Service station in Florida Bay within the
Everglades National Park (Little Madeira Bay; 25.17580∘ N,
80.63269∘ W).
Linear sea level rise (SLR) rates are traditionally estimated from regression
of monthly means with the seasonal cycles removed , and are
shown with dashed lines. However, in the EMD of sea level we do not
alter the data, but use the monthly means as shown in Fig.
with no corrections applied.
The EMD will partition seasonal cycles into appropriate IMFs including
effects from barometric pressure and teleconnections such as ENSO. As
discussed below, this has the advantage of not assuming stationarity of the
seasonal cycle, which is implicit in the standard approach for linear
rate estimates . We also do not alter the data for vertical
land movement, which in the case of South Florida is estimated to be a
small fraction of a millimeter per year .
The resultant EMD residuals are shown in Fig.
with the solid red (Vaca Key), blue (Florida Bay), and green
(Virginia Key) curves, suggesting SLR rates above the linear trend
over the last decade.
Monthly mean sea levels at (a) Vaca Key,
(b) Florida Bay, and (c) Virginia Key, Florida.
Solid lines show the EMD residual (red at Vaca Key, blue in Florida
Bay, green at Virginia Key), dashed lines plot linear regression of
the monthly mean sea level with the annual cycle removed.
Linear models of mean sea level rise. EMD is the linear regression
of the MSL EMD residual with the Florida Current transport estimate
removed.
The Florida Current influences coastal sea levels through
a geostrophic balance of transport and MSL:
Δζ=-fLgV‾s,
where Δζ is the change in sea surface height across the
width of the channel L, f the Coriolis parameter, g the vertical
acceleration, and V‾s the mean transport velocity
. The cross sectional area of the Florida Straits at
27∘ N is approximately 42.96 Mm2 and with a nominal
transport of 32 Sv results in a mean current of
0.745 ms-1 and an equivalent 1 Sv current of
0.023 ms-1. Evaluating
Eq. () at 27∘ N
(L=90km) with a mean current of
V‾s=0.023ms-1 results in a MSL
factor of -1.4 cmSv-1. This of course neglects
nonlinear effects and assumes a barotropic uniform flow. We note that
this result of -1.4 cmSv-1 is similar to model-based
estimates of MOC transport–MSL adjustment by
who found a relation of -1.5 cmSv-1
along the northeast American Atlantic coast north of Cape Hatteras
in the absence of wind forcing.
assessed variability of the Florida Current finding
that while the northward component of flow is not homogeneous
across the strait, the flow is well structured with northward
components more than an order of magnitude stronger than eastward
components, with standard deviations typically 20 % or less of the
mean flow velocity. They also found that barotropic modes dominate over
baroclinic modes, with good coherence between the spatial structure of
density (σT) and temperature across the straits. This suggests
that the assumption of uniform barotropic flow is reasonable for first-order estimates of transport.
Figure a plots sea surface height anomaly (demeaned EMD
residual) at Vaca Key, Florida Bay, and Virginia Key, as well as an
estimated MSL anomaly for the Florida Current at 27∘ N
computed from the Florida Current demeaned EMD residual multiplied by
-1.4 cmSv-1. Subtraction of this Florida Current MSL
anomaly (black curve) from the tide gauge anomalies are shown in
Fig. b providing an estimate of the component of SLR
not attributed to Florida Current transport, where we have
assumed that the transport–MSL relationship deduced at 27∘ N
applies at the tide gauges. We note that found that the
regional dynamic topography of MSL between the cable and tide gauges
is relatively uniform, indicating that this region of the Florida Straits
responds uniformly to dynamic MSL changes. The dashed lines in
Fig. b plot linear regressions to
the MSL changes with the transport changes removed.
Linear rates for the traditional MSL rise
and EMD-based estimates with the transport component removed are
listed in Table .
(a) MSL anomalies at Vaca Key, Florida Bay, and
Virginia Key from EMD residuals, and for the Florida Current at
27∘ N assuming a MSL–transport rate of
-1.4 cmSv-1. (b) Difference of the Florida
Current MSL anomaly from the MSL anomalies at Vaca Key, Florida Bay,
and Virginia Key. Dashed lines are linear regressions to the MSL
differences.
A noticeable feature of MSL with the Florida Current
transport estimate removed is the essentially linear growth of MSL.
This suggests that the recent acceleration of the MSL evident in
Fig. is primarily due to a decline in Florida Current
transport. Specifically, from October 2004 to October 2014
the Vaca Key, Florida Bay, and Virginia Key MSL EMD
residuals increased by 7.4, 7.1 and 5.9 cm,
respectively, while the estimated MSL increase from the EMD residual
of Florida Current
transport is 4.3 cm, which is 58, 60, and 73 % of the
respective MSL rise at each station. Compared to the long-term
linear rates of 3.8 and 3.9 mmyr-1, MSL rises of 7.4, 7.1,
and 5.9 cm over a decade are roughly twice the linear rate.
Seasonal cycle
The seasonal cycle of MSL that is removed from the
monthly means in order to estimate a long-term linear rate
(Fig. ) is computed as an average of the monthly mean
sea level for each month over the period of record, assuming that the
cycle is stationary in frequency and amplitude. Given that the
seasonal anomalies are influenced by atmospheric pressure, ocean
temperature, salinity, and transport, an assumption of stationarity
may be questionable. Regarding Florida Current transport,
found that 9 % of the total variance resides
in the annual timescale, with significant variability in the structure
of the annual cycle across different years. We expect that these annual
cycles should be linked through the transport–MSL relation, and from a
linear perspective cross-correlation of the stationary seasonal
MSL cycle at Vaca Key with IMF 11 of Florida Current
transport which captures the annual timescale, results in a correlation
coefficient of -0.34 significant at the 99.9 % level with a lag of 3
months. This weak anti-correlation suggests some influence between
transport and MSL; however, the 3 month time lag does not clearly indicate
the expected anti-phased timing of MSL and transport over the annual
cycle.
Florida Current transport (NIMF=17) and
Vaca Key MSL (NIMF=9) IMFs with Hilbert
spectrum instantaneous frequencies between 9 and
15 months. (a) March 1992–December 1993; (b)
April 2007–January 2009.
To examine this relation without the assumption of stationarity we identify
Vaca Key MSL and Florida Current transport IMFs with Hilbert instantaneous
frequencies corresponding to periods between 9 and
15 months. A comparison of portions of Vaca Key IMF 4, and
Florida Current IMF 11 which meet this criteria are shown
in Fig. . Here we note a general inverse relationship
between transport and MSL on the annual scale, with cross-correlations
between transport and MSL of -0.86 and -0.92 significant at the
99.9 %
level with a lag of 6 months. Here, the transport and MSL are anti-phased
over the annual cycle as expected, more clearly supporting the idea that
Florida Current transport is an influence on MSL in the Florida Straits on
annual timescales.
Conclusions
Florida Current transport is dynamically linked to coastal sea level
anomalies through geostrophic balance between MSL and mass transport.
Empirical mode decomposition of daily transport estimates indicates
a ±1 Sv variation in mean transport from 1982 to
2004, but a 3 Sv decline from 2004 to 2014; however,
direct measurement of velocities from ship surveys lack this dramatic
decline over the last decade. This conflict needs resolution to
better understand geophysical processes and measurement accuracies
associated with Florida
Current transport. Nonetheless, the decline suggested by the cable
data are consistent with changes in Middle Atlantic Bight MSL
gradient, sea level rise at Vaca Key, Florida Bay, Virginia Key, and
a decline in AMOC since 2004.
Examination of concurrent NAO and Florida Current decadal modes for
1982–2014 is consistent with the analysis of
which considered data for 1982–2007 and
found that outside the 1982–1998 window the anti-correlation between
Florida Current transport and NAO does not appear to hold. However,
their centered-average approach could include effects from longer
period cycles whereas an IMF will not, and we find that decadal IMFs
are approximately in anti-phase synchronization from 1985 to 2006.
While there is some tension between these estimates, they both support
the notion that the NAO and Florida Current transport are not
perennially anti-correlated on decadal timescales.
Regarding sea surface temperatures, we find that decadal modes of the
Florida Current lead the AMO index by an average of 1.7 years.
Since decadal MSL anomalies along the southeastern US coast are
coherent with the AMO index , a connection to Florida
Current transport with a reasonable lead time may prove useful for
prognosis of long-term MSL anomalies along the coast. One must be
cautious that both the NAO and AMO relationships encompass only a few
cycles in the data, there is no reason to warrant that these
relationships will be maintained. Influences of ENSO are confusing at
best. The 1982–1983 El Niño was not observed to be coincident
with a Florida Current anomaly, whereas the 1997 event was.
EMD residuals of monthly mean sea levels at Vaca Key, Florida Bay, and
Virginia Key indicate an acceleration of sea level over the last
decade coincident with the mean transport decline detected in the
cable data. To estimate the transport–MSL relationship at
27∘ N, we neglected nonlinear effects and assumed a barotropic
uniform flow across the channel. We further assumed that the
transport–MSL relationship deduced at 27∘ N applies at the
tide gauges. These assumptions allow for a simple model, but clearly there
are points of deficiency. Further work is needed to validate or
repudiate them.
The transport–MSL relationship of -1.4 cmSv-1 was used
to estimate the MSL anomaly at 27∘ N based on the EMD
residual of Florida Current transport, which when subtracted from the
EMD residual MSL largely recovers the long-term linear trend suggesting
that the recent acceleration of MSL in South Florida is linked to
a decline in Florida Current transport. If this mechanism is causative,
it suggests that 60 % of the roughly 7 cm MSL change in South
Florida over the last decade could be related to a mean decline in
Florida Current transport. Examination of annual modes of transport and
MSL with instantaneous frequencies corresponding to periods between
9 and 15 months finds the expected inverse transport–MSL relationship,
suggesting that the seasonal cycle in MSL is influenced by transport
variability.
It must be noted that even if the 3 Sv mean transport
detected here in the cable data is verified, we cannot say whether or
not this represents a secular climatic shift or the result of natural
variability. reminded us that at least another decade
of MSL observations are needed in the Middle Atlantic Bight to
separate detected accelerations from 20th century variability, and
found that several decades may be required before
acceleration detection methods reveal discernible accelerations in
individual tide gauge records here, primarily due to the considerable
interannual to multidecadal variability of oceanic processes.
Acknowledgements
The Florida Current cable and section data are made freely available
on the Atlantic Oceanographic and Meteorological Laboratory web page
(www.aoml.noaa.gov/phod/floridacurrent/) and are funded by the
NOAA Climate Observation Division.
Edited by: J. M. Huthnance
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