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Western boundary circulation and coastal sea-level variability in Northern Hemisphere oceans

TL;DR: In this paper, the relationship between coastal sea level and the variability of the western boundary currents has been previously studied in each basin separately, but comparison between the two basins is missing.
Abstract: . The northwest basins of the Atlantic and Pacific oceans are regions of intense western boundary currents (WBCs): the Gulf Stream and the Kuroshio. The variability of these poleward currents and their extensions in the open ocean is of major importance to the climate system. It is largely dominated by in-phase meridional shifts downstream of the points at which they separate from the coast. Tide gauges on the adjacent coastlines have measured the inshore sea level for many decades and provide a unique window on the past of the oceanic circulation. The relationship between coastal sea level and the variability of the western boundary currents has been previously studied in each basin separately, but comparison between the two basins is missing. Here we show for each basin that the inshore sea level upstream of the separation points is in sustained agreement with the meridional shifts of the western boundary current extension over the period studied, i.e. the past 7 (5) decades in the Atlantic (Pacific). Decomposition of the coastal sea level into principal components allows us to discriminate this variability in the upstream sea level from other sources of variability such as the influence of large meanders in the Pacific. Our result extends previous findings limited to the altimetry era and suggests that prediction of inshore sea-level changes could be improved by the inclusion of meridional shifts of the western boundary current extensions as predictors. Long-duration tide gauges, such as Key West, Fernandina Beach or Hosojima, could be used as proxies for the past meridional shifts of the western boundary current extensions.

Summary (4 min read)

1 Introduction

  • Western boundary currents (WBCs) are a major feature of global ocean circulation and play an important role in global climate15 by redistributing warm salty waters from the tropics to higher latitudes.
  • Among the typical paths that the Kuroshio can take south of Japan (Fig. 1), the typical large meander is without doubt the most remarkable, and is a major driver of the regional sea level (Kawabe, 2005, 1995, 1985) and atmospheric variability (Sugimoto et al., 2019).
  • From the American coast to 60°W – 55°W,45 northward or southward lateral motions of the Gulf Stream Extension dominate its interannual and seasonal variability.
  • Recent advances have been made on the theoretical underpinning of the relationship between sea level at the western boundaries of ocean and the offshore processes that influence sea level fluctuations (Minobe et al., 2017; Wise et al., 2018).

2.1 Tide gauge selection, treatment, and adjustment for surge variability

  • The authors selected tide gauge stations along the western boundary of the North Atlantic, on the coast of the United States and Canada; and along the western boundary of the North Pacific, on the coast of Japan.
  • To retain only measurements of sufficient quality, length and completeness, historical series with more than 10% of missing monthly values as well as those flagged for quality issues are excluded.
  • The total number of tide gauges retained for the Pacific region is 30 after the criterion of completeness is applied.
  • To correct the records from the effect of local winds and pressure, monthly sea-level pressure and ten meters above sea level wind speeds were obtained from the NCEP/NCAR Reanalysis 1 (Kalnay et al., 1996, NOAA/OAR/ESRL PSL, https: //psl.noaa.gov/).
  • Together with a brief analysis of the results.

2.2 Additional datasets

  • Gridded monthly Sea Surface Height (SSH), Temperature (SST) and Velocities (SSV) derived from satellite altimetry are available from 1993 and were obtained from the Copernicus Marine Environment Monitoring Service website185 (https://marine.copernicus.eu).
  • SSH and SST are obtained from the ARMOR3D product (Guinehut et al., 2012).
  • The authors also retrieved the monthly North Atlantic Oscillation (NAO) index from James Hurrell and National Center for Atmospheric Research Staff (Eds) NAO webpage (https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-index-station-based).
  • The modulus is210 preserved while the phase is randomized.
  • The randomly generated signals are then correlated against B. Significance for zero-lag correlation between A and B is given as the percentage of randomly generated correlations which are less than the correlation between A and B (using absolute values).

2.3 Meridional motions of the Western Boundary Current Extensions

  • At interannual to multidecadal scale, the Gulf Stream Extension and the Kuroshio Extension are quite similarly characterized by strong lateral movements.
  • To produce consistent indices for both oceans, the authors made use of the subsurface sparse temperature observations to derive230 up-to-date indices of the meridional location of the Kuroshio Extension and Gulf Stream Extension, following the GSNW calculation method of Sasaki and Schneider (2011b) and Frankignoul et al. (2001).
  • Given the data availability, the analysis period was restricted to 1960 and 1965 onwards for the Atlantic and Pacific respectively.
  • The leading mode of variability is extracted for each basin by performing an Empirical Orthogonal Function (EOF) decomposition based on correlation (rather than covariance) on the detrended temperature anomaly.
  • The three indices are presented alongside the GSNW (this study) and KEI (this study) in Figure 2 (c) and (d), after detrending is applied.

3 Results

  • The authors propose a scrutiny of the inshore sea level measured by tide gauges using cross-correlation and moving correlation analysis, as well as Empirical Orthogonal Function (EOF) analysis.
  • The authors relate the obtained spatial and temporal patterns to ocean circulation.

3.1 Cross-correlation analysis

  • The resultant correlation patterns suggest groupings of tide gauges across geographic regions, with boundaries defined by changing oceanographic circulation regimes,265 which the authors argue is the fingerprint of ocean circulation on coastal sea level.
  • Three tide gauge groupings are apparent on Figure 3 (a), based on the cross-correlation between Japanese records.
  • All gauges south of Cape Hatteras display almost identical behaviour with correlation average within that group equal to 0.78.
  • Within the groups (a) south and (b) north of Cape Hatteras, the individual correlations (Supplementary Figure S1 (a) and (b), thin grey lines) are high and show little time dependency, with the median never dropping below 0.54 south of Cape Hatteras and below 0.65 north (solid red lines).
  • In the Pacific, the correlations within the two southern groupings feature little time variations (Supplementary Figure S1 (c) and (d)).

3.2 Empirical orthogonal function analysis

  • The authors employ Empirical Orthogonal Function (EOF) analysis to objectively reduce the sea-level anomalies in an ensemble of modes, each composed of a time-varying coefficient α, the Principal Component (PC), and associated spatial-varying coefficients φ, the Empirical Orthogonal Vector or Function (EOF).
  • The Atlantic leading mode explains 60% of the variance and, in a similar way, φ1 features greater amplitudes south of the separation point, Cape Hatteras, and decreasing northward from there (Fig. 5 (b)).
  • 340 Different patterns are found upstream of the separation point.
  • Moreover, it is obvious that the positive velocity pattern (associated with high α1)345 resembles the nearshore NLM (see Figure 1), whereas the negative velocity pattern (associated with low α1) resembles the offshore NLM.

3.2.2 Atlantic and Pacific second modes

  • While similar patterns emerge in both the Atlantic and Pacific leading modes, the same is not true for the second modes.
  • The EOF of the375 Pacific dataset is dominated by the tide gauges on the shores of the Tōkai district.
  • This indicates that, in the region South of Tōkai, the second mode is larger in magnitude to the leading mode.
  • In the Atlantic, amplitudes south of Cape Hatteras are on average −0.8 cm (Figure 6 (a)).
  • As the two modes are different, we390 discuss them separately.

3.2.3 The second mode in the Pacific

  • The principal component α2 obtained with the Pacific gauges is closely linked with the typical large meander of the Kuroshio.
  • The relationship between the tLM periods and the sea-level difference between those two stations is known since the early work of Moriyasu (1958, 1961) and was investigated by Kawabe (1985, 1995, 2005), among others.
  • On the other hand, as was discussed previously, the leading EOF is of same sign and relatively similar magnitude on all of the southern coast of Japan (see the inset of Figure 5 (a) for the amplitude of the leading mode at Kushimoto and Uragami), and the other modes have negligible amplitudes in the region.
  • When the principal component is strongly positive, i.e when the Tōkai coastal sea level is high, the Kuroshio south of Tōkai (135°E – 141°E) is found farther south than when the principal component is negative where it is found much closer to the coast.
  • The negative velocities are also more scattered than their positive counterparts, highlighting that the KE was more stable during period of positive α2 (see also Sugimoto and Hanawa, 2012).

3.2.4 The second mode in the Atlantic

  • The principal component associated with the second EOF in the Atlantic increases from 1948 to the early 1970s, followed by a decrease until the mid-1990s, with interannual deviations from those long-term changes (Figure 6 (d)).
  • The mid-1990s mark an abrupt change, with the interannual variability increasing greatly in amplitude from then onwards.
  • This is shown on Figure 7, which presents the moving standard deviation of α2 obtained with a 15 year running window (solid blue line).
  • As for the difference between Kushimoto and Uragami, substracting the sea level south of Cape Hatteras from north of Cape Hatteras (or reversely) minimizes the influence of the leading mode.
  • The patterns bear some resemblance with the ones obtained with α1 and presented on Figure 5 (b), but the amplitudes of the composite along the Gulf Stream Extension make a strong contrast.

4 Discussion

  • EOF analysis showed similar features of the leading mode of the two basins.
  • This is an important result, because previous465 studies had excluded the Gulf Stream and its extension as plausible drivers of the sea level on the western coast of the North Atlantic basin, on the basis that such drivers were not able to explain coherence across Cape Hatteras (Thompson and Mitchum, 2014; Valle-Levinson et al., 2017).
  • Sasaki et al. (2014) hypothesized that the incoming jet-trapped Rossby waves, which are responsible for the extensions’ shifts, break on the western boundary and propagate equatorwards as Kelvin or other coastally trapped waves, linking the extension variability to coastal sea level.
  • In the Pacific, this second mode is the manifestation of the meandering of the Kuroshio upstream of its separation point, whereas the second EOF in the Atlantic is mainly associated505 with variability north of Cape Hatteras, the separation point.
  • Figure 7 (a) presents the moving600 correlation between the southern and northern gauge averages (dashed orange line) alongside the standard deviation of the principal component α2 computed with a moving 15 years window (solid blue line).

5 Conclusion

  • This study presents a consistent analysis of the two western boundary regions of northern Atlantic and northern Pacific.
  • In the Pacific this relates to upstream meso-scale dynamics (Kuroshio large meander), whereas in the Atlantic, the second mode620 is mainly associated with changes north of Cape Hatteras, the separation point of the Gulf Stream, although weak antivariations exist to the south.
  • The two stations are available respectively from 1965 and 1957 in the PSMSL RLR catalogue.
  • When using these timeseries, please cite the present study appropriately.

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Figures (7)

Content maybe subject to copyright    Report

Western boundary circulation and coastal sea-level variability in
northern hemisphere oceans
Samuel Tiéfolo Diabaté
1
, Didier Swingedouw
2
, Joël Jean-Marie Hirschi
3
, Aurélie Duchez
3
, Philip
J. Leadbitter
4
, Ivan D. Haigh
5
, and Gerard D. McCarthy
1
1
ICARUS, Department of Geography, Maynooth University, Maynooth, Co. Kildare, Ireland
2
Environnements et Paleoenvironnements Oceaniques et Continentaux (EPOC), UMR CNRS 5805 EPOC-OASU-Universite
de Bordeaux, Allée Geoffroy Saint-Hilaire, Pessac 33615, France
3
National Oceanography Centre, Southampton, UK
4
University of East Anglia, Norwich, UK
5
University of Southampton, Southampton, UK
Correspondence: Samuel Tiéfolo Diabaté (samuel.diabate.2020@mumail.ie)
Abstract. The northwest basins of the Atlantic and Pacific oceans are regions of intense Western Boundary Currents (WBC),
the Gulf Stream and the Kuroshio. The variability of these poleward currents and their extension in the open ocean is of major
importance to the climate system. It is largely dominated by in-phase meridional shifts downstream of the points where they
separate from the coast. Tide gauges on the adjacent coastlines have measured the inshore sea level for many decades and
provide a unique window on the past of the oceanic circulation. The relationship between coastal sea level and the variability5
of the western boundary currents has been previously studied in each basin separately but comparison between the two basins is
missing. Here we show for each basin, that the inshore sea level upstream the separation points is in sustained agreement with
the meridional shifts of the western boundary current extension over the period studied, i.e. the past seven (five) decades in the
Atlantic (Pacific). Decomposition of the coastal sea level into principal components allows us to discriminate this variability
in the upstream sea level from other sources of variability such as the influence of large meanders in the Pacific. This result10
suggests that prediction of inshore sea-level changes could be improved by the inclusion of meridional shifts of the western
boundary current extensions as predictors. Conversely, long duration tide gauges, such as Key West, Fernandina Beach or
Hosojima could be used as proxies for the past meridional shifts of the western boundary current extensions.
1 Introduction
Western boundary currents (WBCs) are a major feature of global ocean circulation and play an important role in global climate15
by redistributing warm salty waters from the tropics to higher latitudes. The role of WBCs in the redistribution of heat and salt
in the Atlantic is an integral part of the Atlantic Meridional Overturning Circulation (AMOC), resulting in heat transported
towards the equator in the South Atlantic and the largest heat transport of any ocean northwards in the North Atlantic (Bryden
and Imawaki, 2001). WBCs also interact strongly with the atmosphere, influencing regional and global climate variability
(Imawaki et al., 2013; Kwon et al., 2010; Czaja et al., 2019) and impact the sea level of the coastlines they are adjacent to20
(Little et al., 2019; Sasaki et al., 2014; Woodworth et al., 2019; Collins et al., 2019).
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Figure 1. (a) Kuroshio region circulation: The three Kuroshio paths the typical Large Meander (tLM), the near-shore Non-Large Meander
(nNLM) and the offshore Non-Large Meander (oNLM) are indicated upstream of the Izu-Ogasawara Ridge. The mean location of the KE
is indicated offshore of this point showing the location of the quasi-stationary meanders. The Oyashio current is shown in blue. On land, K
indicates Ky
¯
ush
¯
u, Hon stands for Honsh
¯
u and Ho indicates Hokkaid
¯
o. (b) Gulf Stream region circulation from the Florida Current to the Gulf
Stream Extension. The Northern Recirculation Gyre is also indicated. On land, M.-A. B. stands for Mid-Atlantic Bight and N.S. indicates
Nova Scotia. Markers in (a) and (b) indicate the location of the tide gauges used in this study. The colour and shape of the markers in (a) and
(b) indicate the angle used to rotate the wind stress in an alongshore/across-shore coordinate system for the removal of sea-level variability
driven by local atmospheric effect (See Supplementary Table S1 and Supplementary Table S2). Shadings in (a) and (b) indicate bathymetry.
In the Pacific, north of 30°N, the Kuroshio flows northeastwards along the coast of mainland Japan before leaving the coast
at approximately 35°N and becoming a separated boundary current known as the Kuroshio Extension (KE, Figure 1 (a)). The
Kuroshio and KE have variable flow regimes including decadal timescale variability, with the KE following either a stable
and northern path, or an unstable and southern path (Qiu et al., 2014; Imawaki et al., 2013; Kawabe, 1985). This variability25
is driven by the wind stress curl over the central North Pacific which generates Sea Surface Height (SSH) anomalies. These
anomalies progress westward as jet-trapped waves, shifting meridionally the KE before reaching the Kuroshio Oyashio
confluence (Sugimoto and Hanawa, 2009; Sasaki et al., 2013; Sasaki and Schneider, 2011a; Ceballos et al., 2009). Southeast
of Japan, negative (positive) SSH anomalies ultimately displace the Kuroshio southward (northward) above the shallower
(deeper) region of the Izu-Ogasawara Ridge (IOR). Interaction of the Kuroshio with the bathymetry when it is shifted above30
the shallower region of the IOR is possibly the cause of an unstable Kuroshio Extension (Sugimoto and Hanawa, 2012). In any
case, when the KE is unstable, it has a more southern mean position, and the Kuroshio follows the offshore Non-Large Meander
(oNLM) path (see Fig. 1 (a)). When unstable the Kuroshio has a lower overall transport (Sugimoto and Hanawa, 2012), which
has an impact on the associated ocean heat transport. When the KE is stable, it exhibits a quasi-stationary meanders and a
more northern mean position, and the Kuroshio south of Japan tends to follow either the typical Large Meander (tLM) or the35
near-shore Non-Large Meander (nNLM) (Sugimoto and Hanawa, 2012; Qiu et al., 2014; Usui et al., 2013).
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Among the typical paths that the Kuroshio can take south of Japan (Fig. 1), the typical large meander is without doubt
the most remarkable, and is a major driver of the regional sea level (Kawabe, 2005, 1995, 1985) and atmospheric variability
(Sugimoto et al., 2019). Large meanders (LM) occur when two stationary eddies strengthen south of Japan. One is located
southeast of Ky
¯
ush
¯
u and associated with an anticyclonic circulation; and the other one is located south of T
¯
okai and associated40
with a cyclonic circulation. The front bounded by the two eddies becomes the Kuroshio large meander, and thus the cyclonic
anomaly is inshore between the Kuroshio path and the southern coasts of T
¯
okai.
In the Atlantic, the Gulf Stream has its origins in the eponymous Gulf of Mexico, flowing past the Florida coastline as the
Florida Current before leaving the boundary at Cape Hatteras near 35°N. From here it flows eastward as a meandering, eddying,
free current in the Gulf Stream Extension, and eventually the North Atlantic Current. From the American coast to 60°W 55°W,45
northward or southward lateral motions of the Gulf Stream Extension dominate its interannual and seasonal variability. This
notable intrinsic variability follows closely the main mode of Atlantic atmospheric variability: the North Atlantic Oscillation
(NAO) (Joyce et al., 2000; McCarthy et al., 2018). The abrupt transition from warm subtropical waters to cold subpolar waters
marks a ‘North Wall’ of the Gulf Stream (Fuglister, 1955). This Gulf Stream North Wall (GSNW) is a convenient marker of
the lateral motions of the Gulf Stream Extension (Frankignoul et al., 2001; Joyce et al., 2000; Sasaki and Schneider, 2011b).50
The horizontal circulation of separated western boundary current interact closely with the vertical circulation. The vertical
circulation in this region is part of the AMOC which can be simplified as northward flowing Gulf Stream waters and southward
flowing deep waters as part of the Deep Western Boundary Current (DWBC). One paradigm of the interaction of vertical
and horizontal circulation in the region is that an enhanced DWBC, enhanced AMOC, ‘pushes’ the GSNW to the south, and
expands the Northern Recirculation Gyre (NRG). However, diverse behavior has been found in models, with some supporting55
this paradigm (Zhang and Vallis, 2007; Zhang, 2008; Sanchez-Franks and Zhang, 2015) and some finding the opposite: an
enhanced AMOC, northward shifted GSNW (De Coetlogon et al., 2006; Kwon and Frankignoul, 2014). Alternatively, as in
the Pacific with the KE, the Gulf Stream Extension has been linked to the mechanism of remote wind stress curl forcing the
westward propagation of large-scale jet undulations (Sasaki and Schneider, 2011b). Finally, Andres et al. (2013) highligted that
the coastal sea level on the large shelf north of Cape Hatteras was in agreement with the location of the Gulf Stream Extension60
west of 69 °W and suggested that the shelf transport ‘pushes’ the Gulf Stream, whereas Ezer et al. (2013) hypothesized that a
more inertial Gulf Stream south of the separation point may ‘overshoot’ to the north when leaving the coastline at 35°N and
control, at least to some extent, the location of the extension.
While the Gulf Stream and Kuroshio are western boundary currents driven by the closure of the Sverdrup balance (Stommel,
1948; Munk, 1950), even the brief introduction presented here highlights both differences and similarities between the currents.65
Upstream of separation point, the currents behave quite differently. The Kuroshio takes a number of distinct paths, whereas
the Gulf Stream hugs the coast tightly. The separation point at the B
¯
os
¯
o peninsula and Cape Hatteras has a remarkably similar
latitude both at 35°N. Downstream, the Gulf Stream Extension flows northeastward, whereas the Kuroshio Extension is mainly
flowing eastward. The meandering of the Kuroshio in its extension region is much more defined than that of the Gulf Stream
Extension, with no named quasi-stationary meanders in the Gulf Stream Extension (until farther downstream at the Mann70
eddy). The north south shifts of the extensions are remarkable features of both basins and account for an important part the
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extensions’ variability. It is well established that these lateral shifts are caused by the propagation of long jet-trapped waves
forced by downstream wind in the Pacific, whereas the mechanisms driving the GSNW are not completely clear, with plausible
role of a similar mechanism of wind-forced jet undulation. These jet-trapped waves are possible thanks to the sharp background
velocity gradient induced by WBC Extensions, comparable or greater to the meridional gradient of planetary vorticity within75
the mid-latitude band. Hence, the jet-trapped waves are essentially Rossby waves, but they propagate in the waveguide formed
by the WBC extension, which allow their meridional narrowing as they progress westward, and their southwestward flow in
the Atlantic (Sasaki et al., 2013; Sasaki and Schneider, 2011a, b). It is however important to note that, in the Atlantic, the
lateral shifts of the Gulf Stream Extension have been more often linked with the DWBC and the NRG. In the Pacific, southern
(northern) shifts of the Kuroshio Extension are known to be concurrent with periods of instability (stability), whereas, until80
recent years (prior to 2000), the Gulf Stream Extension has been much more stable (Andres, 2016; Gangopadhyay et al.,
2019). The interaction with the cold currents to the north is also quite different. The continent north of the Gulf Stream to
Newfoundland lends to a topographical constraint on the gyre circulation, whereas the Oyashio is much less constrained by
land. Conversely, the upstream Kuroshio is much more constrained than the upstream Gulf Stream, due to the presence of the
Izu-Ogasawara Ridge. Additionally, there is no Pacific equivalent to the coastal circulation on the prominent shelf north of Cape85
Hatteras (Peña-Molino and Joyce, 2008). The AMOC is a notably Atlantic-specific feature but there is not a distinct feature of
the horizontal circulation that identifies clearly with the presence of the AMOC in the Atlantic basin that is not present in the
Pacific basin. While a decline in the AMOC is robust in climate projections, WBCs are also expected to change. WBCs have
been observed to be shifting polewards (Wu et al., 2012; Stocker et al., 2013) and becoming more unstable (Andres, 2016; Beal
and Elipot, 2016; Gangopadhyay et al., 2019).90
Tide gauges estimate relative sea level at the coast and have done so since the 18
th
century in certain locations (e.g. Amster-
dam, Stockholm, Kronstadt, Liverpool, Brest). Tide gauges have long been used to investigate ocean circulation in regions such
as the Gulf Stream where the impact of strong ocean circulation on coastal sea level is apparent (Montgomery, 1938). However,
ocean circulation is far from the only impact on sea level at the coast. The effects of land motion (including glacial isostatic
adjustment), thermosteric expansion, terrestrial freshwater changes (including river runoff and ice melt), and gravitational fin-95
gerprints all feature in sea level variations at the coast (Meyssignac et al., 2017). In addition, the local forcing of the atmosphere
drives an important part of the coastal sea-level variability, particularly in shelf environments. Variations in wind stress can
force water to travel toward (or away from) the coastline, consequently raising (lowering) the sea level at tide gauge locations.
Both across-shore and alongshore wind stresses can impact sea level as can variations in the local air pressure through the
Inverse Barometer (IB) effect. On the American northeast coast, the inverted barometer greatly influences interannual change100
in the mean sea level, dominates most extreme interannual changes, and is not negligible on multidecadal timescales (Piecuch
and Ponte, 2015), while the alongshore wind is also believed to play a role (Andres et al., 2013; Woodworth et al., 2014;
Piecuch et al., 2019). This contribution of the atmosphere to the mean sea level is particularly challenging to disentangle from
the contribution of ocean dynamics, because the two share similar range of timescales. Hence great care is needed to interpret
coastal sea level fluctuations, as measured by tide gauges, as representative of ocean circulation patterns.105
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A number of approaches have been developed to investigate ocean circulation using tide gauge data. The cross-stream
gradient of sea level can be estimated by using an onshore tide gauge and an offshore island tide gauge (Montgomery, 1938;
Kawabe, 1988; Ezer, 2015; Marsh et al., 2017) providing a direct estimate of a boundary current flowing between the gauges
via the geostrophic relationship. This type of estimate is restricted to locations where suitable offshore island tide gauges exist.
Apart from the limited number of such locations, the offshore estimate is located in the eddy-filled ocean interior which can110
experience sea level fluctuations driven by the ocean mesoscale (Sturges and Hong, 1995; Firing et al., 2004) that are not
representative of the large-scale ocean circulation. In the Atlantic, a number of studies (e.g. Bingham and Hughes, 2009; Ezer,
2013; McCarthy et al., 2015) have used long tide gauge records to estimate the strength of the AMOC, which has only been
continuously observed since 2004 (Cunningham et al., 2007). In the Pacific, the difference between the sea level either side
of the Kii peninsula (Fig. 1) has been extensively used to diagnose past occurrence of the typical large meander (Moriyasu,115
1958, 1961; Kawabe, 1985, 1995, 2005), despite the causal relationship not being fully understood.
Recent advances have been made on the theoretical underpinning of the relationship between sea level at the western bound-
aries of ocean and the offshore processes that influence sea level fluctuations (Minobe et al., 2017; Wise et al., 2018). The rule
of thumb of Minobe et al. (2017) for a western boundary of the northern hemisphere is as follows: the sea level at a point on
the coastline is influenced by (1) long Rossby waves (or any other mass input from the east) incident on that point and (2)120
coastally trapped waves, transmitting equatorward the sea level signal from points farther to the north which, equally, can be
influenced by incidental long Rossby waves. It follows that the alongshore gradient of the coastal sea level at a given latitude
is proportional to the sea level input from the east at the same latitude (Minobe et al., 2017),
y
ζ
f
x
W
=
β
f
2
ζ
x
I
, (1)
where ζ is the sea-level anomaly, evaluated at the coast (x
W
) and at the frontier between the boundary layer and the ocean125
interior (x
I
), and β is the meridional gradient of the Coriolis frequency f. In the real ocean, the mass input into the western
boundary region is more accurately described by the jet-trapped Rossby wave framework than by the direct westward propa-
gation of linear long Rossby waves (Sasaki et al., 2013; Sasaki and Schneider, 2011a; Taguchi et al., 2007). Therefore, pairing
the jet-trapped theory with Minobe et al. (2017) framework is expected to better estimate the sea level on the coast of western
boundaries. In accordance with this idea, the coastal sea level south of Japan is known to be in agreement with the Kuroshio130
location above the Izu-Ogasawara Ridge (Kuroda et al., 2010), the KE meridional shifts during the satellite era (Sasaki et al.,
2014) and the regime shifts in North Pacific mid-latitude (Senjyu et al., 1999). Simply put, the mechanism is that jet-trapped
long waves, originating from the east and responsible for the meridional shifts of the WBC extension, break, when reaching
the coastline, in coastally trapped waves that propagate equatorward (Sasaki et al., 2014).
Globally, the mean sea level has shown an increased rate of rise in the last decades (Dangendorf et al., 2019; Nerem et al.,135
2018) induced by anthropogenic emission of greenhouse gases in the atmosphere, which is a major issue for coastal communi-
ties and environments. Understanding the relationship between sea level and ocean circulation is a component of understand-
ing coastal vulnerability to changing sea levels. Many densely populated regions border WBCs and large changes in WBCs
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Citations
More filters
01 Dec 2006
TL;DR: In this article, low-frequency variability of the Kuroshio Extension (KE) is studied using observations and a multidecadal (1950-2003) hindcast by a high-resolution (0.1°) eddy-resolving, global ocean general circulation model for the Earth Simulator (OFES).
Abstract: Low-frequency variability of the Kuroshio Extension (KE) is studied using observations and a multidecadal (1950–2003) hindcast by a high-resolution (0.1°), eddy-resolving, global ocean general circulation model for the Earth Simulator (OFES). In both the OFES hindcast and satellite altimeter observations, low-frequency sea surface height (SSH) variability in the North Pacific is high near the KE front. An empirical orthogonal function (EOF) analysis indicates that much of the SSH variability in the western North Pacific east of Japan is explained by two modes with meridional structures tightly trapped along the KE front. The first mode represents a southward shift and to a lesser degree, an acceleration of the KE jet associated with the 1976/77 shift in basin-scale winds. The second mode reflects quasi-decadal variations in the intensity of the KE jet. Both the spatial structure and time series of these modes derived from the hindcast are in close agreement with observations. A linear Rossby wave...

187 citations

01 Dec 2018
TL;DR: In this paper, the authors explore the relation between observed river discharge and sea level on the United States Atlantic and Gulf coasts over interannual and longer periods, and show that river-discharge and sea-level changes are significantly correlated, such that sea level rises between 0.01 and 0.08 cm for a 1 km3 annual river discharge increase, depending on region.
Abstract: Significance River discharge exerts an important influence on coastal ocean circulation but has been overlooked as a driver of historical coastal sea-level change and future coastal flood risk. We explore the relation between observed river discharge and sea level on the United States Atlantic and Gulf coasts over interannual and longer periods. We formulate a theory that predicts the observed correspondence between river discharge and sea level, demonstrating a causal relation between the two variables. Our results highlight a significant but overlooked driver of coastal sea level, indicating the need for (1) improved resolution in remote sensing and modeling of the coastal zone and (2) inclusion of realistic river runoff variability in climate models. Identifying physical processes responsible for historical coastal sea-level changes is important for anticipating future impacts. Recent studies sought to understand the drivers of interannual to multidecadal sea-level changes on the United States Atlantic and Gulf coasts. Ocean dynamics, terrestrial water storage, vertical land motion, and melting of land ice were highlighted as important mechanisms of sea-level change along this densely populated coast on these time scales. While known to exert an important control on coastal ocean circulation, variable river discharge has been absent from recent discussions of drivers of sea-level change. We update calculations from the 1970s, comparing annual river-discharge and coastal sea-level data along the Gulf of Maine, Mid-Atlantic Bight, South Atlantic Bight, and Gulf of Mexico during 1910–2017. We show that river-discharge and sea-level changes are significantly correlated (p<0.01), such that sea level rises between 0.01 and 0.08 cm for a 1 km3 annual river-discharge increase, depending on region. We formulate a theory that describes the relation between river-discharge and halosteric sea-level changes (i.e., changes in sea level related to salinity) as a function of river discharge, Earth’s rotation, and density stratification. This theory correctly predicts the order of observed increment sea-level change per unit river-discharge anomaly, suggesting a causal relation. Our results have implications for remote sensing, climate modeling, interpreting Common Era proxy sea-level reconstructions, and projecting coastal flood risk.

38 citations

Journal ArticleDOI
TL;DR: In this paper , the authors synthesize the understanding of contemporary decadal variability in the AMOC, bringing together evidence from observations, ocean reanalyses, forced models and AMOC proxies, revealing periods of decadal-scale weakening and strengthening that differ between the subpolar and subtropical regions.
Abstract: The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the climate through its transport of heat in the North Atlantic Ocean. Decadal changes in the AMOC, whether through internal variability or anthropogenically forced weakening, therefore have wide-ranging impacts. In this Review, we synthesize the understanding of contemporary decadal variability in the AMOC, bringing together evidence from observations, ocean reanalyses, forced models and AMOC proxies. Since 1980, there is evidence for periods of strengthening and weakening, although the magnitudes of change (5–25%) are uncertain. In the subpolar North Atlantic, the AMOC strengthened until the mid-1990s and then weakened until the early 2010s, with some evidence of a strengthening thereafter; these changes are probably linked to buoyancy forcing related to the North Atlantic Oscillation. In the subtropics, there is some evidence of the AMOC strengthening from 2001 to 2005 and strong evidence of a weakening from 2005 to 2014. Such large interannual and decadal variability complicates the detection of ongoing long-term trends, but does not preclude a weakening associated with anthropogenic warming. Research priorities include developing robust and sustainable solutions for the long-term monitoring of the AMOC, observation–modelling collaborations to improve the representation of processes in the North Atlantic and better ways to distinguish anthropogenic weakening from internal variability. The Atlantic Meridional Overturning Circulation (AMOC) has a key role in the climate system. This Review documents AMOC variability since 1980, revealing periods of decadal-scale weakening and strengthening that differ between the subpolar and subtropical regions.

38 citations

01 Dec 2012
TL;DR: In this paper, the authors examined interannual to decadal variability of the Kuroshio Extension (KE) jet using satellite altimeter observations from 1993 to 2010 and found that the leading empirical orthogonal function (EOF) mode of sea level variability in the KE region represents the meridional shift of the KE jet, followed by its strength changes with a few month lag.
Abstract: This study examines interannual to decadal variability of the Kuroshio Extension (KE) jet using satellite altimeter observations from 1993 to 2010. The leading empirical orthogonal function (EOF) mode of sea level variability in the KE region represents the meridional shift of the KE jet, followed by its strength changes with a few month lag. This shift of the KE jet lags atmospheric fluctuations over the eastern North Pacific by about three years. Broad sea level anomalies (SLAs) emerge in the eastern North Pacific 3–4 years before the upstream KE jet shift, and propagate westward along the KE jet axis. In the course of the propagation, the meridional scale of the SLAs gradually narrows, and their amplitude increases. This westward propagation of SLAs with a speed of about 5 cm s−1 is attributed to the westward propagation of the meridional shift of the jet, consistent with the thin-jet theory, whose importance has been suggested by previous numerical studies. In addition, the westward-propagatin...

12 citations

Journal ArticleDOI
TL;DR: In this article , a high-resolution ocean model and a CMIP6 model were used to investigate the impact of Greenland Ice Sheet (GrIS) melting, a key uncertainty for past and future AMOC changes.
Abstract: The Atlantic Meridional Overturning Circulation (AMOC) is a crucial element of the Earth climate. It is a complex circulation system difficult to monitor and to model. There is considerable debate regarding its evolution over the last century as well as large uncertainty about its fate at the end of this century. We depict here the progress since the IPCC SROCC report, offering an update of its chapter 6.7. We also show new results from a high-resolution ocean model and a CMIP6 model to investigate the impact of Greenland Ice Sheet (GrIS) melting, a key uncertainty for past and future AMOC changes. The ocean-only simulation at 1/24° resolution in the Arctic-North Atlantic Ocean performed over the period 2004–2016 indicates that the spread of the Greenland freshwater runoff toward the center of the Labrador Sea, where oceanic convection occurs, seems larger in this model than in a CMIP6 model. Potential explanations are related to the model spatial resolution and the representation of mesoscale processes, which more realistically transport the freshwater released around the shelves and, through eddies, provides strong lateral exchanges between the fine-scale boundary current and the convective basin in the Labrador Sea. The larger freshening of the Labrador Sea in the high-resolution model then strongly affects deep convection activity. In the simulation including GrIS melting, the AMOC weakens by about 2 Sv after only 13 years, far more strongly than what is found in the CMIP6 model. This difference raises serious concerns on the ability of CMIP6 models to correctly assess the potential impact of GrIS melting on the AMOC changes over the last few decades as well as on its future fate. To gain confidence in the GrIS freshwater impacts on climate simulations and therefore in AMOC projections, urgent progress should be made on the parameterization of mesoscale processes in ocean models.

9 citations

References
More filters
Journal ArticleDOI
17 Aug 2007-Science
TL;DR: The vigor of the Atlantic meridional overturning circulation (MOC) is thought to be vulnerable to global warming, but its short-term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain this article.
Abstract: The vigor of Atlantic meridional overturning circulation (MOC) is thought to be vulnerable to global warming, but its short-term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain. We combine continuous measurements of the MOC (beginning in 2004) using the purposefully designed transatlantic Rapid Climate Change array of moored instruments deployed along 26.5°N, with time series of Gulf Stream transport and surface-layer Ekman transport to quantify its intra-annual variability. The year-long average overturning is 18.7 ± 5.6 sverdrups (Sv) (range: 4.0 to 34.9 Sv, where 1 Sv = a flow of ocean water of 106 cubic meters per second). Interannual changes in the overturning can be monitored with a resolution of 1.5 Sv.

785 citations

Journal ArticleDOI
TL;DR: In this paper, a nonparametric method is suggested to estimate the statistical significance of a computed correlation coefficient when serial correlation is a concern, and the method compares favorably with conventional methods.
Abstract: When analyzing pairs of time series, one often needs to know whether a correlation is statistically significant. If the data are Gaussian distributed and not serially correlated, one can use the results of classical statistics to estimate the significance. While some techniques can handle non-Gaussian distributions, few methods are available for data with nonzero autocorrelation (i.e., serially correlated). In this paper, a nonparametric method is suggested to estimate the statistical significance of a computed correlation coefficient when serial correlation is a concern. This method compares favorably with conventional methods.

721 citations

Journal ArticleDOI
TL;DR: Simple extrapolation of the quadratic implies global mean sea level could rise 65 ± 12 cm by 2100 compared with 2005, roughly in agreement with the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5) model projections.
Abstract: Using a 25-y time series of precision satellite altimeter data from TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3, we estimate the climate-change-driven acceleration of global mean sea level over the last 25 y to be 0.084 ± 0.025 mm/y2 Coupled with the average climate-change-driven rate of sea level rise over these same 25 y of 2.9 mm/y, simple extrapolation of the quadratic implies global mean sea level could rise 65 ± 12 cm by 2100 compared with 2005, roughly in agreement with the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5) model projections.

671 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products to find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean cooling rate.
Abstract: Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake1, 2, 3, 4. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns5, 6, 7, but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

564 citations

Frequently Asked Questions (11)
Q1. What have the authors contributed in "Western boundary circulation and coastal sea-level variability in northern hemisphere oceans" ?

Tide gauges on the adjacent coastlines have measured the inshore sea level for many decades and provide a unique window on the past of the oceanic circulation. The relationship between coastal sea level and the variability 5 of the western boundary currents has been previously studied in each basin separately but comparison between the two basins is missing. Here the authors show for each basin, that the inshore sea level upstream the separation points is in sustained agreement with the meridional shifts of the western boundary current extension over the period studied, i. e. the past seven ( five ) decades in the Atlantic ( Pacific ). This result 10 suggests that prediction of inshore sea-level changes could be improved by the inclusion of meridional shifts of the western boundary current extensions as predictors. 

Hence, further work is required on the matter. The authors showed that dissimilarities between Japanese and American inshore sea level emerge in the second mode of variability. Because the tide gauge networks in both oceans extend further back in time than the period analysed in this study, inshore sea level has potential for reconstruction of the variability of the ocean circulation mode of variability. Although the causal630 link between the upstream sea level and the meridional shifts of WBC extensions is not yet completely understood, their results suggest that upstream inshore tide gauges, such as Key West ( available from 1913 in the PSMSL revised local reference ( RLR ) database ), Fernandina Beach ( 1897 ) or Hosojima ( 1930 ) could be used as proxies for the extension meridional shifts and, by extension, the forcing responsible for such meridional shifts. 

Because the tide gauge networks in both oceans extend further back in time than the period analysed in this study, inshoresea level has potential for reconstruction of the variability of the ocean circulation mode of variability. 

the surge correction reduces the variance north of Cape Hatteras, which better constrains the EOF analysis and reduces undesired compensation between modes. 

Both across-shore and alongshore wind stresses can impact sea level as can variations in the local air pressure through theInverse Barometer (IB) effect. 

The role of WBCs in the redistribution of heat and salt in the Atlantic is an integral part of the Atlantic Meridional Overturning Circulation (AMOC), resulting in heat transported towards the equator in the South Atlantic and the largest heat transport of any ocean northwards in the North Atlantic (Bryden and Imawaki, 2001). 

The randomly generated signals are then correlated against B. Significance for zero-lag correlation between A and B is given as the percentage of randomly generated correlations which are less than the correlation between A and B (using absolute values). 

For each year up to 2019, the available sparse subsurface temperature observations were interpolated at the climatological position of the Gulf Stream and KuroshioExtensions using an inverse distance weighting technique with power parameter p= 2 and a search radius of 400 km, allowing235construction of an along-jet temperature matrix. 

In addition, the local forcing of the atmosphere drives an important part of the coastal sea-level variability, particularly in shelf environments. 

The authors make use of the GSNW index from Joyce et al. (2000) and of the Kuroshio Extension indices from Qiu et al. (2016), and also derive in Sect. 2.3 indices for the variability of the two WBC extensions. 

In the615absence of such information, the mechanism proposed by Sasaki et al. (2014) is, so far, the only linking the upstream sea-level and the WBC extensions’ meridional shifts.