Western boundary circulation and coastal sea-level variability in Northern Hemisphere oceans
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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Frequently Asked Questions (11)
Q2. What are the future works in "Western boundary circulation and coastal sea-level variability in northern hemisphere oceans" ?
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.
Q3. What is the potential for reconstruction of the ocean circulation mode of variability?
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.
Q4. What is the effect of the surge correction on the EOF analysis?
the surge correction reduces the variance north of Cape Hatteras, which better constrains the EOF analysis and reduces undesired compensation between modes.
Q5. What is the effect of wind stress on sea level?
Both across-shore and alongshore wind stresses can impact sea level as can variations in the local air pressure through theInverse Barometer (IB) effect.
Q6. What is the role of the western boundary current in the redistribution of heat and salt?
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).
Q7. What is the significance of the correlation between A and B?
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).
Q8. How was the inverse distance weighting technique used?
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.
Q9. What is the main effect of the local forcing of the atmosphere on sea level?
In addition, the local forcing of the atmosphere drives an important part of the coastal sea-level variability, particularly in shelf environments.
Q10. What are the indices used to determine the variability of the two WBC extensions?
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.
Q11. What is the mechanism proposed by Sasaki et al.?
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.