Meteorologically Driven Trends in Sea Level Rise Alexander S

GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L23616, doi:10.1029/2007GL031814, 2007 Click Here for Full Article

Meteorologically driven trends in sea level rise Alexander S. Kolker1 and Sultan Hameed2 Received 24 August 2007; revised 10 October 2007; accepted 6 November 2007; published 15 December 2007.

[1] Determining the rate of global sea level rise (GSLR) changes in the rate of rise. The cause of this variability is during the past century is critical to understanding recent largely unknown, although it has been linked to storms, changes to the global climate system. However, this is winds and floods [Zhang et al., 2000], wind driven Rossby complicated by non-tidal, short-term, local sea-level waves [Hong et al., 2000], shifts in major ocean currents variability that is orders of magnitude greater than the such as the Gulf Stream [Emery and Aubrey, 1991], volca- trend. While the non-dimensional North Atlantic Oscillation nically induced ocean heat content variations [Church et al., (NAO) index can explain some of this variability in the 2005], and in the Pacific Ocean, the El Nino Southern Atlantic, significant results have been largely restricted to Oscillation [Douglas, 2001]. Rates of relative sea-level rise Europe. We show that dimensional indices of the position and (RSLR) in the coastal zone are more variable, both spatially intensity of the atmospheric centers of action (COAs) and temporally, than rates of GSLR. They range from 10 comprising the NAO are correlated with a major fraction of to +10 mm/yr [Douglas, 2001], with differences often the variability and trend at 5 Atlantic Ocean tide gauges since ascribed to factors including subsidence, uplift, tectonics, 1900. COA fluctuations are shown to influence winds, freshwater fluxes, and regional thermosteric effects [Cabanes pressure and sea-surface temperatures, thereby influencing et al., 2001; Emery and Aubrey, 1991; Gornitz et al., 1982]. sea level via a suite of coastal oceanographic processes. [4] Many previous attempts to calculate rates of GSLR These findings reduce variability in regional sea level rise coped with the problem of interannual and decadal scale estimates and indicate a meteorological driver of sea-level variability by averaging large or long-term data sets [Antonov trends. Citation: Kolker, A. S., and S. Hameed (2007), et al., 2005; Cabanes et al., 2001]. An alternative approach is Meteorologically driven trends in sea level rise, Geophys. Res. to determine the cause of this variability as a means of Lett., 34, L23616, doi:10.1029/2007GL031814. distinguishing between competing signals in climate records [Church et al., 2004]. In this paper we show that a major 1. Introduction fraction of the variability and the trend in mean sea level at key sites along the Atlantic Ocean are driven by shifts in the [2] Changes in the position of global mean sea level are a position and intensity of the major atmospheric pressure key indicator of the nature and scale of global climate centers that reside over the Atlantic Ocean, the Azores High changes [Intergovernmental Panel of Climate Change (AH) and the Icelandic Low (IL). (IPCC), 2007]. Estimates of recent rates of global sea level [5] One potential driver of Atlantic Ocean sea level is the rise (GSLR) vary considerably. Miller and Douglas [2004] North Atlantic Oscillation (NAO) [Woolf et al., 2003]. It is determined rates of 20th Century GSLR rates to be 1.5 – often described by a non-dimensional measure of the 2.0 mm/yr, Wadhams and Munk [2004] suggested a lower pressure difference between two stations chosen to represent rate of 1.1 mm/yr for the same period, and the IPCC [2007] the mean state of the IL (Reykjavik, Iceland) and the AH calls for higher rates for the period 1993–2003: 3.1 ± 0.7. (Lisbon, Portugal) [Hurrell, 1995]. While this ‘‘NAO in- The difference in these calculations is non trivial, amount- dex’’ provides a reasonable first approximation of atmo- 3 ing to 360 km /yr of fresh water per 1 mm/yr of GSLR spheric conditions in the North Atlantic, a detailed [Wadhams and Munk, 2004]. Debate has centered on the decomposition of these atmospheric Centers of Action relative contribution of fresh water fluxes [Alley et al., 2006; (COA) is warranted because they are physically distinct in IPCC, 2007; Gornitz et al., 1997], thermal expansion origin and move about the basin in space and time [Rossby, [Antonov et al., 2005] and anomalies in Earth’s rotation 1939]. The position and intensity of the IL is a result of the [Wadhams and Munk, 2004]. Furthermore, GSLR estimates track of extra-tropical cyclones, while the corresponding from tide gauges appear to be 1 mm/yr higher than rates AH properties are governed by the downward fluxes of the determined from volumetric or salinity measurements [Miller Hadley cell. Since these systems are non-stationary, pres- and Douglas, 2004; Munk, 2002]. sure variations at a fixed location may reflect changes in a [3] Determining GSLR rates is complicated by non-tidal, COA’s position and not its intensity. To quantify changes in year-to-year variability in local mean sea level that is one to the COAs, objective indices of the pressure, latitude and two orders of magnitude greater than the long-term trend longitude of each COA (Figure 1) were calculated using [Douglas, 2001; Hong et al., 2000], potentially masking gridded sea level pressure data [Hameed et al., 1995] (Text S1 of the auxiliary material).1 While previous studies attempted to link sea-level variability to the NAO index, 1Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana, USA. significant results have been limited primarily to northern 2Marine Sciences Research Center, Institute for Terrestrial and Europe, leaving variability in the much of the Atlantic basin Planetary Atmospheres, State University of New York at Stony Brook, unexplained [Woolf et al., 2003]. Since the AH and the IL Stony Brook, New York, USA.

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Figure 1. Long term changes in the state of the COAs for months November through March, as determined by Hameed et al. [1995]. While these centers of action (COAs) have been defined as an index of the normalized pressure difference between two fixed stations, there are long-term changes in their position as well as their pressure. Therefore, describing the COAs via a suite of dimensional indices is warranted. Long-term trends are significant (p < 0.05) in the Azores High latitude and longitude and Icelandic Low longitude. can influence winds, atmospheric pressure and temperature movements are a key meteorological driver of sea level distributions worldwide [Hurrell, 1995; Rossby, 1939], we change at these locations, and often more important than hypothesize that shifts in their position and intensity should similar IL properties (Text S2) However, changes in the AH affect sea level by controlling coastal set up and regional Pressure (AHP) are only significantly correlated with sea- inverse barometer and thermosteric effects. level changes in the two European Stations; at Cascias the correlation is not significant on the IB corrected data. Sea- 2. Methods level changes are correlated with changes in the position of the IL at Charleston, Halifax (RAW, GIA and GIA+IB) and [6] We considered tide gauge records at key locations Stockholm (GIA and IB + GIA), while changes in its chosen to be representative of the regional extremes over intensity are correlated with sea level at Cascais (RAW the North Atlantic: Halifax, NS, CA; New York, NY USA; and GIA) and Stockholm (GIA and IB + GIA). Charleston, SC, USA; Stockholm, SWE; and Cascais, PRT [8] The strongest evidence for a north-south dipole, as (Table 1). Previous research has indicated that a tide gauge predicted from NAO theory [Hurrell, 1995], appears in can be representative of both the variability and trend in Europe. Sea level is negatively correlated with AH Press regional (100s–1000s km) sea level processes [Emery and and positively correlated with ILP at Cascais, while the Aubrey, 1991; Holgate, 2007; Hong et al., 2000]. Revised inverse is true at Stockholm, suggesting that these correla- local reference data (RLR) of annual mean sea level were tions reflect known physical phenomena [Hurrell, 1995]. obtained from the Permanent Service for Mean Sea Level However, there is less evidence for a north-south dipole on (PSMSL) [Woodworth and Player, 2003]. Four data permu- the American Atlantic coast; e.g. sea level at Charleston and tations are presented here: raw annual means provided by Halifax are both positively correlated with IL Long and AH PSMSL (RAW), means in which the trend was adjusted for Lat and AH Long. GIA adjustments increase the strength glacial isostatic adjustment (GIA), means that were cor- and number of univariate correlations most at Stockholm, an rected for the inverse barometer effect (IB), and means that area of pronounced isostatic uplift [Peltier, 2004]. The IB were adjusted for IB and GIA (IB+GIA) (Table 1, Text S2). correction appears to have little impact on many univariate GIA rates were determined using the recent and respected correlations, though its importance is increased in multiple Ice5G (VM2) model [Peltier, 2004]. Since GIA models can regressions discussed later on. yield different rates, calculations based on alternative GIA [9] To assess the influence of the COAs on sea level we models are available in Texts S2, S3, and S4. examined pressure, wind and sea surface temperature (SST) composite anomalies during periods of anomalously 3. Regressions and Correlations high AH Lat and years of anomalously eastern AH Long using NCEP/NCAR reanalysis data [Kalnay et al., 1996] [7] Changes in the position of the AH, i.e. its latitude (Figure 2). We see that northward shifts in AH Lat are (AH Lat) and/or longitude (AH Long), are significantly associated with wind anomalies of up to 7 m/sec directed correlated with sea-level changes at all the stations, with and towards Scandinavia and central North America and away without the GIA and IB corrections, suggesting that AH

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Table 1. Summary of Sea Level Rise Rates and Regression Outputs Stockholm Cascais Halifax New York Charleston Location 59° 190N, 38° 410N, 44° 400N, 40° 20N, 32° 470N, 17° 370E 09° 250E 63° 350E 74° 010E 79° 560E Years, (% Complete) 1905–2004 1905–1993 1920–2002 1905–2003 1920–2003 (100) (90) (93.5) (96.9) (100) RSLR (RAW)a,d 3.91 1.70 3.37 3.04 3.23 R2 of Regression 0.60 0.79 0.59 0.57 0.69 Regression Sloped 2.38 1.32 2.11 1.92 2.26 % of Raw Trend 61 78 63 63 70 Residual RSLRd 1.53 0.38 1.26 1.12 0.97

RSLR (GIA Adjustedb,d) 0.21 1.70 2.15 1.40 2.40 R2 of Regression 0.33 0.79 0.51 0.32 0.68 Regression Sloped 0.23 1.32 1.20 0.53 1.75 % of Adjusted Trend 110 78 56 38 73 Residual RSLRd 0.02 0.38 0.95 0.87 0.65

RSLR (IB Adjusted)c,d 3.91 1.77 3.40 2.95 3.24 R2 of Regression 0.59 0.75 0.48 0.54 0.73 Regression Sloped 2.29 1.41 1.62 1.78 2.43 % adjusted trend 59 80 48 60 75 Residual RSLRd 1.62 0.36 1.78 1.17 0.81

RSLR (IBb + GIAc Adjusted)d 0.22 1.70 2.19 1.34 2.42 R2 of Regression 0.24 0.79 0.71 0.38 0.67 Regression Sloped 0.31 1.32 1.56 0.68 1.77 % of Adjusted trend 71 78 71 51 73 Residual RSLRd 0.09 0.38 0.66 0.66 0.65 aSource: Permanent Service for Mean Sea Level. bSource: Peltier [2004]. cAir pressure source: [Allan and Ansell, 2006]. dRates are in mm/yr. from Iberia. Years in which the AH is located further in Texts S2 and S4. Individual regression terms, all signif- eastward also yield wind anomalies up to 5 m/sec in a icant at the 95% level, contribute 10% of the total variance similar direction. These wind anomalies are hypothesized in local sea level. These regressions suggest that sea-level to effect wind-driven coastal setup and wind-driven Rossby change is a complex, multi-factorial phenomena with con- waves that can also influence coastal sea level [Hong et al., tributions from long-term shifts in the position and intensi- 2000]. The high pressure anomalies of 2–10 mbar that ties of the COAs. extend from western Europe to North America during northward shifts in the AH (and to a lesser extent eastward 4. Discussion shifts in the AH) likely lower sea level via the inverse barometer effect. The converse presumably occurs in years [11] These regressions suggest that a large fraction of the of low AH pressure. Finally, in years when the AH is annual mean sea-level variability at these five Atlantic shifted northward, there are SST anomalies of 0.4–1.2 P°C Ocean stations is correlated with shifts in the position and that extend from eastern North America to Europe, which intensity of the COAs (Table 1, Texts S2 and S4). Overall, may affect thermosteric sea level. Thermosteric sea-level our multiple regressions can account for 59–79% of the change depends on the integral over the water column of variability in the RAW sea level data and 24–79% of the temperature change. In shallow waters this is likely to be IB + GIA adjusted data (Table 1). These findings suggest small. that meteorological processes drive coastal sea-level vari- [10] Understanding the controls on GSLR requires dif- ability by redistributing water, heat, and the response of the ferentiating between processes that redistribute water across ocean to atmospheric pressure across the ocean basin ocean basins and changes in ocean volume. A critical (Figures 2 and 3). COA driven sea-level change is consis- component of this problem is determining the cause of tent with earlier studies on this topic [Church et al., 2004; inter-annual and decadal scale oscillations in sea level with Emery and Aubrey, 1991; Hong et al., 2000], but provides a cm to dm scale amplitudes [Hong et al., 2000]. To demon- single comprehensive theory that accounts for much last strate that much of these local sea-level changes are century’s Atlantic Ocean sea level variability. For example, correlated with shifts in the position and intensity of the we find lags between local sea level and the COAs of up to COAs, we constructed a series of multiple regressions six years, which is consistent with work demonstrating a relating the COA indices to sea level at each of the five six-year travel time between NAO-related heat flux in the tide gauges. To look for interactions between COA forcing Labrador Sea and heat flux and sea level at Bermuda [Curry of sea level and deep-ocean processes, we lagged the data et al., 1998]. The link between sea level and the IL Long at for up to six years. Regression outputs are summarized in Charleston and Halifax is intriguing because the IL Long Table1,andTextS3,whilethecompleteregression affects the position of the Gulf Stream northwall [Hameed equations and effective number of degrees of freedom are and Piontkovski, 2004], and oceanographers have long

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Figure 2. Wind, pressure, and sea surface temperature anomalies comparing years when the Azores High has shifted northward vs. years when it has shifted southward and below and years when the Azores High has shifted eastward vs. years when it is westward. A key result is that shifts in the AH influence winds that can drive Rossby waves that affect coastal sea- level variability [e.g., Hong et al., 2000]. Also potentially important are shifts in the latitude of the Azores high that appear to affect sea surface temperatures (top right) which may influence thermosteric sea level anomalies. Data and image source: NCEP/NCAR; Kalnay et al. [1996]. L23616 L23616 KOLKER AND HAMEED: METEOROLOGICAL TRENDS IN SEA LEVEL RISE L23616

Figure 3. Multiple regressions of sea level and COAs, which predict a major fraction of the variability in sea level and a majority of the long-term trends in sea level (see also Table 1). (left) In red are the RAW data as provided by PSMSL, and in black are the regression outputs. (right) The IB + GIA corrected data are in red and while the regression outputs of this data are in black. Y axis units are in mm. Data on the left are standardized to the PSMSL reference, and were obtained from http://www.pol.ac.uk/psmsl/. speculated that changes in the Gulf Stream’s position could (Figure 1) is associated with an increase in the mid-latitude affect coastal sea level [Emery and Aubrey, 1991]. North- eastward wind anomaly (Figure 2) that can affect coastal set ward shifts in the AH (Figure 2) are coupled to a SST up and thereby sea level’s position in central North America anomaly that corresponds with the general position of the (Figure 3). Northward AH movements also appear to Gulf Stream, further suggesting a link between COAs and increase the flow of water into Scandinavia and away from the ocean. COA shifts also affect global wind fields, which Iberia, potentially contributing to a sea-level rise in northern are thought to be responsible for Rossby waves that influ- Europe and sea-level fall in southern Europe, in spite of ence sea level when they interact with the coast [Hong et al., isostatic or tectonic motions in the opposite direction. For 2000]. the regressions of the RAW data, the regression slope [12] There are trends in the multiple regressions relating ranges from 2.38 mm/yr in Stockholm to 2.26 mm/yr in (Table 1, Figure 3), suggesting that long-term changes in Charleston, which accounts for 61–78% of the raw RSLR local sea level are associated with changes in the position or (Table 1, Figure 3). This value changes to 38–110% when intensity of the COAs. Analysis of historical climate data GIA is accounted for and 51–78% when both GIA and IB [e.g., Kalnay et al., 1996] strongly suggests a physical basis are considered (Table 1). These finding suggest that the for these correlations. The northward shift in the AH long-term movement of the COAs, and particularly the AH,

5of7 L23616 KOLKER AND HAMEED: METEOROLOGICAL TRENDS IN SEA LEVEL RISE L23616 have led to large scale atmospheric and oceanographic al., 2005]. Our results yield rates of recent sea level rise that shifts that effected the volume of water along both coasts are closer to Wadhams and Munk’s [2004] 1.1 mm/yr than of the Atlantic basin. Miller and Douglas’s 1.5–2.0 mm/yr. This result suggests [13] The difference between the raw rate of RSLR and the that meteorological trends are an important driver of sea- rate of RSLR determined from the regressions yields a level change, which could close the enigmatic gap in GSLR residual rate of local sea-level rise, which may provide a rates if our results were to be replicated worldwide. While useful measure of local sea-level trends because it is largely these findings may downgrade rates of warming induced free from the confounding effects of isocracy and COA- sea-level change along Atlantic shorelines, they might not influenced processes. Results are particularly encouraging reduce estimates of climate change impacts, as it is often for the three North American tide gauges. Using the Ice5G meteorologically driven sea-level changes that are most (VM2) model [Peltier, 2004], the residual rate of RSLR is damaging to coastal ecosystems and infrastructure [Scavia 0.66 mm/yr in Halifax, 0.66 mm/yr in New York and et al., 2002]. 0.65 mm/yr in Charleston (Table 1). This suggests a regional rate of residual sea level rise with substantially [17] Acknowledgments. Comments from two anonymous reviewers less variability than the 1.40–2.15 mm/yr range yielded by improved the manuscript. T. U. supported ASK as a McWilliams Scholar. a GIA correction alone. Furthermore, the inferred rate of sea-level rise is higher in western Atlantic tide gauges than References Allan, R. J., and T. J. 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