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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C04021, doi:10.1029/2009JC005767, 2010 Annual cycle in coastal sea level from tide gauges and altimetry Sergey V. Vinogradov 1

Annual cycle in coastal sea level from tide gauges and altimetry

Sergey V. Vinogradov 1 and Rui M. Ponte 1

Received 28 August 2009; revised 17 November 2009; accepted 4 December 2009; published 24 April 2010.

[1 ] Tide gauges provide a unique data set extending many decades back in time, but coverage is restricted to continental boundaries and a few oceanic islands and the extent to which the tide gauge records can be used to infer low frequency, large scale sea level behavior remains unclear. Since 1992, satellite altimetry provides near global coverage of sea level variability, including coastal regions. We compare variability at 345 continental and island tide gauge coastal locations and adjacent shallow and deep oceans, as inferred from altimetry. Initial focus is on the dominant annual cycle. On average, annual amplitudes in tide gauges are comparable to but larger than those in the nearby shallow ocean (<200 m). Substantial differences are found in areas adjacent to strong river outflows and narrow coastal currents. The annual cycle in shallow areas is usually enhanced relative to the open ocean, apart from areas with strong western boundary currents offshore. Differences of 1 2 months in annual phases for coastal, shallow, and deep ocean are typical. Our analysis points to the presence of considerable spatial variability in the annual cycle across deep, shallow, and coastal regions, and to the importance of both tide gauge and altimeter measurements for proper resolution and interpretation of such variability.

Citation: Vinogradov, S. V., and R. M. Ponte (2010), Annual cycle in coastal sea level from tide gauges and altimetry, J. Geophys. Res., 115, C04021, doi:10.1029/2009JC005767.

1. Introduction

[ 2] Given its direct importance to coastal populations, sea level variability has been observed and documented over decadal and even centennial time scales at many coastal and island locations [e.g., Douglas , 1992], generally associated with major sea ports and shipping routes. These records provide an opportunity to study the low frequency vari- ability in sea level, including the prominent seasonal cycle, an important climate signal representing a dominant fraction of the nontidal total sea level variance. The factors con- tributing to the sea level variability include density changes due to temperature (thermosteric) and salinity (halosteric) variations of the water column, along with mass changes due to oceanic mass redistribution and mass input from precipitation, evaporation and river runoff. In contrast to the deep ocean, mass changes in coastal regions become sig- nificant as compared to steric contributions [e.g., Ponte , 1999; Vinogradova et al. , 2007]. Wind and wave setup, windinduced upwelling/downwelling, complex bathyme- try, forcing by rivers and atmospheric pressure, and other factors can be important for near shore ocean dynamics. [ 3] Pattullo et al. [1955] first described the sea level seasonal variability on the global scale using tide gauge

1 Atmospheric and Environmental Research, Inc., Lexington, Massachusetts, USA.

Copyright 2010 by the American Geophysical Union. 01480227/10/2009JC005767

observations sparsely distributed along the continental coasts and on some islands. They noticed an annual phase reversal between the northern and southern hemispheres, and about a 3 month phase difference between subtropical and subpolar locations, as well as significant differences in annual amplitudes at the opposite sides of even small land areas. Based on a few density records taken in deep waters, Pattullo et al. concluded that ste ric heights are generally in a good agreement with seasonal cycle in sea level as mea- sured by the tide gauges; however, a few locations had noticeable discrepancies (Ore gon/Washington coast, Sea of Japan, Indonesia). Tsimplis and Woodworth [1994] updated the Pattullo et al. findings using all available tide gauge data; they concluded that the coastal and open ocean annual sea level signals can differ significantly, and that the coastal tide gauges should be used i n conjunction with satellite altimetry to infer the annual cycle in the open ocean. [ 4 ] In the last decade, space borne satellite altimetry has been used to estimate the details of the sea level variability on a global scale [e.g., Stammer , 1997; Fukumori et al. , 1998; Vinogradov et al., 2008]. The along track satellite altimetry data in conjunction with tide gauge time series can provide unprecedented insight on the spatial variability and structure of the annual sea level in coastal and nearby shallow and deep oceans. In this work, we take advantage of the new satellite sea level observations to evaluate to what extent the tide gauge records can be used to infer the large scale annual cycle in sea level, what is the relation between shallow and open ocean annual sea level signal, and what is the relative usefulness of tide gauge and altimetry data for




AND PONTE: COASTAL VS OPEN OCEAN ANNUAL SEA LEVEL C04021 Figure 1. Locations of the tide

Figure 1. Locations of the tide gauge stations used in this study.

dealiasing corrections, promise to deliver cleaner coastal al- timeter products in the near future. Here we assess present standard products, which can provide a baseline for analysis

of forthcoming improved coastal data sets.

[ 7 ] Mean annual amplitudes and phases were computed for monthly averaged altimetric sea level time series at every

point along the track in a way similar to the tide gauges annual fit. All suitable T/P along track data were collected in the proximity of every tide gauge (TG). The selected spatial radius for this work was 134 km for every TG, a somewhat optimal coverage to include enough nearby altimetry tracks. The collected T/P data were split into

shallow and deep groups relative to the 200 m isobath, a typical outer limit of the continental shelf (Figure 2). Alongtrack annual cycles within each T/P group were averaged as

a sum of sine waves to produce mean shallow and deep

annual cycles. As a result, annual cycle estimates of coastal (either continental or island TG), shallow, and deep sea level

have been computed and collected in the vicinity of every TG.

possible use in constraining models of the oceanic coastal regions.

2. Data and Methods

[ 5] For the temporal period of analysis, we used the initial years of the TOPEX/POSEIDON (T/P) altimetry mission (1 January 1993 to 7 December 2001), during which the satellite was in its nominal orbit. Spanning this period, monthly records from 345 tide gauges from the Permanent Service for Mean Sea Level (http://www.pol.ac.uk [e.g., Woodworth and Player , 2003]) yielded valid mean annual cycles. Based on their geographical location relative to the continental shelf, 204 tide gauges are defined as conti- nentaland 134 as islandstations (Figure 1), depending on whether a shelf break is present between the location and a nearest continent. As the focus is on dynamically relevant signals in sea level, the inverted barometer (IB) correction has been applied by using the National Centers for Envi- ronmental Prediction/National Center for Atmospheric Research reanalysis sea level pressure fields [Kalnay et al., 1996]. Each sea level time series was detrended and 12 mean months were computed by averaging all the available values for January, December; the mean annual fit was

, found from this set of 12 mean months. We only used the records containing a complete mean annual fit, that is, at least one mean month should exist for this time series for each of the 12 months. [ 6] Altimetric T/P data obtained from the Physical Oceanography Distributed Active Archive Center, NASA Jet Propulsion Laboratory, cover the ocean from 66°S to 66°N, with repeat period of 10 days and alongtrack sampling of 7 km. All standard environmental corrections (including the IB correction) are applied to the raw T/P data [e.g., Benada, 1997]. The IB correction is essentially the same as that used for the tide gauge records. We do not attempt any special treatment of nearland retrievals, which are known to suffer from noisier radar backscatter and radiometer readings. Ongoing efforts to improve radar tracking and wet tropo- spheric delay algorithms, along with better tidal and nontidal

3. Analysis

[ 8 ] The scatter plots in Figures 3 6 show the amplitudes and phases of the annual cycle in both TG and corresponding shallow and deep T/P locations. The range associated with each T/P value in Figures 3 6 denotes its respective standard deviation and provides a measure of how spatially variable the annual cycle is along track. The correlation coefficients

annual cycle is along track. The correlation coefficients Figure 2. An example of Humboldt Bay, California,

Figure 2. An example of Humboldt Bay, California, tidal station and averaging of the nearby altimetric data. Yellow

diamond is the location of the tide gauge. Color background

is the bathymetry. Blue (green) dots are along track T/P ob-

servations that are in water deeper (shallower) than 200 m. Annual signals at blue and green dots are averaged within each group to produce mean deep and shallow offshore annual sea level cycles as observed by the altimeter. Dashed red circle shows the extent of the spatial averaging.

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AND PONTE: COASTAL VS OPEN OCEAN ANNUAL SEA LEVEL C04021 Figure 3. Scatter plots of the

Figure 3. Scatter plots of the mean annual sea level amplitudes in TG (x axis) versus average (a) shallow and (b) deep ocean annual cycles in T/P (y axis). Range plotted for each T/P value represents ±1 standard deviation of the along track T/P shallow and deep values and can be interpreted as a measure of the re- spective spatial variability of the annual cycle as observed by T/P. The 1:1 line is also plotted.

inferred from the amplitude and phase scatter plots are sum- marized in Table 1.

3.1. Annual Amplitudes

[ 9] Tide gauges annual amplitudes are comparable with nearby shallow altimetry data with correlation coefficient R = 0.85 (Figure 3a). The observed amplitudes generally range from just a few mm to 0.2 m, with some TG signals reaching 0.4 0.5 m. The mean T/P annual amplitudes do not exceed 0.26 m. The standard deviation of the estimated mean T/P values may reach 60 mm but is usually smaller than 50 mm.

[ 10] Continental TG amplitudes (red dots in Figure 3a) are usually higher than nearby shallow T/P, particularly in places where amplitudes are larger than 100 mm. The largest discrepancies are found in the vicinity of the Ganges River delta, where coastal amplitudes range from 0.3 to 0.5 m due to extremely high monsoonal driven river outflow, whereas altimetry observes amplitudes of only 0.20.25 m over the shallow continental shelf. The TG amplitudes are signifi- cantly larger than the shallow ocean amplitudes along the US West Coast (up to 80 mm differences, with TG amplitudes ranging from 100 to 150 mm), where the strong along shore California Current isolates coastal seasonal variability from

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AND PONTE: COASTAL VS OPEN OCEAN ANNUAL SEA LEVEL C04021 Figure 4. Annual amplitudes in T/P

Figure 4. Annual amplitudes in T/P observations over the shelf and nearby open ocean. Range bars are plotted as in Figure 3, depicting spatial variability associated with shallow and deep T/P values.

the open ocean. Some TG stations in Singapore (the south- ernmost tip of Malaysia Peninsula) observe 140 mm am- plitudes whereas nearby altimetry measures only 6070 mm annual signal in Malacca/Singapore Strait. There is a no- ticeable latitudinal change of the difference between coastal and continental shelf annual amplitudes along the US East Coast, associated with Gulf Stream moving further offshore; TG amplitudes exceed nearby shallow T/P annual signal in most TG stations in Florida, but this difference reverses starting from Fort Pulaski, GA, and northward. Other areas where offshore sea level has noticeably larger annual ampli- tudes relative to TG records are found at some stations in the Gulf of Mexico, where persistent coastal jets separate rela- tively stable nearshore environments from the significant mesoscale seasonal variability. [ 11] Most island TG stations are located in the open ocean, and they do not have a shallow T/P counterpart for this analysis, because the ocean in proximity is mostly deeper than 200 m. Those island locations that have shallow T/P data in their vicinity are plotted as blue dots in Figure 3a. Annual amplitudes do not exceed 150 mm. With a few exceptions, shallow T/P annual mean amplitudes near island TG stations have standard deviations smaller than those near most continental TG; however the statistics for the former are less significant due to a smaller number of along track points that constitute the shallow T/P groups near island TG. [ 12] Comparison of TG amplitudes with T/P data degrades with increase in depth (R = 0.53). As expected, continental TG have much smaller correlation (0.36) with deep than with

shallow T/P amplitudes. Most continental TG have annual amplitudes higher than nearby deep ocean (red dots in Figure 3b). The relative differences between continental TG and deep T/P amplitudes have geographical patterns similar to the previous comparison with shallow T/P, but in some areas like California/Oregon/Washington coast, the dis- crepancies become larger. Another area of substantial dif- ferences is the South Australian coast, where deep ocean has much smaller annual variability than coastal TG. [ 13] The overall correlation between island TG (blue dots in Figure 3b) and deep T/P values (0.79) is much higher than that for continental TG. The most noticeable outlier is the station at Minami Torishima Atoll (153.98°E, 24.30°N); TG records at this small Pacific island indicate an amplitude of 162 mm compared to a T/P value no larger than 50 mm, therefore implying a possible land lockingof the tide gauge, probably due to geomorphologic changes of the instrument location (e.g., changes in geometry of the basin and connections with open ocean), or some sort of hardware problem. A similar discrepancy is found for Benoa station, Indonesia (115.21°E, 8.74°S), which is positioned within a shallow harbor and not expected to represent deep ocean measurements, whereas midocean atolls like Minami Torishima are usually considered suitable for direct mea- surements of sea level variability in the open ocean. [ 14] Comparing mean T/P annual amplitudes over shallow and deep areas in vicinity of TG locations yields a corre- lation of 0.64, with shallower waters usually having larger amplitudes (Figure 4). From the standard deviations dis- played in Figure 4, spatial variability of annual cycle over

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Figure 5. Same as in Figure 3 but for annual phases, shown as the time of the annual maximum. Dashed lines show 3 month phase difference, whereas solid lines show 6 month phase difference. Axis tick marks correspond to the middle of the month.

likely due to land contamination from small islands in shallow altimetric data.

3.2. Annual Phases

[ 15] In most places, the difference between mean sea level annual phases in TG and nearby shallow T/P data does not exceed 1 month (Figure 5a). The total correlation is 0.92. There is more phase discrepancy near continental TG than near island TG, with some outliers having phase differences of 3 5 months. Among continental stations (red dots in Figure 5a, R = 0.92), most notable outlier is Puerto Quetza on the Pacific coast of Guatemala (the leftmost point in Figure 5a), where the time of the annual maximum changes from September offshore to early March at the coast and nearby shallow waters. Similar changes are observed at two

South African stations (Saldanha Bay on the Atlantic coast, and East London on the Indian Ocean coast). For the island stations (blue dots in Figure 5a, R = 0.94), the largest dis- crepancy of about 3 months between TG and shallow T/P annual phases is found at Jolo Island, Philippines, which essentially reflects mean annual sea level phase difference between Sulu and Celebes Seas. Other large outliers are found near the Antarctic Peninsula, where altimetry values show large standard deviations, probably due to ice return contamination. [ 16] Larger phase discrepancies are seen between conti- nental TG and deep T/P values (red dots in Figure 5b, R = 0.88), reflecting high spatial variability at places like Charleston and Tofino in west North American coast, Durban in South African East coast, Termisa in Brazil, in addition to locations mentioned above in the coastal versus shallow ocean comparison. Most island locations correlate very well (0.93, blue dots in Figure 5b). Notable exception

is San Felix Island off the coast of Chile (80.13°W 26.28°S), which lies near an area of large phase changes in the annual cycle [e.g., see Vinogradov et al., 2008, Figures 3b and 3d]. Another large outlier is Macquarie Island midway between Antarctica and New Zealand (158.96°E 54.48°S), in the area of the Southern Ocean that has significant small scale fluc- tuations in sea level annual phase as observed by altimetry.

the shelf is generally larger than over the deep ocean. The largest difference in the annual amplitudes between shallow and deep data are found near the continental TG stations (correlation is 0.58). Most notable differences occur at the U.S./Canadian Pacific Northwest, where amplitudes can decrease (on the scales of selected spatial radius) from 120 mm at the TG, to 80 mm in shallow areas, and to less than 10 mm in deep ocean. Other sharp differences between shallow and deep annual amplitudes are found along the South Australian coast (e.g., for Esperance, amplitudes decrease from 107 mm at the coast to 92 mm in shallow T/P data, and to 47 mm in deep waters). The correlation between shallow and deep amplitudes near islands (0.85) is much higher than that for nearcontinental locations. Large spatial variability over the continental shelf at Cilacap, South Indonesian coast (standard deviation of 45 mm), is due to inclusion of a few T/P data points on the north side of Indonesia and reflects the difference of about 60 mm in mean annual amplitudes on the opposite shores. Locations with higher shallow T/P standard deviation (60+ mm) are

with higher shallow T/P standard deviation (60+ mm) are Figure 6. Same as in Figure 4

Figure 6. Same as in Figure 4 but for annual phases. Axis tick marks correspond to the middle of the month.

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Table 1. Correlations in Annual Amplitudes and Phases Between Tide Gauges and Nearby Altimetry

Shallow T/P Versus TG

Deep T/P Versus TG

Shallow Versus Deep T/P









































[ 17] Scatter plot in Figure 6 shows the discrepancies and relative spatial variability in shallow and deep sea level annual phases from T/P. Overall correlation is 0.92; along track standard deviations are mostly higher in shallow waters than in deep ocean. Most locations identified in previous comparisons of altimetry with coastal sea level as having noticeable differences or large spatial variability in the annual phase also stand out in Figure 6. Large dis- crepancies in mean sea level annual phases are found near some continental TG stations (R = 0.90, red dots in Figure 6), in particular, along the US/Canada West coast, some near South African and Kenyan TG, Imbituba in Brazil, and San Juan station in Peru. Deep and shallow areas near island TG are better correlated (R = 0.97, blue dots in Figure 6), and they mostly have smaller along track phase variations. Largest discrepancies between deep and shallow phases are found near some Indonesian islands, accompanied by large spatial variability in both shallow and deep estimates.

4. Discussion and Conclusions

[ 18] Our findings indicate significant variability in annual sea level cycle across the coastal ocean, from the immediate coastline to the adjacent shallow and deep waters. The complexity of coastal annual sea level patterns can be illustrated by the example of the US/Canadian West Coast (Figure 7). On the scale of 7 km sampled by the altimeter, along track amplitudes and phases exhibit consistent and gradual changes that are still quite different from the coastal TG. Approaching from the ocean, annual amplitudes in the altimetry (Figure 7a) tend to increase to 70 80 mm in the vicinity of 100 200 m isobaths, and decrease below 40 mm closer to the coast, whereas all TG in this region north of San Francisco have consider ably larger amplitudes (90 140 mm). The annual cycle on the coast and over the Californian continental shelf peaks during winter (Figure 7b), but there is a distinct smallscale complete phase reversal in altimetry data transitioning from the open ocean to shallow waters just 7 to 30 km offshore. Although some of the fea- tures in the alongtrack data can be attributed to instrument errors, such spatial variability is consistent with the complex ocean circulation in the area, including a combination of the strong alongshore California Current system and winddriven coastal upwelling, which has been a subject of detailed study [Strub and James, 2000; Veneziani et al., 2009]. The large difference between TG and altimeter annual cycles right near the coast is, however, quite striking and has not yet been addressed in any detail. [ 19] The annual cycle in sea level appears to have mostly larger amplitudes and spatial variability in shallow areas, with largest amplitudes right at the coast, except in the vicinity of strong western boundary currents located just offshore. These and other differences across the coastal

oceans can be attributed to many physical factors, resulting from a combination of local and remote atmospheric, oce- anic and terrestrial processes. The land/ocean boundary exhibits sharp gradients in atmospheric fluxes and wind patterns, the important drivers for sea level variability. The differences in the atmosphere over land and water occur on all scales, and can provide different annual forcing for the ocean in the immediate vicinity of land and just offshore [e.g., Haack et al. , 2005]. Surface heating and cooling have different impact on shallow (mixed from top to bottom) circulation and offshore (mor e stratified) dynamics. The seasonal upwelling also provides fine spatial details to the offshore profile of annual sea level and does not necessarily occur right at the coastline. Upwelling associated with mesoscale ocean circulation may exhibit quasi annual periodicity (e.g., Gulf of Mexico), and appear at some dis- tance from the coast, depending on eddy dynamics and shape of the continental shelf [Vinogradov et al., 2004]. The river input is another significant factor, which relates to hydro- logical and atmospheric regimes far upstream (and inland) from the river mouths. A good example is the Ganges River delta with the largest annual amplitudes observed in TG data. In addition to hydrological and atmospheric factors, the dynamics of inner harbors, fjords and basins where TG are sometimes located can be very different from the nearby open ocean. Terrestrial impacts through sedimentation, coastline changes, harbor construction, etc., can also affect the coastal sea level measurement, but they are likely to be less important than the hydrological and atmospheric factors mentioned above. [ 20] Observational errors in the TG and altimetry esti- mates contribute to some of the differences highlighted in Figures 3 6. The two systems have very different sampling properties in time, and the various data processing steps (e.g., original smoothing, gap filling, and filtering of the TG hourly records) can affect th e annual fit computations. In addition, the TG measures sea level with respect to a fixed geolocation, whereas altimetry measures absolute sea level relative to the geoi d. Any land motions with an annual component (e.g., from tidal or atmospheric pres- sure loading) could give rise to differences between the TG and altimeter measurements. Annual amplitudes of a few millimeters are possible [ van Dam et al. , 2007]. [ 21] The typical RMS errors for TG monthly mean data are believed to be within 10 mm [ Pugh , 2004]. Instrument errors may also have a seasonal dependence. As for altim- etry observations, besides instrument noise, a plethora of corrections are applied to the raw data, including wet tro- posphere correction, IB removal, and tidal dealiasing that are important for the coastal sea level observations. Ponte et al. [2007] offer space dependent RMS estimates of 2 4 cm for the total altimetry error; uncertainties for the annual period per se are likely to be much smaller. Present work to

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AND PONTE: COASTAL VS OPEN OCEAN ANNUAL SEA LEVEL C04021 Figure 7. California, Oregon, and Washington

Figure 7. California, Oregon, and Washington coast: (a) annual amplitudes (mm) and (b) time of annual maximum in TG and T/P. Isobaths shown are 200 m (gray curve) and 1000 m (black curve).

improve corrections and dealiasing models and techniques promises to deliver in the near future better altimetric esti- mates in the nearcoastal waters. [ 22] Taking the uncertainty in the mean annual cycle estimates from TG and altimeter data to be 12 cm, much of the differences seen in Figures 3 6 are hard to explain simply in terms of data noise and represent to some degree the true spatial variability of the annual cycle in the coastal ocean. We notice also that, in most of the areas studied, the standard deviation of T/P mean amplitudes exceeds 2 3 cm. Thus, the spatial variability sensed by the altimeter instru-

ment particularly in the shallow continental shelves seems to be a robust feature of the annual sea level patterns. [ 23] Land contamination and aliasing effects, typically a concern in altimetric coastal measurements, are less of an issue for our study of the annual cycle. The altimetric orbits were designed to specifically reduce tidal aliasing at the annual period and Ponte and Lyard [2002] show that the annual cycle in sea level is probably less contaminated (aliased) by other variability. Similarly, although we did not apply any special algorithms to detect land contamination in addition to what was performed on the original T/P data

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processing, we have found only a few locations where land returns may contribute outliers to the altimetry data, mostly in the areas with many small islands (e.g., Indonesia, South Florida). The results in Figures 3 6 and Table 1 suggest that the altimeter measurements are still very useful in most shallow regions, even very close to the land boundaries. [ 24] The differences in annual sea level cycle between TG and nearby altimetry limit the use of TG data for inferring mean annual cycle in the adjacent shallow and open ocean. One implication is that TG data can only provide a weak constraint on coarse resolution ocean circulation models, as they fail to capture physical processes behind fine spatial gradients in annual sea level cycle that we found in along track altimetry data. Highresolution ocean data assimilation systems will benefit most from TG input, if they are capable of resolving both oceanic and atmospheric short scale dynamics along the sea/land boundary. On the other hand, along track T/P sea level data have quite robust, stable and consistent annual cycles in shallow waters, and can provide strong constraints for both coarseand highresolution models of the coastal regions. [ 25] High spatial variability of the annual sea level cycle in shallow areas indicates strong seasonally varying features in local ocean and atmospheric circulation that need to be studied in detail in a coupled highresolution modeling framework. Coastal modeling systems that combine ocean, atmosphere and terrestrial input (hydrology, inner basins, land motion) can benefit from both altimetry and tide gauge data in simulating the complexity of the nearcoastal cir- culation on annual and lower frequencies. Such integrated modeling efforts will be essential for predictions of the coastal environment on climate time scales.

[ 26] Acknowledgments. This work is supported by NASA Physical Oceanography program through cont ract NNH08CD67C. The authors thank Charmaine King from the Massachusetts Institute of Technology for helping with initial data processing.


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