GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L12610, doi:10.1029/2006GL026304, 2006
Near 5-day nonisostatic response of the Atlantic Ocean to atmospheric surface pressure deduced from sub-surface and bottom pressure measurements Jae-Hun Park1 and D. Randolph Watts1 Received 14 March 2006; revised 11 May 2006; accepted 22 May 2006; published 29 June 2006.
[1] Nonisostatic ocean responses to atmospheric pressure have been observed in tropical regions of the Pacific and Atlantic Oceans at periods near 5 days. Barotropic ocean model simulations coupled to atmospheric forcing predicted that this nonisostatic ocean response is driven not regionally but globally by the Rossby-Haurwitz wave. To date little observational evidence has been provided to support the model simulations, especially at extratropical latitudes. Here we present the basin-scale nature of the nonisostatic response in the Atlantic Ocean near 5-day periods using four historical long-term (1.5 year) sub-surface and bottom pressure measurements spanning from 16°S to 37°N. Joint analysis of them together with global-gridded atmospheric pressure reveals a basin-scale nonisostatic sea level fluctuation in the North and tropical Atlantic Oceans near 5-day periods with almost uniform phase. It also confirms that the driving force for this near 5-day fluctuation is the westward propagating Rossby-Haurwitz wave. Citation: Park, J.-H., and D. R. Watts (2006), Near 5-day nonisostatic response of the Atlantic Ocean to atmospheric surface pressure deduced from sub-surface and bottom pressure measurements, Geophys. Res. Lett., 33, L12610, doi:10.1029/ 2006GL026304.
1. Introduction [2] The inverted barometer (IB) response is an isostatic sea level adjustment to the local fluctuations of atmospheric surface pressure (Patm) loading, that is, approximately a 1 cm increase (decrease) of sea level in response to a 1 mbar decrease (increase) of local Patm, which results in no changes of ocean bottom pressure [Wunsch and Stammer, 1997; Mathers and Woodworth, 2001]. Using observations and numerical simulations, many studies have demonstrated that the IB response is characteristic of the open ocean response to Patm generally at periods longer than 2 days [Brown et al., 1975; Ponte, 1993]. However, spectral analysis of scattered island and coastal tide gauge data in the tropical Pacific Ocean showed a coherent westward propagating sea level fluctuation near 5-day periods with the sea level response to Patm different from 1 cm/mbar ratio, that is, non-IB response [Luther, 1982]. The RossbyHaurwitz wave, a 5-day period global-scale oscillation in
1
Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA.
barometric pressure [Madden and Julian, 1972, 1973], was suggested as a probable forcing to generate this non-IB sea level fluctuation. Spectral analysis of sub-surface pressure (Pss) data at Ascension and St. Helena Islands in the tropical South Atlantic Ocean showed that Pss was significantly coherent with local Patm with nearly zero phase difference near 5-day periods, that is, non-IB response just as observed in the tropical Pacific [Woodworth et al., 1995]. [3] Using a barotropic ocean model simulation, Ponte [1993] predicted a non-IB response near 5-day periods in the tropical regions, with larger departures from IB response in the Atlantic Ocean than in the Pacific Ocean. In another simulation experiment, Ponte [1997] predicted that the 5-day Rossby-Haurwitz wave could drive the non-IB ocean response at periods near 5 days not regionally but globally. In that simulation, the non-IB near 5-day response was predicted to have nearly constant amplitude and phase in the tropical and North Atlantic Oceans, but complex behaviors of amplitude and phase in the Pacific Ocean. Further simulations have confirmed this non-IB near 5-day ocean response [Mathers and Woodworth, 2001; Carre`re and Lyard, 2003; Mathers and Woodworth, 2004]. Despite the limitation of approximately 10-day sampling cycle of Topex/Poseidon (T/P) measurements, the assumption of spatial homogeneity inside the basin allowed the use of basin-averaged sea level from T/P satellite altimetry measurements north of 30°S to investigate the nature of the non-IB near 5-day signals [Hirose et al., 2001]. However, they could not extract from the T/P data evidence of non-IB near 5-day fluctuations at extratropical latitudes. Moreover, a pioneering bottom pressure (P bot ) measurement study conducted by deploying a small array of bottom gauges about 300 km southwest of Bermuda (near 28°N, 70°W) revealed incoherence between Pbot and Patm at Bermuda, consistent with near-complete IB on the timescales of 2 days to 1 month [Brown et al., 1975]. Global tide-gauge data revealed IB response to local Patm near 5-day periods in extratropical latitudes, because strong synoptic weather systems mask the small amplitude of non-IB near 5-day response [Mathers and Woodworth, 2004]. To date little observational evidence has been reported for non-IB near 5-day ocean response at extratropical latitudes. [4] The previous studies [Ponte, 1993, 1997] suggest that the shape and depth of ocean basins control the ocean response at periods near 5 days, and hence the Atlantic Ocean is a proper basin to investigate the nature of non-IB sea level fluctuations with limited in situ observations. Here we report in situ observational evidence including the extratropical region indicating the basin-scale nature of
Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL026304
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Table 1. Summary of Sub-Surface and Bottom Pressure Dataa Name
Location
Depth, m
Synoptic Ocean Prediction Experiment (SYNOP) Ascension (ASC) St. Helena (StH) FP1
36.84°N, 67.46°W 07.92°S, 14.42°W 15.97°S, 05.70°W 0°, 20.00°W
4980 14.5 1.4 2700
Duration (Length) 06/88 – 08/90 06/88 – 08/90 06/88 – 08/90 02/83 – 09/84
(807 (807 (807 (600
days) days) days) days)
Deployed by URI POL POL POL
a
URI, University of Rhode Island; POL, Proudman Oceanography Laboratory.
non-IB sea level response to Patm near 5-day periods in the Atlantic Ocean.
2. Data and Methods [5] We use four historical long-term (1.5 year) Pss and Pbot measurements spanning from 16°S to 37°N (Table 1, Figure 1). One is from the Synoptic Ocean Prediction Experiment (SYNOP Pbot) conducted by University of Rhode Island (URI) [Watts et al., 1995], and the other two are from sub-surface pressure gauges at Ascension (ASC Pss) and St. Helena Islands (StH Pss) deployed through the UK ACCLAIM (Antarctic Circumpolar Current Levels by Altimetry and Island Measurements) Programme of the Proudman Oceanography Laboratory (POL) [Spencer et al., 1993; Woodworth et al., 1995]. The ASC and StH Pss measurements were deployed at the same time interval as the SYNOP Pbot measurements, allowing cross-spectral analysis between them. The fourth one is Pbot deployed on the equator (FP1 Pbot) by POL during Feb 1983 – Sep 1984 [Cartwright et al., 1987]. [6] We also use global-gridded Patm and wind stress data with 2.5° latitude 2.5° longitude and 1.905° latitude 1.875° longitude spatial resolutions, respectively, within 60°S– 60°N. These atmospheric data are available from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis project. For the joint analysis with the NCEP/NCAR data, the hourly Pbot measurements are subsampled once every 6 hours. [7] The cross-spectral analyses between times series were conducted as follows. All input time series had their mean and a linear trend removed. The time series were divided into 50% overlapped equal-length blocks each having 128 data points (32 days), and block averaging in the frequency domain was used to smooth the spectra [Emery and Thomson, 2001].
[9] Coherence squared (hereafter simply referred to as coherence) between the SYNOP Pbot and the StH Pss shows high values near 5-day periods (Figure 2). In addition, it also shows high values at 2 – 4-day periods with a maximum value near 2.3-day periods. The phase relationship between these two time series shows no phase lag near 5-day periods, while the SYNOP Pbot leads the StH Pss with gradually increasing phase lag from 0 to 150° at 5 – 2-day periods. These coherence and phase values suggest that the high coherence near 2.3-day periods may be related to a basin mode oscillation with out-of-phase relation between North and South Atlantic Oceans. The SYNOP Pbot is also highly coherent with remote Patm at the equator at periods near 5 days, where it leads the Equator Patm by 10°, while the SYNOP Pbot shows no significant coherences with local Patm in all frequency bands. This result indicates that the SYNOP region responds to the local Patm isostatically, which is consistent with the result of a previous study from Bermuda Pbot measurements [Brown et al., 1975]. [10] These responses revealed by the Pss and Pbot measurements and Patm led us to investigate spatial and temporal structures of the driving force (Patm) in the Atlantic Ocean. We computed frequency-wavenumber spectra at the equator (0°) and at an extratropical latitude (40°N), as shown in the top two plots of Figure 3. For each spectrum, a 2-dimensional periodogram of Patm (latitude-time domain) was calculated using 2-dimensional fast Fourier transform, and then smoothed using a 3 3 box filter on the 2-dimensional periodogram. Generally, the Patm power spectrum at the equator is much less energetic than at the extratropical latitude in all frequency and wavenumber bands except the diurnal and semi-diurnal frequency bands. In the equatorial Patm spectrum, a clear peak of energy can
3. Results [8] Figure 2 exhibits band-pass filtered time series of SYNOP Pbot, StH Pss, Patm at the equator (0°, 30°W), and Patm near SYNOP (37.5°N, 67.5°W), which are superimposed on each original time series. We used a 3rd-order Butterworth filter with cutoffs at 4.0 and 5.5 days. The filtering was carried out in the forward direction, and then the filtered sequence was reversed and run again through the filter in order to eliminate all phase shifts. The band-pass filtered results of the first three time series show similar patterns, though the SYNOP Pbot is 8500 km and 5700 km away from the sites of StH Pss and the Equator Patm, respectively. In contrast, SYNOP Patm shows no similarity with the others.
Figure 1. Locations of sub-surface and bottom pressure measurements.
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Figure 2. (left) Time series (gray lines) of SYNOP Pbot, StH Pss, Equator Patm (0°, 30°W), and SYNOP Patm (37.5°N, 67.5°W). Bandpass filtered (4.0– 5.5 days) time series (blue lines) are superimposed on these time series. (right) Coherence squared and phase between StH Pss and SYNOP Pbot (red line), and Equator Patm and SYNOP Pbot (green line). Coherence squared only is shown between SYNOP Patm and SYNOP Pbot (cyan line). Vertical and horizontal dashed lines indicate 5-day period and 95% confidence level, respectively. Gray-shaded zone indicates 4.0– 5.5-day bandwidth. be seen at periods near 5 days with long wavelength, typical of the Rossby-Haurwitz wave, while it is not distinguishable in the extratropical latitude spectrum. The 5-day Rossby-Haurwitz wave has meridionally-symmetric structure about the equator with predominant zonal wavenumber 1 (westward propagating) as shown in the bottom plot of Figure 3 [Madden and Julian, 1972, 1973]. Its amplitude varies with time, 1 to 2 mbar in extratropical regions, and 0.5 to 1.0 mbar in the tropics [Madden and Julian, 1973]. The frequency-wavenumber spectra in Figure 3 illustrate that energetic synoptic-scale P atm variations obscure the near 5-day Rossby-Haurwitz signal at extratropical latitudes. [11] To examine the barotropic ocean response to Patm, we conducted cross-spectral analysis between the Pss and Pbot measurements and the global-gridded NCEP/NCAR Patm and wind stress data in 100°W – 30°E and 60°S – 60°N. The first three columns of Figure 4 exhibit coherence, gain and phase maps for the Pss and Pbot measurements with Patm at 4.6-day period. Some significantly coherent features were obtained at two additional spectral estimates near 4.6-day period (5.3-day and 4.0-day periods), but we show here only the maps at 4.6-day period where the most significant coherences were obtained. Moreover, gains and phases at the neighboring two periods were very similar to those at 4.6-day period. The fourth column exhibits coherence maps for the four Pss and Pbot measurements with u- and v-component wind stress at 4.6-day period. The larger of the u- and v-component coherences is plotted. Coherence, gain and phase are omitted in mapping if coherence is lower than the 95% confidence level. [12] The coherence map for the SYNOP Pbot exhibits no coherence with local Patm near the SYNOP region (lower than confidence level of 0.11, see Figure 1). However, the coherence map exhibits that the SYNOP Pbot is significantly coherent with remote Patm mainly between 30°S and 30°N
with maximum coherence near the equator (Figure 4a). This is a surprising feature because SYNOP is about 6000 km away from the region of maximum coherence. The gain map of the SYNOP Pbot exhibits a meridionally symmetric distribution increasing equatorward. Maximum gain near the equator is larger than 1.3. Phase relationship between the SYNOP Pbot and Patm exhibits that the eastern part of the tropical region leads the western part with zero phase lag near 20°W. In addition, the phase increases westward with zonally uniform phase. The phase lag between 90°W and 0°W is approximately 90°, indicating zonal wavenumber 1. This phase relationship exhibits the characteristics of the 5-day Rossby-Haurwitz wave. Lack of coherence between the SYNOP Pbot and wind stress confirms that only Patm loading drives this near 5-day Pbot fluctuation at SYNOP. [13] To extract the global-scale 5-day Rossby-Haurwitz wave signal obscured by the energetic synoptic-scale Patm variations (see Figure 3), we carried out zonal low-pass filtering with cutoff wave number 120° on the Patm data. Using these zonally low-pass filtered Patm time series, we conducted cross-spectral analysis with the SYNOP Pbot, as shown in Figure 4b. The coherence map exhibits extended regions of significant coherence, including the SYNOP site. The meridionally symmetric distribution of gain and regular westward increasing phase is similar to those of Figure 4a with increased gain values near the equator (>1.5). [14] The ASC and StH Pss measurements have been demonstrated to have significant coherence with local Patm at periods near 5 days [Woodworth et al., 1995], which is reconfirmed in our coherence maps (Figures 4c and 4d). However, our coherence and gain maps exhibit that the maximum coherences and gains for these two Pss measurements are near the equator, consistent with the features shown in the SYNOP Pbot coherence and gain maps. The phase relationship, regularly increasing from east to west, is also consistent with that of SYNOP. Unlike the SYNOP
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the equator. The reduced amplitude of the Rossby-Haurwitz wave at the equator explains the maximum gain there. The meridionally-symmetric gain distributions in the tropical region also support that the non-IB near 5-day fluctuations are driven by the meridionally-symmetric Rossby-Haurwitz wave. The coherence maps with wind stress confirm that local wind effects on the non-IB near 5-day fluctuations are negligible. [16] The results from the SYNOP Pbot indicate that the near-IB response to energetic synoptic-scale P atm
Figure 3. (top) Frequency-wavenumber spectra computed using the NCEP/NCAR Patm data at the equator (0°) and the extratropical region (40°N). The data span from June 1988 to August 1990 with 6-hour interval and from 100°W to 30°E with 2.5° longitude resolution. Red arrow points out the 5-day Rossby-Haurwitz wave spectral band. Spectral densities [mbar2/(day 1 km 1)] are plotted on a logarithmic scale. (bottom) Theoretical spatial structure of the 5-day Rossby-Haurwitz wave in Patm with zonal wavenumber 1 [Madden and Julian, 1972; Ponte, 1997]. Contour interval is 0.2 mbar. Pbot, local coherences with wind stress are shown for both ASC and StH Pss, but they are negligible compared to the significant coherences with Patm. For comparison, Figure 4e also maps cross-spectral analysis results of FP1 Pbot, which was deployed on the equator during 2/83 – 9/84. The crossspectral analysis results of the other three measurements are remarkably similar to those of the FP1 Pbot.
4. Discussion [15] The most striking result from cross-spectral analysis is the highly-coherent near 5-day fluctuations with no phase lag between SYNOP Pbot and StH Pss measurements, though the distance between them is about 8500 km across the equator. This agrees well with the predicted non-IB response in the tropical and North Atlantic Oceans (nearly constant amplitude and phase) caused by the 5-day RossbyHaurwitz wave [Ponte, 1997; Mathers and Woodworth, 2004]. The equator Patm has a fluctuation near 5-day periods, the Rossby-Haurwitz wave, as shown in Figure 3, and hence the basin-scale non-IB responses detected by the Pss and Pbot measurements reveal maximum coherence at
Figure 4. Cross-spectral analysis results at 4.6-day period of (a) SYNOP Pbot, (c) ASC Pss, (d) StH Pss, and (e) FP1 Pbot with the NCEP/NCAR Patm and wind stress data. (b) Same results of SYNOP Pbot with the zonally low-pass filtered NCEP/NCAR Patm with cutoff wave length 120°. The first three columns represent coherence-squared, gain and phase maps with the Patm data. Gain is defined (Pss or Pbot)/Patm, and positive phase indicates Patm lags Pss or Pbot. The fourth column represents coherence-squared maps with the wind stress data. The larger of the u- and v-component coherences is plotted. The Pss and Pbot measurements are summarized in Table 1. The Patm and wind stress data have 2.5° 2.5° and 1.905° 1.875° resolutions, respectively, in 100°W – 30°E and 60°S – 60°N. Coherence squared, gain and phase are omitted in mapping if coherence is lower than 95% confidence level. Contour intervals for coherencesquared, gain and phase maps are 0.05, 0.2 and 20°, respectively. Solid triangles indicate Pss and Pbot sites.
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fluctuations in extratropical regions can obscure the basinscale-generated non-IB response to the Rossby-Haurwitz wave, which results in no significant coherence of the SYNOP Pbot with the local Patm. This is confirmed by significant coherence of the SYNOP Pbot with the zonally low-pass filtered local Patm, which filtered out the synopticscale fluctuations. [17] The precise measurements of satellite altimetry covering the global ocean have been conducted long enough to investigate interannual or interdecadal scale ocean sea level variabilities. The basin-scale non-IB response near 5 days can alias energy in the altimetry measurements into long periods because of their coarse temporal samplings (e.g., 590-day period in 9.9156-day sampling T/P altimetry), which could be falsely interpreted as interannual variability [Wunsch and Stammer, 1997]. Removing the non-IB effects from the satellite measurements is essential for improved utilization of the global sea surface height observations [e.g., Carre`re and Lyard, 2003]. [18] Acknowledgments. Two sub-surface tide gauge data (ASC and StH) were provided by the ACCLAIM Programme of the Proudman Oceanography Laboratory, from their web site at http://www.pol.ac.uk/ psmsl/programmes/acclaim.info.html. One bottom pressure data (FP1) were provided by the Global Undersea Pressure (GLOUP) web site at http:// www.pol.ac.uk/psmslh/gloup/gloup.html. NCEP/NCAR Reanalysis data were provided by the NOAA-CIRES ESRL/PSD Climate Diagnostics branch, Boulder, Colorado, USA, from their Web site at http:// www.cdc.noaa.gov/. We thank N. Hirose for suggesting the zonal filtering of atmospheric pressure data (Figure 4b). We also thank two anonymous reviewers for their valuable comments on the original manuscript. This work was supported by the National Science Foundation grant OCE-0453681.
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Carre`re, L., and F. Lyard (2003), Modeling the barotropic response of the global ocean to atmospheric wind and pressure forcing: Comparisons with observations, Geophys. Res. Lett., 30(6), 1275, doi:10.1029/ 2002GL016473. Cartwright, D. E., R. Spencer, and J. M. Vassie (1987), Pressure variations on the Atlantic equator, J. Geophys. Res., 92, 725 – 741. Emery, W. J., and R. E. Thomson (2001), Data Analysis Methods in Physical Oceanography, 2nd ed., Elsevier, New York. Hirose, N., I. Fukumori, and R. M. Ponte (2001), A non-isostatic global sea level response to barometric pressure near 5 days, Geophys. Res. Lett., 28, 2441 – 2444. Luther, D. S. (1982), Evidence of a 4 – 6 day barotropic, planetary oscillation of the Pacific Ocean, J. Phys. Oceanogr., 12, 644 – 657. Madden, R., and P. Julian (1972), Further evidence of global-scale 5 day pressure waves, J. Atmos. Sci., 29, 1464 – 1469. Madden, R., and P. Julian (1973), Reply, J. Atmos. Sci, 30, 935 – 940. Mathers, E. L., and P. L. Woodworth (2001), Departures from the local inverse barometer model observed in altimeter and tide gauge data and in a global barotropic numerical model, J. Geophys. Res., 106, 6957 – 6972. Mathers, E. L., and P. L. Woodworth (2004), A study of departures from the inverse-barometer response of sea level to air-pressure forcing at a period of 5 days, Q. J. R. Meteorol. Soc., 130, 725 – 738. Ponte, R. M. (1993), Variability in a homogeneous global ocean forced by barometric pressure, Dyn. Atmos. Oceans, 18, 209 – 234. Ponte, R. M. (1997), Nonequilibrium response of the global ocean to the 5-day Rossby-Haurwitz wave in atmospheric surface pressure, J. Phys. Oceanogr., 27, 2158 – 2168. Spencer, R., P. R. Foden, C. McGarry, A. J. Harrison, J. M. Vassie, T. F. Baker, M. J. Smithson, S. A. Harangozo, and P. L. Woodworth (1993), The ACCLAIM programme in the South Atlantic and Southern oceans, Int. Hydrogr. Rev., 70, 7 – 21. Watts, D. R., J. M. Bane Jr., K. L. Tracey, and T. J. Shay (1995), Gulf Stream path and thermocline structure near 74°W and 68°W, J. Geophys. Res., 100, 18,292 – 18,312. Woodworth, P. L., S. A. Windle, and J. M. Vassie (1995), Departures from the local inverse barometer model at periods of 5 days in the central South Atlantic, J. Geophys. Res., 100, 18,281 – 18,290. Wunsch, C., and D. Stammer (1997), Atmospheric loading and the oceanic ‘‘inverted barometer’’ effect, Rev. Geophys., 35, 79 – 107. J.-H. Park and D. R. Watts, Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197, USA. (
[email protected];
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