|Laurence Padman, ESR|
|Lana Erofeeva, OSU|
This modeling effort is a collaboration between ESR (L. Padman) and Oregon State University (G. Egbert and S. Erofeeva). We have created two models: the Arctic Ocean Dynamics-based Tide Model (AODTM-5) and the Arctic Ocean Tidal Inverse Model (AOTIM-5). Each is described below. The models are coded on a 5-km cartesian grid, and contain tide height and velocity information for 8 constituents: 4 major tides (M2, S2, K1, O1); and 4 lesser tides (N2, K2, P1, Q1).
The models are available to the public. Two access packages are provided: a Fortran-based package; and a Matlab-based GUI (Graphical User Interface) which also includes batch processing scripts ("M-files") for many common processes such as extracting amplitude and phase fields for a specific tidal harmonic, and making predictions. For most users we expect that the inverse model AOTIM-5 will be the more useful model, since its accuracy is improved through assimilation of coastal and benthic tide gauges, plus TOPEX/Poseidon and ERS satellite radar altimetry. Thus, at this time the AOTIM-5 model is the only one available at this site. However, users interested in AODTM-5 can obtain the model by contacting L. Padman (ESR).
A PDF version of a paper describing these models is also available.
Back to Top
This work was funded by the National Science Foundation grant OPP-0125252 (ESR) and NASA JASON-1 grant NCC5-711 (OSU). We appreciate the assistance of A. Proshutinsky and G. Kivman in developing the data base of Arctic tide gauge harmonic constants used in this study.
If you experience problems with this page, or need further information, contact Laurie Padman (ESR).
Back to Top
It is now widely recognized that tidal currents contribute significantly to general patterns of hydrography and circulation in the world ocean through their effect on mixing in the ocean interior [e.g., Munk and Wunsch, 1998; Wunsch, 2000; Egbert and Ray, 2000]. This also holds true in high-latitude seas. Padman  summarizes many of the processes by which tides contribute to the larger-scale structure of the Arctic Ocean. These include elevated mixing in the benthic layer and also in the ocean interior through baroclinic tide generation, and ocean influences on the concentration and properties of sea ice leading to modifications to ocean-atmosphere exchanges of heat and fresh water. Kowalik and Proshutinsky  (hereinafter denoted KP-94) used a barotropic tidal model (14-km grid) with simple coupling of an ice cover to demonstrate that the increased open-water ("leads") fraction associated with tides provided a significant component of the total ocean-to-atmosphere heat loss in winter and a corresponding increase in net ice growth, locally between 10 and 100 cm per year due to tides alone. This process has been further discussed (for the Antarctic) by Padman and Kottmeier . Another consequence of tides was suggested by Robertson et al. , who noted that increased benthic friction associated with tidal flow could significantly lower the net wind-forced transport of the Weddell Gyre. Strong tidal flows may also, however, contribute to "mean flow" through tidal rectification effects; see, e.g., Loder , Padman et al. , and Padman .
Back to Top
The model domain (Figure 1) uses the International Bathymetric Chart of the Arctic Ocean ("IBCAO") [Jakobsson et al., 2000] digitized on a uniform 5-km grid. The domain includes all of the central Arctic Ocean, the Greenland Sea, the Labrador Sea and Baffin Bay, and the Canadian Arctic Archipelago ("CAA"). There are two features of this domain that will be relevant to further discussions. First, the Arctic Ocean is dominated by the broad continental shelf seas of the eastern (or Eurasian) Arctic. Second, the passages of the CAA are narrow, and the flow of tidal energy through these passages (including Nares Strait at the northern end of Baffin Bay) critically depends on adequate resolution of these passages. As with other tide modeling efforts, the quality of dynamics-based models depends to a large extent on the accuracy of the bathymetry grid.
Back to Top
From previous studies we have identified 310 coastal and benthic tide gauge records in the model domain that provide tidal coefficients for at least a few of the most energetic tidal harmonics [KP-94; Tidal tables, 1941, 1958]; for locations, see Figure 1. Additional data are available from the T/P and ERS satellite radar altimeters: T/P measures sea surface height (SSH) for ice-free ocean to ~66oN, while ERS measures SSH for ice-free ocean to ~82oN. The T/P orbit was specifically chosen to allow tides to be accurately measured [Parke et al., 1987]; however, individual ERS height measurements are of lower accuracy, the orbit is unfavorable for resolving solar constituents (e.g., S2 and K1), and the higher-latitude data must frequently be discarded because of ice cover within the radar's footprint. Nevertheless, for some constituents the ERS data set provides useful information about ocean tides above the T/P turning latitude.
The tide gauge data set ("TG") has been divided into 7 regions (see Figure 1) in order for us to present general comparisons between various tide models and data. This division is roughly based on the amplitudes of semidiurnal and diurnal tides.
Back to Top
The 5-km Arctic Ocean Dynamics-based Tide Model (AODTM-5) is the numerical solution to the shallow water equations (SWE), which are essentially linear. Following Egbert and Erofeeva  (hereinafter denoted EE), we solve the SWE by direct matrix factorization for 8 tidal constituents: M2, S2, N2, K2, K1, O1, P1, and Q1. Potentially significant simplifications to the SWE dynamics include our use of: tidal loading and self attraction computed from a global model ( TPXO6.2 Global Tide Model); and the use of linear benthic friction to parameterize the dissipation term. We use a constant friction velocity value of 0.5 m s-1 for semi-diurnal constituents and 2 m s-1 for diurnals. The larger value for diurnal constituents is required because their currents are frequently strongest in relatively deep water along the shelf break. We assume that the errors introduced by the simplified dynamics can be corrected by the data assimilation. The AODTM-5 uses elevations taken from the latest Ό degree global solution ( TPXO6.2 as open ocean boundary conditions. Additional forcing is provided by the specified astronomical tide potentials.
Back to Top
The 5-km Arctic Ocean Tidal Inverse Model (AOTIM-5) was created following the data assimilation scheme described by Egbert et al. , hereinafter denoted EBF], Egbert , and EE. Only the 4 most energetic tides (M2, S2, O1, and K1) were simulated with AOTIM-5: for prediction purposes we use N2, K2, P1, and Q1 from AODTM-5. Assimilated data consists of coastal and benthic tide gauges (between 250 and 310 gauges per constituent), and 364 cycles of T/P and 108 cycles of ERS altimeter data from a modified version of the "Pathfinder" database [Koblinsky et al., 1999] with no tidal corrections applied [B. Beckeley, personal communication, 2003]. We used T/P altimeter data for 11178 data sites with a spacing of ~7 km, and ERS altimeter data for 18224 data sites, illustrated in Figure 1. Data assimilation was done for 4 major constituents only: TG+T/P+ERS for M2 and O1, TG+T/P for K1 and TG only for S2. These choices are based on the ability of the satellite data to resolve specific constituents depending on orbit characteristics; see, e.g., Parke et al., 1987, Smith , and Smith et al. .
The dynamical error covariance was defined following the considerations outlined in EBF, using the "prior" solution (AODTM-5) to estimate the spatially varying magnitudes of errors in the momentum equations. The correlation length scale for the dynamical errors was set to 50 km (10 grid cells). The continuity equation was assumed to be exact.
To compute the inverse solution we used the single-constituent reduced basis approach (EBF) to calculate the representer coefficients. The efficient calculation scheme described by EE was applied. Figure 2 shows elevations for the inverse solution for the M2, S2, K1 and O1 constituents. The most significant changes from the prior solution for the semi-diurnals were in the Barents Sea near the entrance into the White Sea (amplitude changes >60 cm for M2 and >30 cm for S2), in the White Sea (amplitude changes >40 cm for M2 and >10 cm for S2) and in the northern part of Baffin Bay (amplitude changes >20 cm for M2 and >10 cm for S2). The most significant changes from the prior solution for the diurnals were in the Baffin Bay and the Gulf of Boothia in the CCA (maximum amplitude changes ~20 cm for K1 and ~5 cm for O1), in the Barents Sea near the entrance to the White Sea (maximum amplitude changes ~10 cm for both K1 and O1), and in the Greenland Sea (maximum amplitude changes ~10 cm for K1 and ~5 cm for O1).
Back to Top
Maps of tide height amplitude and phase for the 4 most energetic tides (Figure 2) are qualitatively similar to previously published maps (e.g., figures 2-5 in KP-94). The M2 amplitude exceeds 1 m in the southern Barents Sea near the entrance to the White Sea, in the Labrador Sea, and at the northern end of Baffin Bay. The distribution of S2 amplitude is similar to M2, but about a factor of 3 smaller. Diurnal tide amplitudes are largest in Baffin Bay and the Labrador Sea ("BBLS") and in the Gulf of Boothia in the southern CAA. Maximum K1 amplitudes are ~0.4 m, and about 0.2 m for O1.
We calculate errors for each location averaged over the "in-phase" and "quadrature" components for each constituent, i.e.,
where are the measured harmonic constants and are the modeled harmonic constants for the constituent l at location xi, and the sum is over the N data sites. For conciseness, we present averaged error values for the 7 subdomains described in section 2.2 (see Figure 1). Table 1 lists the value of RMSRI for the comparison of TG data with 4 models; KP-94, AODTM-5, TPXO6.2, and AOTIM-5. Note that both TPXO6.2 and AOTIM-5 have assimilated these tide gauges, hence the fits in these cases represent the assigned uncertainty in data coefficients and the effect of other assumptions in the inverse calculation.
Averaged over the entire domain, AODTM-5 performs significantly better than KP-94 for the M2 tide, which contains most of the total tide height signal (see Figure 2). The values of RMSRI averaged over the entire domain for S2, K1, and O1 are similar for both models. The improvement from KP-94 to AODTM-5 is most pronounced for M2 in the CAA and Nares Strait: we attribute this result primarily to the higher resolution (5 km for AODTM-5 vs. 14 km for KP-94) of the passages through the CAA and Nares Strait. The diurnal constituents are slightly worse in AODTM-5 than in KP-94 for the North Atlantic and Eastern Arctic seas, but are significantly better for the CAA, BBLS, and Nares Strait. Our studies indicate that dynamics-based models of diurnal tides are sensitive to the choice of the boundary location south of Davis Strait: the location in AODTM/AOTIM is chosen to minimize RMSRI in AODTM-5 for the BBLS region.
As expected, the errors for the 2 inverse models TPXO6.2 and AOTIM-5 are smaller than for either dynamics-based model. For semidiurnal tides, AOTIM-5 is a significantly better fit to the tide gauge data than is TPXO6.2. This occurs because the coastline of the Barents and White Seas are less well resolved in the Ό degree global model, semidiurnal tide elevations there approach 2 m, and more than 40% of all tide gauges are located in the Barents Sea. Furthermore, the decorrelation length scale for TPXO6.2 is 250 km, which does not allow as accurate a fit to closely spaced tidal data as can be achieved with the 50 km decorrelation length scale in AOTIM-5. For diurnal tides, errors for the 2 inverse models are comparable throughout the domain.
The best way to look at the output from AOTIM-5 is to download the model and play with it. Two software bases are available: Fortran-based (OSU) and Matlab-based (ESR). However, maps of amplitude and phase of sea surface elevation for each tidal harmonic are available for a quick look.
Back to Top
While the new models have been validated against tide height data (see Table 1), most practical oceanographic interest in tides is related to the strength and gradients of tidal currents rather than height variations. We assume that the high-resolution data assimilation model AOTIM-5, which best fits the available tide height data (Table 1), also best represents tidal currents. This is a reasonable assumption since the inverse model is consistent with the shallow-water wave equations to within the assumed accuracy of the bathymetry-based and dissipation terms. For each grid node in the model domain we calculate the time-averaged speed based on all 8 modeled constituents:
In (2), i=1,2,...T is the summation is of hourly currents over a time interval of 14 days, which encompasses the beat periods of the 4 major constituents. The components of the current vector
are calculated as:
and α l(t) gives the slowly modulated periodic ("nodal": ~18.6 y period) time variations for constituent l (for details see, e.g., EE).
The map of ū (Figure 3) is comparable to the map of "maximum tidal current" umax plotted by KP-94 (their Plate 1): in general, umax≈ ū . The largest currents are over the broad and shallow Eurasian shelf seas, with typical values for ū are ~5-15 cm s-1. Values of ū >80 cm s-1 are found in the western Barents Sea south of Bear Island [Kowalik and Proshutinsky, 1995] and in the southern Barents Sea near the entrance to the White Sea. Strong currents are also found in Davis Strait in the Labrador Sea, and in Nares Strait and various locations within the CAA. Currents are weak over the deep basins and along the northern coast of Alaska.
We have ignored the possible effects of sea-ice cover in the present models. KP-94 note that sea ice may change tidal amplitudes by up to 10% and phases by 1-2 h, presumably leading to quasi-seasonal variability in tidal coefficients.
The inverse model (AOTIM-5) is the most accurate Arctic tide model available at this time, as judged by comparisons with tide gauge data and satellite altimetry. Nevertheless, the long-term goal will be to develop dynamics-only models with comparable accuracy, through further improvements in resolution, addition of sea-ice interactions, and more sophisticated dissipation parameterizations including benthic friction and the conversion of barotropic tidal energy to internal tides.
Instructions for accessing the model are provided here.
Back to Top
Back to Top
Back to Top
Figure 1: Model domain, showing locations of tide gauge data (red squares), and ERS and TOPEX/Poseidon radar altimetry (magenta and yellow dots, respectively). The domain is partitioned into 7 regions for model-data comparisons (see Table 1). These regions are: (1) North Atlantic; (2) Barents Sea (including the White Sea); (3) the Russian shelf seas (Kara, Laptev, East Siberian, Chukchi); (4) the northern coast of Alaska and the Canadian Northwest Territories; (5) the Canadian Arctic Archipelago ("CAA"); (6) Baffin Bay and the Labrador Sea ("BBLS"); and (7) Nares Strait. Click on the figure for a clearer (*.png) image.
Back to Top
Figure 2: Amplitude (color shading, in m) and phase (white contours) for the 4 most energetic tidal harmonics, M2, S2, K1, and O1, for the Arctic Ocean Tidal Inversion Model (AOTIM-5). Peak M2 amplitudes are close to 1.8 m in the southern Barents Sea near the entrance to the White Sea. Click on the figure for a clearer (*.png) image.
Back to Top
Figure 3: Mean tidal current speed (cm s-1) based on simulating 14 days of hourly total tidal speed from the 8-constituent inverse solution AOTIM-5. The largest values exceed 80 cm s-1 in the southern Barents Sea near the entrance to the White Sea and around Bear Island in the western Barents Sea south of Svalbard. Click on the figure for a clearer (*.png) image.
Back to Top