Understanding tidally and atmospherically forced resonances in an Ocean Circulation Model in terms of free oscillations
Final Report Abstract
In this project we used observational, numerical and analytical approaches in order to understand and simulate transient signal in the global barotropic and baroclinic ocean tides. A global analysis of long term tide gauge records, revealed a secular change in tidal amplitudes and phases. The North Atlantic tides show the largest trends and further, show a rapid change of semidiurnal tides after 1980, which brings into question whether the change in tides are driven by climate related processes. It will be important to understand the underlying processes of secular tidal trends, because in some region tidal trends are significant with the rise in mean sea level and further, if there is a connection between global warming and changing tides, long term tide gauge records might contain much more information than simply sea level. With a high-resolution ocean circulation and tide model we present the first map of barotropic and baroclinic tidal variability on seasonal timescales. This variability can be in the range of 1-10% of tidal amplitude and 1-5° in tidal phase and, so far, it is not considered in any of the correction processes of geodetic measurements. We found that on seasonal timescales, the largest variability in barotropic ocean tides, which is not implicitly induced by gravitational forcing, is dominant in coastal and polar regions, with relative changes of 5-10% of the tidal amplitude. We explored the global pattern of the seasonal cycle of the M 2 tidal constituent using a global numerical highresolution model, analysis of 19 years of satellite altimetry data and long-term tide gauge records, and theoretical models. It shows that the ocean circulation and tide model captures the seasonal pattern of the M2 tide reasonably well. There are two main processes leading to this inter-annual variability. First, seasonal changes in stratification on the continental shelf affect the vertical profile of eddy viscosity and, in turn, the vertical current profile. Second, the frictional effect between sea-ice and the surface ocean layer leads to seasonally varying tidal transport. Interestingly, the largest seasonal tide signals appear in regions where large M2 residuals in GRACE2 gravity products are also found and it will be worthwhile to include the seasonal tidal variability in the correction process of satellite gravity and altimetry. The inter-annual variability of the global low mode internal tide field is compared with satellite altimeter data, as well. We show an complex interplay between the low frequency wind forced ocean circulation, seasonal varying solar forcing and the generation and propagation of the internal tide in the South China Sea. These results are consistent with other observational data analyses, where large transient signals in the internal tide in the South China Sea have been found. It will be important to use both, results from ocean circulation and tide models and from observations, in order to understand and predict inter-annual variability of the internal tide field on a global scale. This will improve the prediction of internal tides and the estimation of the uncertainties involved in the correction of future satellite altimeter and gravity data. This will be especially important for the planed satellite altimeter missions, which will observe surface elevations with a unprecendented spatial resolution. In order to postprocess their data, it will be important to understand and simulate the baroclinic tides and their deviation from stationarity. Analytical and simplified numerical model aproaches have been used to understand the effect of stratification on barotropic tides. This study showed that stratification in coastal region has an considerable impact on barotropic tides, and for highly accurate tide model simulations in will be important to allow for sophisticated eddy turbulence model appoaches. Seasonal changes in tides, induced by stratification are 1-5 % and a 10 meter change in mixed layer depth can induce a change in barotropic transport of about 1-2 %. The high-resolution model approach further allowed us to compute a global of barotropic-tobaroclinic tidal conversion rates, which has not been done before in a model which combines a realistic ocean circulation and stratification with tidal motions. This map can now be used to parameterize tidal mixing in a physically consistent way.
Publications
- (2010) Seasonal modulation of M2 tide in shallow seas from TPJ data and numerical model. Altimetry for Oceans and Hydrology meeting, Lisbon, Portugal, October 18-22, 2010
Cherniawsky, J., Müller, M., and Foreman, M.
- (2011) Rapid change in semi-diurnal tides in the North Atlantic since 1980. Geophys. Res. Lett.38, L11602
Müller, M.
- (2011) Secular trends in ocean tides: Observations and model results, Journal Geophysical Research 116, C05013
Müller, M., Arbic, B. K., and Mitrovica, J. X.
- (2012) Sensitivity of North Atlantic ocean tides to climate related processes. Eos Trans. AGU, Ocean Sciences Meeting Supplement
Müller, M.
- (2012) The influence of changing stratification conditions on barotropic tidal transport and its implications for seasonal and secular changes of tides. Cont. Shelf Res.
Müller, M.
(See online at https://doi.org/10.1016/j.csr.2012.07.003)
