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1.
For the purpose of coastal planning and management, especially under changing climatic conditions, enhanced knowledge about the evolution of extreme sea levels in the past, present, and future is required. This paper presents statistical analyses of high seasonal water level percentiles of 13 tide gauges in the German Bight, spanning over a period of up to 109 years throughout the twentieth and twenty-first centuries. Seasonal and annual high percentile time series of water levels were investigated in comparison to the mean sea level (MSL) for changes on seasonal, inter-annual, and decadal timescales. While throughout the first half of the twentieth century extreme water levels generally followed changes in MSL, during the second half of the century, linear extreme sea level trends exceeded those in MSL in the order of 9–64 cm per century. The largest, although insignificant, contribution to the magnitude of these trends occurs in the winter season (January to March), while smaller but, due to the generally lower atmospheric variability, significant changes are observed during spring (April to June). The observed multi-decadal trends are generally in good agreement with multi-decadal trends in the corresponding percentiles of local zonal surface winds. Only small parts of the trends remain unexplained. It is suggested that these remaining trends result from modifications in the local tidal regime. For the aspects of coastal planning, the findings clarify that in the German Bight, in addition to changes in MSL, potential changes in storminess and in the tidal regime significantly contribute to the development of extreme water levels. Since these factors have influenced the characteristic of extremes throughout the recent past, they also have to be taken into account when estimating design water levels for, e.g., dikes (in a warming climate) under changing greenhouse gas emissions.  相似文献   

2.
This study examines connections between mean sea level (MSL) variability and diurnal and semidiurnal tidal constituent variations at 17 open-ocean and 9 continental shelf tide gauges in the western tropical Pacific Ocean, a region showing anomalous rise in MSL over the last 20 years and strong interannual variability. Detrended MSL fluctuations are correlated with detrended tidal amplitude and phase fluctuations, defined as tidal anomaly trends (TATs), to quantify the response of tidal properties to MSL variation. About 20 significant amplitude and phase TATs are found for each of the two strongest tidal constituents, K1 (diurnal) and M2 (semidiurnal). Lesser constituents (O1 and S2) show trends at nearly half of all gauges. Fluctuations in MSL shift amplitudes and phases; both positive and negative responses occur. Changing overtides suggest that TATs are influenced by changing shallow water friction over the equatorial Western Pacific and the eastern coast of Australia (especially near the Great Barrier Reef). There is a strong connection between semidiurnal TATs at stations around the Solomon Islands and changes in thermocline depth, overtide generation, and the El Niño Southern Oscillation (ENSO). TATs for O1, K1, and M2 are related to each other in a manner that suggests transfer of energy from M2 to the two diurnals via resonant triad interactions; these cause major tidal variability on sub-decadal time scales, especially for M2. The response of tides to MSL variability is not only spatially complex, it is frequency dependent; therefore, short-term responses may not predict long-term behavior.  相似文献   

3.
The convection of plasma in the high-latitude ionosphere is strongly affected by the interplanetary magnetic field (IMF) carried by the solar wind. From numerous statistical studies, it is known that the plasma circulation conforms to patterns that are characteristic of particular IMF states. Following a change in the IMF, the convection responds by reconfiguring into a pattern that is more consistent with the new IMF. Some early studies reported that the convection first begins to change near noon while on the dawn and dusk flanks and on the nightside it remains relatively unaffected for tens of minutes. Work by Ridley et al. (J. Geophys. Res. 103 (1998) 4023–4039) and Ruohoniemi and Greenwald (Geophys. Res. Lett. 25 (1998) 2913–2916) that was based on measurements with more global sets of instruments challenged this view. A debate ensued as to the true nature of the convection response. We follow the arguments of Lockwood and Cowley (J. Geophys. Res. 104 (1999) 4387–4391) and Ridley et al. (J. Geophys. Res. 104 (1999) 4393–4396) by reviewing recent results on the timing of the onset of the convection response to the changed IMF. We discuss the timing problem from the perspectives of observations and modeling. In our view, the onset of the ionospheric response to changed IMF is globally simultaneous on time scales of a few minutes. A physical basis for the rapid communication of effects in the dayside convection to the nightside has been demonstrated in magnetohydrodynamic simulations. We also offer some cautionary notes on the timing of convection changes and the use of global assimilative techniques to study local behavior.  相似文献   

4.
Summary

The Hooghly is a tidal river and Calcutta one of the most important ports of India is situated on it. The tidal effect stretches to over 175 miles length up to Nabadwip where two nonperennial rivers Bhagirathi and Jalangi, both taking off at different sites from the Gangesmeet and discharge their rain water (freshets) into the Hoogly all through the summer months. Other rivers join it also downstream. Annual tide tables are published for three places on the river viz Sagar, Diamond Harbour and Garden Reach.

In this paper lunitidal intervals, durations of rise and fall, mean high water, mean low water, mean tide level and mean sea level planes and mean ranges have been shown to go through periodic seasonal changes. It has been shown that different tidal planes have changed differently in the intrior of the river indicating changes in tidal regimes and in the river bed. Whereas variations in yearly sea levels since 1882 have not been appreciable at Sagar, these at Garden Reach have changed considerable at indicating sometimes an improvement in the channel, other times worsening of the channel. Four 19 yearly cycles of mean sea-level at Garden Reach have been analysed to give phaselags different from Zero and large variations in amplitudes. To carry this study further one period of 19 years each for Sagar, Madras and Vishakhapatnam on East Coast and 4 periods each for Bombay and Aden in the Arabian Sea have also been analysed to see the possibility of nodal variations being constant in amplitude and phase over large areas of the ocean. 19-yearly cycles in the differences between MTL &; MSL have been examined for Bombay, Madras and Garden Reach. The effects of freshets on various planes have been analysed and the curves of annual freshets and MTLs at Garden Reach are found to run closely parallel. The variations in yearly high water and law water planes above MTLs of the corresponding years after corrections for the longitude of the moons' node have been found to be inappreciable. Theoretical extreme high water and low water planes have been found to be more extreme than actually recorded planes which are very near M.H.W.S. and M.L.W.S. planes indicating there are no predominant effects of winds and storms on the tides in the river. The mean ranges at Garden Reach have been found to be steadily increasing. Theoretical effects of increase in range and lowering of MSL on MHW and MLW plane has been found to agree with actual values. Monthly average temperatures and salinities show distinct annual cycles. High temperatures occur at the same times as low salinities and high mean sea levels. Vertical density structure may be responsible for a large part of annual variation in the sea level at Sagar which is a region of particular interest on account of opposite influences of seasonal monsoons, drainage of rivers and oceanic currents. Seasonal variations in average monthly pressure have been related to variations in mean sea levels and it is found that as the pressures decrease, the height of MSL increases. Tide in Hooghly has been found to be progressive type of wave. Rates of travel of tidal streams outside Sagar and rates of streams in the river have been given also, stating the effects of freshets on tidal streams. Bores in Hooghly have been examined. Shapes of tidal curves and profile of the bore at Garden Reach have been drawn. Rates of propagation of the bore from Doodsons' formulae have been compared with actual values and a remarkable agreement has been found. The rise of tide in 10 minutes and 30 minutes which is the time taken for the steep rise to disappear has been correlated with range of tide and a useful table has been included. Tides in Hooghly are predicted by applying harmonic shallow water corrections to open sea primary predictions at Sagar. As the freshets are unpredictable an approximate relationship has been worked out between the departures of actual freshets from num freshets of the years of analyses and departures of actual from predicted tides. From a knowledge of actual freshets the predictions can be corrected almost, daily.

A large number of tables and only the essential graphs have been included to illustrate the contents.  相似文献   

5.
《Journal of Atmospheric and Solar》2002,64(12-14):1531-1543
Jicamarca unattended long term investigations of the ionosphere and atmosphere radar observations of equatorial spread F (ESF) plasma irregularities made between August 1996 and April 2000 are analyzed statistically. Interpretation of the data is simplified by adopting a taxonomy of echo types which distinguishes between bottom-type, bottomside, topside, and post-midnight irregularities. The data reveal patterns in the occurrence of ESF in the Peruvian sector that are functions of season, solar flux, and geomagnetic activity. We confirm earlier work by Fejer et al. (J. Geophys. Res. 104 (1999) 19,859) showing that the quiet-time climatology of the irregularities is strongly influenced by the climatology of the zonal ionospheric electric field. Under magnetically quiet conditions, increasing solar flux implies greater pre-reversal enhancement amplitudes and, consequently, irregularity appearances at earlier times, higher initial altitudes, and higher peak altitudes. Since the post-reversal westward background electric field also grows stronger with increasing solar flux, spread F events also decay earlier in solar maximum than in solar minimum. Variation in ESF occurrence during geomagnetically active periods is consistent with systematic variations in the electric field associated with the disturbance dynamo and prompt penetration described by Fejer and Scherliess (J. Geophys. Res. 102 (1997) 24,047) and Scherliess and Fejer (J. Geophys. Res. 102 (1997) 24,037). Quiet-time variability in the zonal electric field contributes significantly to variability in ESF occurrence. However, no correlation is found between the occurrence of strong ESF and the time history of the zonal electric field prior to sunset.  相似文献   

6.
Initially the development of shallow sea three-dimensional barotropic tidal models is briefly reviewed with a view to determining what were the key measurements that allowed progress in this field and rigorous model validation. Subsequently this is extended to a brief review of baroclinic tidal models to try to determine a “way forward” for baroclinic model development. The difficulty of high spatial variability, and wind influence are identified as possibly important issues that must be considered in validating baroclinic tidal models. These are examined using a three-dimensional unstructured grid model of the M2 internal tide on the shelf edge region off the west coast of Scotland. The model is used to investigate the spatial variability of the M2 internal tide, and associated turbulence energy and mixing in the region. Initial calculations are performed with tidal forcing only, with subsequent calculations briefly examining how the tidal distribution is modified by down-welling and up-welling favourable winds. Calculations with tidal forcing only, show that there is significant spatial variability in the internal tide and associated mixing in the region. In addition, these are influenced by wind effects which may have to be taken into account in any model validation exercise. The paper ends by discussing the comprehensive nature of data sets that need to be collected to validate internal tidal models to the same level currently attained with three dimensional barotropic tidal models.  相似文献   

7.
We present sea level observations derived from the analysis of signal-to-noise ratio (SNR) data recorded by five coastal GPS stations. These stations are located in different regions around the world, both in the northern and in the southern hemisphere, in different multipath environments, from rural coastal areas to busy harbors, and experience different tidal ranges.The recorded SNR data show periodic variations that originate from multipath, i.e. the interference of direct and reflected signals. The general assumption is that for satellite arcs facing the open sea, the rapid SNR variations are due to reflections off the sea surface. The SNR data recorded from these azimuth intervals were analyzed by spectral analysis with two methods: a standard analysis method assuming a static sea level during a satellite arc and an extended analysis method assuming a time dependent sea level during a satellite arc.The GPS-derived sea level results are compared to sea level records from co-located traditional tide gauges, both in the time and in the frequency domain. The sea level time series are highly correlated with correlation coefficients to the order of 0.89–0.99. The root-mean-square (RMS) difference is 6.2 cm for the station with the lowest tidal range of 165 cm and 43 cm for the station with the highest tidal range of 772 cm. The relative accuracy, defined as the ratio of RMS and tidal range, is between 2.4% and 10.0% for all stations.Comparing the standard analysis method and the extended analysis method, the results based on the extended analysis method agree better with the independent tide gauge records for the stations with a high tidal range. For the station with the highest tidal range (772 cm), the RMS is reduced by 47% when using the extended analysis method. Furthermore, the results also indicate that the standard analysis method, assuming a static sea level, can be used for stations with a tidal range of up to about 270 cm, without performing significantly worse than the extended analysis method.Tidal amplitudes and phases are derived by harmonic analysis of the sea level records. Again, a high level of agreement is observed between the tide gauge and the GPS-derived results. Comparing the GPS-derived results, the results based on the extended analysis method show a higher degree of agreement with the traditional tide gauge results for stations with larger tidal ranges. Spectral analysis of the residuals after the harmonic analysis reveals remaining signal power at multiples of the draconitic day. This indicates that the observed SNR data are to some level disturbed by additional multipath signals, in particular for GPS stations that are located in harbors.  相似文献   

8.
Behavior of semidiurnal tides in the north and south polar MLT regions simulated by Middle Atmosphere Circulation Model at Kyushu University is described. Summertime enhancement of westward propagating semidiurnal tide with zonal wavenumber s=1 is found, which is consistent with the observed result at the South Pole (Ann. Geophys. 16 (1998) 828). Additional numerical simulations show that the non-migrating semidiurnal tide is mainly generated by the nonlinear interactions between stationary planetary waves with zonal wavenumber s=1 and the migrating semidiurnal tide in the stratosphere and mesosphere as suggested by Forbes et al. (Geophys. Res. Lett. 22(23) (1995) 3247).  相似文献   

9.
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10.
《Journal of Atmospheric and Solar》2002,64(12-14):1525-1529
Almost no new theoretical work has been conducted in the area of mid-latitude F-region plasma instabilities since Perkin's (J. Geophys. Res. 78 (1973) 218) linear theory. New experimental data now suggest that the nonlinear development of mid-latitude F-region structures includes large polarization electric fields which dominate the final state. Airglow and radar data show that ΣP is greatly depleted in some regions, which is in agreement with a polarization hypothesis. We hope these new results will inspire new simulations with some anticipation of solving this perplexing but fascinating problem.  相似文献   

11.
《Continental Shelf Research》2008,28(18):2565-2573
A numerical model is used to determine the resonant period and quality factor Q of Chesapeake Bay and explore physical mechanisms controlling the resonance response in semi-enclosed seas. At the resonant period of 2 days, the mouth-to-head amplitude gain is 1.42 and Q is 0.9, indicating that Chesapeake Bay is a highly dissipative system. The modest amplitude gain results from strong frictional dissipation in shallow water. It is found that the spatial distribution of energy dissipation varies with forcing frequency. While energy at tidal frequencies is dissipated around topographic hotspots distributed throughout the Bay, energy dissipation at subtidal frequencies is mainly concentrated in the shallow-water lower Bay. An analytic calculation shows that the bottom friction parameter is much larger in Chesapeake Bay than in other coastal systems with strong resonance response. The model-predicted amplitude gains and phase changes agree well with the observations at semidiurnal and diurnal tidal frequencies. However, the predicted amplitude gain in the resonant frequency band (34–54 h period) falls below that inferred from band-passed sea level observations. This discrepancy can be attributed to the local wind forcing which amplifies the sea level response in the upper Bay. The model is also used to show that rising sea levels associated with global warming will shift the resonance period of Chesapeake Bay closer to the diurnal tides and thus exacerbate flooding problems by causing an increase in tidal ranges.  相似文献   

12.
Absolute paleointensity experiments were carried out using Shaw’s method [Geophys. J. Res. Astr. Soc. 39 (1974) 133] and its modifications [Geophys. J. Res. Astr. Soc. 54 (1978) 241; Geophys. J. Int. 80 (1985) 773; Geophys. J. Int. 118 (1994) 781] on 49 samples belonging to six Late Quaternary basaltic flows from central Mexico. Samples were selected from a large collection because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. Moreover, they previously yielded high quality Thellier paleointensity results, which makes them good candidates to assess the reliability of Shaw’s paleointensity method. Only 13 samples yielded acceptable results using Shaw’s original method (ARM2/ARM1 ratio varies from 0.95 to 1.05 for accepted determinations) although 6 samples do not pass the validity test proposed by Tsunakawa and Shaw [Geophys. J. Int. 118 (1994) 781] and thus should be rejected for paleointensity analyses. Rolph and Shaw’s [Geophys. J. Int. 80 (1985) 773] method gives reliable determination only in one case and no single determination was obtained by Kono’s [Geophys. J. Res. Astr. Soc. 54 (1978) 241] modification. Our results indicate an extremely low success rate of Shaw’s paleointensity method, which may be due to magneto-chemical changes that occurred during heating of samples above their Curie temperatures.  相似文献   

13.
Precise measurements of seismological Q are difficult because we lack detailed knowledge on how the Earth’s fine velocity structure affects the amplitude data. In a number of recent papers, Morozov (Geophys J Int 175:239–252, 2008; Seism Res Lett 80:5–7, 2009; Pure Appl Geophys, this volume, 2010) proposes a new procedure intended to improve Q determinations. The procedure relies on quantifying the structural effects using a new form of geometrical spreading (GS) model that has an exponentially decaying component with time, e ?γt·γ is a free parameter and is measured together with Q. Morozov has refit many previously published sets of amplitude attenuation data. In general, the new Q estimates are much higher than previous estimates, and all of the previously estimated frequency-dependence values for Q disappear in the new estimates. In this paper I show that (1) the traditional modeling of seismic amplitudes is physically based, whereas the new model lacks a physical basis; (2) the method of measuring Q using the new model is effectively just a curve fitting procedure using a first-order Taylor series expansion; (3) previous high-frequency data that were fit by a power-law frequency dependence for Q are expected to be also fit by the first-order expansion in the limited frequency bands involved, because of the long tails of power-law functions; (4) recent laboratory measurements of intrinsic Q of mantle materials at seismic frequencies provide independent evidence that intrinsic Q is often frequency-dependent, which should lead to frequency-dependent total Q; (5) published long-period surface wave data that were used to derive several recent Q models inherently contradict the new GS model; and (6) previous modeling has already included a special case that is mathematically identical to the new GS model, but with physical assumptions and measured Q values that differ from those with the new GS model. Therefore, while individually the previous Q measurements have limited precision, they cannot be improved by using the new GS model. The large number of Q measurements by seismologists are sufficient to show that Q values in the Earth are highly laterally variable and are often frequency dependent.  相似文献   

14.
Following the 2007, August 15th, M w 8.0, Pisco earthquake in central Peru, Sladen et al. (J Geophys Res 115: B02405, 2010) have derived several slip models of this event. They inverted teleseismic data together with geodetic (InSAR) measurements to look for the co-seismic slip distribution on the fault plane, considering those data sets separately or jointly. But how close to the real slip distribution are those inverted slip models? To answer this crucial question, the authors generated some tsunami records based on their slip models and compared them to DART buoys, tsunami records, and available runup data. Such an approach requires a robust and accurate tsunami model (non-linear, dispersive, accurate bathymetry and topography, etc.) otherwise the differences between the data and the model may be attributed to the slip models themselves, though they arise from an incomplete tsunami simulation. The accuracy of a numerical tsunami simulation strongly depends, among others, on two important constraints: (i) A fine computational grid (and thus the bathymetry and topography data sets used) which is not always available, unfortunately, and (ii) a realistic tsunami propagation model including dispersion. Here, we extend Sladen’s work using newly available data, namely a tide gauge record at Callao (Lima harbor) and the Chilean DART buoy record, while considering a complete set of runup data along with a more realistic tsunami numerical that accounts for dispersion, and also considering a fine-resolution computational grid, which is essential. Through these accurate numerical simulations we infer that the InSAR-based model is in better agreement with the tsunami data, studying the case of the Pisco earthquake indicating that geodetic data seems essential to recover the final co-seismic slip distribution on the rupture plane. Slip models based on teleseismic data are unable to describe the observed tsunami, suggesting that a significant amount of co-seismic slip may have been aseismic. Finally, we compute the runup distribution along the central part of the Peruvian coast to better understand the wave amplification/attenuation processes of the tsunami generated by the Pisco earthquake.  相似文献   

15.
This paper addresses the role of meteorological forcing on mean sea level (MSL) variability at the tide gauge of Cuxhaven over a period from 1871 to 2008. It is found that seasonal sea level differs significantly from annual means in both variability and trends. The causes for the observed differences are investigated by comparing to changes in wind stress, sea level pressure and precipitation. Stepwise regression is used to estimate the contribution of the different forcing factors to sea level variability. The model validation and sensitivity analyses showed that a robust and timely independent estimation of regression coefficients becomes possible if at least 60 to 80 years of data are available. Depending on the season, the models are able to explain between 54 % (spring, April to June) and 90 % (winter, January to March) of the observed variability. Most parts of the observed variability are attributed to changes in zonal wind stress, whereby the contribution of sea level pressure, precipitation and meridional wind stress is rather small but still significant. On decadal timescales, the explanatory power of local meteorological forcing is considerable weaker, suggesting that the remaining variability is attributed to remote forcing over the North Atlantic. Although meteorological forcing contributes to linear trends in some sub-periods of seasonal time series, the annual long-term trend is less affected. However, the uncertainties of trend estimation can be considerably reduced, when removing the meteorological influences. A standard error smaller than 0.5 mm/year requires 55 years of data when using observed MSL at Cuxhaven tide gauge. In contrast, a similar standard error in the meteorologically corrected residuals is reached after 32 years.  相似文献   

16.
A global experimental model for gravity tides of the Earth   总被引：1，自引：0，他引：1  
The long-term, continuous and high-quality tidal gravity data, recorded with the superconducting gravimeters (SGs) at 19 stations in the Global Geodynamics Project (GGP), were simultaneously used to investigate the global pattern of the tidal gravity variations. The atmospheric effects were removed from the gravity observations by using the simultaneous pressure records at the stations. A total of six global co-tidal models were employed to remove the loading effects of oceanic tides. The resonance parameters of the Earth's free core nutation (FCN), as well as the spheroidal constant components in the gravimetric factors of waves O1 and M2, were accurately retrieved. As a result, a global experimental model for gravity tides (GEMGT) was developed, considering the nearly diurnal resonance and the latitude-dependence of the gravimetric amplitude factors. The final results indicate that the mean discrepancy of the four main tidal waves (i.e. O1, K1, M2 and S2) between the GEMGT and SG observations is less than 0.2% on average. The GEMGT is in good agreement with the theoretical models based on the inelastic non-hydrostatic equilibrium Earth models [Dehant, V., Defraigne, P., Wahr, J., 1999. Tides for a convective Earth. J. Geophys. Res. 104, 1035–1058; Mathews, P.M., 2001. Love numbers and gravimetric factor for diurnal tides. J. Geodetic Soc. Jpn. 46 (4), 231–236] with a mean discrepancy less than 0.15%. However, the GEMGT is in closer accordance with the theoretical model given by Mathews [Mathews, P.M., 2001. Love numbers and gravimetric factor for diurnal tides. J. Geodetic Soc. Jpn. 46 (4), 231–236] for the diurnal tides while it is in closer agreement with one obtained by Dehant et al. [Dehant, V., Defraigne, P., Wahr, J., 1999. Tides for a convective Earth. J. Geophys. Res. 104, 1035–1058] for the semi-diurnal tides.  相似文献   

17.
We present new archeointensity results obtained from 127 potsherds and baked brick fragments dated from the last four millennia BC which were collected from different Syrian archeological excavations. High temperature magnetization measurements were carried out using a laboratory-built triaxial vibrating sample magnetometer (Triaxe), and ancient field intensity determinations were derived from the experimental procedure described by Le Goff and Gallet [Le Goff and Gallet. Earth Planet. Sci. Lett. 229 (2004) 31–43]. As some of the studied samples were previously analyzed using the classical Thellier and Thellier [Thellier and Thellier . Ann. Geophys. 15 (1959) 285–376] method revised by Coe [Coe. J. Geophys. Res. 72 (1967) 3247–3262], a comparison of the results is made from the two methods. The differences both at the fragment and site levels are mostly within ± 5%, which strengthens the validity of the experimental procedure developed for the Triaxe. The new data help to better constrain the geomagnetic field intensity variations in Mesopotamia during archeological times, with the probable occurrence of an archeomagnetic jerk around 2800–2600 BC.  相似文献   

18.
In a tidal channel with adjacent tidal flats, along–channel momentum is dissipated on the flats during rising tides. This leads to a sink of along–channel momentum. Using a perturbative method, it is shown that the momentum sink slightly reduces the M2 amplitude of both the sea surface elevation and current velocity and favours flood dominant tides. These changes in tidal characteristics (phase and amplitude of sea surface elevations and currents) are noticeable if widths of tidal flats are at least of the same order as the channel width, and amplitudes and gradients of along–channel velocity are large. The M2 amplitudes are reduced because stagnant water flows from the flats into the channel, thereby slowing down the current. The M4 amplitudes and phases change because the momentum sink acts as an advective term during the fall of the tide, such a term generates flood dominant currents. For a prototype embayment that resembles the Marsdiep–Vlie double–inlet system of the Western Wadden Sea, it is found that for both the sea surface elevation and current velocity, including the momentum sink, lead to a decrease of approximately 2% in M2 amplitudes and an increase of approximately 25% in M4 amplitudes. As a result, the net import of coarse sediment is increased by approximately 35%, while the transport of fine sediment is hardly influenced by the momentum sink. For the Marsdiep–Vlie system, the M2 sea surface amplitude obtained from the idealised model is similar to that computed with a realistic three–dimensional numerical model whilst the comparison with regard to M4 improves if momentum sink is accounted for.  相似文献   

19.
The Arctic plays a fundamental role in the climate system and shows significant sensitivity to anthropogenic climate forcing and the ongoing climate change. Accelerated changes in the Arctic are already observed, including elevated air and ocean temperatures, declines of the summer sea ice extent and sea ice thickness influencing the albedo and CO2 exchange, melting of the Greenland Ice Sheet and increased thawing of surrounding permafrost regions. In turn, the hydrological cycle in the high latitude and Arctic is expected to undergo changes although to date it is challenging to accurately quantify this. Moreover, changes in the temperature and salinity of surface waters in the Arctic Ocean and Nordic Seas may also influence the flow of dense water through the Denmark Strait, which are found to be a precursor for changes in the Atlantic meridional overturning circulation with a lead time of around 10 years (Hawkins and Sutton in Geophys Res Lett 35:L11603, 2008). Evidently changes in the Arctic and surrounding seas have far reaching influences on regional and global environment and climate variability, thus emphasizing the need for advanced quantitative understanding of the ocean circulation and transport variability in the high latitude and Arctic Ocean. In this respect, this study combines in situ hydrographical data, surface drifter data and direct current meter measurements, with coupled sea ice–ocean models, radar altimeter data and the latest GOCE-based geoid in order to estimate and assess the quality, usefulness and validity of the new GOCE-derived mean dynamic topography for studies of the ocean circulation and transport estimates in the Nordic Seas and Arctic Ocean.  相似文献   

20.
Sea level variability along the US West Coast is analyzed using multi-year time series records from tide gauges and a high-resolution regional ocean model, the base of the West Coast Ocean Forecast System (WCOFS). One of the metrics utilized is the frequency of occurrences when model prediction is within 0.15 m from the observed sea level, F. A target level of F?=?90% is set by an operational agency. A combination of the tidal sea level from a shallow water inverse model, inverted barometer (IB) term computed using surface air pressure from a mesoscale atmospheric model, and low-pass filtered sea level from WCOFS representing the effect of coastal ocean dynamics (DYN) provides the most straightforward approach to reaching levels F>80%. The IB and DYN components each add between 5 and 15% to F. Given the importance of the DYN term bringing F closer to the operational requirement and its role as an indicator of the coastal ocean processes on scales from days to interannual, additional verification of the WCOFS subtidal sea level is provided in terms of the model-data correlation, standard deviation of the band-pass filtered (2–60 days) time series, the annual cycle amplitude, and alongshore sea level coherence in the range of 5–120-day periods. Model-data correlation in sea level increases from south to north along the US coast. The rms amplitude of model sea level variability in the 2–60-day band and its annual amplitude are weaker than observed north of 42 N, in the Pacific Northwest (PNW) coast region. The alongshore coherence amplitude and phase patterns are similar in the model and observations. Availability of the multi-year model solution allows computation and analysis of spatial maps of the coherence amplitude. For a reference location in the Southern California Bight, relatively short-period sea level motions (near 10 days) are incoherent with those north of the Santa Barbara Channel (in part, due to coastal trapped wave scattering and/or dissipation). At a range of periods around 60 days, the coastal sea level in Southern California is coherent with the sea surface height (SSH) variability over the shelf break in Oregon, Washington, and British Columbia, more than with the coastal SSH at the same latitudes.  相似文献   

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