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The 18.6 yr nodal modulation in the tides of Southern European Coasts
No full textThe nodal modulation of the diurnal (K1 and O1) and semi-diurnal (M2, and K2) tidal constituents at the coasts of the Mediterranean Sea and the eastern Atlantic is estimated and its spatial variability mapped. Fourteen hourly tide gauge records each spanning more than 18 years are considered in this analysis. Ten tide gauges are located in the Mediterranean Sea and four in the Bay of Biscay. The nodal modulation of the most energetic tidal constituent (M2) reaches up to 5 cm at the eastern Atlantic coasts, while within the Mediterranean Sea its modulation is in general less than 1.2 cm. The largest K2 nodal modulation found is 3.7 cm in the eastern Atlantic coasts. In the Mediterranean Sea, smaller modulation amplitudes, ranging between 0.4 and 1.4 cm are found. The K1 tide constituent has the largest amplitude nodal modulation within the Mediterranean Sea of 1.9 cm in the north Adriatic Sea, which is also larger than the modulation of this constituent at the eastern Atlantic coasts. The O1 tide constituent has the highest amplitude nodal modulation (1.4 cm) at the eastern Atlantic coasts. In the Mediterranean Sea the maximum value is 1 cm in the north Adriatic Sea.The derived nodal modulations of the diurnal and semi-diurnal constituents follow, in general, the equilibrium tidal theory. The tidal amplitudes for all four components do not indicate significant secular trends for most tide gauges. The tidal phases indicate significant negative trends for all four tidal constituents within the central and eastern Mediterranean Sea.<br/
A front-following algorithm for AVHRR SST imagery
No full textA Front-Following Algorithm provides a new approach to determining the position and characteristics of thermal oceanic fronts using Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) imagery. This algorithm differs from standard line enhancement, threshold edge detection, or classical contour techniques. Instead it utilizes a hyperbolic tangent function in a surface-fitting technique to follow an oceanic front. It has the advantage of describing the characteristics of an oceanic thermal front (including mean SST, SST difference, width and gradient across the front) and extracting information on the position and characteristics of the front into parameter form. Thus, the algorithm has the added benefit of recording the changes in the characteristics of the thermal front as it tracks along the front. The algorithm was applied to AVHRR SST imagery on part of the Subtropical Front known as the Southland Front (SF) off the east coast of the South Island of New Zealand, where subantarctic surface waters and subtropical surface waters converge. The algorithm was tested on examples of the SF and also compared with a standard gradient operator. The results showed that the algorithm performed well when following the SF, with low standard errors of parameter estimates, good visual verification of tracking, and consistent standards of accepted data. The algorithm estimated gradients better than a gradient operator
Measurements of an oceanic front using a front-following algorithm for AVHRR SST imagery
No full textThe Subtropical Front (STF) is a global front which extends around the Southern Ocean and is the boundary where the Subantarctic Surface Water mass (SAW) converges with the Subtropical Surface Water mass (STW). The Southland Front (SF) is part of the STF, which lies off the east coast of the South Island, New Zealand. The SF is narrow, approximately 8 km, with a temperature difference of approximately 1.8°C which can be detected using remote sensing Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data. The work presented here is an application of remote sensing for the first detailed study of the surface spatial and temporal variability of the SF. The variability of the SF was quantified using an algorithm developed to follow the Front using AVHRR SST imagery. The algorithm used a new approach to determine the position, mean SST, SST difference, width, and gradient across the Front. Three time scales of variability were examined: long-term (3 years), annual, and seasonal. The algorithm efficiently followed the SF and consistently showed the 3-year mean position was stable and constrained by the bathymetry of the continental slope. Seasonally, the front moved inshore during summer. The temperature gradient across the front was strongest and the front narrowest in winter. The decrease in SST gradient during the 3-year data set coincided with the decrease in Southern Oscillation Index (SOI)
The influence of the North Atlantic Oscillation on sea-level variability in the North Atlantic region
No full textSatellite altimeter (Topex/Poseidon, 1992–2001) and tide-gauge measurements are used to explore the relationship of the sea level of the North Atlantic and neighbouring seas and coastlines to the North Atlantic Oscillation (NAO). Altimeter measurements suggest significant gyre-scale influence of the NAO in the North Atlantic, but also stronger influences on the continental shelf and inland seas of Europe. A north–south dipole in sea-level anomaly consistent with a hydrostatic response to the NAO sea-level pressure dipole is evident, but there are also large non-hydrostatic effects. The strongest response on the European Shelf is in the southeastern part of the North Sea where sea level is positively correlated to NAO Index. The sea level in two semi-enclosed seas, the Baltic Sea positively and the Mediterranean Sea negatively, is also strongly influenced by the NAO. A weak negative correlation is apparent around the northeastern coastline of North America. These features are confirmed by contemporary coastal tide-gauge data, but the tide-gauge data also show that the influence of the NAO was weaker early in the Twentieth Century (20C) on parts of the Northwest European coastline. Inter-annual sea-level variability associated with fluctuations in the NAO are generally much larger than those associated with secular trends. Inferred multi-decadal fluctuations associated with the NAO are very substantial compared to the 15(35) cm estimated for 20C global sea-level rise (Church, J.A., Gregory, J.M., Huybrechts, P., Kuhn, M., Lambeck, K., Nhuan, M.T., Qin, D. and Woodworth, P.L. (2001). Changes in sea level. Chapter 11 of the Intergovernmental Panel on Climate Change Third Assessment Report, pp. 639–694. Cambridge University Press, Cambridge.) and scenario forecasts for the 21C (350 cm). Therefore, the behaviour of the NAO in the next few decades will be a major regional factor in sea-level rise and coastal vulnerability in some European regions
Seasonal and inter-annual variability of SST off the east coast of the South Island, New Zealand
No full textThis study examined the temporal variability of sea surface temperature (SST) of three water masses east of the South Island, New Zealand, where two of these water masses are part of the global Subtropical Front (STF). The STF is well defined and can be detected easily using remote sensing Advanced Very High Resolution Radiometer (AVHRR) SST data. Ten sites were used from a ten year AVHRR SST data set and they were compared with a manually sampled 45 year SST data set from Portobello Marine Laboratory, Dunedin.This investigation decomposed each SST series into a periodic seasonal component and an interannual trend. The latter component was further decomposed into low and mid-frequency bands using smoothing techniques. The results showed that the periodic component of offshore waters were in phase and reached their maximum temperature 10 days after the inshore neritic waters. The amplitudes of seasonal variations of Subantarctic and Subtropical water masses were similar while the neritic waters showed considerably higher seasonal variation. El Niño has a clear effect and lowers the temperatures of all water masses by 1.4ºC at Portobello and 2.2-3.4ºC for all
offshore waters
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Study on reverse calculation for unidirectional waves from shallow water
No full textIn the coastal region, some wave measurements are usually deployed in shallow water. However, the knowledge of deep-water waves is more required in some cases. Targeting the east coast of Taiwan with a steep slope, we experimentally detect the capability of the linear spectrum theory in reversely estimating unidirectional deep-water waves from the shallow-water waves. Tank experiments of irregular waves show that the nonlinearity of wave-wave interactions significantly contributes to the total energy transfer. The energy of incident waves around peak frequency is transferred to lower and higher frequency domains. Even though waves are travelling through a short distance under a steep slope, the nonlinear wave-wave interactions cannot be neglected. In the application of the linear spectrum theory, the reversely calculated deep-water spectrum is found to be overestimated in frequency over 2Hz and in total energy of spectrum, but underestimated around peak frequency. There is evidence to show that the ratio of H 1/3 /Hs is correlated with the spectral bandwidth and Kurtosis
Seasonal sea level extremes in the Mediterranean Sea and at the Atlantic European coasts
Full text linkHourly sea level data from tide gauges and a barotropic model are used to explore the spatial and temporal variability of sea level extremes in the Mediterranean Sea and the Atlantic coasts of the Iberian peninsula on seasonal time scales. Significant spatial variability is identified in the observations in all seasons. The Atlantic stations show larger extreme values than the Mediterranean Sea primarily due to the tidal signal. When the tidal signal is removed most stations have maximum values of less than 90 cm occurring in winter or autumn. The maxima in spring and summer are less than 60 cm in most stations. The wind and atmospheric forcing contributes about 50 cm in the winter and between 20–40 cm in the other seasons. In the western Mediterranean the observed extreme values are less than 50 cm, except near the Strait of Gibraltar. Direct atmospheric forcing contributes significantly to sea level extremes. Maximum sea level values due to atmospheric forcing reach in some stations 45 cm during the winter. During the summer the contribution of the direct atmospheric forcing is between 10–20 cm. The Adriatic Sea shows a resonant behaviour with maximum extreme observed sea level values around 200 cm found at the northern part. Trends in the 99.9% percentiles are present in several areas, however most of them are removed when the 50% percentile is subtracted indicating that changes in the extremes are in line with mean sea level change. The North Atlantic Oscillation and the Mediterranean Oscillation Index are well correlated with the changes in the 99.9% winter values in the Atlantic, western Mediterranean and the Adriatic stations. The correlation of the NAO and the MOI indices in the Atlantic and western Mediterranean is significant in the autumn too. The correlations between the NAO and MOI index and the changes in the sea level extremes become insignificant when the 50% percentile is removed indicating again that changes in extremes have been dominated by changes in the mean sea level
The use of AVHRR data in a pilot study for investigating and modelling regional ocean circulation
No full textRegional oceanographic studies rely on in situ data. However, sampling design strategies for the collection of oceanographic data are not always straightforward, particularly in regions of complex bathymetry. Here we present a pilot study methodology that uses remotely sensed sea surface temperature data to generate coarse resolution surface temperature fields suitable for forcing a regional numerical circulation model. The model is then used to help identify the major surface and interior oceanographic features in the region and hence determine the most suitable locations to deploy instrumentation. Such pilot studies are uncommon in regional oceanographic research and here we demonstrate an effective, low-cost pilot study methodology
Distribution and biomass of two squid species off southern New Zealand: Nototodarus sloanii and Moroteuthis ingens
No full textThe distribution and biomass of two species of squid, the ommastrephid arrow squid Nototodarus sloanii and the onychoteuthid squid Moroteuthis ingens, were analysed off southern New Zealand. These two species are the most important and abundant species in this region of the South Pacific Ocean. Data were obtained from extensive NIWA research cruises over 10 years. There was a sharp demarcation between the distribution of the two species, with N. sloanii occurring predominantly shallower than 600 m, with the greatest biomass less than 300 m. In contrast, M. ingens had the highest biomass between 650 and 700 m and occurred down to 1400 m. The biomass of N. sloanii reached more than 3500 kg · km-2, with an average catch rate of over 186 kg · km-2. In contrast, the biomass of M. ingens was more than an order of magnitude less, with all catch weights less than 200 kg · km-2 and an average catch rate less than 17 kg · km-2. The separation of these two species appeared to be related to depth, temperature and, possibly, salinity. N. sloanii occurred predominantly in warmer, shallower subtropical waters while M. ingens occurred in deeper, cooler subantarctic and antarctic intermediate water masses. The Subtropical Front formed a major barrier between the distribution of these two squid species
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