Validation of Recent Altimeter Missions at Non-Dedicated Tide Gauge Stations in the Southeastern North Sea

Consistent calibration and monitoring is a basic prerequisite for providing a reliable time series of global and regional sea-level variations from altimetry. The precisions of sea-level measurements and regional biases for six altimeter missions (Jason-1/2/3, Envisat, Saral, Sentinel-3A) are assessed in this study at 11 GNSS-controlled tide gauge stations in the German Bight (SE North Sea) for the period 2002 to 2019. The gauges are partly located at the open water, and partly at the coast close to mudflats. The altimetry is extracted at virtual stations with distances from 2 to 24 km from the gauges. The processing is optimized for the region and adjusted for the comparison with instantaneous tide gauge readings. An empirical correction is developed to account for mean height gradients and slight differences of the tidal dynamics between the gauge and altimetry, which improves the agreement between the two data sets by 15–75%. The precision of the altimeters depends on the location and mission and ranges from 1.8 to 3.7 cm if the precision of the gauges is 2 cm. The accuracy of the regional mission biases is strongly dependent on the mean sea surface heights near the stations. The most consistent biases are obtained based on the CLS2011 model with mission-dependent accuracies from 1.3 to 3.4 cm. Hence, the GNSS-controlled tide gauges operated operationally by the German Waterway and Shipping Administration (WSV) might complement the calibration and monitoring activities at dedicated CalVal stations.

Indian Ocean Dynamic Sea Level, Variability And Projections In CMIP6 Models

The regional sea level variability and its projection amidst the global sea level rise is one of the major concerns for coastal communities. The dynamic sea level plays a major role in the observed spatial deviations in regional sea level rise from the global mean. The present study evaluates 27 climate model simulations from the sixth phase of the coupled model intercomparison project (CMIP6) for their representation of the historical mean states, variability and future projections for the Indian Ocean. Most models reproduce the observed mean state of the dynamic sea level realistically, however consistent positive bias is evident across the latitudinal range of the Indian Ocean. The strongest sea level bias is seen along the Antarctic Circumpolar Current (ACC) regime owing to the stronger than observed south Indian Ocean westerlies and its equatorward bias. Further, this equatorward shift of the wind field resulted in stronger positive windstress curl across the southeasterly trade winds in the southern tropical basin and easterly wind bias along the equatorial waveguide. These anomalous easterly equatorial winds cause upwelling in the eastern part of the basin and keeps the thermocline shallower in the model than observed, resulted in enhanced variability for the dipole zonal mode or Indian Ocean dipole in the tropics. In the north Indian Ocean, the summer monsoon winds are weak in the model causing weaker upwelling and positive sea level bias along the western Arabian Sea. The high-resolution models compare better in simulating the sea level variability, particularly in the eddy dominated regions like the ACC regime in interannual timescale. However, these improved variabilities do not necessarily produce a better mean state likely due to the enhanced mixing driven by parametrizations set in these high-resolution models. Finally, the overall pattern of the projected dynamic sea level rise is found to be similar for the mid (SSP2-4.5) and high-end (SSP5-8.5) scenarios, except that the magnitude is higher under the high emission situation. Notably, the projected dynamic sea level change is found to be milder when only the best performing models are used compared to the full ensemble.

Geological stories from the journey of mollusks fossils in Java

The journey began in the Eocene with the presence of mollusk fossil in the Nanggulan Formation (near Yogyakarta) in Central Java. Many experts believe this was the early part of the Tethys system which might still be connected to the Tethys system in Europe.The oldest mollusk fossils type locality after Nanggulan is the Early Miocene Jonggrangan Formation in Kulon Progo near the city of Yogyakarta, which is dominated by the gastropod Haustator specimen. Molluscan paleontological studies of this type of locality reflect a restricted environment with less influence of the Tethyan system. Haustator are considered as the ancestor of the Turritellidae group, which is found mostly on Java Island, during the younger Tertiary to Quaternary Periods.The story continued to the Middle Miocene where the Tethyan realms indication was clearly observed by the presence of some typical Tethys species such as Volema and Babylonia from Nyalindung Formation, West Java. The regional sea level rise in this epoch (around 12 Ma) that was indicated by the presence of Vicarya as an index fossil, which occurrence was due to land submerging to become mangroves area. The fossil then quickly become extinct when the sea level dropped back.Late Miocene to Pliocene was like the transition period from the Tethyan realm to the Pacific realm, where the Tethyan fauna was no longer present. Only evolutional traces of the Middle Miocene mollusk fossils were observed. This continuous evolution is most clearly seen in Turritella cramatensis (late Miocene), Turritella acuticarinata (early Pliocene) and Turritella cikumpaiensis (late Pliocene) which was interpreted to have originated from Turritella angulata as their ancestors.Earth cooling environment that happened in the late Pliocene/early Pleistocene has led the diversity and evolution of a new group of mollusks, most clearly observed from the abundance of Turritella bantamensis in the Bojong Formation, Banten. The new Turritella group has a curved whorl that different from its predecessor with an angled whorl shape.Plio-Pleistocene tectonics event has ended the period of Java marine mollusks domination, then only freshwater mollusk fossils can be found in almost all Quaternary mollusks-bearing deposits.

Validation of Recent Altimeter Missions at Non-dedicated Tide Gauge Stations in the Southeastern North Sea

Consistent calibration and monitoring is a basic prerequisite for providing reliable time series of global and regional sea level variations from altimetry. The precision of sea level measurements and regional biases for six altimeter missions (Jason-1/2/3, Envisat, Saral, Sentinel-3A) is assessed at eleven GNSS-controlled tide gauge stations in the German Bight (SE North Sea) for the period 2002 to 2019. The gauges are partly located at the open water, partly at the coast close to mudflats. The altimetry is extracted at virtual stations with distances from 2 to 24 km from the gauges. The processing is optimized for the region and adjusted for the comparison with instantaneous tide gauges readings. An empirical correction is developed to account for mean height gradients and slight differences of the tidal dynamics between gauge and altimetry which improves the agreement between the two data sets by 15-75%. The precision of the altimeters is depending on location and mission and is shown to be at least 1.8 to 3.7 cm based on an assumed precision of 2 cm for the gauges. The accuracy of the regional mission biases is strongly dependent on the mean sea surface heights near the stations. The most consistent biases are obtained based on the CLS2011 model with mission dependent accuracies from 1.3 to 3.4 cm. Hence, the GNSS-controlled tide gauges operated operationally by WSV might complement the calibration and monitoring activities at dedicated CalVal stations.

Multidecadal Sea-level Rise in the Southeast Indian Ocean: The Role of Ocean Salinity Change

Regional sea-level rise in the Southeast Indian Ocean (SEIO) exerts growing threats to the surrounding Australian and Indonesian coasts, but the mechanisms of sea-level rise have not been firmly established. By analyzing observational datasets and model results, this study investigates multidecadal steric sea-level (SSL) rise of the SEIO since the mid-20th century, underscoring a significant role of ocean salinity change. The average SSL rising rate from 1960 through 2018 was 7.4±2.4 mm decade−1, and contributions of the halosteric and thermosteric components were ~42% and ~58%, respectively. The notable salinity effect arises primarily from a persistent subsurface freshening trend at 400-1000 m depths. Further insights are gained through the decomposition of temperature and salinity changes into the Heaving (vertical displacements of isopycnal surfaces) and Spicing (density-compensated temperature and salinity change) modes. The subsurface freshening trend since 1960 is mainly attributed to the Spicing mode, reflecting property modifications of the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the southern Indian Ocean. Also noteworthy is a dramatic acceleration of SSL rise (20.3±7.0 mm decade−1) since ~1990, which was predominantly induced by the thermosteric component (16.3±5.5 mm decade−1) associated with the Heaving mode. Enhanced Ekman downwelling by surface winds and radiation forcing linked to global greenhouse-gas warming mutually caused the depression of isopycnal surfaces, leading to the accelerated SSL rise through thermosteric effect. This study highlights the complexity of regional sea-level rise in a rapid-changing climate, in which the role of ocean salinity is vital and time-varying.

Ocean mass, sterodynamic effects, and vertical land motion largely explain US coast relative sea level rise

Regional sea-level changes are caused by several physical processes that vary both in space and time. As a result of these processes, large regional departures from the long-term rate of global mean sea-level rise can occur. Identifying and understanding these processes at particular locations is the first step toward generating reliable projections and assisting in improved decision making. Here we quantify to what degree contemporary ocean mass change, sterodynamic effects, and vertical land motion influence sea-level rise observed by tide-gauge locations around the contiguous U.S. from 1993 to 2018. We are able to explain tide gauge-observed relative sea-level trends at 47 of 55 sampled locations. Locations where we cannot explain observed trends are potentially indicative of shortcomings in our coastal sea-level observational network or estimates of uncertainty.

Decadal sea-level variability in the Australasian Mediterranean Sea

Strong regional sea-level trends, mainly related to basin-wide wind stress anomalies, have been observed in the western tropical Pacific over the last 3 decades. Analyses of regional sea level in the densely populated regions of the neighbouring Australasian Mediterranean Sea (AMS; also called tropical Asian seas) are hindered by its complex topography and respective studies are sparse. We used a series of global eddy-permitting ocean models, including a high-resolution configuration that resolves the AMS with 120∘ horizontal resolution, forced by a comprehensive atmospheric forcing product over 1958–2016 to characterize the patterns and magnitude of decadal sea-level variability in the AMS. The nature of this variability is elucidated further by sensitivity experiments with interannual variability restricted to either the momentum or buoyancy fluxes, building on an experiment employing a repeated-year forcing without interannual variability in all forcing components. Our results suggest that decadal fluctuations of the El Niño–Southern Oscillation (ENSO) account for over 80 % of the variability in all deep basins of the region, except for the central South China Sea (SCS). Changes related to the Pacific Decadal Oscillation (PDO) are most pronounced in the shallow Arafura and Timor seas and in the central SCS. On average, buoyancy fluxes account for less than 10 % of decadal SSH variability, but this ratio is highly variable over time and can reach values of up to 50 %. In particular, our results suggest that buoyancy flux forcing amplifies the dominant wind-stress-driven anomalies related to ENSO cycles. Intrinsic variability is mostly negligible except in the SCS, where it accounts for 25 % of the total decadal SSH variability.