Description: The ocean is key to understanding societal threats including climate change, sea level rise, ocean warming, tsunamis, and earthquakes. Because the ocean is difficult and costly to monitor, we lack fundamental data needed to adequately model, understand, and address these threats. One solution is to integrate sensors into future undersea telecommunications cables. This is the mission of the SMART subsea cables initiative (Science Monitoring And Reliable Telecommunications). SMART sensors would “piggyback” on the power and communications infrastructure of a million kilometers of undersea fiber optic cable and thousands of repeaters, creating the potential for seafloor-based global ocean observing at a modest incremental cost. Initial sensors would measure temperature, pressure, and seismic acceleration. The resulting data would address two critical scientific and societal issues: the long-term need for sustained climate-quality data from the under-sampled ocean (e.g., deep ocean temperature, sea level, and circulation), and the near-term need for improvements to global tsunami warning networks. A Joint Task Force (JTF) led by three UN agencies (ITU/WMO/UNESCO-IOC) is working to bring this initiative to fruition. This paper explores the ocean science and early warning improvements available from SMART cable data, and the societal, technological, and financial elements of realizing such a global network. Simulations show that deep ocean temperature and pressure measurements can improve estimates of ocean circulation and heat content, and cable-based pressure and seismic-acceleration sensors can improve tsunami warning times and earthquake parameters. The technology of integrating these sensors into fiber optic cables is discussed, addressing sea and land-based elements plus delivery of real-time open data products to end users. The science and business case for SMART cables is evaluated. SMART cables have been endorsed by major ocean science organizations, and JTF is working with cable suppliers and sponsors, multilateral development banks and end users to incorporate SMART capabilities into future cable projects. By investing now, we can build up a global ocean network of long-lived SMART cable sensors, creating a transformative addition to the Global Ocean Observing System.

Description: We propose to establish a new mean sea level observing system that consists of a global array of thousands of drifting buoys tracked by a Global Navigation Satellite System. The recorded height of the sea surface along such Lagrangian trajectories, when averaged over geographical areas, will provide daily estimates of regional and global mean sea levels. This new system will be independent of existing systems, resilient, sustainable, and comparatively economical. Using the example of the historical trajectories of the drifters of the NOAA Global Drifter Program (GDP), we have demonstrated that with the current configuration of the GDP array, global-mean sea-level decadal linear trend estimates with an uncertainty less than 0.3 mm per year could be achieved with daily random error of 1.6 m or less in the vertical direction for each individual drifter daily estimate. To test this requirement, we conducted a pilot project by deploying drifters in a moored configuration at two coastal locations in close proximity of reference tide gauges. We found that a standard GDP drifter equipped with a single-frequency u-blox GPS chipset and onboard processing do not always meet the required vertical accuracy. In contrast, a gold standard drifter equipped with a geodetic-grade GPS chipset continuously recording its dual-frequency raw tracking data from four global navigation satellite systems achieves the required accuracy. As such, a pathway exists to deliver the envisioned new global mean sea-level observing system as an added value to the NOAA Global Drifter Program.

Description: The meridional heat transport (MHT) estimated at 26N in the Atlantic Ocean as part of the RAPID-MOCHA project is one of the prominent observable oceanic indices of the global climate. Understanding the linear relationship between the MHT and other global climate indicators is important for improving our understanding of climate dynamics and providing benchmarks for assessing earth system reanalysis products. The MHT time series exhibits significant correlation with various climate indices but the dynamical interpretation of these correlations is difficult since these indices are typically correlated among themselves. To clarify the implications of these correlations, we investigate the regression patterns between the MHT and field variables from the ERA5 monthly reanalysis dataset. We use a method that allows us to decompose these regression patterns into statistically significant spatio-temporal components that can be subsequently linked to known global climate indices. Our preliminary results suggest that up to 40% of the monthly variance of the MHT can be linearly attributed to three global modes. These modes, explaining 19, 13, and 8 percent of the MHT variance, are respectively strongly related to the Arctic Oscillation index, the Southern Oscillation index, and the Antarctic Oscillation index, which are three indices uncorrelated with each other. Yet, each of these three modes exhibits air-sea heat flux and 2-m atmospheric temperature patterns over northern hemisphere latitudes that are markedly different. This implies that the net impacts on regional climates of MHT changes at any time can vary depending on the global state of the climate system.

Description: The time series of the Atlantic Meridional Overturning Circulation (AMOC) at 26°N has been extended to December 2020 and is close to 17 years long. During the period from 2004 to 2008 the AMOC was about 2.5 Sv stronger than in the following years. Since then, there has been significant interannual variability, but the AMOC has remained relatively weak compared with the first four years of observations. The design of the array was changed in 2020 so that continuous measurements are no longer made over the mid-Atlantic Ridge and in the deep eastern basin. Instead, it is proposed to use data from quinquennial hydrographic surveys to quantify changes in these locations. Here, the extended time series is presented and the impact of the design change on the accuracy of the RAPID timeseries is examined. Other possible design changes are considered too. It is shown that, although the mid-Atlantic ridge measurements have been important in determining the mean structure of the overturning streamfunction, the impact upon the variability of the streamfunction maximum has been small. It is hoped that these changes will enable the measurement of the AMOC at 26°N to be sustained in the future.