Weighing the Ocean

How much water is entering the oceans? This proposal is aimed at providing an answer to this question. Knowledge of mass exchange between the continents and the oceans is fundamental to understanding and interpreting sea level rise. However, constraining this value has been very difficult until the launch of the Gravity Recovery and Climate Experiment (GRACE) satellite mission. GRACE provides a unique global view of the motion of water across the surface of the planet. Several studies have used GRACE data to estimate the mass gain in the oceans over its lifetime (2002-present). Unfortunately, the gravity changes that GRACE measures are due not only to water entering the ocean, but also due to the ongoing motion of the solid earth caused by the last ice age, a process called glacial isostatic adjustment (GIA). In an odd coincidence, the ocean-averaged, apparent mass trend caused by GIA is nearly equal in magnitude and opposite in sign to the trend of freshwater entering the ocean from the continents. As the ocean-averaged GIA value can only be modelled, it introduces a large uncertainty to the interpretation of the ocean-averaged GRACE results.

Another independent method of measuring the mass change is to observe the change in pressure at the bottom of the ocean. The mass component of water entering the ocean spreads quickly worldwide, thus increasing bottom pressure everywhere. There are two complications, however. First, there are many dynamic processes in the ocean that also impact bottom pressure. For sea level, these dynamic changes often are much larger than the average change in sea level height caused by the water flux. For bottom pressure, though, the variation due the dynamic signals is much smaller than that of sea level, and in certain regions, such as near the equator, it is expected they may even be smaller than the pressure change due to addition of mass into the ocean. Second, the water entering the oceans does not distribute uniformly because the shifting position of the mass causes crustal motion and changes in gravity. These changes produce unique patterns of bottom pressure change, which we refer to as the static ocean response to differentiate it from the dynamic processes.

This proposal will test the hypothesis that bottom pressure measurements can provide a useful measure of the mass entering the ocean. To assess this possibility, we will complete four tasks. First, we gather existing bottom pressure data and generate bottom pressure fields from the GRACE satellite mission data. Second, we compare a number of dynamic ocean model results to gauge our knowledge of the dynamic bottom pressure signal and the variability of this measurement. Third, we will generate a number estimates of the static bottom pressure field, caused by change of water on land and the ongoing response of the earth to the last ice age, to explore the spatial and temporal variability expected due to these sources. Finally, we will combine the previous results to evaluate our ability to extract the globally-averaged mass change at the short time scales (monthly to annually) that are accessible with the available data. In addition, this task will also determine the ideal set of bottom pressure measurements that are needed to estimate the global average on longer time scales. If we find that the bottom pressure measurements are useful, we will create a proposal to the global observing systems that bottom pressure should be added as a component that complements the GRACE mission in the same manner as tide gauges complement altimetry.

The Academic Beneficiaries section details the academic impacts of the proposed work. They will be summarised at the end for completeness. In addition, wider communities will benefit from the research.

i) Feedback to policy makers regarding sea level change: Sea level change will have a significant impact on society. The results from this study will help scientists to better understand the processes contributing to sea level rise. We will ensure that the information gained from this project, placed in appropriate context, is communicated to policy makers through two channels. First, we will ensure that the results are known to the IPCC assessments. Tamisiea has been involved with the Fourth Assessment Report, and has participated in meetings leading up to the Fifth Assessment. The aggregation of our results with those of the wider community greatly increases the value of the research to policy makers. Second, we will incorporate the results into the ongoing communication of groups at NOC, such as the National Sea Level and Tidal Facility (NTSLF), with government departments, such as DEFRA and the Environment Agency. NOC also contributes to the Marine Climate Change Impacts Partnership's Annual Report Card.
In May 2010, Hughes advised a researcher from the Parliamentary Office of Science and Technology who was preparing a high-level summary from MPs on sea level change.

ii) Teachers: The videos and animations we develop will be aimed at a general level and will make excellent supplementary material for teachers who want to address the subject of sea level change. Both Hughes and Tamisiea have recent experience in passing information on current areas of research onto `A' Level teachers, and will utilise these past experiences and contacts to publicise the availability of these products.

iii) General Public: While the media frequently focus on the global average of sea level rise, the regional values of sea level change are what will impact society. The videos produced will introduce these issues and show some of the systems put in place to observe sea level change. The videos will be distributed via YouTube and be available from websites of NOC and the Research Centre for Marine Sciences and Climate Change at the University of Liverpool. We will work with the NOC Communications and Public Engagement department to publicise the availability of the videos.

Summary of the Academic Beneficiaries:
i) The sea level community obtains a more robust estimate of the mass entering the ocean, which provides greater confidence on interpreting the regional sea level variations that have the greatest impact on society.
ii) The geodetic community can use the estimate of the oceans changing mass in studies to better understand geocenter motion. Results from these studies could improve GPS and altimetry results, which are impacted by geocenter motion.
iii) The oceanographic community can use the bottom pressure data and variability derived from the static and dynamic ocean models in process studies and data assimilation procedures.
For i) - iii), the results of the study will be distributed via conference presentations and journal articles.
iv) The global observing systems can use the proposed bottom pressure measurement system to cheaply and quickly obtain a new observation that contributes to achieving the goals of the overall observing system. As a representative to GGOS for the PSMSL, Tamisiea can feed the results of the research into the system.
v) The Permanent Service for Mean Sea Level (PSMSL) will gain a set of processed and quality controlled bottom pressure data for distribution the scientific communities. The PSMSL will widely publicise to various communities that these data sets are available.
vi) Scientists in general will benefit from animations produced as part of the pathways to impact to better illustrate the causes behind regional sea level change. We will publicise the availability on various mailing lists.