Satellite Altimetry Calibration and Validation
High precision satellite altimeter missions including TOPEX/Poseidon (T/P), Jason-1 and OSTM/Jason-2, have contributed fundamental advances in our understanding of oceans role in the Earth's climate system. Altimeters measure the height of the ocean surface at unparalleled spatial and temporal resolution – these measurements capture global to regional scale variability in sea level, yielding information relating to ocean circulation and sea level rise.
The determination of changes in global mean sea level (GMSL) is of fundamental importance in understanding the response of the ocean to a continuing warming climate – both through thermal expansion of the ocean, discharge and melting of the major ice sheets in Greenland and Antarctica, melting of mountain glaciers, and through the redistribution of water over the continents and atmosphere. As with all scientific observations, it is vital that the measurement tool is operating within its specifications – calibration and validation is therefore an important component that is overseen by the mission agencies (NASA, CNES, NOAA, EUMETSAT) via the Ocean Surface Topography Science Team (OSTST).
This IMOS sub-facility provides the sole southern hemisphere calibration data stream directly to the OSTST. This contribution, importantly, involves in situ data that spans multiple altimeter missions, thus enabling the assimilation of multi-mission satellite data to determine ongoing changes in sea level with flow-on effects to other uses of this data. Also provided is a data stream generated by the comparison of altimeter data with the global tide gauge network. These data are focused on characterising the evolution of the GMSL trend observed by satellite altimetry. These data assist the OSTST to collectively oversee the operation and validation of the precision altimetry missions – this ensures each of these missions is performing as accurately as possible in order to meet mission objectives.
Instrumentation and Data
The IMOS supported calibration site in Australia is one of four primary in situ calibration/validation sites that contribute to the Ocean Surface Topography Science Team (OSTST). The remaining sites include Harvest (USA), Corsica (France), and Gavdos (Greece).
The Australian calibration site includes two comparison points where in situ data is compared against the altimeter – Bass Strait and Storm Bay. These two locations both lie on descending (N -> S) pass 088 of the satellite altimeter, and thus share similar satellite orbit characteristics. The use of these two sites allows detailed investigation into the accuracy of the altimeter over two regions with distinctly different oceanographic conditions.
Instrumentation at each site consists of moored oceanographic sensors that observe ocean bottom pressure, temperature, salinity and current velocity through the water column. Episodic deployment of GPS equipped buoys at each site enables the realisation of an absolute datum that enables the sea level data to be directly compared to the altimeter. The primary data stream provided to the OSTST is a time series of “absolute bias” – this records the evolving differences between altimeter and in situ sea level. A second data stream known as “bias drift” investigates the accuracy of the trend in the sea level by comparing altimeters against the global tide gauge network.
Application of Data
- In June 2017, our satellite altimeter time series was again the focus of a paper in the journal Nature Climate Change, with a commentary also published in The Conversation. In this paper, the altimeter records from various laboratories, including ours, were analysed in an attempt to improve the instantaneous closure of the sea-level budget (i.e. the agreement between the total observed signal and the sum of the observed constituent components), over the satellite era. The main findings were:
- The mass contributions to global mean sea level (i.e. the contributions from the Antarctic and Greenland ice sheets, mountain glaciers, and terrestrial water storage) increase from 45% in 1993 to 70% of the total in 2014.
- The largest and statistically significant increase in contributions to GMSL comes from the contribution from the Greenland ice sheet, which is less than 5% of the GMSL rate during 1993 but more than 25% during 2014.
- This approximate but improved closure of the sea-level budget over the satellite era is progress with respect to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. The result increases confidence in our altimetry observations (including the associated calibration and validation supported by IMOS) and improves our understanding of recent changes and emerging increases to the rate of sea-level rise.
- In May 2015, our bias drift contribution to the OSTST was published in a new paper in the journal Nature Climate Change , together with a commentary in The Conversation. This work investigates the altimeter derived trend in global mean sea level by comparing altimeter data with sea level observations from the global tide gauge network. There are three main findings:
- First, we confirm that sea levels have risen faster over the altimeter era compared with proceeding decades over the twentieth century as observed by tide gauges.
- Second, after taking into account land motion, our results suggest that the first six years of the altimeter record slightly overestimates the sea-level trend. Our revised estimate over the full altimeter period (1993-mid 2014) is +2.6-2.9 mm/yr (with the exact value depending on the method used to estimate vertical land motion), slightly less than the previous estimate of +3.2 mm/yr.
- Third, without corrections for bias-drift, the previous record showed a slower rate of sea-level rise over the most recent decade, relative to the proceeding decade. Our revised record is significantly different and suggests that the rate has actually increased in line with accelerating contributions from the Greenland and West Antarctic ice sheets. Our acceleration is also consistent with the acceleration determined from the IPCC sea level projections.
The IMOS team is one of a number of international groups that actively participate in the process of validating and further understanding this important climate data record.
Satellite Remote Sensing Facility Publication Report - If you have any questions regarding the data, or corrections, or would like to add a publication or presentation that uses IMOS data please contact the IMOS office via email: publication(at)emii.org.au.
Dr Christopher Watson (UTAS)
Dr Benoit Legresy (CSIRO)
Your access to raw data from the IMOS oceanographic moorings deployed by the satellite altimetry calibration and validation sub-facility is through the Australian Ocean Data Network (AODN) Portal.