Aquarius Salinity Validation Abstracts

 

Aquarius Salinity Validation

Aquarius SSS optimum interpolation analysis for global and regional studies
Contact author: Oleg Melnichenko, <oleg@hawaii.edu> All Authors:
Oleg Melnichenko, IPRC/SOEST, University of Hawaii
Peter Hacker, HIGP/SOEST, University of Hawaii
Nikolai Maximenko, IPRC/SOEST, University of Hawaii
Gary Lagerloef, Earth and Space Research, Seattle, Washington
James Potemra, HIGP/SOEST, University of Hawaii

A new high-resolution sea surface salinity (SSS) analysis has been produced at the International Pacific Research Center of the University of Hawaii using Aquarius Level-2 (swath) data from September 2011 to August 2014. The primary product is a weekly SSS analysis on a nearly-global 0.5-degree grid. The analysis method is optimum interpolation (OI) that takes into account analyzed errors on the observations, specific to the Aquarius instrument. The method also includes a large-scale correction for satellite biases, filtering of along-track SSS data prior to OI, and the use of realistic correlation scales of SSS anomalies. All these features of the analysis are shown to result in more accurate SSS maps, free from spurious structures. In particular, the method reduces the effects of relative biases between the Aquarius beams, which, if not properly accounted for, result in artificial north-south-striped patterns in mapped SSS fields. Also, the method eliminates most of the large-scale, space- and time-varying satellite biases relative to in-situ data, including spurious annual signals. A statistical comparison between the weekly OI SSS maps and concurrent buoy data demonstrates that the global root-mean-square error of the analysis is smaller than 0.2 psu for nearly all weeks for the 3-year period of comparison. The utility of the OI SSS analysis is also exemplified by the derived patterns of regional SSS variability.

Evaluation of sea surface salinity variability in the East China Sea observed by the Aquarius instrument
Contact author: Seungbum Kim, <seungbum.kim@jpl.nasa.gov>
All Authors:
Seungbum Kim, Jet Propulsion Laboratory
Jaehak Lee, Korea Institute of Ocean Science and Technology
Paolo de Matthaeis, NASA Goddard Space Flight Center

This study demonstrates that the spaceborne Aquarius instrument is able to monitor the sea surface salinity (SSS) variations in the East China Sea (ECS) with the spatial resolution of about 150 km at 7-day interval, where routine observations are difficult. The ascending orbits appear to be affected by unfiltered radiofrequency interference (RFI) much less than the descending tracks. The Aquarius SSS along the ascending tracks is low over the ECS by 0.40 to 0.93 psu (with respect to the in situ CTD data collected during the two separate 7-day periods) and is biased low by 0.41 to 1.07 psu (accuracy, or the time-mean of difference from the regional model along three Aquarius tracks over a 18-month period). The Aquarius data describe well the temporal and spatial variability of the ECS SSS. The temporal trend and magnitude of salinity changes agree remarkably between Aquarius and a regional numerical model, during both the freshwater discharge season from the Yangtze river and the rest of the year. The precision of the Aquarius observation in the ECS is comparable with the Aquarius mission requirement (0.2 psu one-sigma for a monthly average over the open ocean).

Comparison analysis between Aquarius sea surface salinity and World Ocean Database in situ analyzed sea surface salinity
Contact author: James Reagan, <james.reagan@noaa.gov>
All Authors:
James Reagan, CICS-MD/NODC
Tim Boyer, NODC
John Antonov, UCAR/NODC
Melissa Zweng, NODC

A new monthly sea surface salinity (SSS) product calculated from profile data within the World Ocean Database (WOD) is compared and contrasted with Aquarius SSS, both standard and Combined Active-Passive (CAP) products, from September 2011 through September 2013. The comparison examination first compares the new WOD SSS product to Argo-only gridded data where we find reasonable agreement between the two. Next, the gridded WOD SSS monthly fields are compared to the Aquarius fields where zonal and regional biases are found and discussed. In addition to a direct comparison, we also examine the differences in annual cycles between WOD SSS and Aquarius. Furthermore, we examine differences in interannual changes seen by the WOD product and the Aquarius products. Finally, we show that utilizing supplementary hydrographic data (in addition to Argo profiling floats), like that from CTDs, is extremely important in validating Aquarius in regions where little to no Argo data exists (i.e., high latitudes). It is also shown that in some regions data from these supplementary sources are the only reliable SSS measurements we have.

Aquarius SSS space/time biases with respect to Argo data
Contact author: Peter Hacker, <phacker@hawaii.edu> All Authors:
Peter Hacker, U. Hawaii, SOEST, HIGP/IPRC
Oleg Melnichenko, U. Hawaii, SOEST, IPRC
Nikolai Maximenko, U. Hawaii, SOEST, IPRC
James Potemra, U. Hawaii, SOEST, HIGP/IPRC

The Aquarius/SAC-D satellite provides an opportunity to observe near-global sea surface salinity (SSS) with unprecedented space and time resolution not available by other components of the Global Ocean Observing System. In order to evaluate and quantify the potential utility of the SSS data for global and regional studies of SSS variability, our research group has been using the Level-2, three-beam swath data and Argo data to characterize and quantify systematic space/time biases and random errors on a global grid. Despite continuing Level-2 product improvement of Aquarius data over the past three years, significant ascending/descending and inter-beam space/time biases with respect to Argo data persist. Time-mean and annual biases are particularly significant. For the present version 3.0 data, our analyses include quantifying the mean spatial biases for ascending and descending data for each of the three beams (typical range of +/- 0.25 psu), and 3-year mean and time-varying biases for the standard Aquarius and SST-adjusted SSS products from September 2011 to August 2014 (typical range of =/- 0.25 psu). An EOF analysis provides the amplitude and spatial structure for the first three significant components, which account for ~50% of the time-varying Aquarius minus Argo SSS bias. The first two components are annually varying with a 3-month lag; the third component is primarily semi-annual. The amplitude of the annual cycle in the bias field varies spatially from 0-0.2 psu and can be a significant bias compared to the Argo-derived annual cycle regionally.