Search Results

You are looking at 1 - 4 of 4 items for

  • Author or Editor: Nadya T. Vinogradova x
  • All content x
Clear All Modify Search
Nadya T. Vinogradova and Rui M. Ponte

Abstract

Calibration and validation efforts of the Aquarius and Soil Moisture and Ocean Salinity (SMOS) satellite missions involve comparisons of satellite and in situ measurements of sea surface salinity (SSS). Such estimates of SSS can differ by the presence of small-scale variability, which can affect the in situ point measurement, but be averaged out in the satellite retrievals because of their large footprint. This study quantifies how much of a difference is expected between in situ and satellite SSS measurements on the basis of their different sampling of spatial variability. Maps of sampling error resulting from small-scale noise, defined here as the root-mean-square difference between “local” and footprint-averaged SSS estimates, are derived using a solution from a global high-resolution ocean data assimilation system. The errors are mostly <0.1 psu (global median is 0.05 psu), but they can be >0.2 psu in several regions, particularly near strong currents and outflows of major rivers. To examine small-scale noise in the context of other errors, its values are compared with the overall expected differences between monthly Aquarius SSS and Argo-based estimates. Results indicate that in several ocean regions, small-scale variability can be an important source of sampling error for the in situ measurements.

Full access
Nadya T. Vinogradova and Rui M. Ponte

Abstract

The Aquarius/Satelite de Aplicaciones Cientificas-D (SAC-D) salinity remote sensing mission is intended to provide global mapping of sea surface salinity (SSS) fields over the next few years. Temporal and spatial averages of the satellite salinity retrievals produce monthly mean fields on 1° grids with target accuracies of 0.2 psu. One issue of relevance for the satellite-derived products is the potential for temporal aliasing of rapid fluctuations into the climate (monthly averaged) values of interest. Global daily SSS fields from a data-assimilating, eddy-resolving Hybrid Coordinate Ocean Model (HYCOM) solution are used to evaluate whether the potential aliasing error is large enough to affect the accuracy of the SSS retrievals. For comparison, salinity data collected at a few in situ stations over the tropical oceans are also used. Based on the HYCOM daily series, over many oceanic regions, a significant part of the total salinity variability is contributed by rapid fluctuations at periods aliased in the satellite retrievals. Estimates of the implicit aliasing error in monthly mean salinity estimates amount to 0.02 psu on average and >0.1 psu in some coastal, tropical, western boundary current, and Arctic regions. Comparison with in situ measurements suggests that HYCOM can underestimate the effect at some locations. While local aliased variance can be significant, the estimated impact of aliasing noise on the overall Aquarius system noise is negligible on average, when combined with effects of other instrument and geophysical errors. Effects of aliased variance are strongest at the shortest periods (<6 months) and become negligible at the annual period.

Full access
Nadya T. Vinogradova and Rui M. Ponte

Abstract

Unprecedented changes in Earth’s water budget and a recent boom in salinity observations prompted the use of long-term salinity trends to fingerprint the amount of freshwater entering and leaving the oceans (the ocean water cycle). Here changes in the ocean water cycle in the past two decades are examined to evaluate whether the rain-gauge notion can be extended to shorter time scales. Using a novel framework it is demonstrated that there have been persistent changes (defined as significant trends) in both salinity and the ocean water cycle in many ocean regions, including the subtropical gyres in both hemispheres, low latitudes of the tropical Pacific, the North Atlantic Subpolar Gyre, and the Arctic Ocean. On average, the ocean water cycle has amplified by approximately 5% since 1993, but strong regional variations exist (as well as dependency on the surface freshwater flux products chosen). Despite an intensified ocean water cycle in the last two decades, changes in surface salinity do not follow expected patterns of amplified salinity contrasts, challenging the perception that if it rains more the seas always get fresher and if it evaporates more the seas always get saltier. These findings imply a time of emergence of anthropogenic hydrological signals shorter in surface freshwater fluxes than in surface salinity and point to the importance of ocean circulation, salt transports, and natural climate variability in shaping patterns of decadal change in surface salinity. Therefore, the use of salinity measurements in conjunction with ocean salt fluxes can provide a more meaningful way of fingerprinting changes in the global water cycle on decadal time scales.

Full access
Nadya T. Vinogradova, Rui M. Ponte, Katherine J. Quinn, Mark E. Tamisiea, Jean-Michel Campin, and James L. Davis

Abstract

The oceanic response to surface loading, such as that related to atmospheric pressure, freshwater exchange, and changes in the gravity field, is essential to our understanding of sea level variability. In particular, so-called self-attraction and loading (SAL) effects caused by the redistribution of mass within the land–atmosphere–ocean system can have a measurable impact on sea level. In this study, the nature of SAL-induced variability in sea level is examined in terms of its equilibrium (static) and nonequilibrium (dynamic) components, using a general circulation model that implicitly includes the physics of SAL. The additional SAL forcing is derived by decomposing ocean mass anomalies into spherical harmonics and then applying Love numbers to infer associated crustal displacements and gravitational shifts. This implementation of SAL physics incurs only a relatively small computational cost. Effects of SAL on sea level amount to about 10% of the applied surface loading on average but depend strongly on location. The dynamic component exhibits large-scale basinwide patterns, with considerable contributions from subweekly time scales. Departures from equilibrium decrease toward longer time scales but are not totally negligible in many places. Ocean modeling studies should benefit from using a dynamical implementation of SAL as used here.

Full access