Key Science Results Abstracts

 Key science results abstracts

Aquarius Satellite Salinity Measurement Mission Status, and Science Results from the initial 3-Year Prime Mission
Contact author: Gary Lagerloef, <Lager@esr.org>
All Authors:
Gary Lagerloef, Earth and Space Research
Hsun-Ying Kao, Earth and Space Research

The Aquarius measurement objectives are to describe unknown features in the sea surface salinity (SSS) field, and document seasonal and interannual variations on regional and basin scales. This presentation will first describe the structure of the mean annual global salinity field compared with the previous in situ climatology and contemporary in situ measurements , including small persistent biases of opposite sign in high latitudes versus low latitudes, currently under intense investigation, as well as global and regional error statistics. Then we summarize highlights of various studies and papers submitted to the JGR-Oceans special section on satellite salinity (2014). The most prominent seasonal variations, most notably the extant and variability of the SSS signature of the Atlantic and Pacific inter-tropical convergence zones, Amazon-Orinoco and other major rivers, and other important regional patterns of seasonal variability. Lastly we will examine the trends observed during the three Sep-Aug measurement years beginning Sep2011, Sep2012 and Sep2013, respectively, in relation to ENSO and other climate indices, as the first step in analyzing interannual SSS variability. An outline for extended mission operations beyond the initial three-year prime mission will be presented.

Broadly Sharing the Importance of Ocean Salinity
Contact author: Annette deCharon, <annette.decharon@maine.edu>
All Authors:
Annette deCharon, University of Maine
Carla Companion, University of Maine
Ryan Cope, University of Maine
Lisa Taylor, University of Maine

NASA’s Aquarius instrument and Salinity Processes in the Upper Ocean Regional Study (SPURS) have given the scientific community unprecedented insight into salinity’s role in the earth system. Synergistic efforts by the University of Maine-based public engagement team have focused on themes of the water cycle, ocean circulation and climate. Resources highlighting salinity’s ties to these familiar topics have been disseminated through Aquarius and SPURS webinars and workshops. The direct involvement of research scientists and engineers has been key to the success of these endeavors, collaboratively working with experienced communicators to produce content that is appropriate for nonscientists.
Deconstruction in various forms, from mapping out complex science to simplifying visual materials, has been instrumental in aiding audience understanding of salinity science, technology, engineering, and mathematics (STEM) concepts. As a result, classroom educators and their students have been given direct access to innovative STEM content, cost-free software, and vetted learning resources. Moreover, these audiences have acquired insights into the critical thinking used by scientists and engineers to solve real-world problems.
By transitioning from in-person workshops to webinars, Aquarius and SPURS communication teams have reached broad audiences. English- and Spanish-language webinar events directly engaged 511 people in 38 U.S. states/territories and 13 non-U.S. countries, 8 of which are located in Central or South America. Argentina had the highest percentage (41%) of non-U.S. participation, likely because NASA’s salinity sensor, Aquarius, is onboard SAC-D, which was built and is operated by Argentina’s Space Agency (CONAE). The webinars’ videos and archived pages have been viewed over 22,700 times by people in almost 80 countries, providing long-term access and greatly expanding the reach of salinity-related content.

Aquarius SSS-max and SSS-min views of the Sea
Contact author: Arnold L. Gordon, <agordon@ldeo.columbia.edu>
All Authors:
Arnold Gordon, Lamont-Doherty Earth Observatory

For decades oceanographers have had global near synoptic time series views of the sea surface from space, including, in addition to visible images, temperature, height, color, sea ice, wind and marine rainfall patterns, all of which have dramatically expanded our knowledge of the ocean condition, enabling quantitative investigating of ocean processes and their coupling to the atmosphere/climate and marine ecosystems. But only recently have we gathered from orbiting satellites views of the sea surface salinity (SSS), with the advent of ESA’s Soil Moisture and Ocean Salinity (November 2009 launch) and of NASA/CONAE Aquarius/SAC-D satellite mission (June 2011 launch). As salinity is a seawater equation-of-state parameter, an active component of ocean dynamics, and an indicator of the marine hydrological cycle, the SSS global near synoptic time series is revolutionizing our understanding of the ocean and earth’s climate system. The number of research papers utilizing satellite SSS data is expanding at an ever increasing rate; discoveries are being made and many more will be forthcoming as the SSS time series grows, capturing the interannual and decadal scales of variability. The satellite data reveal river plume patterns within the ocean; detail the ocean response to evaporation/precipitation forcing, e.g. the SSS-max driven by subtropical evaporation, the SSS-min induced by ITCZ downpours, enabling investigation of the ocean processes coupled to such air-sea forcing. The atmospheric and oceanic processes governing SSS are influenced by regional ocean basin configuration, e.g. subtropical SSS-max patterns and locations display marked differences between the 5 subtropical regimes (North and South Atlantic and Pacific; southern Indian Ocean), each is unique in its own way*. The seas of Southeast Asia, from the Bay of Bengal to the South China Sea standout in global SSS maps by their low SSS, owing to excess precipitation and large river outflow. The low SSS water is exported into the surrounding ocean, to balance the net evaporation of the Arabian Sea and into the tropical zone, to interact with the Indonesian throughflow.
*The differences of the 5 subtropics regions are covered in more detail in an article for the SPURS Oceanography issue: “Differences between the Subtropical Surface Salinity Patterns” by A. L. Gordon, C. F. Giulivi, J. Busecke and F. M. Bingham [in revision]

Where can ocean salinity be used as a rain gauge?
Contact author: Lisan Yu, <lyu@whoi.edu>
All Authors:
Lisan Yu, Woods Hole Oceanographic Institution

The concept of using ocean salinity as a rain gauge to detect the change of the global water cycle has long been proposed but yet materialized. The main obstacle is that ocean salinity and the freshwater flux (namely, the E-P flux) are related through complex upper ocean dynamics, and the ocean advective/mixing processes are often found to have a more influential role than the freshwater flux in explaining the observed SSS variability on seasonal and longer timescales. The correlation between the E-P flux and ocean salinity is typically weak and insignificant in most global oceans but with exception for limited regions under the influence of the ITCZ. In this study I will show that the better correlation in the tropical oceans is due to the existence of the intertropical low-salinity converge zone (LSCZ), identified recently from the satellite sea surface salinity (SSS) retrievals and Argo subsurface salinity observations. The LSCZ is a narrow zone band on either side of the equator, with a SSS front as its surface manifestation. I will show that although the LSCZ owes its source to the ITCZ precipitation, its generation and seasonal migration are dictated by the wind-driven Ekman dynamics. One key evidence is that the center of the LSCZ is locked to the poleward edge of the Ekman convergence zone and moves poleward progressively after the LSCZ is formed at the equatorial latitudes in early spring. In the tropical north Pacific, the LSCZ and ITCZ rain band collocate only during August-October, the time that the Ekman convergence aligns with the rain band. Lastly, I will discuss the connection between the ITCZ precipitation and the low salinity in the LSCZ and the potential implications of the findings for the global water study.

Linking information from sea surface salinity to oceanic freshwater fluxes using near-surface salinity budgets
Contact author: Nadya Vinogradova, <nadya@aer.com>
All Authors:
Nadya Vinogradova, Atmospheric and Environmental Research (AER)
Rui Ponte, Atmospheric and Environmental Research (AER)
Martha Buckley, George Mason University (GMU)

Characterizing freshwater fluxes (FWF) over the oceans is a key element in advancing our understanding of climate change. One way to improve knowledge of these fluxes is to use variations in sea-surface salinity (SSS). However, the problem is challenging because the relationship between SSS and FWF can depend on complex upper-ocean dynamic processes. A common approach to describe such dependencies is to use salinity budget framework and consider the nature of oceanic fluxes in relation to forcing and salinity tendency. Once properly formulated, salinity budget allows one to explore how freshwater fluxes can be expressed as a combination of ocean variables, including SSS. Here we examine global, near-surface salinity budgets computed within dynamically-consistent, constrained estimation of the oceanic state during the last two decades. Dynamical consistency of the solution ensures physically realistic correspondence between FWF and SSS tendencies. The focus is to explore budget closure and the effect of a realistic real freshwater surface boundary condition. Preliminary results show that ocean contribution to salinity variations is particular important in the tropics, high latitudes, and many coastal regions, which may complicate the use of salinity observations as a direct proxy of FWF, at least on timescales from months to years.

Aquarius sea surface salinity measurements bring new understanding to intraseasonal variability in tropical oceans
Contact author: Tong Lee, <tlee@jpl.nasa.gov>
All Authors:
Tong Lee, Jet Propulsion Laboratory

The primary objectives of the Aquarius/SAC-D mission are to map month-to-month variability of sea surface salinity (SSS) to facilitate the studies of the relationships between SSS, ocean circulation, climate variability, and the water cycle with an emphasis on seasonal-to-interannual time scales. However, Aquarius SSS measurements have enabled the investigations of intraseasonal variability, especially those on time scales closer to a month. These include the studies of tropical instability waves (TIWs) in the Pacific and Atlantic Oceans and SSS signature associated with the Madden-Julian Oscillation (MJO) in the tropical Indo-Pacific oceans that are important to ocean dynamics, climate variability, and marine ecosystem. This presentation highlights the new understanding of TIWs and MJO enabled by the applications of Aquarius data. For the Pacific, Aquarius data provided a better view of TIW at the equator because of the large meridional gradient of SSS there, and revealed a shorter dominant period and faster propagation of TIWs at the equator than those at off-equatorial latitudes. These features have significant implications to the physics associated with TIWs at and away from the equator and TIW-mean flow interactions. In the Atlantic, SSS is found to play an essential role in TIW-mean flow interaction. Not accounting for the influence of SSS would underestimate the perturbation potential energy at the surface by a factor of three. For MJO-related oceanic response in the Indo-Pacific oceans, SSS is found to be as important as sea surface temperature in regulating the surface density anomalies. This has strong implications to MJO-related modeling and coupled forecast.

Seasonal to Interannual Variations of the Salinity Fronts Under the Pacific Intertropical Convergence Zone
Contact author: Hsun-Ying Kao, <hkao@esr.org>
All Authors:
Hsun-Ying Kao, Earth and Space Research
Gary Lagerloef, Earth and Space Research

The Pacific Intertropical Convergence Zone (ITCZ) is a zonal band of atmospheric convective instability, clouds and rainfall near the equator. High-resolution sea surface salinity (SSS) measurements from the Aquarius satellite reveals more detail in the band of lower salinity and a sharp front that aligns with the strong ITCZ atmospheric convection. In the upper ocean, the strong salinity gradient (i.e. the salinity front, SF) can be the main contributor for sharp surface density fronts where the sea surface temperature in near homogeneous under the ITCZ. To understand the variations of the SF will help us better understand the variations of the upper ocean dynamics, such as the formation of barrier layers. In this presentation, we analyze the seasonal to interannual variations of the SF with 3 years of data. The associated precipitation and surface velocity are also discussed.

Sea surface salinity variability and ENSO
Contact author: Tangdong Qu, <tangdong@hawaii.edu>
All Authors:
Tangdong Qu, University of Hawaii

This study investigates the sea surface salinity (SSS) variability in the equatorial Pacific using recently available Aquarius and Argo data. For the period of observation from August 2011 to present, Aquarius nicely resolves the SSS front and its zonal displacement along the equator. The on-going collection of Argo data shows high correlations between this SSS front and the existing indices of El Nino, suggesting its potential important role in ENSO evolution. Further analysis reveals that SSS variability in the southeastern tropical Pacific is crucial to identify the type of El Nino. The longitudinal location of the SSS front and the SSS variability in the southeastern tropical Pacific can be used as indices to characterize ENSO events and their types. The processes that possibly control these SSS indices are also discussed.

Estimation of the Barrier Layer Thickness in the Indian Ocean using satellite derived salinity
Contact author: Subrahmanyam Bulusu, <sbulusu@geol.sc.edu>
All Authors:
Subrahmanyam Bulusu, University of South Carolina
Clifford Felton, University of South Carolina
VSN Murty, National Institute of Oceanography
Jay Shriver, Naval Research Laboratory

Monthly barrier layer thickness (BLT) estimates are derived from satellite-derived salinity using a multilinear regression model (MRM) within the Indian Ocean. Sea surface salinity (SSS) from the recently launched Soil Moisture and Ocean Salinity (SMOS) and Aquarius salinity missions are utilized to estimate the BLT. The MRM derived BLT estimates are compared to gridded Argo and Hybrid Coordinate Ocean Model (HYCOM) BLTs. It is shown that different mechanisms are important for sustaining the BLT variability in each of the selected regions in the Indian Ocean. Sensitivity tests show that sea surface salinity is the primary driver of the BLT within the MRM. Results suggest that salinity measurements obtained from Aquarius and SMOS can be useful for tracking and predicting the BLT in the Indian Ocean. BLT estimations using HYCOM simulations display large errors that are related to model layer structure and the selected BLT methodology. The formation of a Barrier Layer can lead to possible feedbacks that impact the atmospheric component of the Madden-Julian Oscillation (MJO), the Arabian Sea mini warm pool and the Indian Ocean Dipole.

Salinity Structure of the Indian Ocean Dipole: Perspectives from Aquarius and SMOS satellite missions
Contact author: Ebenezer Nyadjro, <ebenezer.nyadjro@noaa.gov>
All Authors:
Ebenezer Nyadjro, NOAA PMEL
Bulusu Subrahmanyam, University of South Carolina

We present results on Aquarius and the Soil Moisture and Ocean Salinity (SMOS) observations of the sea surface salinity (SSS) structure during an Indian Ocean Dipole (IOD) event. Comparisons with Argo data show that the satellites are able to resolve the observed SSS pattern in the Indian Ocean despite some challenges in the northern Indian Ocean. Results of box averages for the Java Sumatra Coast (JSC) and South Central Indian Ocean (SCIO) regions show low SSS anomalies in the former and high SSS anomalies in the latter during the 2010 negative IOD event. Analyses of salt flux and salt budget terms suggest that, in the JSC region, salt tendency is an interplay between freshwater forcing and horizontal advection terms, with increased precipitation having a higher impact in driving SSS anomalies than advection. In the SCIO region, advection seems to be more important than the freshwater forcing term.

Modeling Skin-Layer Salinity with an Extended Surface-Salinity Layer
Contact author: Y. Tony Song, <Tony.Song@jpl.nasa.gov>
All Authors:
Y. Tony Song, Jet Propulsion Laboratory
Tong Lee, Jet Propulsion Laboratory
Jae-Hong Moon, Jet Propulsion Laboratory
Tangdong Qu, University of Hawaii
Simon Yueh, Jet Propulsion Laboratory

Due to near-surface salinity stratification, it is problematic to compare satellite-measured surface salinity within the first few centimeters (skin-layer) of the ocean with Argo-measured top-level salinity at 5 m, or with ocean models that do not resolve the skin layer. Although an instrument can be designed to measure the surface salinity, a global scale measurement is currently not available. A regional model can be configured to have a vertical grid in centimeters, but it would be computationally prohibited on a global scale due to time step constraints. Here, we propose an extended surface- salinity layer (ESSL) within a global ocean circulation model to diagnose skin SSS without increasing the computational cost, while allowing comparable solutions with both satellite and Argo salinity at the respective depths. Cross-comparisons with Aquarius and Argo data show that the gridded Aquarius surface salinity has a much stronger seasonal variability than the gridded Argo top-level salinity at 5 m or 10 m, particularly in regions of high precipitation variability, suggesting the exist of strong near-surface vertical salinity stratification. The near-surface stratification is well reproduced by the proposed ESSL model. In comparisons with data-assimilated HYCOM results, the ESSL provides much stronger seasonal variability of SSS, similar to the Aquarius observations. The ESSL solution also provides a useful reference for the global mean SSS to constrain the global calibration constants in Aquarius SSS retrieval.

Interannual Caribbean salinity in satellite data and model simulations
Contact author: Semyon Grodsky, <senya@atmos.umd.edu>
All Authors:
Semyon Grodsky, University of Maryland
Benjamin Johnson, University of Maryland
James Carton, University of Maryland
Frank Bryan, NCAR

Aquarius sea surface salinity (SSS) reveals the presence of interannual salinity variations in the Caribbean with about 0.5 psu change between salty and fresh events. Anomalous SSS propagates westward across the basin at an average speed of 11 cm/s. Fresh and salty events in the Caribbean are preceded by corresponding SSS anomalies east of the Lesser Antilles. These upstream SSS anomalies are produced by interannual changes in the Amazon/Orinoco plume. Their presence is verified using in-situ salinity measurements from the Northwest Tropical Atlantic Station (NTAS). In contrast to salinity, which displays the westward propagation patterns, SST changes almost immediately across the Caribbean, thus suggesting the primarily role of the atmosphere in forcing the interannual SST. A global 1/10 deg mesoscale ocean model is used to quantify possible origination mechanisms of the Caribbean salinity anomalies and their fate. Simulations confirm that they are produced by anomalous horizontal salt advection, which conveys salinity anomalies from the area located east of the Lesser Antilles towards the west across the Caribbean. Anomalous horizontal advection is dominated by mean currents acting on anomalous salinity. The model suggests that Caribbean salt anomalies propagate further, entering the Florida Current and reaching the Gulf Stream in about 6 to 12 months after crossing the central Caribbean. Previous studies link the origin of salinity anomalies in the Amazon/Orinoco plume to variations in the annual freshwater discharge from the continent. Despite the fact that model river discharge doesn’t include interannual variations, the simulated SSS variability is in line with observations. This suggests that interannually forced ocean dynamics plays a key role in river plume variability and its spatial dispersion.

Relating river discharges to salinity changes
Contact author: Xiaosu Xie, <xiaosu.xie@jpl.nasa.gov>
All Authors:
Xiaosu Xie, Jet Propulsion Laboratory
W. Timothy Liu, Jet Propulsion Laboratory
Elizabeth Clark, University of Washington
Yuan Xing, Princeton University

New river discharge data recently produced from terrestrial hydrology models and river gauge measurements, as part of the NASA Energy and Water studies, are brought together with spacebased sea surface salinity measurements by Aquarius and SMOS to demonstrate the role of river discharge in salinity changes near three river mouths: the Mississippi, the Ganges, and the Amazon. The characteristics of the seasonal cycle and the year-to-year changes of the river runoff are described. Various versions of the satellite salinity data are compared. The relative roles of river discharge, surface water flux, and horizontal advection in changing surface salinity in regions near the river mouths are examined. Satellite measurements of SSS clearly track movements of the fresh water from river discharges. Besides the river discharge, E-P plays an important role in the seasonal salinity variation near the Ganges and Irrawaddy River mouths. For the Mississippi and Amazon river mouths, the central and eastern Atlantic ITCZ, E-P contributes very little to the salinity seasonal change. In the central and eastern Atlantic ITCZ, contribution of advection to the salinity tendency is clearly identified. Both salinity and salinity tendency are dominated by semi-annual cycle in the Atlantic ITCZ between 5ºN to 9ºN, whereas annual cycle dominates at other latitudes.

Freshwater Flux from Bay of Bengal, South China Sea and Eastern Indian Ocean and Its Impacts on the Indonesian Throughflow
Contact author: R. Dwi Susanto, <dwi@atmos.umd.edu>
All Authors:
R. Dwi Susanto, University of Maryland
Quanan Zheng, University of Maryland

The proposed research is a pioneering work to determine the freshwater flux from the Bay of Bengal, the South China Sea, and the Eastern Indian Ocean (BBSCSEIO) and its impacts on the Indonesian Throughflow (ITF), using in situ observations and satellite data, especially the Aquarius/SAC-D satellite ocean salinity data. We will collaborate with numerical model scientists to validate the modeled results with our analysis results derived from in situ and remotely sensed data. We hypothesize that freshwater flux from BBSCSEIO play an important role in controlling the vertical stratification and mixing of the ITF in the Makassar Strait and all ITF exit passages into the Indian Ocean. We can make an analogy as if the ITF is a cup of coffee, then the freshwater flux from BBSCSEIO is creamer. Although it is much weaker than the main ITF through the Makassar Strait, it is a dominant factor to adjust the taste & color (stratification and mixing) of the ITF. The ongoing in situ observations in the Karimata Strait will capture freshwater flux from the Bay of Bengal and the South China Sea. Meanwhile, moorings in the Sunda Strait, the Lombok Strait and an array of the Indian Ocean RAMA and Java Upwelling moorings will capture that from the Eastern Indian Ocean (south and west of Java/Sumatra). All the observations will be used to validate and calibrate the satellite ocean salinity data. Because in situ observation is not sustainable due to logistical challenge and high costs, satellite Aquarius/SAC-D data and the follows up mission will be used as a proxy of the heat flux and freshwater flux from the BBSCSEIO. Given the importance of ITF in the global ocean circulation and climate as well as water cycle, it is necessary and important to determine the heat flux and freshwater flux from the BBSCSEIO.

On the Seasonal Cycle of Sea Surface Salinity in the Gulf Stream from Aquarius/SAC-D: A data comparison
Contact author: Jorge Vazquez, <jorge.vazquez@jpl.nasa.gov>
All Authors:
Jorge Vazquez, Jet Propulsion Laboratory
Wenqing Tang, Jet Propulsion Laboratory
Akiko Hayashi
Vardis Tsontos

Three different sea surface salinity (SSS) data sets were used for comparing the annual signal of salinity in the Gulf Stream. These were both the Project version 3.0 SST-adjusted and non-adusted data sets as well as SSS based on the v3.0 Combined Active-Passive (CAP) algoirthm. A simple empirical orthogonal function (EOF) analysis was applied to all three data sets to resolve and attribute key components of the observed variability. All three data sets showed that the first mode accounted for more than 90% of the variability and was associated with the Gulf Stream. All three data sets indicated a magnitude for the annual signal of around 0.5 PSU, with a high in the late winter-early spring time frame. The phase of the annual cycle was compared with SST gradients, OSCAR current speeds, and Wind Speed from CAP to determine if the annual cycle was being driven by advection or air-sea coupling. Maxima in wind speed, SST gradients, and SSS all occurred in the winter to spring period. This indicates that variation in SSS within the Gulf Stream is most likely being driven by changes in evaporation and not advection from the South. An EOF analysis of the covariance between the SSS and wind speed yielded correlations of 0.70, indicative of the coupling between wind speed and salinity. Future work will compare directly with evaporation and precipitation data.