Four years ago we reported on the first of a series of 5 oceanographic cruises over the period 2012 - 2013 seeking to investigate the salinity structure and balance in a high salinity, high evaporation, and low rainfall region of the subtropical North Atlantic (Figure 1). Findings from this SPURS1 field study, focused within a 900 x 800-mile square study area centered at 25N, 38W, have yielded fundamental new insights into the physical processes underlying the maintenance of this subtropical surface salinity maximum 1 but also their role in influencing the water cycle and patterns of rainfall much further afield 2,3. All SPURS1 data and associated documentation have since been archived and are available from the PO.DAAC.
Recently, on 19 August, the R/V Revelle began survey operations that will run through the middle of September 2016 at the site of the SPURS2 study region in a high rainfall region the Eastern Tropical Pacific aiming to make an equally important scientific splash. This is the second phase of the NASA-funded Salinity Processes in the Upper Ocean Regional Study (SPURS) project, a science process study and series of oceanographic field campaigns that aim to elucidate key mechanisms responsible for near-surface salinity variations in the oceans. Employing a suite of advanced sampling technologies coupled to regional circulation models, SPURS seeks to quantify the relative significance of circulation, evaporation, precipitation over a range of scales for representative areas of the open ocean. In so doing, it addresses the fundamental role the ocean plays in global water cycle budgets and its relationship to climate.
The ongoing Revelle cruise is the first of up to three SPURS2 cruises spanning a complete annual cycle in 6-month intervals focused within a 3x3 degree survey areas centered at 10N, 125W. It aims to better understand physical processes that influence upper-ocean salinity in a low-salinity, precipitation dominated region of the eastern tropical Pacific (ETP). This area is heavily influenced by rainfall associated with the intertropical convergence zone (ITCZ; Figure 2). It has a shallow thermocline and strong seasonally-varying currents, making sampling a challenge. SPURS leverages satellite salinity measurements from SMAP, SMOS and other remote sensing datasets (e.g. SST, altimetry, scatterometry, precipitation). A nested sampling design will examine processes on three different scales:
- Large Scale: The purpose here is to characterize processes that freshen and salinity the upper ocean in the ITCZ region and connect it to the eastern Pacific fresh pool.
- Mesoscale: A smaller observational region surrounding the central SPURS-2 site will resolve scales from 10-300 km. This scale includes eddies which act to stir the salinity field. Numerical modeling studies will be especially important at this scale.
- Small-scale: The smallest scales to be observed are less than 10 km. Precipitation is expected to be locally patchy at this scale. SPURS-2 especially focuses on how intense rainfall bursts are dispersed vertically and horizontally.
The objective of the SPURS2 field campaign is to answer the following key science questions:
- What is the fate of the rainwater, and by what oceanic processes is the freshwater transformed from shallow, patches of freshwater into the large-scale mean distribution of salinity that makes up the Eastern Pacific Fresh Pool?
- What impact does horizontal and vertical patchiness in salinity have on the performance of satellite-based measurements of sea surface salinity (SSS)?
- What are the local and broader-scale effects of the freshwater flux on the ocean?
- How does the ocean SSS impact the feedbacks on the atmosphere?
A suite of in situ (field) observations will be guided by a data-assimilating, regional circulation model to address these questions within this high variability, dynamic region. This integrated sensor web is comprised of the following components (Figures 3 & 4):
Complementing ongoing observations from the TAO/TRITON mooring array, SPURS2 will deploy three moorings arranged meridionally along 125°W. The WHOI Central mooring at 10N, 125W and 2 PMEL mooring (north & south) will provide air-sea flux measurements (freshwater, heat, and momentum) and data on upper-ocean temperature, salinity, and velocity structure.
Lagrangian instruments including ARGO profiling floats, neutrally buoyant floats, SVPS Drifters, will be deployed within the broader survey domain to resolve salinity gradients
Autonomous Mobile (Eulerian) Assets
Wave Gliders and Seagliders with their onboard CTDs will be used to monitor the meridional thermohaline structure within an inner 70x70km sampling area of the SPURS2 domain that they will be patrolling.
Research vessel observations will include measurements of ocean temperature, salinity, and velocity structure plus rainfall and atmospheric flux measurements at a variety of scales. They include the use of conductivity-temperature-depth (CTD) sections, thermosalinographs (TSG), a meterological flux package and some novel techniques for surface water and atmosphere monitoring: “Salinity Snake”, towed Surface Salinity Profiler, LTAIRS (Lighter-than-Air IR System (balloon), CFT Controlled Flux Technique (CO2 laser heating surface patch viewed with IR camera). A further innovative aspect of SPURS2 is the use of the sailing boat “Lady Amber” (Figure 5) for on-going monitoring of the domain (7 trips planned during SPURS2) and agile sensor deployment and retrieval.
Regional and Local Modeling
SPURS2 will be a fully integrated observing and modeling program. Before and during the field experiment, SPURS2 will use models to perform Observing System Simulation Experiments (OSSEs) to aid the observing system design. Coupled ocean-atmosphere models on a variety of scales to will be used in support of SPURS2 synthesis activities.
SPURS2 is the next chapter of an innovative NASA field campaign program that will provide a further important piece of the informational puzzle relating to oceanic salinity variability and its implication for the earth’s water cycle. Follow the story of the SPURS2 field campaign as it evolves with its discoveries in the making via the cruise blog of NASA physical oceanography program manager, Dr. Eric Lindstrom, as he reports daily from aboard the R/V Revelle through late September. And visit the SPURS project website and PO.DAAC SPURS mission page for periodic updates on SPURS2 and access to the data.
PO.DAAC Science Team