Introduction

NOAA supports the collection of sea surface salinity (SSS) and sea surface temperature (SST) data from thermosalinographs (TSG) installed on ships of the NOAA fleet and ships of the Ship Of Opportunity Program (SOOP). Both SSS and SST observations from TSGs are part of the Global Ocean Observing System.

The global atmospheric and oceanic observations, including TSG observations from ship under the SOOP and research vessels, have been the foundation for understanding long-term changes in marine climate and are essential input to climate and weather forecast models.

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Objectives within the Global Ocean Observing System

NOAA is improving meteorological measurement capabilities on the SOOP and NOAA fleet for enhance marine weather and climate forecasting in general, and concentrate on a specific subset of high accuracy SOOP transects to be frequently repeated and sampled at high resolution for systematic upper ocean and atmospheric measurement. This task will support climate services by providing ocean and atmosphere measurements needed to:

• Document heat uptake, transport, and release by the ocean.
• Determine ocean carbon sources and sinks (carbon sampling instrumentation is detailed under a separate task below)
• Study air-sea exchange of water and the ocean's overturning circulation.
• Validate future SSS satellite missions.

(From the NOAA Program Plan "Building a Sustained Ocean Observing System for Climate", Office of Climate observations, March 2006).

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Sea surface salinity

Sea surface salinity is a critical parameter that needs to be investigated to meet climate research goals because:

• Approximately 90% of global evaporation and 80% of global precipitation occur over the oceans. Therefore, SSS is the key variable for understanding how the variability of global precipitation, evaporation, and the water cycle. with SSS variability being the key tracer for fresh water input to -and output from- the ocean associated with precipitation, evaporation, ice melting, and river runoff.
• With temperature, salinity determines seawater density and buoyancy, driving the extent of ocean stratification , mixing , and water mass formation SSS will aid to identify how climate variation induces changes in global ocean circulation as SSS and sea surface temperature together determine seawater density, regulate ocean circulation and the formation of water masses.
• It is utilized to initialize climate forecast models, particularly when salinity profiles are not available, and as computer models evolve, a more adequate measure of the variability of sea surface salinity will provide the essential SSS data needed to link the two major components of the climate system : the water cycle and ocean circulation.
• It is linked to the amount of dissolved CO₂ in the oceans through its relationship with alkalinity.
• It can be used to calibrate and/or assess paleo-salinity time series.
• It can be used to assess numerical model performance.

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How TSG and float observations complement each other

Profiling floats currently provide the largest number of sea surface (5 m deep) salinity observations with global coverage in an approximate 3x3 grid. Thermosalinographs are simple, automatically operated, able to transmit data in real-time, long lasting, inexpensive instruments that measure SSS with an accuracy of 0.02. TSG data consist of alongtrack SSS and SST, usually recorded every 10 seconds.

TSG-derived SSS and SST observations complement data from profiling floats since:

1. TSG observations have excellent spatial resolution (approximately 3 km for quality controlled data) along the track of the ship, providing the only means to precisely locate surface ocean fronts based on simultaneous salinity and temperature observations.
2. Ship transects, and therefore the location of TSG observations, are not biased by a low number of observations in regions of ocean divergence
3. TSGs do not provide unusually large number of observations in regions of high eddy activity as profiling floats do, since floats sometimes remain in warm or cold rings in regions of high eddy activity.
4. TSGs can be calibrated once a drift (or trend) is identified in their signal potentially providing more reliable observations than profiling floats, whose data need to be calibrated using climatology observations once a drift is detected

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