SMAP sea surface salinity (RSS and JPL)¶

Soil Moisture Active Passive (SMAP) Sea Surface Salinity — RSS SMAP Level 2C/Level 3 SSS V6.0 and JPL SMAP CAP Level 2B/Level 3 SSS V5.0 (distributed by NASA PO.DAAC)

Available

At a glance¶

Note on period: SMAP science observations began in April 2015. The first RSS V6.0 8-day L3 file is centred on 2015-04-01 (28 March to 5 April 2015) and monthly files start in April 2015; JPL CAP V5.0 8-day L3 files start with the one centred on 2015-05-04. Safe-mode events left no data from 2019-06-19 to 07-23 and from early August to late September 2022 (return to the nominal orbit on 10-06). The RSS release notes give the gaps as 2019-06-17 to 07-25 and 2022-08-06 to 09-21 (no 8-day files), while the PO.DAAC announcements give 2019-06-19 to 07-23 and 2022-08-09 to 10-06, so the documents differ slightly. RSS has no monthly files for July 2019 or August and September 2022.

Overview¶

Sea surface salinity (SSS) from the L-band (1.41 GHz) radiometer on NASA's SMAP satellite. SMAP is a soil moisture mission, but after Aquarius ended in June 2015 it took over NASA's satellite salinity record, and two teams process the same brightness temperatures with different algorithms. The Remote Sensing Systems (RSS) V6.0 product uses an algorithm from the Aquarius family with CCMP winds for the roughness correction, while JPL's CAP (Combined Active-Passive) V5.0 uses a combined algorithm developed for Aquarius that retrieves salinity and wind speed together. Both provide 8-day running-mean and monthly L3 fields on a 0.25° grid through PO.DAAC.

What changes with the successor¶

RSS SMAP SSS V5.0 → RSS SMAP SSS V6.0¶

Transition: 2024-03

Switches the input antenna temperatures to SMAP L1B Version 6 (Piepmeier et al., 2023) and refines several bias corrections. Released in March 2024, with back-processing of the full record finished in September 2024. Until then, users were told to use the V5.3 evaluation release (through the end of 2023), which is identical to V6.0 apart from its V5 L1B input.

Sources: RSS SMAP Salinity V6.0 Release Notes (PDF) · PO.DAAC — V6.0 release announcement (2024-03-26) · PO.DAAC — V5.3 evaluation release announcement (2024-06-07)

Available variables¶

Variable names and units follow the NASA Earthdata catalogue (PO.DAAC) variable tables. Salinity in 1e-3 is practical salinity (same magnitude as psu). RSS and JPL use different file structures and variable names, so take care when combining them.

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

RSS SMAP L3 V6.0 (8-day running mean and monthly, 0.25°)¶

JPL SMAP CAP L3 V5.0 (8-day running mean and monthly, 0.25°)¶

The full variable list is in the official documentation.

How to get the data¶

• NASA PO.DAAC — RSS SMAP L3 8-day V6.0 (SMAP_RSS_L3_SSS_SMI_8DAY-RUNNINGMEAN_V6) — Earthdata Search / earthaccess / OPeNDAP / podaac-data-subscriber
  • Monthly files are SMAP_RSS_L3_SSS_SMI_MONTHLY_V6, swaths SMAP_RSS_L2_SSS_V6 and near-real-time swaths SMAP_RSS_L2_SSS_NRT_V6. A free NASA Earthdata Login is required.
• NASA PO.DAAC — JPL SMAP CAP L3 8-day V5.0 (SMAP_JPL_L3_SSS_CAP_8DAY-RUNNINGMEAN_V5) — Earthdata Search / earthaccess / OPeNDAP / podaac-data-subscriber
  • Monthly files are SMAP_JPL_L3_SSS_CAP_MONTHLY_V5, swaths SMAP_JPL_L2B_SSS_CAP_V5, and near-real-time swaths SMAP_JPL_L2B_NRT_SSS_CAP_V5 and SMAP_JPL_L2B_NRT2_SSS_CAP_V5 (2-hour latency).
• RSS data server (HTTPS) — Direct HTTPS download (netCDF-4, e.g. V06.0/FINAL/L3/8day_running/2026/) No login needed
  • The V6.0 document (Release_V6.0.pdf) and the lists of unused orbits (bad_orbits) are on the same server. FTP access requires free registration (register.remss.com).
• RSS SMAP salinity page — Web (version description, known issues, citation text, preview images)

Example code¶

Downloads a month of RSS V6.0 8-day L3 files with earthaccess and selects the standard salinity (sss_smap) around the Korean Peninsula. Run earthaccess.login() once with a NASA Earthdata account. For JPL CAP, change short_name to SMAP_JPL_L3_SSS_CAP_8DAY-RUNNINGMEAN_V5, the variable to smap_sss, and the coordinates to latitude and longitude.

# pip install earthaccess xarray netCDF4
import earthaccess
import xarray as xr

earthaccess.login()
results = earthaccess.search_data(
    short_name="SMAP_RSS_L3_SSS_SMI_8DAY-RUNNINGMEAN_V6",  # monthly: SMAP_RSS_L3_SSS_SMI_MONTHLY_V6
    temporal=("2025-08-01", "2025-08-31"),
)
files = earthaccess.download(results, "./smap_sss")

ds = xr.open_dataset(files[0])
box = (ds.lat >= 25) & (ds.lat <= 40) & (ds.lon >= 120) & (ds.lon <= 135)  # Korea and the East China Sea
sss = ds["sss_smap"].where(box, drop=True)
print(float(sss.mean()), int(ds["nobs"].where(box, drop=True).sum()))

Practical tips¶

• RSS treats the ~70 km smoothed salinity (sss_smap) as the official product and the 40 km salinity (sss_smap_40km) as experimental, and advises using the much less noisy 70 km fields for most open-ocean work.
• RSS notes that undetected radio-frequency interference (RFI) near Japan, China and Taiwan during certain periods can make salinity too fresh. Treat unusually low values in East Asian coastal seas with suspicion.
• Cells near the coast are dropped or degraded by land contamination. RSS provides no salinity where land contamination is strong and excludes moderate contamination from the standard smoothed product, so narrow seas such as the Yellow Sea and East China Sea have many gaps.
• Right after rain the surface layer stratifies, and SMAP reads fresher than Argo (about 5 m depth) or HYCOM. This can be a difference in measurement depth rather than an error; compare with the rain-filtered sss_smap_RF.
• RSS reports an increasing salty bias near 60°S since 2020, so be careful with long-term trends in the Southern Ocean. Waters colder than 5°C are also flagged as degraded.
• RSS and JPL values differ for the same SMAP observations because their algorithms and ancillary data differ. Do not mix them in one time series, and state in papers which line you used (RSS V6.0 or JPL CAP V5.0).
• Besides the 2019-06 to 07 and 2022-08 to 09 safe-mode gaps, RSS lists days with no L2 files (2015-12-03 and 04, 2016-04-15 and 16, 2017-09-27 and 28, 2017-11-25, 2018-12-16) and days without retrievals because CCMP winds were missing (2017-05-25 and 26, 2020-10-22 and 23). 8-day files that include these days have fewer observations.
• When checked on 2026-10-13, the RSS V6.0 8-day L3 files on PO.DAAC centred between early May and early September 2026 had been regenerated on 2026-09-12, and the May to August 2026 monthly files on 09-29 (overlapping the reprocessing of the SMAP 2026-05-14 to 07-28 geolocation error). If you downloaded these periods earlier, downloading them again is the safe choice.

Notices and change history¶

References for your paper¶

Dataset citation¶

RSS SMAP L3 8-day V6.0 (PO.DAAC) — DOI 10.5067/SMP60-3SPCS

@misc{rss2024smapsss8day,
  author    = {{Remote Sensing Systems}},
  title     = {{SMAP} Sea Surface Salinity Products, Level 3 8-Day Running Mean, Version 6.0},
  year      = {2024},
  publisher = {NASA Physical Oceanography Distributed Active Archive Center},
  version   = {6.0},
  doi       = {10.5067/SMP60-3SPCS}
}

RSS SMAP L3 monthly V6.0 (PO.DAAC) — DOI 10.5067/SMP60-3SMCS

@misc{rss2024smapsssmonthly,
  author    = {{Remote Sensing Systems}},
  title     = {{SMAP} Sea Surface Salinity Products, Level 3 Monthly, Version 6.0},
  year      = {2024},
  publisher = {NASA Physical Oceanography Distributed Active Archive Center},
  version   = {6.0},
  doi       = {10.5067/SMP60-3SMCS}
}

RSS recommended citation (V6.0, product DOI suffixes 2SOCS, 3SPCS and 3SMCS)

@misc{meissner2024smapv6,
  author    = {Meissner, T. and Wentz, F. J. and Manaster, A. and Lindsley, R. and Brewer, M. and Densberger, M.},
  title     = {Remote Sensing Systems {SMAP} Ocean Surface Salinities [Level 2C, Level 3 Running 8-day, Level 3 Monthly], Version 6.0 validated release},
  year      = {2024},
  publisher = {Remote Sensing Systems},
  address   = {Santa Rosa, CA, USA},
  note      = {doi: 10.5067/SMP60-xxxxx (2SOCS: L2C, 3SPCS: L3 8-day, 3SMCS: L3 monthly)}
}

JPL SMAP CAP L3 8-day V5.0 (PO.DAAC) — DOI 10.5067/SMP50-3TPCS

@misc{jpl2020smapcap8day,
  author    = {{JPL}},
  title     = {{JPL} {CAP} {SMAP} Sea Surface Salinity Products, Level 3 8-Day Running Mean, Version 5.0},
  year      = {2020},
  publisher = {NASA Physical Oceanography Distributed Active Archive Center},
  version   = {5.0},
  doi       = {10.5067/SMP50-3TPCS}
}

JPL SMAP CAP L3 monthly V5.0 (PO.DAAC) — DOI 10.5067/SMP50-3TMCS

@misc{jpl2020smapcapmonthly,
  author    = {{JPL}},
  title     = {{JPL} {CAP} {SMAP} Sea Surface Salinity Products, Level 3 Monthly, Version 5.0},
  year      = {2020},
  publisher = {NASA Physical Oceanography Distributed Active Archive Center},
  version   = {5.0},
  doi       = {10.5067/SMP50-3TMCS}
}

Reference papers¶

RSS salinity algorithm (Aquarius V5 and SMAP V3) — DOI 10.3390/rs10071121

@article{meissner2018salinity,
  author  = {Meissner, Thomas and Wentz, Frank J. and Le Vine, David M.},
  title   = {The Salinity Retrieval Algorithms for the {NASA} {Aquarius} Version 5 and {SMAP} Version 3 Releases},
  journal = {Remote Sensing},
  volume  = {10},
  number  = {7},
  pages   = {1121},
  year    = {2018},
  doi     = {10.3390/rs10071121}
}

RSS retrievals near the sea-ice edge — DOI 10.3390/rs13245120

@article{meissner2021seaice,
  author  = {Meissner, Thomas and Manaster, Andrew},
  title   = {{SMAP} Salinity Retrievals near the Sea-Ice Edge Using Multi-Channel {AMSR2} Brightness Temperatures},
  journal = {Remote Sensing},
  volume  = {13},
  number  = {24},
  pages   = {5120},
  year    = {2021},
  doi     = {10.3390/rs13245120}
}

JPL CAP algorithm — DOI 10.1109/TGRS.2016.2601486

@article{fore2016cap,
  author  = {Fore, Alexander G. and Yueh, Simon H. and Tang, Wenqing and Stiles, Bryan W. and Hayashi, Akiko K.},
  title   = {Combined Active/Passive Retrievals of Ocean Vector Wind and Sea Surface Salinity With {SMAP}},
  journal = {IEEE Transactions on Geoscience and Remote Sensing},
  volume  = {54},
  number  = {12},
  pages   = {7396--7404},
  year    = {2016},
  doi     = {10.1109/TGRS.2016.2601486}
}

Acknowledgements¶

For RSS data, include "SMAP salinity data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Salinity Science Team. Data are available at www.remss.com." For data from PO.DAAC, cite the product-specific dataset DOI and your access date.

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• RSS — SMAP sea surface salinity page
• PO.DAAC — SMAP salinity overview
• PO.DAAC — RSS SMAP L3 monthly V6.0
• PO.DAAC — JPL SMAP CAP L3 monthly V5.0
• RSS SMAP Salinity V6.0 Release Notes (PDF)
• JPL SMAP-SSS CAP V5.0 user guide (PDF)
• SMAP master events log (safe modes and manoeuvres)
• This site — SMAP soil moisture (land products from the same satellite)
• This site — SMOS sea surface salinity
• This site — ESA CCI sea surface salinity (merged SMAP, SMOS and Aquarius)
• This site — CCMP ocean winds (used in the RSS roughness correction)