Surface salinity variations and processes affecting surface salinity in the high-salinity region of the subtropical North Atlantic (the SPURS-1 area) are investigated by combining data from in situ observations and satellite remote-sensing measurements. On temporal average, the surface freshwater flux term (evaporation minus precipitation) in the SPURS-1 region increases mixed-layer salinity. Oceanic advection plays the largest role in compensating this salinity increase. On seasonal time scales, mixed-layer salinity increases from April to August and decreases from September to March. This seasonal evolution of the mixed-layer salinity is largely controlled by the freshwater flux term, with vertical entrainment playing a secondary role. The domain-averaged oceanic advection and diffusion terms do not show significant seasonal cycles. The sum of all estimated salinity budget terms largely captures salinity variations on interannual time scales. Unlike the seasonal cycle, variations in freshwater flux, oceanic advection, and vertical entrainment all contribute to interannual variations in surface salinity. Oceanic advection plays a larger role in salinity changes during 2008–2012, whereas the surface freshwater flux term dominates surface salinity evolution during 2004–2007 and in 2013. Although evaporation in the SPURS-1 region dominates the mean freshwater flux, precipitation plays a larger role in interannual variations of the freshwater flux. Separating the advection term into geostrophic and Ekman components indicates that the Ekman component dominates the total advection term. The effect of Ekman advection on salinity changes in the SPURS-1 region is closely linked to the spatial distribution of salinity anomalies. Therefore, it is important to understand large-scale forcing changes.
Adler, R.F., G.J. Huffman, A. Chang, R. Ferraro, P.-P. Xie, J. Janowiak, B. Rudolf, U. Schneider, S. Curtis, D. Bolvin, and others. 2003. The version-2 global precipitation climatology project (GPCP) monthly precipitation analysis (1979–present). Journal of Hydrometeorology 4:1,147–1,167, https://doi.org/10.1175/1525-7541(2003)004<1147:TVGPCP>2.0.CO;2.
Bindoff, N.L., and T.J. McDougall. 2000. Decadal changes along an Indian Ocean section at 32°S and their interpretation. Journal of Physical Oceanography 30:1,207–1,222, https://doi.org/10.1175/1520-0485(2000)030<1207:DCAAIO>2.0.CO;2.
Bingham, F.M., and R. Lukas. 1996. Seasonal cycles of temperature, salinity and dissolved oxygen observed in the Hawaii Ocean Time-series. Deep Sea Research Part II 43:199–213, https://doi.org/10.1016/0967-0645(95)00090-9.
Bingham, F.M., G.R. Foltz, and M.J. McPhaden. 2012. Characteristics of the seasonal cycle of surface layer salinity in the global ocean. Ocean Science 8:915–929, https://doi.org/10.5194/os-8-915-2012.
Boyer, T.P., and S. Levitus. 2002. Harmonic analysis of climatological sea surface salinity. Journal of Geophysical Research 107:1–14, https://doi.org/10.1029/2001JC000829.
Broecker, W.S. 1997. Thermohaline circulation, the Achilles heel of our climate system: Will man-made CO2 upset the current balance? Science 278:1,582–1,588, https://doi.org/10.1126/science.278.5343.1582.
Curry, R., and C. Mauritzen. 2005. Dilution of the northern North Atlantic Ocean in recent decades. Science 308:1,772–1,774, https://doi.org/10.1126/science.1109477.
Curry, R., B. Dickson, and I. Yashayaev. 2003. A change in freshwater balance of the Atlantic Ocean over the past four decades. Nature 426:826–829, https://doi.org/10.1038/nature02206.
de Boyer Montégut, C., G. Madec, A.S. Fischer, A. Lazar, and D. Iudicone. 2004. Mixed layer depth over the global ocean: An examination of profile data and a profile-based climatology. Journal of Geophysical Research 109, C12003, https://doi.org/10.1029/2004JC002378.
de Boyer Montégut, C., J. Mignot, A. Lazar, and S. Cravatte. 2007. Control of salinity on the mixed layer depth in the world ocean: Part 1. General description. Journal of Geophysical Research 112, C06011, https://doi.org/10.1029/2006JC003953.
Dee, D.P., S.M. Uppala, A.J. Simmons, P. Berrisford, P. Pali, S. Kobayshi, U. Andrae, M.A. Balmaseda, G. Balsamo, P. Bauer, and others. 2011. The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quarterly Journal of the Royal Meteorological Society 137:553–597, https://doi.org/10.1002/qj.828.
Dong, S., S.L. Garzoli, and M.O. Baringer. 2009. An assessment of the seasonal mixed-layer salinity budget in the Southern Ocean. Journal of Geophysical Research 114, C12001, https://doi.org/10.1029/2008JC005258.
Ducet, N., P.-Y. Le Traon, and G. Reverdin. 2000. Global high resolution mapping of ocean circulation from TOPEX/POSEIDON and ERS-1 and -2. Journal of Geophysical Research 105:19,477–19,498, https://doi.org/10.1029/2000JC900063.
Durack, P.J., and S.E. Wijffels. 2010. Fifty-year trends in global ocean salinities and their relationship to broad-scale warming. Journal of Climate 23:4,342–4,362, https://doi.org/10.1175/2010JCLI3377.1.
Durack, P.J., S.E. Wijffels, and R.J. Matear. 2012. Ocean salinities reveal strong global water cycle intensification during 1950 to 2000. Science 336:455–458, https://doi.org/10.1126/science.1212222.
Fedorov, A.V., R.C. Pacanowski, S.G. Philander, and G. Boccaletti. 2004. The effect of salinity on the wind-driven circulation and the thermal structure of the upper ocean. Journal of Physical Oceanography 34:1,949–1,966, https://doi.org/10.1175/1520-0485(2004)034<1949:TEOSOT>2.0.CO;2.
Foltz, G.R., and M.J. McPhaden. 2008. Seasonal mixed layer salinity balance of the tropical North Atlantic Ocean. Journal of Geophysical Research 113, C02013, https://doi.org/10.1029/2007JC004178.
Godfrey, J.S., E.F. Bradley, P.A. Coppin, L.F. Pender, T.J. McDougall, E.W. Schulz, and I. Helmond. 1999. Measurements of upper ocean heat and freshwater budgets near a drifting buoy in the equatorial Indian Ocean. Journal of Geophysical Research 104:13,269–13,302, https://doi.org/10.1029/1999JC900045.
Gregory, J.M., K.W. Dixon, R.J. Stouffer, A.J. Weaver, E. Driesschaert, M. Eby, T. Fichefet, H. Hasumi, A. Hu, J.H. Jungclaus, and others. 2005. A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration. Geophysical Research Letters 32, L12703, https://doi.org/10.1029/2005GL023209.
Häkkinen, S. 1999. Variability of the simulated meridional heat transport in the North Atlantic for the period 1951–1993. Journal of Geophysical Research 104:10,991–11,007, https://doi.org/10.1029/1999JC900034.
Huang, B., V.M. Mehta, and N. Schneider. 2005. Oceanic response to idealized net atmospheric freshwater in the Pacific at the decadal time scale. Journal of Physical Oceanography 35:2,467–2,486, https://doi.org/10.1175/JPO2820.1.
Johnson, E.S., G.S.E. Lagerloef, J.T. Gunn, and F. Bonjean. 2002. Surface salinity advection in the tropical oceans compared with atmospheric freshwater forcing: A trial balance. Journal of Geophysical Research 107(C12), 8014, https://doi.org/10.1029/2001JC001122.
Josey, S.A., and R. Marsh. 2005. Surface freshwater flux variability and recent freshening of the North Atlantic in the eastern subpolar gyre. Journal of Geophysical Research 110, C05008, https://doi.org/10.1029/2004JC002521.
Lukas, R., and E. Lindstrom. 1991. The mixed layer of the western equatorial Pacific Ocean. Journal of Geophysical Research 96:3,343–3,357, https://doi.org/10.1029/90JC01951.
O’Connor, B.M., R.A. Fine, and D.B. Olson. 2005. A global comparison of subtropical underwater formation rates. Deep Sea Research Part I 52:1,569-1,590, https://doi.org/10.1016/j.dsr.2005.01.011.
Qu, T., S. Gao, and I. Fukumori. 2011. What governs the North Atlantic salinity maximum in a global GCM? Geophysical Research Letters 38, L07602, https://doi.org/10.1029/2011GL046757.
Qu, T., S. Gao, and I. Fukumori. 2013. Formation of salinity maximum water and its contribution to the overturning circulation in the North Atlantic as revealed by a global general circulation model. Journal of Geophysical Research 118:1,982–1,994, https://doi.org/10.1002/jgrc.20152.
Rahmstorf, S. 1995. Multiple convection patterns and thermohaline flow in an idealized OGCM. Journal of Climate 8:3,028–3,039, https://doi.org/10.1175/1520-0442(1995)008<3028:MCPATF>2.0.CO;2.
Roemmich, D., and J. Gilson. 2009. The 2004–2008 mean and annual cycle of temperature, salinity, and steric height in the global ocean from the Argo program. Progress in Oceanography 82:81–100, https://doi.org/10.1016/j.pocean.2009.03.004.
Schmitt, R.W. 2008. Salinity and the global water cycle. Oceanography 21(1):12–19, https://doi.org/10.5670/oceanog.2008.63.
Schmitt, R.W., and A.Blair. 2015. A river of salt. Oceanography 28(1):40–45, https://doi.org/10.5670/oceanog.2015.04.
Sprintall, J., and M. Tomczak. 1992. Evidence of the barrier layer in the surface layer of the tropics. Journal of Geophysical Research 97:7,305–7,316, https://doi.org/10.1029/92JC00407.
Stocker, T.F., and A. Schmittner. 1997. Influence of CO2 emission rates on the stability of the thermohaline circulation. Nature 388:862–865.
Vinogradova, N.T., and R.M. Ponte. 2013. Clarifying the link between surface salinity and freshwater fluxes on monthly to interannual time scales, Journal of Geophysical Research 118:3,190–3,201, https://doi.org/10.1002/jgrc.20200.
Wong, A., N.L. Bindoff, and J.A. Church. 1999. Large-scale freshening of intermediate waters in the Pacific and Indian Oceans. Nature 400:440–443, https://doi.org/10.1038/22733.
Yu, L. 2011. A global relationship between the water cycle and near-surface salinity. Journal of Geophysical Research 116, C10025, https://doi.org/10.1029/2010JC006937.
Yu, L., and R.A. Weller. 2007. Objectively analyzed air-sea heat fluxes for the global ice-free oceans (1981–2005). Bulletin of the American Meteorological Association 88:527–539, https://doi.org/10.1175/BAMS-88-4-527.
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