Ocean Observing and Climate
OVAC Theme 2 - Trends in Freshwater Forcing of the North Atlantic
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The climatological mean precipitation minus evaporation field determined from the SOC Flux Dataset for 1980-1997. (Click to enlarge) Upper ocean salinity has declined across much of the mid-latitude North Atlantic since the 1960s, with possible implications for the large-scale ocean circulation. Within this theme we are exploring the relation of such changes to variability in the freshwater forcing of the ocean. The North-East Atlantic is a region where precipitation dominates evaporation as is evident from the climatological mean field for 1980-1997 from the SOC climatology (left). |
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The area integrated annual mean freshwater flux anomaly for the region outlined in white on the climatological field. (Click to enlarge) The graph (right) shows a time series of the area integrated net freshwater flux from the NCEP/NCAR reanalysis for the box outlined on the map (above). A strong freshening trend equivalent to an extra 5000 km3 of freshwater since about 1970 is evident in the time series. The magnitude of this change in the surface forced freshwater flux is similar to that for the Great Salinity Anomaly which was an advective feature that occurred in the 1960s. |
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We have attempted to put the recent trend into context by reconstructing the area integrated flux for the period since 1900 using a regression model in which the independent variables are rain gauge measured precipitation rates from southwest Iceland, the Faeroe Islands, Shetland and west Scotland1. The reconstructed time series is shown as the blue line in the lower panel, it suggests that the recent trend is unusual in the context of the century long freshwater forcing of this region. Future work in this area will explore to what extent the recent trend can explain the observed decline in upper ocean salinity in the mid-latitude North Atlantic.
1 these measurements are available for the earlier period and are taken from the Climatic Research Unit, UEA precipitation climatology.
Further reading:
Josey, S. A., E. C. Kent and B. Sinha, 2001: Can a state of the art
atmospheric general circulation model reproduce recent NAO related
variability at the air-sea interface? Geophys. Res. Lett., 28, 4543-4546.
Links to OVAC Theme 2 Research
Rainfall Variability Atlantic Freshwater SST Anomalies Flux Parameterisations
