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The influence of the Sub Polar Gyre on the thermohaline circulation

The influence of the Sub Polar Gyre on the thermohaline circulation. Anne Britt Sandø, Hjálmar Hátún, Helge Drange, Bogi Hansen and Hedinn Valdimarsson. Salinity of Atlantic Inflow – Thermohaline Circulation (THC). Observations show decreasing salinities since 1950

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The influence of the Sub Polar Gyre on the thermohaline circulation

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  1. The influence of the Sub Polar Gyre on the thermohaline circulation Anne Britt Sandø, Hjálmar Hátún, Helge Drange, Bogi Hansen and Hedinn Valdimarsson

  2. Salinity of Atlantic Inflow – Thermohaline Circulation (THC) • Observations show decreasing salinities since 1950 • Recent observations show record-high salinities in the Atlantic Inflow to the Nordic Seas • Why is the salinity increasing? Dickson et al. 2002:

  3. Häkkinen and Rhines (2004): Decline of the Subpolar North Atlantic Circulation During the 1990s • May this decline also have influenced the north-eastern parts of the North Atlantic? • Can the model be used to extend the time-series of the observed Sea Surface Height (SSH)? Observed and simulated EOF modes: Häkkinen and Rhines mode based on altimetry SSH mode based on model results

  4. Is there any relation between the salinity of the Atlantic Inflow and the strength of the Sub Polar Gyre (SPG)? • Gyre circulation low salinity high • Similarity between gyre mode in observations and model • Model is used to extend times-series back to 1960ies Gyre index: Principal component associated with the leading North Atlantic SSH mode

  5. Possible links between the SPG strength and the salinity of the inflowing waters (i) Changes in the air-sea flux of freshwater (E-P) (ii) Increasing salinities of the Sub Tropical Gyre (STG) water (iii) Increasing salinities of the SPG water (iv) Dynamic changes in the relative contributions from the two gyres

  6. (ii)-(iii) Increasing salinities of STG and SPG waters: • Salinity variations in the Atlantic Inflow do not correspond to salinity variations in the STG or SPG upper layers (i) Changes in the air-sea flux of freshwater (E-P): • Air-sea flux cannot explain the record high Atlantic Inflow salinities

  7. (iv) Dynamic changes in the relative contributions from the two gyres Simulated spatial distribution of typical SPG water, STG water and a mixture of both water masses: Reduced SPG strength Reduced eastward extent of fresh SPG waters Increased salinity of Atlantic Inflow waters HOW is the Atlantic Inflow salinity affected?

  8. The relative contributions of SPG and STG waters Gyre index low STG transport high SPG transport low Atlantic Inflow salinity high The effect of the gyre circulation is to modulate the influence of the relatively fresh SPG wateron the Atlantic Inflow

  9. Conclusions • On inter-annualto inter-decadal time scales: Variability of SPG controls the salinity of the Atlantic Inflow • On longer time scales: STG salinity may become important • Observed record-high salinities in the Atlantic Inflow waters are not caused by increased salinities in the tropics

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