North Atlantic and North Sea Climate Change: curl up, shut down, NAO and Ocean Colour

Abstract

The strength of the North Atlantic Current (NAC) (based on sea-surface elevation slopes derived from altimeter data) is correlated with westerly winds (based on North Atlantic Oscillation [NAO] Index data over a nine year period [1992–2002] with 108 monthly values). The data time window includes the major change in climate forcing over the last 100 years (1995 to 1996). It is shown that the NAO Index can be used for early warning of system failure for the NAC. The correlation response or early warning time scale for western Europe and south England is six months. The decay scale for the NAC and Subtropical Gyre circulation is estimated as three years. Longer period altimeter elevation/circulation changes are discussed. The sea-surface temperature (SST) response of the North Sea to negative and positive NAO conditions is examined. The overall temperature response for the central North Sea to NAO index forcing, reflecting wind induced inflow, shelf circulation and local climate forcing, is ∼5 months. In years with strong North Atlantic winter wind induced inflow, under marked NAO positive conditions, mean temperatures (∼10.5°C) are about 1°C warmer than under negative conditions. In 1996 under extreme negative winter NAO conditions, the North Sea circulation stopped, conditions near the Dogger Bank became more continentally influenced and the winter (March) temperature fell to 3.1°C whereas in 1995 under NAO positive winter conditions the minimum temperature was 6.4°C (February). Seasonal advance of North Atlantic and North Sea temperature is derived in relation to temperature change. Temperature change and monthly NAO Index are discussed with respect to phytoplankton blooms, chlorophyll-a measurements, ocean colour data and the anomalous north-eastern Atlantic 2002 spring/summer bloom SeaWiFS chlorophyll concentrations.