Abstract
AbstractThe Gulf Stream (GS) is central to the global redistribution of heat due to the transport of large volumes of warm water from the tropics to high latitudes and the extreme ocean heat loss to the atmosphere. This study assesses the extent to which winter surface heat fluxes and wind stress curl can drive interannual variations of the full‐depth GS transport. Intensification of the GS has been observed (e.g., April 1977) immediately after a winter of frequent cold air outbreaks that led to a deepening of the mixed layer and subsequent steepening of meridional temperature gradients to the south of the GS. This forcing process is further investigated here using the ORCA12 hindcast (1978–2010) from the global, eddy‐resolving, ocean‐only Nucleus for European Modeling of the Ocean model in order to understand GS forcing mechanisms. Lagrangian analysis is also undertaken to examine the impact on the southern recirculation gyre. Results show that surface heat fluxes and wind stress curl can act in concert to effect year‐to‐year changes of up to 38% of the spring GS transport at 70°W. However, anomalous heat losses (∼200 Wm‐2) over the western Subtropical Gyre are found to be the dominant cause of peaks in GS transport via two mechanisms: (1) a strengthening of cross‐stream density gradients on the northern flank of the GS from an intense cooling (up to 4°C) in the Slope Water and (2) a westward intensification of the southern recirculation, which can also limit the formation of deeper mixed layers to the south of the GS near 70°W.
Highlights
The Gulf Stream (GS) is the Western Boundary Current of the North Atlantic subtropical gyre and facilitates transport of warm, saline water from the subtropics to higher latitudes by rapid advection (Rossby, 1996)
Results show that surface heat fluxes and wind stress curl can act in concert to effect year-to-year changes of up to 38% of the spring GS transport at 70°W
Anomalous heat losses (∼200 Wm-2) over the western Subtropical Gyre are found to be the dominant cause of peaks in GS transport via two mechanisms: (1) a strengthening of cross-stream density gradients on the northern flank of the GS from an intense cooling in the Slope Water and (2) a westward intensification of the southern recirculation, which can limit the formation of deeper mixed layers to the south of the GS near 70°W
Summary
The Gulf Stream (GS) is the Western Boundary Current of the North Atlantic subtropical gyre and facilitates transport of warm, saline water from the subtropics to higher latitudes by rapid advection (Rossby, 1996). Worthington (1977) after the severe winter of 1976/1977, when cold air outbreaks led to large heat losses over the western subtropical gyre This led to anomalously deep mixed layers and a high renewal of mode water to the south of the GS, which strengthened the meridional temperature gradients across the current. A study by Zheng et al (1984) proposed that cooling over the Sargasso Sea is not the sole driving force for an increase in GS transport They suggest that this mechanism is reinforced by the heat loss observed in the Slope Water, that is, the combination of isotherms upwelling to the north of the GS and downwelling to the south. Worthington [1977]; Zheng et al [1984]), and propose a mechanism that suggests westward intensification of the SRG
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