Deep temperature variability in Drake Passage

• Published in: Journal of geophysical research. Oceans 2017-01, Vol.122 (1), p.713-725
• Main Authors: Firing, Yvonne L., McDonagh, Elaine L., King, Brian A., Desbruyères, Damien G.
• Format: Article
• Language: English
• Subjects: Altimeters; Annual variations; Antarctic Circumpolar Current; Antarctic front; Antarctica; Atmosphere; Components; Cooling; Density; Enthalpy; Frameworks; Fronts; Geophysics; Heat content; Heave; Heaving; Hydrographic data; Ice; Interannual variability; Isopycnals; Marine; Occupation; Ocean circulation; Ocean surface; Ocean temperature; Oceans; Polar fronts; Position (location); Remote sensing; Satellite altimetry; Satellites; Sea surface; Ships; Southern Ocean; Spaceborne remote sensing; Surface velocity; temperature; Temperature (air-sea); Temperature effects; Temperature variability; Trends; Variability; Variance
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1002/2016JC012452

Observations made on 21 occupations between 1993 and 2016 of GO‐SHIP line SR1b in eastern Drake Passage show an average temperature of 0.53°C deeper than 2000 dbar, with no significant trend, but substantial year‐to‐year variability (standard deviation 0.08°C). Using a neutral density framework to decompose the temperature variability into isopycnal displacement (heave) and isopycnal property change components shows that approximately 95% of the year‐to‐year variance in deep temperature is due to heave. Changes on isopycnals make a small contribution to year‐to‐year variability but contribute a significant trend of −1.4 ± 0.6 m°C per year, largest for density (γn) > 28.1, south of the Polar Front (PF). The heave component is depth‐coherent and results from either vertical or horizontal motions of neutral density surfaces, which trend upward and northward around the PF, downward for the densest levels in the southern section, and downward and southward in the Subantarctic Front and Southern Antarctic Circumpolar Current Front (SACCF). A proxy for the locations of the Antarctic Circumpolar Current (ACC) fronts is constructed from the repeat hydrographic data and has a strong relationship with deep ocean heat content, explaining 76% of deep temperature variance. The same frontal position proxy based on satellite altimeter‐derived surface velocities explains 73% of deep temperature variance. The position of the PF plays the strongest role in this relationship between ACC fronts and deep temperature variability in Drake Passage, although much of the temperature variability in the southern half of the section can be explained by the position of the SACCF. Plain Language Summary We investigated how the temperature of the deep Southern Ocean changes over time using two decades of measurements in Drake Passage, between South America and Antarctica. In this region, ocean temperature increases from south to north not smoothly but in steps, or fronts. When a front moves across a location (this may be driven by the wind, or by internal instabilities), the temperature there will change. We found that a large fraction of the observed variability is due to this mechanism; because the fronts have an effect on the height of the sea surface, this means that we can use satellites to tell us about some of the changes in temperature even deep below the ocean surface. Over the last two decades, average deep temperature in Drake Passage has decreased, and about half of this trend