Ocean Circulation and Climate Dynamics

Mean zonal velocity (top, a) and oxygen content (bottom, b) as obtained from meridional ship sections taken between 28.5° and 23°W during 1999-2008. Eastward current bands, marked by reddish colors, are generally associated with elevated oxygen content (bottom). Grey lines mark neutral densities (Brandt et al. 2010).

Ventilation of oxygen minimum zones

Regions with low dissolved oxygen content in the ocean exist in the eastern tropical Pacific and Atlantic as well as in the northern Indian Ocean. Models proposed and observations showed changes in the expansion of oxygen-poor regions, especially a vertical expansion concurrently with decreasing oxygen content. The future ocean may experience major biogeochemical shifts triggered by oxygen changes leading to positive and negative feedbacks on the climate. To investigate the related processes in 2008 the Collaborative Research Centre SFB-754 ‘Climate-biogeochemistry interactions in the tropical ocean’ started, and the SFB-754 was recently funded until the end of 2015. Within the SFB-754 among others the ventilation of the oxygen minimum zones is investigated by the Physical Oceanography research group. 

Oxygen sources for the ocean are the atmosphere and photosynthesis. Ocean circulation and mixing transport oxygen from the near surface layers to greater depth and towards the low oxygen areas and hence lead to a ventilation of the oxygen minimum zones. Mixing is described in a separate chapter; here the ventilation of ocean movements is presented.  

Horizontal distribution of (a) ADCP velocity vectors at 400 m depth recorded between 5 January and 14 February 2009 with current bands indicated by arrows and the mean climatological oxygen distribution at 400 m in molkg-1 and (b) altimeter-derived sea level anomaly (in cm) on 4 February 2009 with the ship track shown as black lines (Czeschel et al. 2011).

In the equatorial Atlantic and Pacific a complicated system of eastward and westward directed zonal currents exist. In the Atlantic latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second contribute to the ventilation of the oxygen minimum zone. Changes in the strength of a zonal jet affect mean oxygen levels in the oxygen minimum zone.  A decrease in the core of the oxygen minimum zone of the tropical eastern Atlantic in the last 30 years could be related to a reduction of the system of zonal jets acting on the background gradients (Brandt et al. 2010). 

In the eastern equatorial Pacific the differences between eastward and westward flows are typically 10-50 mmolkg-1 (Stramma et al. 2010). As in the equatorial Atlantic the oxygen level at the equator is higher than in the surrounding oxygen-poor regions. Particularly in the upper thermocline, the Equatorial Undercurrent is responsible for the strongest supply of oxygen. Different to the Atlantic, however, the off-equatorial eastward currents don’t supply oxygen to the oxygen minimum zones in the eastern tropical Pacific. During 2 Meteor cruise legs in the eastern tropical South Pacific in early 2009 as well as in late 2012 cyclonic and anticyclonic eddies were observed transporting high oxygen water in the cyclonic eddy and low oxygen water in the anticyclonic eddy (Czeschel et al. 2011; Stramma et al. 2013). Hence also eddies are responsible for oxygen changes in the oxygen minimum zones.

Rough estimates from the values derived in these publications and listed for the Pacific in Brandt et al. (2015) resulted in the following contributions:                          

 

Supply by

 

 

Atlantic, µmol/kg/year

 

 

Pacific, µmol/kg/year

 

 

Jet advection

 

 

0.6 (30%)

 

 

1.2 (33%)

 

 

Eddy mixing

 

 

0.75 (35%)

 

 

1.6 (45%)

 

 

Vertical mixing

 

 

0.65 (34%)

 

 

0.8 (22%)

 

 

Total ventilation

 

 

2.0

 

 

3.6

 

  

Contact: Peter Brandt, Lothar Stramma

 

Literature

P. Brandt et al. (2010): Changes in the ventilation of the oxygen minimum zone of the tropical North Atlantic. Journal of Physical Oceanography, 40, 1784-1801.
  

L. Stramma et al. (2010): Eastern Pacific oxygen minimum zones: Supply paths and multidecadal changes. Journal of Geophysical Research, 115, doi:10.1029/2009JC005976. 655–658.
  

R. Czeschel et al. (2011): Middepth circulation of the eastern tropical South Pacific and its link to the oxygen minimum zone. Journal of Geophysical Research, 116, doi:10.1029(2010JC006565.
 

P. Brandt et al. (2015): On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic Biogeosciences (BG), 12 . pp. 489-512. DOI 10.5194/bg-12-489-2015.
 

L. Stramma et al.(2013): On the role of mesoscale eddies for the biological productivity and biogeochemistry in the eastern tropical Pacific Ocean off Peru. Biogeosciences (BG), 10 (11). pp. 7293-7306. DOI 10.5194/bg-10-7293-2013.