Tuesday, April 21, 2015

One of the most striking features that has developed in our observations since Tahiti is the shallow (~500-1000 m) lens of low oxygen water that is decreasing in concentration as we move toward the equator (see figure below).  Around 11 deg S, the water began to plummet from 100 umol/kg to nearly 10 umol/kg at times around 6 deg S.  This feature coincides with a similar low oxygen feature from this line in 2006, but it is lower in concentration, in general, than in 2006.  Why does it change from high to low oxygen, in that depth range, at 11 deg S?  Why would oxygen be lower now than in 2006?

To answer that, we have to think about what water masses are influencing us in the South Pacific.  Antarctic Intermediate Water (AAIW) is an influential water mass in the South Pacific. AAIW can be found as far north as 10-20 degrees North, but in the equatorial regions, other water masses are influential, as well, including the North Pacific Intermediate Water (NPIW) and Equatorial Pacific Intermediate Water (EqPIW).  AAIW lies above deep water but below the tropical thermocline, has been characteristically identified as a salinity minimum (34-34.5 psu) north of the Antarctic Circumpolar Current. AAIW forms in the southeast Pacific and spreads northward toward the equator bringing with it highly oxygenated waters. Prior work has observed freshening, shoaling and warming of AAIW over the past several decades. These other water masses have properties as well - for example, EqPIW is very low in oxygen and a bit saltier than AAIW (34.5-35).  EqPIW has contributions from AAIW, Pacific Deep Water, and is advected along the equator from the East Pacific toward the West Pacific.  EqPIW brings with it low oxygen waters formed in the Oxygen Minimum Zone off the coast of Peru.

AAIW can be seen in the P16N leg 1 section plot of salinity along our cruise track so far between 500-1000 m.  Its presence can be seen through station 11 (11.5 deg S) in that depth range in both oxygen and salinity.  AAIW is typically high in oxygen, so its absence is part of the reason for the oxygen decline.  This is similar to the prior occupation in 2006.  The low oxygen water interlaced with the AAIW is most likely EqPIW in some combination with NPIW - both low in oxygen.  The oxygen is low in those water masses because it has a respiration signal which accumulated over the water masses' path as it collected falling organic-rich particles - both water masses are "older".  Older water also means it has been a long time since those water masses have been at the surface, and thus, ventilated by the atmosphere.

Because the water is warming and becoming more stratified, we would expect the already low oxygen waters to become lower in oxygen in the warming world for two reasons. First, warm water holds less gas.  Second, increased stratification will reduce the rate (flux) at which oxygen can diffuse into the low oxygen water mass from the higher oxygen water masses lying above and below the oxygen minimum.

These results are all preliminary, but exciting because the changes in oxygen are consistent with our expectations for how the system changes in a warming world.  Whether or not they are attributed to anthropogenic changes or natural variability will be a question for the scientific community to determine in the coming years.
Oxygen cross section (depth in meters vs latitude) along our cruise track for P16N (2015).  The white contours show the salinity along the same cruise track and the black dots identify the bottles sampling the water column along the way.  The depth axis has been stretched in the region of interest in order to highlight the features blogged about here. 

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