All specific oxygen concentration values discussed in this section can be found in Table 1, which is found at the end of this article.
Table 1
Basin | Subsection | 500-1000 | 1000-1500 | 1500-2000 | 2000-2500 | 2500-3000 | 3000-3500 | 3500-Bottom |
N. Atlantic | Northwest North Atlantic | -0.160 | -0.404 | -0.538 | -0.277 | -0.062 | 0.032 | -0.034 |
| Northeast North Atlantic | -0.317 | 0.083 | -0.194 | -0.047 | 0.000 | -0.048 | -0.062 |
| North Central North Atlantic | 0.151 | 0.310 | -0.232 | -0.395 | -0.083 | -0.066 | -0.086 |
| West North Atlantic | -0.012 | 0.439 | 0.038 | 0.038 | 0.034 | 0.043 | -0.028 |
| Central North Atlantic | -0.104 | 0.198 | -0.055 | -0.046 | -0.031 | -0.037 | -0.045 |
| East North Atlantic | -0.094 | 0.044 | -0.014 | -0.048 | -0.065 | -0.053 | -0.044 |
| Caribbean | -0.306 | -0.144 | -0.007 | -0.021 | -0.033 | -0.059 | -0.094 |
| West Tropical North Atlantic | -0.046 | -0.163 | -0.187 | -0.102 | -0.124 | -0.083 | -0.090 |
| Central Tropical North Atlantic | -0.176 | -0.125 | -0.235 | -0.081 | -0.071 | -0.057 | -0.062 |
| East Tropical Atlantic | -0.084 | -0.111 | -0.230 | -0.117 | -0.085 | -0.064 | -0.056 |
| Average | -0.115 | 0.013 | -0.165 | -0.110 | -0.052 | -0.039 | -0.060 |
S. Atlantic | West Central South Atlantic | -0.174 | -0.224 | -0.169 | -0.119 | -0.125 | -0.142 | -0.095 |
| East Central South Atlantic | -0.133 | -0.225 | -0.137 | -0.154 | -0.132 | -0.105 | -0.132 |
| South Central South Atlantic | -0.378 | -0.241 | -0.238 | -0.164 | -0.116 | -0.104 | -0.101 |
| Southeast South Atlantic | -0.334 | -0.247 | -0.127 | -0.100 | -0.103 | -0.132 | -0.122 |
| Farsouth South Atlantic | -0.211 | -0.158 | -0.153 | -0.066 | -0.101 | -0.092 | 0.000 |
| Weddell | -0.124 | -0.169 | -0.309 | -0.118 | 0.098 | -0.066 | -0.272 |
| Average | -0.226 | -0.211 | -0.189 | -0.120 | -0.080 | -0.107 | -0.120 |
N. Pacific | Bering | -0.165 | 0.060 | 0.040 | 0.010 | -0.029 | -0.048 | 0.065 |
| Northwest North Pacific | -0.307 | -0.086 | -0.035 | -0.025 | -0.057 | -0.069 | -0.088 |
| North Central North Pacific | -0.157 | -0.075 | -0.079 | -0.098 | -0.077 | -0.057 | -0.050 |
| Northeast North Pacific | -0.224 | -0.063 | -0.005 | 0.018 | 0.028 | 0.007 | -0.033 |
| Far West North Pacific | -0.247 | -0.243 | -0.224 | -0.057 | -0.053 | -0.091 | -0.069 |
| West Central North Pacific | -0.264 | 0.018 | 0.005 | -0.006 | -0.015 | -0.023 | -0.043 |
| East Central North Pacific | -0.221 | -0.042 | -0.021 | -0.047 | -0.074 | -0.049 | -0.044 |
| Far East North Pacific | -0.104 | -0.056 | -0.068 | -0.055 | -0.052 | -0.064 | -0.041 |
| Average | -0.211 | -0.061 | -0.048 | -0.033 | -0.041 | -0.049 | -0.038 |
S. Pacific | West Tropical Pacific | 0.119 | -0.027 | -0.047 | 0.025 | 0.006 | 0.004 | -0.023 |
| Central Tropical Pacific | 0.134 | -0.044 | -0.028 | -0.032 | -0.011 | -0.013 | -0.075 |
| East Tropical Pacific | 0.024 | 0.059 | 0.024 | -0.005 | -0.025 | -0.034 | -0.044 |
| Northwest South Pacific | 0.306 | 0.089 | -0.022 | -0.037 | -0.039 | -0.033 | -0.017 |
| Central South Pacific | 0.143 | 0.096 | 0.015 | 0.002 | -0.009 | -0.035 | -0.067 |
| Northeast South Pacific | 0.016 | 0.104 | 0.032 | -0.026 | 0.025 | 0.021 | -0.019 |
| East South Pacific | -0.314 | -0.099 | 0.001 | -0.004 | 0.008 | -0.005 | -0.067 |
| Southwest South Pacific | -0.019 | 0.075 | 0.000 | -0.026 | -0.035 | -0.043 | -0.061 |
| Southeast South Pacific | -0.236 | -0.020 | -0.002 | 0.018 | 0.017 | -0.015 | -0.077 |
| Farsouth South Pacific | 0.002 | -0.035 | -0.053 | -0.057 | -0.078 | -0.215 | -0.285 |
| Drake | -0.659 | -0.326 | -0.247 | -0.333 | -0.364 | -0.335 | -0.257 |
| Average (w/o Drake) | 0.017 | 0.020 | -0.008 | -0.014 | -0.014 | -0.037 | -0.073 |
Indian | West North Indian | -0.050 | -0.022 | -0.128 | -0.099 | -0.076 | -0.068 | -0.017 |
| West Central Indian | 0.051 | -0.070 | -0.096 | -0.056 | -0.053 | -0.081 | -0.022 |
| Central Indian | 0.151 | 0.138 | 0.007 | 0.022 | -0.051 | -0.091 | -0.035 |
| East Central Indian | -0.043 | 0.037 | 0.020 | 0.031 | 0.044 | 0.020 | 0.005 |
| Southwest Indian | 0.046 | -0.214 | -0.375 | -0.342 | -0.209 | -0.128 | -0.106 |
| Southeast Indian | -0.107 | -0.051 | -0.005 | 0.020 | -0.026 | -0.059 | -0.050 |
| West Far South Indian | -0.062 | -0.104 | -0.060 | -0.045 | -0.040 | -0.078 | -0.054 |
| Central Far South Indian | -0.140 | -0.040 | 0.025 | 0.035 | 0.022 | -0.030 | -0.111 |
| Antarctic Far South Indian | 0.141 | 0.102 | 0.033 | -0.034 | -0.095 | -0.183 | -0.163 |
| East Far South Indian | 0.000 | 0.002 | 0.000 | 0.012 | 0.011 | -0.015 | -0.044 |
| Average | -0.001 | -0.022 | -0.058 | -0.046 | -0.047 | -0.071 | -0.060 |
Overall | By Volumes | -0.091 | -0.040 | -0.073 | -0.052 | -0.042 | -0.057 | -0.066 |
North Atlantic Ocean (excluding Labrador Sea)
We examined 10 subregions of the North Atlantic Ocean. DO levels in the waters below 1500 dbar---the layers relevant to the North Atlantic Deep Water---showed a consistent negative trend over time. Both the Northeast and North Central North Atlantic subsections had particularly strong deoxygenation in their 1500-2000 dbar and 2000-2500 dbar layers.
The 500-1000 dbar layer shows consistent deoxygenation throughout the North Atlantic, apart from North Central North Atlantic. Strongest negative trends for this layer occurred in the Northeast North Atlantic (-0.317 µmol kg−1 year−1) and the Caribbean (-0.306 µmol kg−1 year−1) regions. However, this layer does show a broad range of DO changes (+0.151 µmol kg−1 year−1 to -0.317 µmol kg−1 year−1).
We observe the principal exception to the otherwise consistent negative DO trends in all layers of the North Atlantic in the 1000-1500 dbar layer. Although equatorial-bordering regions of the North Atlantic showed a negative trend in this layer, there is an overall positive trend in DO concentrations in the 1000-1500 dbar layer in the remainder of the North Atlantic, averaging +0.080 µmol kg−1 year−1. We observed particularly notable increases in the North Central North Atlantic (+0.310 µmol kg−1 year−1) and West North Atlantic (+0.439 µmol kg−1 year−1).
While there are differences, these North Atlantic’s results align with those of Stendardo et al. [13]. Their investigation in the North Atlantic found deoxygenation, especially in the 500-1000 dbar layer.
Labrador Sea (Northwest North Atlantic)
The Labrador Sea matched 2016-minus-1990 cruise pair (‘Northwest North Atlantic’ in Figure 1) shows consistent deoxygenation in all layers except for the 3000-3500 dbar layer. In fact, in the 1000-1500 and 1500-2000 dbar layers, the Northwest North Atlantic experienced some of the largest individual rates of decrease in DO concentrations (-0.404 and -0.538 µmol kg−1 year−1, respectively) we found for these layers in all the global subregions we examined. This is unlike the rest of the North Atlantic subregions, especially in the 1000-1500 dbar layer.
The Labrador Sea is oceanographically unique among the North Atlantic subregions due to winter convection reaching to ca. 2000 dbars plus both interannual [19] and decadal (AMOC and NAO-driven) [20] variability. Thus, varying upper layer characteristics and deep winter convection combine to bring temporal changes well into the Labrador Sea interior. Excepting single-cruise fluctuations, a time series of cruise-mean dissolved oxygen data in 500 dbar thick layers from 500 to 3500 dbar from 28 Labrador Sea cruises conducted between 1990 and 2016 (Figure 2) shows mostly consistent multi-year patterns of increases and decreases of layer mean oxygen concentrations. There are abrupt, shorter-term changes, but strong deoxygenation between 1000 to 2500 dbars is evident over the 27-year record. There is evidence in the last years of the record for reoxygenation of these layers, which warrants future exploration.
South Atlantic Ocean & Weddell Sea
We examined 6 subregions of the South Atlantic Ocean, with results showing the most consistent temporal DO trend among the five major basins we studied. All layers in all subsections of the South Atlantic experienced deoxygenation since the 1980s (Figure 1), with especially large deoxygenation in Antarctic-bordering waters.
Each layer shows stronger deoxygenation magnitudes than those in the other principal ocean regions, with the 500-1000 dbar layer especially strong at -0.226 µmol kg−1 year−1. In most subregions, greatest deoxygenation is observed in the upper one or two layers.
The Weddell Sea is a crucial element of the global overturning circulation because of involvement in Antarctic Deep Water formation [21]. We found one spatially matched bottle DO section pair, though separated by only 9 years. Results from that pair suggest that the subregion experienced deoxygenation rates consistent with those of the South Atlantic, with especially noticeable deoxygenation in the 1500-2000 dbar layer and the layer below 3500 dbar. The 2500-3000 dbar layer of the Weddell Sea is the only layer in all the South Atlantic showing an increase in DO, at a rate of +0.098 µmol kg−1 year−1. Interestingly, this is the depth range of impact from Weddell Sea polynyas [22]. One or more of these likely ventilated this layer locally during the interval between the cruises.
North Pacific Ocean & Bering Sea
We examined 8 subregions of the North Pacific Ocean. The North Pacific shows consistent decreases in DO in nearly all layers, like results from earlier research [17]. Note that we included the tropical Pacific Ocean subregions in the South Pacific due to regional similarities.
The 500-1000 dbar layer shows particularly large and consistent deoxygenation throughout the North Pacific, averaging -0.218 µmol kg−1 year−1 (excluding the Bering Sea). Most North Pacific layers below 1500 dbar also showed consistent deoxygenation, with rates clustered near the average values. There is unusually large deoxygenation in the Far West North Pacific subregion, perhaps stemming from large-scale upwelling and continual remineralization of organic matter [12].
In the Bering Sea, unlike the rest of the North Pacific, we observed DO increases in the deeper layers in the Bering Sea. The layers between 1000 and 2500 dbar and below 3500 dbars all show temporal increases in DO. These observations appear to be mostly consistent with those in Sun et al. [23].
Equatorial & South Pacific Ocean
We examined 11 subregions of the Equatorial and South Pacific Ocean.
In the tropical and mid-latitude South Pacific, waters in the 500-1000 dbar layer consistently showed increases in DO, differing from the North Pacific’s strong, consistent negative temporal DO trend in the same layer. But in the southern South Pacific, like the southern portions of the South Atlantic and Indian basins, DO mostly decreased over time, especially in the East and Southeast South Pacific.
As is the case with nearly every ocean subregion we studied, South Pacific waters below 3500 dbar showed a consistent negative temporal DO trend. For South Pacific layers between 1000 and 3500 dbar, noteworthy results include: (1) the large increase in DO in the 1000-1500 dbar layer in mid-latitude South Pacific, with Northwest, Central, and Northeast South Pacific showing an average +0.096 µmol kg−1 year−1 increase; (2) deoxygenation continuously increasing with depth in the Farsouth South Pacific, with especially large values for oceanic layers below 3000 dbar; and (3) contrasting trends between the upper layers of the South Pacific’s northeast, which show small DO increases, and southeast, where there were large DO decreases, subregions.
The Drake Passage showed strong deoxygenation in every layer between 1990 and 2009, at times off-scale in Figure 1. In fact, the deoxygenation observed in the Drake Passage is the largest in magnitude in the layers between 500 and 3500 dbars across all subsections in the five basins studied (excluding the Labrador Sea). Due to these anomalously large changes, which may be the result of an unresolved data quality issue, the Drake Passage DO change results were not included in relevant average value calculations.
Indian Ocean
We examined 10 subregions of the Indian Ocean, excluding only the northeast Indian Ocean due to lack of spatially coincident data meeting our criteria. Temporal trends in DO in the Indian Ocean are not as strongly spatially coherent as those we found in the Pacific and Atlantic Oceans. There are also no clearly observable layers that stand out over several subregions with particularly strong and/or consistent increasing or decreasing DO content.
A continuity occurs in the two layers below 3000 dbar, which display negative DO trends in every subregion but one. Four out of the 10 Indian Ocean subregions display DO increases in the 500-1000 dbar layer, with no marked geographical/regional grouping of DO changes. There is also little similarity between the behavior of the 500-1000 dbar layer and the layers below 1000 dbar, which may be a sign of independence of the waters above 1000 dbar from those below.