Volume transport and residence time
To illustrate how the particles move, the positions of the particles at several times are shown in Fig. 1. The movie of particles is also shown in the supplemental material (Movie S1). Some of the particles immediately rise to the deep layer (2000-3500 m depth) and return to the Southern Ocean through the Southwest Pacific Basin (Figs. 1a and 1b). The rest of the particles are transported to the North Pacific basins (the Central, Northwest, and Northeast Pacific Basins) by the horizontal currents in the bottom layer which are constrained by the bottom topography (Figs. 1b and 1c). These particles eventually ascend to the intermediate and shallow layers (< 1000 m depth) and reach the Indian and Arctic Oceans through the Indonesian Archipelago and the Bering Strait, respectively (Fig. 1d).
Almost all (~ 99.8%) of the particles reach the other basins (the Southern, Indian, and Arctic Oceans) or evaporate after 3000 years (Fig. 2a). 3.50, 2.17, and 1.05 Sv (46.5, 28.8, and 13.9%) of bottom water originated from the Samoan Passage (and its adjacent passages) is transported to the Southern, Indian, and Arctic Oceans, respectively, and 0.80 Sv (10.3%) evaporates within the Pacific Ocean (Table 1). Several observation-based studies2,5,9 assumed that all (~ 10 Sv) of the Pacific deep water returns to the Southern Ocean in the deep layer. On the other hand, Talley3 estimated about 9, 4, and 1 Sv of the Pacific deep water originated from the Southern Ocean are exported to the Southern, Indian, and Arctic Oceans, respectively. Our result is qualitatively consistent with this estimate3 although the modeled export to the Southern and Indian Oceans is smaller. The modeled export to the Arctic Ocean is a bit larger than the observational estimate20of the net volume transport through the Bering Strait (~ 0.8 Sv).
Table 1
Number and ratio of particles, volume transport, and residence time of particles for each destination. The mean, median, and mode are listed for the residence time. The result is obtained from the tracking for 3000 years. The total number of particles is 2,342,025.
Destination
|
Number of particles
|
Ratio of particle (%)
|
Volume transport (Sv)
|
Residence time (year)
|
mean
|
median
|
mode
|
Southern Ocean
|
1,080,079
|
46.5
|
3.50
|
533
|
409
|
40
|
Indian Ocean
|
675,128
|
28.8
|
2.17
|
672
|
559
|
318
|
Arctic Ocean
|
326,424
|
13.9
|
1.05
|
728
|
620
|
421
|
Atmosphere
|
248,321
|
10.6
|
0.80
|
804
|
696
|
501
|
residual
|
4,073
|
0.2
|
0.01
|
-
|
-
|
-
|
The residence time (seawater age) of the deep Pacific water is equivalent to the time required for particles to reach other basins. Its statistics are summarized in Table 1. The mean required time to reach the Southern, Indian, and Arctic Oceans and the atmosphere (evaporation) is 533, 627, 728, and 804 years, respectively. The median time is shorter than the mean time by about 100 years for each destination, and the mode time is even shorter because of the right-skewed frequency distribution in the arrival time (Fig. 2b). The residence time of particles is less than 1000 years and is consistent with an observation-based estimate of water age in the deep Pacific Ocean21.
Main pathway
The main pathways, defined in terms of particle tracking, are the routes where a large number of particles pass through. Therefore, the main pathways are visualized by superimposing the trajectories of the particles (left panels in Figs. 3 and 4). To identify the shortest pathways, the trajectories of the earliest 10% of the particles that leave the Pacific Ocean are extracted for each destination together with the horizontal positions where these particles first cross the depths dividing the bottom, deep, intermediate, and shallow layers (“ascending points” hereafter) (Fig. 3).
Most of the particles reaching the Southern Ocean ascend near the Samoan Passage (e.g., Melanesian Basin) and are transported southward in the Southwest Pacific Basin without crossing the equator (Figs. 3a and 3b). Some particles ascend in the Melanesian Basin or Southwest Pacific Basin and reach the Southern Ocean through the South Fiji Basin and the Tasman Sea (Figs. 3a and 3b), which corresponds to the Tasman leakage22.
Except for the shallow layer (< 500 m depth), a majority of particles reaching the Indian Ocean through the Indonesian Archipelago are transported in the western North Pacific Ocean, not in the east (Fig. 3c). The particles are transported in the Central Pacific Basin or the Melanesian Basin in the bottom layer, then join and ascend to the deep layer at the Izu-Ogasawara Ridge where turbulent vertical mixing is strong (Figs. 3c and 3d). These particles ascend to the intermediate layer (500-1000 m depth) at the Luzon Strait and around the Ryukyu Islands and are transported to the Indonesian Archipelago (Figs. 3c and 3d). The particles on the secondary pathway ascend to the deep layer in the Melanesian Basin and to the intermediate layer in the Solomon Sea or North/South Fiji Basin and reach the Indonesian Archipelago (Figs. 3c and 3d).
The trajectories and ascending points of most of the particles reaching the Arctic Ocean are similar to those reaching the Indian Ocean in the deep and bottom layers (Figs. 3e and 3f). In the shallow and intermediate layers, the particles are transported by the Kuroshio, its extension, and the North Pacific subarctic gyre, and reach the Bering Strait (Fig. 3e). The particles on the secondary pathway ascend around the Kuril and Aleutian Islands, where the turbulent vertical mixing is locally enhanced, from the bottom layer to the shallow layer (Fig. 3f).
The trajectories and ascending points of particles evaporating within the Pacific Ocean are similar to those reaching the Arctic Ocean below the intermediate layer (> 1000 m depth). Evaporation of particles mainly takes place in the equatorial Pacific Ocean (figure not shown). The particles are transported to the low-latitude region by the North Pacific subtropical gyre and the Equatorial Counter Current in the shallow layer (Figs. 3g and 3h).
The particles which reside longer in the North Pacific Ocean follow different pathways. Here, the trajectories are extracted for the particles with around the median residence time (between 45 and 55 percentiles) in the Pacific Ocean. (Fig. 4). The trajectories and ascending points of the particles that reach the Southern Ocean spread over the whole Pacific Ocean (Fig. 4a). The returning routes in the deep layer to the Southern Ocean are localized in the western part of the Southwest Pacific Basin for the short-staying particles (Fig. 3a), while the routes of the medium-term-staying particles are distributed over the whole South Pacific Ocean (Fig. 4a). In summary, the main pathway of particles transported to the Southern Ocean in the deep layer is found in the western part of the Southwest Pacific Basin, and the secondary is found in the Southeast Pacific Basin. This main pathway differs from previous suggestions inferred from the distribution of temperature and salinity: along the eastern edge of the Southwest Pacific Basin2 or eastern boundary of the Pacific Ocean9.
The pathway of the short-staying particles that reach the Indian Ocean is localized in the western Pacific Ocean (Fig. 3c), while the pathway of medium-term staying particles spreads over the Pacific Ocean (Fig. 4c). The ascending points are also distributed in the whole Pacific Ocean in the bottom and deep layers (Fig. 4d). These features of the pathway are similar for the particles reaching the Arctic Ocean and the atmosphere (figure not shown).
Dependency on flow field
Because the northward transport of bottom water at the Samoan Passage (and its adjacent passages) in the case 1D is larger than that in the case 3D, the number of released particles is also larger in the case 1D (Figs. 5a and 5c). Almost all the particles leave the Pacific Ocean after 3000 years (Table 2; Fig. 2c). The ratio of the particles reaching the Southern Ocean (57.9%) is larger than that in the case 3D (46.5%), while the ratio of particles reaching the Indian (19.1%) and Arctic Oceans (2.2%) are smaller (Tables 1 and 2). The residence time of the particles reaching the Southern Ocean is smaller than that in the case 3D, while that for the Indian and Arctic Oceans is larger (Tables 1 and 2, and Figs. 2b and 2d). This behavior of the particles in the case 1D is associated with the small vertical mixing in the deep layer.
Table 2
Same as Table 1 but for the case 1D. The total number of particles is 3,475,625.
Destination
|
Number of particles
|
Ratio of particle (%)
|
Volume transport (Sv)
|
Residence time (year)
|
mean
|
median
|
mode
|
Southern Ocean
|
2,013,649
|
57.9
|
6.47
|
497
|
399
|
219
|
Indian Ocean
|
662,513
|
19.1
|
2.13
|
772
|
681
|
589
|
Arctic Ocean
|
78,129
|
2.2
|
0.25
|
827
|
737
|
571
|
Atmosphere
|
719,078
|
20.7
|
2.31
|
855
|
765
|
602
|
residual
|
2,256
|
0.1
|
0.01
|
-
|
-
|
-
|
The ascending points of particles are localized around the ridges, seamounts, and boundaries in the case 3D, while they are homogeneously distributed in horizontal in the case 1D as a result of the horizontally uniform vertical diffusivity (Fig. 6). The strong vertical mixing localized in the western North Pacific Ocean in the case 3D causes the efficient ascending of particles and short required time to reach the Indian and Arctic Oceans (Fig. 2d, Tables 1).