Interannual Variation of Annual Subduction Rate in the North Pacific Estimated from a Gridded Argo Product

Spatiotemporal variability of the subduction rate in the North Pacific from 2005 to 2012 is examined based on the Argo observational data. The subduction rate in the subtropical North Pacific varies significantly from year to year between 25 and 50 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1), and it is well correlated with the Pacific decadal oscillation. The temporal change of the subduction rate is largely determined by that of the late winter mixed layer depth through the lateral induction term. The increase (decrease) in the subduction rate in the subtropical mode water areas accompanies densification (lightening) of the mode density class of the subducted water. The subduction rate variability in the central mode water and eastern subtropical mode water regions is anticorrelated as found in the previous study using the output from an ocean GCM. The subduction rate in the central mode water density range changes dramatically, which is very large in 2005 and 2010 but almost disappears in 2009. The subduction rate variability in the western subtropical mode water regions seems to be correlated with the Pacific decadal oscillation with a lag of a few years.

Formation and Subduction of Central Mode Water Based on Profiling Float Data, 2003–08

Temperature and salinity data from Argo profiling floats in the North Pacific during 2003–08 have been analyzed to study the structure of winter mixed layer north of the Kuroshio Extension and the subsurface potential vorticity distribution in the subtropical gyre in relation to the formation and subduction of the central mode water (CMW). In late winter, two zonally elongated bands of deep mixed layer extend at 33°–39° and 39°–43°N, from the east coast of Japan to 160°W. These correspond to the formation region of the lighter variety of CMW (L-CMW) and that of the denser variety of CMW (D-CMW) and the recently identified transition region mode water (TRMW), respectively. In the western part of the L-CMW and D-CMW–TRMW formation regions west of 170°E, the winter mixed layer becomes deeper and lighter to the east (i.e., to the downstream). As a result, the formed mode water is reentrained into the mixed layer in the farther east in the following winter and modified to the lighter water and is thus unable to be subducted to the permanent pycnocline. In the eastern part of the formation regions between 170°E and 160°W, on the other hand, the winter mixed layer becomes shallower and lighter to the east. From these areas, the L-CMW with potential density of 25.7–26.2 kg m−3 and the D-CMW–TRMW (mostly the former) of 26.1–26.4 kg m−3 are subducted to the permanent pycnocline, and they are then advected anticyclonically in the subtropical gyre. These results imply that during the analysis period large-scale subduction to the permanent pycnocline occurs in the density range up to 26.4 kg m−3 in the open North Pacific, whereas the winter mixed layer density reaches the maximum of 26.6 kg m−3. This is supported by the vertical distribution of apparent oxygen utilization in a hydrographic section in the subtropical gyre.

Distributions of mixed layer properties in North Pacific water mass formation areas: comparison of Argo floats and World Ocean Atlas 2001

Winter mixed layer characteristics in the North Pacific Ocean are examined and compared between Argo floats in 2006 and the World Ocean Atlas 2001 (WOA01) climatology for a series of named water masses, North Pacific Tropical Water (NPTW), Eastern Subtropical Mode Water (ESTMW), North Pacific Subtropical Mode Water (NPSTMW), Light Central Mode Water (LCMW) and Dense Central Mode Water (DCMW). The WOA01 is found to be in good agreement with the Argo data in terms of water mass volumes, average temperature-salinity (T-S) properties, and outcrop areas. The exception to this conclusion is for the central mode waters, DCMW and LCMW, whose outcropping is shown to be much more intermittent than is apparent in the WOA01 and whose T-S properties vary from what is shown in the WOA01. Distributions of mixed layer T-S properties measured by floats are examined within the outcropping areas defined by the WOA01 and show some shifting of T-S characteristics within the confines of the named water masses. In 2006, all the water masses were warmer than climatology on average, with a magnitude of about 0.5°C. The NPTW, NPSTMW and LCMW were saltier than climatology and the ESTMW and DCMW fresher, with magnitudes of about 0.05. In order to put these results into context, differences between Argo and WOA01 were examined over the North Pacific between 20 and 45° N. A large-scsale warming and freshening is seen throughout this area, except for the western North Pacific, where results were more mixed.

Distributions of mixed layer properties in North Pacific water mass formation areas: comparison of Argo floats and World Ocean Atlas 2001

Winter mixed layer characteristics in the North Pacific Ocean are examined and compared between Argo floats in 2004 and 2005 and the World Ocean Atlas 2001 (WOA01) climatology for a series of named water masses, North Pacific Tropical Water (NPTW), Eastern Subtropical Mode Water (ESTMW), North Pacific Subtropical Mode Water (NPSTMW), Light Central Mode Water (LCMW) and Dense Central Mode Water (DCMW). The WOA01 is found to be in good agreement with the Argo data in terms of water mass volumes, average temperature-salinity (T-S) properties, and outcrop areas. The exception to this conclusion is for the central mode waters, especially DCMW, whose outcropping is shown to be much more intermittent than is apparent in the WOA01 and whose T-S properties vary from what is shown in the WOA01. Distributions of mixed layer T-S properties measured by floats are examined within the outcropping areas defined by the WOA01 and show some shifting of T-S characteristics within the confines of the named water masses. In 2005, all the water masses were warmer than climatology on average, with DCMW being highest at about 1°C. Similar results were found for the 2004 Argo data except ESTMW and DCMW which were slightly cooler than climatology. Differences between float data and climatology were examined for the entire North Pacific in order to put the above results into context. This analysis showed the winter North Pacific mixed layer to be warmer and fresher than climatology in both 2004 and 2005, with magnitudes of about 0.3–0.4°C and 0.06–0.07. This warming and freshening was apparent throughout a large area of the tropics and northeastern North Pacific, but in the mode water formation areas the trends were less clear.