Shipboard Observations to Clarify the Formation and Subduction of Central Mode Water in Relation to Fronts and Eddies

Abstract:

Large-scale picture of formation and subduction of Central Mode Water (CMW) and Transition Region Mode Water (TRMW) in the North Pacific has been presented by recent analyses of Argo float data. We further need to update the picture by clarifying the relation of the mode water formation and subduction to thermohaline fronts and mesoscale eddies. For such purpose, synoptic observation data with enough horizontal resolution are indispensable.

We analyzed hydrographic data east of Japan from five research cruises and Argo floats in spring 2003 to examine the relation of the formation of CMW and TRMW in the late winter of 2003 to fronts and eddies. TRMW and the denser variety of CMW (D-CMW) were formed continuously just south of the subarctic frontal zone between 155ºE and 165ºW with little relation to eddies, suggesting that the absence of the permanent thermocline and halocline in this area is essential for the formation. The lighter variety of CMW (L-CMW) was formed south of the Kuroshio bifurcation front and east of 165ºE, partly in an anticyclonic eddy associated with the Kuroshio Extension. Some portion of D-CMW and L-CMW likely had been subducted to the permanent pycnocline by crossing southward the Kuroshio bifurcation front and the Kuroshio Extension front, respectively. In contrast, the formation of these waters in the western regions was inactive and was significantly different from that described previously using multiyear Argo float data. West of 155ºE, TRMW and D-CMW were formed only in two anticyclonic eddies that had been detached from the Kuroshio Extension one to two years ago. L-CMW was hardly formed west of 165ºE, which might be related to the upstream Kuroshio Extension being in its stable state characterized by low regional eddy activity.

We also conducted a hydrographic observation in the TRMW/D-CMW formation region in April 2013 using R/V Hakuho-maru. The KH-13-3 cruise was disrupted by cyclones seven times, and we had to focus on the westernmost part of the TRMW formation region where deepest winter mixed layers in the North Pacific have been observed. Along a zonal section between 42ºN, 151ºE and 41ºN, 161ºE, we observed thick TRMW and D-CMW just after formation at an interval of 10’ in longitude. We revisit the L-CMW and D-CMW/TRMW formation regions in June 2015 to complement the 2013 observation.