A global analysis of subtropical underwater (STUW) using drifter and tracer methods

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Rana Fine, Committee Chair

Second Committee Member

Donald Olson, Committee Member


The STUW, a component of the Central Waters, is an upper thermocline water mass present in the subtropics and tropics. It is distinguished as a vertical salinity maximum below the mixed layer. The STUW is formed by subduction within the subtropical gyres, where Ekman convergence, and the ambient evaporation-precipitation (E-P) are large. As part of the shallow subtropical cell, it has potentially important implications for climate, due to feedback on the atmosphere of upwelled water property anomalies near the equator, and poleward heat and salt fluxes in the Western Boundary Currents. The STUW is not symmetric about the equator in any of the oceans, but seems to be symmetric about the ITCZ.An analysis of the fate of STUW shows that in the Pacific and Atlantic oceans, the southern STUW dominates the equatorial region. This is not the case in the Indian Ocean. The STUW in the South Indian Ocean does not seem to reach the equator, possibly due to the influence of the Indonesian Throughflow. The strong mixing in the pathway of the fresher Pacific water carried by the Throughflow, destroys the salinity maximum and seems to confine the STUW in the South Indian Ocean. In the North Atlantic, about 2/3 of the STUW circulates in the Gulf Stream recirculation, increasing salinity in the formation region of Subtropical Mode Water (STMW). Most of the STUW in the North Atlantic that flows north in the Gulf Stream is convected into STMW as suggested by Talley et al. (1996).The first calculations of subduction and formation rates for STUW are presented using: (1) drifter data (1988--1998) and (2) CFC-12 data (1987--1995). The two methods are independent except for the calculation of the outcrop area. The use of two independent methods allows us to put bounds on the subduction rate. The subduction rate in the North Atlantic is 36--44 m/yr, in the South Pacific it is 32--33 m/yr, in the South Indian it is 22--25 m/yr and in the North Pacific it is 17--26 m/yr. The formation rates correlate with outcrop area: South Pacific 7 Sv, North Pacific 5 Sv, North Atlantic 2 Sv, and South Indian Ocean <1 Sv. The tracer and drifter methods agree remarkably well (within the estimated uncertainties), with the exception of the North Pacific.To estimate the effect of mixing on the subduction (not included in the drifter calculation), diapycnal salt fluxes are calculated.Effects of interannual variability on the STUW subduction rates are examined in the Pacific, and North Atlantic. (Abstract shortened by UMI.)


Biology, Oceanography

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