A method to estimate dynamic height and geostrophic transport relative to deeper levels from 400m expendable bathythermograph in the Indonesian throughflow

Abstract

The TOGA/WOCE Expendable Bathythermograph (XBT) Programme used a mix of T4 (450 m) and T7 (750 m) XBT's during the pre-TOGA periods. Studies are needed to determine how to use the T4/T7 data together, in particular with regard to a reference level for calculation of dynamic height and geostrophic currents. Temperature profiles to 750 m collected from 1986 through 1989 on the trackline across the Indonesian throughflow between NW Australia and Java are used to show the relations between dynamic height and geostrophic flow using reference levels at 400 db and 750 db. A very high temporal correlation between vertically averaged temperature in the upper 400 m and dynamic height at 50 m relative to 750 db was found. The corresponding regression relationships are presented for all one degree latitude bins along the section and can be used for dynamical calculation of currents in the upper 400 m relative to 750 db. An attempt is made to estimate volume transport relative to 750 db from 400 m profiles. Problems which make estimation of transport more difficult than estimation of dynamic height are identified. A method which overcomes the problems is proposed and it appears to work extremely well for longterm mean temperature sections. When the method was applied to individual cruise data, errors in estimation of the net transport through the section were occasionally as large as 4 Sv (1 Sv=106 m3/s) and often about 2 Sv. We believe the errors are largely due to high-frequency internal waves and eddies, which strongly affect transports at the stations at the ends of a section. An experiment showed that if the temperature profiles down to 750 m at the two ends of the section were available, the estimated 750 m transport using the regression method with temperature of the upper 400 m in the middle of the section would be very accurate for both the longterm mean and individual cruise data. Future design of the Global Ocean Observing System needs more upgrading of the XBT network with temperature moorings at the ends of sections, especially where they end near the continental shelf.