Wang, Matear, and Trull, JGR-oceans, accepted for publication.

This paper presents an assessment of resupply of phosphate across the

seasonal thermocline in the Sub-Antarctic and Polar Frontal Zones, and

finds that it approximately doubles the amount of carbon export estimated

from seasonal nutrient depletion in surface waters.

Modeling the seasonal phosphate export and resupply in the Subantarctic and

Polar Frontal Zones in the Australian sector of the Southern Ocean

Xiujun Wang1,2, Richard J. Matear1,3, and Thomas W. Trull1

1. Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart, 7001,

Tasmania, Australia.

2. Institute of Antarctic and Southern Ocean Studies, University of

Tasmania, GPO Box 252-77, Hobart, 7001, Tasmania, Australia.

3. CSIRO Division of Marine Research, GPO Box 1538, Hobart, 7001, Tasmania,

Australia.

Revised on 19 April , 2001

Abstract

A one-dimensional biophysical model was developed to simulate the seasonal

phosphate export production and resupply in the Subantarctic Zone (SAZ) and

the Polar Frontal Zone (PFZ). The model was calibrated using the seasonal

phosphate observations, and the Sea-viewing Wide Field-of-view Sensor

(SeaWiFS) chlorophyll a estimates of phytoplankton. The model was capable

of simulating the seasonal amplitudes of sea surface temperature and mixed

layer depth. The modeled annual export production was significantly

higher in the PFZ (8.6 - 10.1 mol C m-2) than in the SAZ (6.3 - 7.2 mol C

m-2) despite the PFZ having lower primary production and lower seasonal

nutrient depletion. The higher export production in the PFZ was

accomplished by having much larger resupply of nutrients to the upper ocean

(53 - 63 %) than the SAZ (22 - 32 %) for the period of September to March.

In the SAZ, vertical mixing is the dominant process responsible for the

phosphate resupply while in the PFZ, vertical mixing supplied only 37 % of

the phosphate, while horizontal transport supplied 63 % of the phosphate

into the euphotic zone. Our modelling work reconciles the previous apparent

contradiction of low seasonal nutrient depletion in the PFZ [Lourey and

Trull, 2000] in the presence of high export production (as indicated by

mesopelagic Ba concentration [Cardinal et al., 2000]).