Abstract

Subantarctic mode water (SAMW) is a subsurface water mass which is formed through surface heat loss. This leads to thick winter mixed layers which are then subducted resulting in low stratification water-masses. SAMW formation regions are important for the storage and transport of heat and carbon around the globe. In the South Pacific mode water formation region, central and eastern pools of mode water have been found to have wintertime thicknesses that vary strongly interannually and out of phase across the basin. This thickness variability is associated with changes in atmospheric forcing, as evident in peaks in variability in sea level pressure between the central and eastern pools. However, the mechanisms by which the external forcing drives this thickness variability have not been quantified.

To investigate how external forcing affects the properties of SAMW in the central and eastern mode water pools, a set of adjoint sensitivity experiments are conducted. The traditional approach to adjoint sensitivity experiments in the ECC Ov4 state estimate uses a vertical mask that is fixed at all times. Instead, here the adjoint is developed so that a density following mask is employed, which more closely reflects how water masses preferentially spread along density surfaces.

The adjoint sensitivity experiments are used to directly quantify the role of local versus remote forcing in driving the variability in regional mode water properties raised in recent studies. Two separate adjoint sensitivity experiments are completed with horizontal masks in the central and eastern pools of mode water in the south Pacific. The objective function used here is the yearly averaged volume over the pool and the density surfaces. The analysis of the adjoint sensitivities focus on comparing the sensitivities to the north and the south of the Subantarctic Front (SAF) and in winter and summer months. The impact of the sensitivities is provided by multiplying the sensitivities by their respective anomalies and then comparing the central and eastern pools. The analysis reveals the dominant control of the thickness variability comes from local and far field surface heat fluxes. It is also found that there are opposing responses to net heat flux to the north and south of the SAF .