A document by Christopher John Roach. Click on the document to view its contents.

The location of the Subtropical Front (STF), the boundary between Subtropical and Subantarctic Water in the Southern Ocean is proposed to be controlled by the strength and location of the Southern Hemisphere westerly winds. We use a hydrodynamic hindcast model and recent observations to test if changes in the westerly winds can cause meridional shifts in the STF over interannual to decadal time scales by modulating local Ekman transport. We find that increased, or northward, shifted westerly winds lead to an enhanced northward Ekman transport over large parts of the Southern Ocean, resulting in a northward shift in the STF. Conversely for weaker or southward shifted westerly winds. Regions with strong eddy variability, such as western boundary current systems of the Agulhas and East Australian Current behave differently, as the Sverdrup balance causes an opposite shift. In these regions an increase in westerly winds lead to a southward shift in the STF. A southward shift of STF has been observed between 2004-2019. However, the shift is smaller than the latitudinal shifts in the location of the zero wind stress curl and maximum westerly winds (-0.4° latitude/decade). This discrepancy is due to positive Ekman trends resulting from the intensification of the westerly winds, which oppose the southward migration. Changes in the Ekman transport and the overall southward shift of the STF have also resulted in an observed positive trend in chlorophyll-a concentrations south of the STF, which could have ramifications for the biological pump and carbon uptake in the Southern Ocean.

This paper describes the development of New Zealand’s Earth System Model (NZESM) and evaluates its performance against its parent model (United Kingdom Earth System Model, UKESM) and observations. The main difference between the two earth system models is an embedded high-resolution (1/5°) nested region over the oceans around New Zealand in the NZESM. Due to this finer ocean model mesh, boundary currents such as the East Australian Current, East Australian Current Extension, Tasman Front and Tasman Leakage and their transports are better simulated in NZESM. The improved oceanic transports have led to a reduction in upper ocean temperature and salinity biases over the nested region. In addition, net transports through the Tasman Sea of volume, heat and salt in the NZESM agree better with previously reported estimates. A consequence of the increased cross-Tasman transports in the NZESM is increased temperatures and salinity west of Australia and in the Southern Ocean reducing the meridional sea surface temperature gradient between subtropics and sub-Antarctic. This also leads to a weakening of the westerly winds between 60S and 45S over large parts of the Southern Ocean, which reduces the northward Ekman transport, reduces the formation of Antarctic Intermediate Water and allows for a southward expansion of the Super-Gyre in all ocean basins. Connecting an improved oceanic circulation around New Zealand to a basin-wide Super-Gyre response is an important step forward in our current understanding of how local scales can influence global scales in a fully coupled earth system model.

We report the results of two Earth System Model (ESM) configurations which differ in their ocean physics around New Zealand. The first is a global low-resolution configuration of UKESM1.0 while the second model, NZESM has an eddy-permitting ocean embedded around New Zealand. The nominal ocean resolution of the UKESM is 1 degree and that of the NZESM is 0.2 degrees. Near New Zealand, total cloud amount is negatively correlated with temperature. This relationship is reversed near the seasonal sea ice edge where increased evaporation results from open ocean which was previously covered in sea ice. In the simulations, the change to the cloud amount is dominated by changes to stratocumulus cloud and the resulting improvement to shortwave cloud radiative effect - with respect to CERES-EBAF observations - is statistically significant at the 95% level across the Southern Ocean, assuming a normally distributed control ensemble. The near-surface air temperature in the vicinity of the nested ocean model is also improved, when compared to ERA5 reanalysis data. In general, clouds and their radiative effects over the Southern Ocean are not well simulated by Earth System Models and the changes made here improve both near-surface temperature near New Zealand and zonal mean shortwave cloud radiative effect across the Southern Ocean. Noting that the development of climate models always involves an element of ‘tuning’, changing the regional ocean physics without doing any further tuning (as is the case here), will tend to remove some compensating bias and therefore make the model-observation agreement in some regions less good.

Knowledge about the early life history of Antarctic toothfish (Dissostichus mawsoni) is still incomplete, particularly on the spatial and temporal extent of spawning and the subsequent transport of eggs and juveniles from the offshore spawning areas to the continental shelf. This study used a high-resolution hydrodynamic model to investigate the impact of ocean circulation and sea-ice drift on the dispersal of eggs and juvenile Antarctic toothfish. The virtual eggs were released on seamounts of the Pacific-Antarctic ridge in the northern Ross Sea and advected using hydrodynamical model data. Particles were seeded annually over a 14-year period (2002 to 2016) and tracked for three years after release. Spawning success was evaluated based on the number of juveniles that reached known coastal recruitment areas, in the eastern Ross and Amundsen Sea, within three years. Observations show that juveniles (50-100 cm size class) are abundant on the shelf and slope of the Ross and Amundsen Seas. Sensitivities to certain juvenile behaviours were explored and showed that spawning success was reduced by around 70% if juveniles drifted with sea-ice during the second winter season as this carried them into the open ocean away from the shelf region. Spawning success increased during the second winter season if juveniles were entrained in the Ross Gyre circulation or if they actively swam towards the shelf. These modelling results suggest that the ecological advantage of sea-ice association in the early life cycle of toothfish diminishes as they grow, promoting a behaviour change during their second winter.

The meridional variability of the Subtropical Front (STF) and the drivers of variability on interannual time scales in the New Zealand region are analysed using a multi-decadal eddy-resolving ocean hindcast model, in comparison with Argo data. The STF marks the water mass boundary between subtropical waters and subantarctic waters, and is defined as the southern-most location of the 11 degree C isotherm and 34.8 isohaline between 100 m and 500 m. The STF shifts up to 650 km (6 degree) meridionally on seasonal timescales. In addition to seasonal variability, shifts of around 200 km (2 degree) occur on interannual time scales. These shifts are connected to local wind stress curl anomalies in the eastern Tasman Sea, which trigger Ekman convergence/divergence and result in meridional transport of heat and salt into/out of the Tasman Sea. The net transports across the northern boundary of the Tasman Sea show the largest sensitivity to these wind stress curl anomalies. During periods of positive wind stress curl anomalies and Ekman convergence, the heat and salt content increases shifting the position of the STF southward. The opposite tendency occurs during periods of negative wind stress curl anomalies. The migration of the STF does not appear to be directly linked to regional climate oscillations.