Jonny Williams

and 5 more

Erik Behrens

and 5 more

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.

Jonny Williams

and 4 more

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.

Guang Zeng

and 20 more

We quantify the impacts of halogenated ozone-depleting substances (ODSs), methane, N2O, CO2, and short-lived ozone precursors on total and partial ozone column changes between 1850 and 2014 using CMIP6 Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) simulations. We find that whilst substantial ODS-induced ozone loss dominates the stratospheric ozone changes since the 1970s, the increases in short-lived ozone precursors and methane lead to increases in tropospheric ozone since the 1950s that make increasingly important contributions to total column ozone (TCO) changes. Our results show that methane impacts stratospheric ozone changes through its reaction with atomic chlorine leading to ozone increases, but this impact will decrease with declining ODSs. The N2O increases mainly impact ozone through NOx-induced ozone destruction in the stratosphere, having an overall small negative impact on TCO. CO2 increases lead to increased global stratospheric ozone due to stratospheric cooling. However, importantly CO2 increases cause TCO to decrease in the tropics. Large interannual variability obscures the responses of stratospheric ozone to N2O and CO2 changes. Substantial inter-model differences originate in the models’ representations of ODS-induced ozone depletion. We find that, although the tropospheric ozone trend is driven by the increase in its precursors, the stratospheric changes significantly impact the upper tropospheric ozone trend through modified stratospheric circulation and stratospheric ozone depletion. The speed-up of stratospheric overturning (i.e. decreasing age of air) is driven mainly by ODS and CO2; increases. Changes in methane and ozone precursors also modulate the cross-tropopause ozone flux.