A document by Yusuf A. Bhatti. Click on the document to view its contents.

Equilibrium climate sensitivity - ECS - is easily-understood as the global mean atmospheric warming for a doubling of CO2. It is widely applied, has been studied for over 150 years and is therefore appealing as a metric for communication of climate model results. However, here we argue that ECS is not a good metric for comparing different climate models and is no longer appropriate because of expanding model design and conditions. Using brief examples concerning the Pliocene epoch and the Paleocene–Eocene Thermal Maximum, it is further posited that models which produce temperatures towards the higher end of model intercomparisons are useful in spite of recent studies concluding that these models are `too hot'. It is hoped that this brief manuscript generates discussion on how to prioritise the consideration of more useful, and potentially novel, ways of comparing climate models going forward.

In this work we analyse the impact of including a regional, high-resolution ocean model on simulated atmospheric climate in a coupled earth system model. The resolution of the regional, nested ocean model is approximately 0.2 degrees compared to the 1 degree resolution of the global ocean model within which it is embedded and this work complements previously published work on ocean circulation and marine heatwaves using the New Zealand Earth System Model, NZESM. After a discussion of the eddy-permitting capability of the nested ocean and its coupling to the overlaying atmosphere, we study the effects on air temperature, precipitation and evaporation, latent and sensible surface heat balances, zonal wind, the storm track and the effect on total cloud amount. Overall we find that the NZESM provides a better representation of regional atmospheric climate compared to its parent model - UKESM1 - although this improvement is not universal. For example, although the NZESM shows better agreement in surface air temperature within the nested ocean region, there is also some deterioration in the agreement  at high southern latitudes where the seasonal sea ice edge coincides with a transition from negative to positive correlation between air temperature and cloud amount. The lack of additional model tuning in the NZESM after the nested ocean model's inclusion largely accounts for the presence of these improvement-deterioration pairs with respect to observations.

We assess the effective radiative forcing due to ozone-depleting substances using models participating in the Aerosols and Chemistry Model Intercomparison Project (AerChemMIP). A large inter-model spread in this globally averaged quantity necessitates an “emergent constraint” approach whereby we link the radiative forcing to the amount of ozone depletion simulated during 1979-2000, excluding two volcanically perturbed periods. During this period ozone-depleting substances were increasing, and several merged satellite-based climatologies document the ensuing decline of total-column ozone. We use these analyses to come up with effective radiative forcing magnitudes. For all of these satellite climatologies we find an effective radiative forcing outside or on the edge of the previously published “likely” range given by the 5th Assessment Report of IPCC, implying an offsetting effect of ozone depletion and/or other atmospheric feedbacks of -0.4 to -0.25 Wm-2, which is in absolute terms is larger than the previous best estimate of -0.15 Wm-2.

This work provides an updated set of 12 dominant geopotential height fields - or ‘regimes’ - over Aotearoa New Zealand. These regimes were initially produced by Kidson (2000) and have provided the basis for many other subsequent studies. These maps provide a guide to the prevailing weather due to the broad equivalence of 1000hPa geopotential height and mean-sea-level pressure. The results presented here are broadly in agreement with previous work but with some important differences. The most notable of these being the need to average two blocking regimes together to provide good agreement between this work and Kidson (2000). These differences are attributed to the software used, improvements to the underlying dataset itself and to the ‘mixing’ of statistically indistinguishable empirical orthogonal functions - EOFs - in different linear combinations. All data and code used in this work is publicly accessible and it is hoped that this will provide a catalyst for open discussions on this topic, particularly with relation to future perturbations to these regimes under climate change.

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.

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.