CSIRO Oceans and Atmosphere

Most of the world's food is produced in a handful of countries, the so-called breadbaskets of the world. Due to climate change, there is an increasing risk of crop failures, due to compounding hot and dry extremes. Furthermore, certain climate drivers – through teleconnections – have shown to lead to simultaneous crop failures around the globe. This highlights the importance to understand which climate processes drive global crop yield variability. Here we show global crop yield failures (Maize, Soya, Wheat, Rice, and combined) are associated with La Nina-like sea surface temperature (SST) anomalies, using Archetype Analysis. The adverse crop-yield archetypes show simultaneous hot-dry-surface imprints across the world, highlighting these high risk crop failure scenarios are driven by climate extremes. Our results demonstrate the importance in understanding the climate drivers of global crop production, and highlights the deep uncertainty associated with a changing climate. The response of ENSO due to anthropogenic activities is not yet fully understood and climate models often inaccurately reproduce the observed La Nina trends. Thus the fact that our results indicate that simultaneous crop failures are linked to La Nina like SSTs, highlights the deep uncertainty we currently face regarding food security in the future.

Land carbon sinks are responsible for removing about a quarter of anthropogenic CO$_2$ emissions, and make up approximately half of total global carbon sinks. Uncertainty in the response of land carbon sinks to climate and increasing CO$_2$ emissions are large, and dominate the uncertainty in total carbon sinks over the next century. Understanding the carbon cycle response to net-zero and net-negative emissions has important implications for projecting future climate. Experiments in the `flat10' model intercomparison (flat10MIP) were designed for directly estimating key climate metrics that underlie carbon budgeting frameworks. Here we characterize the response of land carbon pools and fluxes from ten emissions-driven Earth system models (ESMs) under positive, net-zero, and net-negative CO$_2$ emissions. Although there are many differences in simulated land carbon pools and fluxes across models, we find some consistent behavior across ESMs. 1) During the positive emissions phase, carbon is gained on land -primarily in vegetation pools- in both the tropics and mid-latitudes. 2) Following net-negative emissions to the point of cumulative zero emissions, vegetation carbon is lost from land. 3) In tropical latitudes, total carbon is lost coming primarily from vegetation pools, but in mid-latitudes nearly all models show net land carbon gain, primarily in soil pools. 4) Following an extended period of net-zero emissions, a majority of models again show carbon gain in mid-latitudes and vegetation carbon loss in the tropics. Under net-negative emissions the timing of vegetation carbon response relative to peak emissions is relatively consistent across ESMs, but timing of soil carbon response varies widely, implying larger intermodel disagreement associated with the longer timescale responses of land carbon.

<strong class="journal-contentHeaderColor">Abstract.</strong> Regime dependencies and Granger causal relationships between tropical and extratropical teleconnections are inferred using Bayesian structure learning. Using ERA5 data, an examination of the differences between the learned graphical structures during particular phases of the Interdecadal Pacific Oscillation (IPO) are used to infer the role of the background state on interactions between the major climate teleconnections. These relationships present a clear regime dependency on the phase of IPO. In the positive phase, IPO autocorrelations are weak whereas Indian Ocean Dipole (IOD) and El Ni&ntilde;o Southern Oscillation (ENSO) autocorrelations and the influence of the Madden Julian Oscillation (MJO) are indicative of an enhanced Walker circulation. In contrast, during the negative phase, IPO autocorrelations are strongest with evidence of an enhanced role for extratropical teleconnections on the tropics. Exclusion of MJO removes important tropical extratropical influences while increasing posterior edge weights between ENSO, the IPO and IOD. Our analysis reveals the dependence of the ENSO autocorrelation on the phase of the background IPO state, and the role of the MJO as being key to link the extra tropical tropospheric modes (PNA, NAO) and equatorial surface ocean temperatures (IOD, ENSO) and as a consequence convection.

Abstract Despite extensive research on the western Pacific crown-of-thorns starfish (CoTS; Acanthaster cf. solaris ), especially from Australia’s Great Barrier Reef (GBR), there are persistent knowledge gaps that constrain understanding and management. Given renewed population irruptions of CoTS on the GBR, alongside escalating climate impacts and direct anthropogenic pressures, a quantitative assessment of knowledge gaps was undertaken to identify research priorities and guide future efforts. In this study, 27 experts independently scored 206 research questions across seven Themes and 39 topics, based on four different criteria ( Knowledge gap , Feasibility , Urgency , and Applicability ). These questions were ultimately rationalized into 170 distinct questions, with scores aggregated across merged questions. Management was the highest scoring Theme for Urgency , Applicability , and overall. All but six of the 20 top-ranked questions were from the Management Theme and mainly related to Culling and/ or Monitoring. Most questions (158 out of 170) also scored highly for Feasibility , suggesting that there are existing methods and technical capability available to advance the extensive and diverse range of research questions that were posed, if resources were made available. This quantitative assessment provides guidance on the prioritization of potential research that could improve management of CoTS populations to improve coral protection outcomes on the GBR. This study also suggests that there are limited constraints to undertaking necessary research to address many of the persistent knowledge gaps relating to CoTS.

Abstract The radioisotope 36 Cl was produced prolifically by nuclear weapons testing, principally at Bikini and Enewetak Atolls in the 1950s. Much of this was injected into the stratosphere, from where it was dispersed worldwide. The fallout of this bomb‐produced 36 Cl has been measured as a function of time in an ice core from a high‐accumulation site in Antarctica. These measurements complement an earlier study in an Arctic ice core. A box model has been developed to describe simultaneously the data from both the two sites. Using an up‐to‐date catalog, the 36 Cl production of the individual tests has been estimated and used as input to the box model. Production is dominated by high‐yield thermonuclear tests carried out on barges moored in the lagoons of the atolls in the 1950s. Exchange times of 3.7 years for gaseous exchange between stratosphere and troposphere and 4.2 years for gaseous exchange between the stratospheres of the two hemispheres are deduced. The 36 Cl bomb pulse constitutes a global tracer with an input function that is well‐defined in space and time, and its exchange between stratosphere and troposphere is dominated by one‐way transport due to its rapid removal once in the troposphere. This latter behavior greatly simplifies the interpretation of the data in terms of an exchange time relative to the 14 C bomb pulse. The stratosphere‐troposphere exchange time derived here from the bomb‐produced 36 Cl offers an independent constraint on the flux of ozone between stratosphere and troposphere which agrees well with other more direct estimates.

Abstract. A new configuration of the Australian Community Climate and Earth System Simulator coupled model, ACCESS-CM2, with a higher resolution ocean-sea ice component at 0.25° is introduced. The higher resolution ACCESS-CM2-025 model was developed to better represent the ocean mesoscale and expand the scope of climate modelling research applications. The individual model components have not been changed compared with ACCESS-CM2-1, the existing lower resolution version of the model at 1°, which was one of Australia's contributions to the World Climate Research Program's Coupled Model Intercomparison Project Phase 6 (CMIP6). This paper assesses the simulated climate for a 500 years present-day run in ACCESS-CM2-025 against observations, the lower resolution ACCESS-CM2-1 model, and two ocean-sea ice models using the same model components and comparable grid resolutions but with prescribed atmospheric forcing. ACCESS-CM2-025 is more energetic and performs better in regions of elevated ocean mesoscale variability such as at western boundary currents. The higher resolution ACCESS-CM2-025 also features a more realistic ENSO life cycle and seasonality, with a reduced biennality, which is common in the lower resolution ACCESS-CM2-1. Both ACCESS-CM2 models share many biases, particularly near the sea surface and also affecting sea ice coverage, reflecting insufficiency in the atmospheric model component. While ACCESS-CM2-025 exhibits improved time-mean deep convection, sea ice, and mixed layer depth in the North Atlantic, it also experiences multidecadal variability, which is evident in many variables, including the Atlantic Meridional Overturning Circulation.

<strong class="journal-contentHeaderColor">Abstract.</strong> We present a new ultra-high resolution 1/30&deg; (&sim;3 km) regional ocean model of the eastern Australian region and evaluate the performance of this model against two 1/10&deg; (&sim;10 km) models as well as a suite of satellite and in situ observations. We evaluate model biases in the context of (i) submesoscale-permitting (&sim;3 km) vs. mesoscale-permitting (&sim;10 km) horizontal resolution and (ii) differences between version 5 and version 6 of the Modular Ocean Model (MOM5 vs. MOM6) to assess the added value in each case and determine the suitability of our higher resolution model for scientific research. There are some consistent biases shared by the two regional MOM6 configurations, and also in the higher resolution configuration that are not seen in the lower resolution models. These biases are further investigated with two sets of sensitivity experiments to understand the effect of submesoscale eddy parameterization and imposed dynamic viscosity at a submesoscale-permitting resolution. The high-resolution simulation has much higher variance compared to the lower resolution simulations across all evaluation metrics, indicating that the greater spectrum of length scales also manifests in more variability in the temporal domain. The two MOM6 regional configurations of differing resolution appear to be more aligned than the regional (MOM6) and global (MOM5) 1/10&deg; configurations in most results, reflecting the substantial changes made to the MOM between version 5 and version 6. Importantly, we also show that higher resolution is not a panacea: in regions where key dynamics are quasi-linear and well-captured captured by coarser grids (e.g., the EAC jet), further refinement may offer limited benefit &ndash; and actually degrade the performance if parameterizations are not appropriately tuned.

Abstract The southwest Pacific Ocean (SWPO), located on the western side of the South Pacific subtropical gyre, has exhibited a notable trend of subsurface freshening over the past several decades. Using observation‐based ocean salinity data sets, we find that SWPO subsurface freshening has slowed in the most recent three decades (1990–2020), to a rate less than one‐fourth of that observed during the prior three decades (1960–1990). To understand the drivers of this subsurface salinity variability, we conduct surface forcing perturbation experiments using a global ocean‐sea ice model, specifically targeting the distinct impacts of changes in surface heat, freshwater, and momentum (wind stress) fluxes. Our study reveals that the subsurface freshening slowdown in the SWPO since the 1990s is largely (75%) attributed to a weakened poleward shift of the westerly winds. This shift induces isopycnal deepening, enhanced poleward Ekman transport, and subsequent subduction of saltier subtropical surface waters into the ocean interior. Surface heat fluxes also contribute to these to a lesser extent (37%) via the spiciness component, as surface cooling at high latitudes shifts isopycnal outcrops equatorward, enabling saltier surface waters to be subducted; these effects are partly offset by freshwater fluxes (−12%). Further analysis suggests that these surface flux changes are at least partly linked to a multidecadal shift in the interdecadal Pacific oscillation around 1990. The mechanisms identified in this study strengthen our understanding of how ocean interior salinity responds to large‐scale climate variability and provide insights for better detecting and quantifying the intensification of the global water cycle.