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• Marine Heatwaves (MHW) can be defined relative to different baselines. • The baselines determine whether long term warming is included or excluded. • Different baselines convey different levels of changing risk for marine species. • The choice of baseline should be carefully considered to best fit the application. Marine heatwaves (MHWs), prolonged periods of unusually high ocean temperatures, significantly impact global ecosystems. However, there is ongoing debate regarding the definition of these extreme events, which is crucial for effective research and communication among marine scientists, decision-makers, and the broader public. Fundamental to all MHW analyses is a clearly defined background oceanic climate – i.e., a temperature 'baseline' against which the MHW is defined. While a single approach to implementing a baseline may not be suitable for all MHW research applications, the choice of a baseline for analysing MHWs must be intentional as it affects research outcomes. This perspective examines baseline choices and discusses their implications for marine organism and ecosystem risks, and their relevance in communicating MHW characteristics and metrics to stakeholders, policymakers, and the public. In particular we analyse five different baseline approaches for computing MHW statistics, assess their technical differences, and discuss their ecological implications. Different baselines suggest widely different trends in MHW characteristics in a warming world. This would, for example, imply differences in future risk, reflective of marine organisms with different adaptive potential, thereby affecting recommendations for management strategies. We also examine the consequences of different baseline choices on ease of implementation and communication with wider audiences. Our analyses highlight the need to clearly specify a chosen baseline in MHW studies, and to be mindful of its implications for MHW statistics, practical considerations, and interpretations concerning the adaptive capacities of marine organisms, ecosystems and human systems. The challenges and implications of different MHW baselines highlighted here have similar relevance in research and communication for other branches of climate extremes. [ABSTRACT FROM AUTHOR]

Western Boundary Currents (WBCs) are important for the oceanic transport of heat, dissolved gases and nutrients. They can affect regional climate and strongly influence the dispersion and distribution of marine species. Using state-of-the-art climate models from the latest and previous Climate Model Intercomparison Projects, we evaluate upper ocean circulation and examine future projections, focusing on subtropical and low-latitude WBCs. Despite their coarse resolution, climate models successfully reproduce most large-scale circulation features with ensemble mean transports typically within the range of observational uncertainty, although there is often a large spread across the models and some currents are systematically too strong or weak. Despite considerable differences in model structure, resolution and parameterisations, many currents show highly consistent projected changes across the models. For example, the East Australian Current, Brazil Current and Agulhas Current extensions are projected to intensify, while the Gulf Stream, Indonesian Throughflow and Agulhas Current are projected to weaken. Intermodel differences in most future circulation changes can be explained in part by projected changes in the large-scale surface winds. In moving to the latest model generation, despite structural model advancements, we find little systematic improvement in the simulation of ocean transports nor major differences in the projected changes. [ABSTRACT FROM AUTHOR]

The Southern Hemisphere (SH) surface westerlies fundamentally control regional patterns of air temperature, storm tracks, and precipitation while also regulating ocean circulation, heat transport and carbon uptake. Wind‐forced ocean perturbation experiments commonly apply idealized poleward wind shifts ranging between 0.5 and 10 degrees of latitude and wind intensification factors of between 10% and 300%. In addition, changes in winds are often prescribed ad hoc as a zonally uniform anomaly that neglects important regional and seasonal differences. Here we quantify historical and projected SH westerly wind changes based on examination of CMIP5, CMIP6, and reanalysis data. We find a significant reduction in the location bias of the CMIP6 ensemble and an associated reduction in the projected poleward shift compared to CMIP5. Under a high emission scenario, we find a projected end of 21st Century ensemble mean wind increase of ∼10% and a poleward shift of ∼0.8° latitude, although there are important seasonal and regional variations. Plain Language Summary: The westerly winds in the Southern Hemisphere have increased in speed and shifted toward Antarctica over the last few decades, with model projections suggesting further poleward intensification in the future. Changes in the westerly winds are of great importance because they control the Southern Ocean circulation, ocean carbon uptake, and ocean heat transport, with ramifications for global climate. To understand the impacts of changes in the westerlies on the Southern Ocean, ocean model simulations are often run by artificially increasing and/or shifting winds toward Antarctica to understand the ocean impact of future changes in the winds. However, there is no consistency in the way these changes are incorporated, with large variations in the applied shift and strengthening. In this study, we quantify recent observed and projected changes in the surface westerlies, aiming to provide guidance as to what wind perturbations should be applied in ocean models. We further show that the latest generation of coupled climate models show a reduction in the equatorward bias in the location of westerly winds as compared to the previous generation of models. Key Points: Recent observational record is strongly affected by internal variability and is not a good indicator of the forced changes in the westerliesCoupled Model Intercomparison Project 6 (CMIP6) models show a reduction in the equatorward bias in the location of the westerlies as compared to CMIP5There are significant regional and seasonal differences in wind changes that need to be considered when simulating past and future trends [ABSTRACT FROM AUTHOR]

Using an ensemble of 28 climate models, we examine hindcasts and 'business as usual' future changes to large-scale South Indian Ocean dynamics. We compare model ensemble seasonal-to-annual volume transports to observations and explore drivers of past and future circulation variability and change. Off the west coast of Australia, models consistently project a weakening of the Leeuwin Current and Undercurrent due to reduced onshore flow and downwelling. The reduced onshore flow is related to changes in the alongshore pressure gradient. While the alongshore pressure gradient change is consistent with the Indonesian Throughflow projected weakening, we found no inter-model relationship between these changes. In the south-western Indian Ocean, the models project a robust weakening of the North East and South East Madagascar Currents, Agulhas Current and transport through the Mozambique Channel. This reduced Indian Ocean western boundary flow is partly associated with a weaker Indonesian Throughflow and overturning circulation, where the latter is related to a decrease in the convergence of deep Southern Ocean waters into the Indian Ocean. In contrast to the weakening of other features, the westward flowing Agulhas Current extension south of Africa is projected to strengthen, which is consistent with an intensification of the Antarctic Circumpolar Current. [ABSTRACT FROM AUTHOR]

The distribution of marine species is often modeled using Eulerian approaches, in which changes to population density or abundance are calculated at fixed locations in space. Conversely, Lagrangian, or individual-based, models simulate the movement of individual particles moving in continuous space, with broader-scale patterns such as distribution being an emergent property of many, potentially adaptive, individuals. These models offer advantages in examining dynamics across spatiotemporal scales and making comparisons with observations from individual-scale data. Here, we introduce and describe such a model, the Individual-based Kinesis , Advection and Movement of Ocean ANimAls model (Ikamoana), which we use to replicate the movement processes of an existing Eulerian model for marine predators (the Spatial Ecosystem and Population Dynamics Model, SEAPODYM). Ikamoana simulates the movement of either individual or groups of animals by physical ocean currents, habitat-dependent stochastic movements (kinesis), and taxis movements representing active searching behaviours. Applying our model to Pacific skipjack tuna ( Katsuwonus pelamis ), we show that it accurately replicates the evolution of density distribution simulated by SEAPODYM with low time-mean error and a spatial correlation of density that exceeds 0.96 at all times. We demonstrate how the Lagrangian approach permits easy tracking of individuals’ trajectories for examining connectivity between different regions, and show how the model can provide independent estimates of transfer rates between commonly used assessment regions. In particular, we find that retention rates in most assessment regions are considerably smaller (up to a factor of 2) than those estimated by this population of skipjack’s primary assessment model. Moreover, these rates are sensitive to ocean state (e.g. El Nino vs La Nina) and so assuming fixed transfer rates between regions may lead to spurious stock estimates. A novel feature of the Lagrangian approach is that individual schools can be tracked through time, and we demonstrate that movement between two assessment regions at broad temporal scales includes extended transits through other regions at finer-scales. Finally, we discuss the utility of this modeling framework for the management of marine reserves, designing effective monitoring programmes, and exploring hypotheses regarding the behaviour of hard-to-observe oceanic animals. [ABSTRACT FROM AUTHOR]

Regional climate projections in the Pacific region are potentially sensitive to a range of existing model biases. This study examines the implications of coupled model biases on regional climate projections in the tropical western Pacific. Model biases appear in the simulation of the El Niño Southern Oscillation, the location and movement of the South Pacific Convergence Zone, rainfall patterns, and the mean state of the ocean-atmosphere system including the cold tongue bias and erroneous location of the edge of the Western Pacific warm pool. These biases are examined in the CMIP3 20th century climate models and used to provide some context to the uncertainty in interpretations of regional-scale climate projections for the 21st century. To demonstrate, we provide examples for two island nations that are located in different climate zones and so are affected by different biases: Nauru and Palau. We discuss some of the common approaches to analyze climate projections and whether they are effective in reducing the effect of model biases. These approaches include model selection, calculating multi model means, downscaling and bias correcting. [ABSTRACT FROM AUTHOR]

This study investigates the impact of Indian Ocean sea surface temperature (SST) anomalies on the atmospheric circulation of the Southern Hemisphere during El Niñño events, with a focus on Australian climate. During El Niñño episodes, the tropical Indian Ocean exhibits two types of SST response: a uniform ''basinwide warming'' and a dipole mode--the Indian Ocean dipole (IOD). While the impacts of the IOD on climate have been extensively studied, the effects of the basinwide warming, particularly in the Southern Hemisphere, have received less attention. The interannual basinwide warming response has important implications for Southern Hemisphere atmospheric circulation because 1) it accounts for a greater portion of the Indian Ocean monthly SST variance than the IOD pattern and 2) its maximum amplitude occurs during austral summer to early autumn, when large parts of Australia, South America, and Africa experience their monsoon. Using observations and numerical experiments with an atmospheric general circulation model forced with historical SST from 1949 to 2005 over different tropical domains, the authors show that the basinwide warming leads to a Gill--Matsuno-type response that reinforces the anomalies caused by changes in the Pacific as part of El Niñño. In particular, the basinwide warming drives strong subsidence over Australia, prolonging the dry conditions during January--March, when El Niñño--related SST starts to decay. In addition to the anomalous circulation in the tropics, the basinwide warming excites a pair of barotropic anomalies in the Indian Ocean extratropics that induces an anomalous anticyclone in the Great Australian Bight. [ABSTRACT FROM AUTHOR]

Purse‐seine fishers using drifting fish aggregating devices (dFADs), mainly built with bamboo, plastic buoys, and plastic netting, to aggregate and catch tropical tuna, deploy 46,000–65,000 dFADs per year in the Pacific Ocean. Some of the major concerns associated with this widespread fishing device are potential entanglement of sea turtles and other marine fauna in dFAD netting; marine debris and pollution; and potential ecological damage via stranding on coral reefs, beaches, and other essential habitats for marine fauna. To assess and quantify the potential connectivity (number of dFADs deployed in an area and arriving in another area) between dFAD deployment areas and important oceanic or coastal habitat of critically endangered leatherback (Dermochelys coriacea) and hawksbill (Eretmochelys imbricata) sea turtles in the Pacific Ocean, we conducted passive‐drift Lagrangian experiments with simulated dFAD drift profiles and compared them with known important sea turtle areas. Up to 60% of dFADs from equatorial areas were arriving in essential sea turtle habitats. Connectivity was less when only areas where dFADs are currently deployed were used. Our simulations identified potential regions of dFAD interactions with migration and feeding habitats of the east Pacific leatherback turtle in the tropical southeastern Pacific Ocean; coastal habitats of leatherback and hawksbill in the western Pacific (e.g., archipelagic zones of Indonesia, Papua New Guinea, and Solomon Islands); and foraging habitat of leatherback in a large equatorial area south of Hawaii. Additional research is needed to estimate entanglements of sea turtles with dFADs at sea and to quantify the likely changes in connectivity and distribution of dFADs under new management measures, such as use of alternative nonentangling dFAD designs that biodegrade, or changes in deployment strategies, such as shifting locations. [ABSTRACT FROM AUTHOR]

Prolonged high-temperature extreme events in the ocean, marine heatwaves, can have severe and long-lasting impacts on marine ecosystems, fisheries and associated services. This study applies a marine heatwave framework to analyse a global sea surface temperature product and identify the most extreme events, based on their intensity, duration and spatial extent. Many of these events have yet to be described in terms of their physical attributes, generation mechanisms, or ecological impacts. Our synthesis identifies commonalities between marine heatwave characteristics and seasonality, links to the El Niño-Southern Oscillation, triggering processes and impacts on ocean productivity. The most intense events preferentially occur in summer, when climatological oceanic mixed layers are shallow and winds are weak, but at a time preceding climatological maximum sea surface temperatures. Most subtropical extreme marine heatwaves were triggered by persistent atmospheric high-pressure systems and anomalously weak wind speeds, associated with increased insolation, and reduced ocean heat losses. Furthermore, the most extreme events tended to coincide with reduced chlorophyll-a concentration at low and mid-latitudes. Understanding the importance of the oceanic background state, local and remote drivers and the ocean productivity response from past events are critical steps toward improving predictions of future marine heatwaves and their impacts. [ABSTRACT FROM AUTHOR]

The Amundsen Sea Low (ASL) is a distinctive feature of the Southern Hemisphere high latitude atmospheric circulation, regulating regional Antarctic climate, meridional heat transport, ocean circulation, and sea‐ice in the Amundsen‐Bellingshausen Seas. Most previous research on the ASL has focused on its variability with only a few studies attempting to understand why the climatological ASL exists. These studies have proposed different hypotheses to explain the presence of the ASL, however, a clear understanding of the mechanisms responsible for the generation of the ASL remains uncertain. Here we use an atmospheric general circulation model to show that the ASL is a consequence of the interaction between Antarctic topography and the westerly wind jet, with negligible influence from low‐latitude teleconnections. A nonrotating fluid flow simulation further suggests that the ASL can be explained by flow separation resulting from the interaction of westerly winds with the topography of Antarctica. Plain Language Summary: The Amundsen Sea Low (ASL) is a permanent atmospheric low‐pressure center located offshore of western Antarctica. The ASL affects air and ocean temperature, precipitation, sea‐ice, and ice shelves in the West Antarctica region. Various hypotheses have been proposed to explain the mechanisms responsible for the existence of the ASL. However, a clear understanding of the generation mechanisms is not yet available. In this study, we use a state‐of‐the‐art climate model to test the previously suggested mechanisms and show that the ASL is generated by an interaction between the westerly winds with Antarctic orography, with negligible influence from low latitudes. Key Points: The Amundsen Sea Low (ASL) owes its existence to an interaction between the westerly wind jet and Antarctic orographyMountains in the West Antarctic region trap the low‐pressure system in the Amundsen‐Bellingshausen Seas regionTeleconnections from low latitudes have little to no role in generating the climatological ASL [ABSTRACT FROM AUTHOR]

Large-scale modes of atmospheric variability in the southern midlatitudes can influence Antarctic sea ice concentrations (SIC) via diverse processes. For instance, variability in both the Southern Annular Mode (SAM) and zonal wave 3 (ZW3) have been linked to the abrupt 2015/16 sea ice decline. While SIC responses to each of SAM and ZW3 have been examined previously, their interaction and synchronous impact on Antarctic sea ice has not. Here, we investigate SAM/ZW3 interactions and their associated combined impacts on Antarctic sea ice using a 1200-yr simulation from a state-of-the-art climate model. Our results suggest that zonal wind anomalies associated with SAM drive SIC anomalies in the marginal ice-zone via advection of ice normal to the ice edge and Ekman drift. In contrast, meridional wind anomalies associated with ZW3 can have opposing dynamic and thermodynamic effects on SIC. Both SAM- and ZW3-related SIC anomalies propagate eastward, likely by the Antarctic Circumpolar Current. The interaction of SAM and ZW3 leads to interesting regional SIC responses. During negative SAM, ZW3-associated meridional wind anomalies across western Antarctica are closer to the ice edge and have a stronger impact on sea ice overall. ZW3 phase affects meridional wind anomalies across the whole ice edge, whereas it affects SIC anomalies mainly over western Antarctica. In parts of eastern Antarctica, SIC anomalies are less sensitive to ZW3 phase, but are sensitive to SAM, particularly in locations where the ice edge has a prominent angle relative to the SAM-related zonal wind anomalies. Significance Statement: The Southern Annular Mode (SAM) and zonal wave 3 (ZW3) are large-scale atmospheric circulation patterns affecting midlatitude east–west and north–south winds, respectively, over the Southern Ocean. Variations in winds can affect sea ice formation, which can feed back to influence Southern Hemisphere climate. We examine how variations in SAM and ZW3 affect Antarctic sea ice due to a combination of wind- and ocean-driven ice movement and sea ice growth or melting. Regional variations in ice concentrations are due both to alternating north–south ZW3 winds and to the interaction of SAM-related east–west winds with the ice edge. SAM and ZW3 can also interact, leading to stronger north–south wind and sea ice responses over western Antarctica when SAM-related midlatitude winds weaken. [ABSTRACT FROM AUTHOR]

A correction to the article "The contribution of Indian Ocean sea surface temperature anomalies on Australian summer rainfall during El Niño events" in the 2011 issue is presented.

Projections of equatorial sea surface temperature from CMIP5 climate models are important for understanding possible future changes in marine habitats, local rainfall and climate processes such as El Niño Southern Oscillation. Interpreting the projected changes in the tropical Pacific is complicated by the systematic cold tongue bias and overly westward location of the Warm Pool edge at the equator in coupled models. Here an index based on the maximum zonal salinity gradient is used to differentiate the Warm Pool from the cold tongue in each of 19 CMIP5 models. Warming is then calculated relative to the dynamic edge of the Warm Pool between the second halves of the 20th and 21st Centuries from the RCP8.5 scenario to provide a bias adjusted SST projection. Based on this definition of the edge, while the Warm Pool edge is projected to warm, it is likely to remain within 10° of its present longitude. This is in stark contrast to the large projected eastward displacements of the isotherms that are usually used to define the edge. Adjusting for the edge, warming within the Warm Pool is projected to be fairly uniform with surface water freshening. Projected warming is enhanced over the cold tongue with the net effect of reducing the zonal SST gradient. In contrast, if the warming is calculated without correcting for the edge of the Warm Pool, the warming signature is dominated by the poorer performing models with an overly westward Warm Pool, resulting in enhanced warming across the equatorial Pacific. Bias adjusting realigns the warming signature and reduces the model spread of projected warming. The biased warming signature also introduces spurious meridional and zonal SST gradients. This will potentially alter the behaviour of the atmospheric convergence zones and the dynamics of ENSO which is influenced by the extent of the Warm Pool and zonal SST gradients. [ABSTRACT FROM AUTHOR]

Multi-disciplinary approaches that combine oceanographic, biogeochemical, ecosystem, fisheries population and socio-economic models are vital tools for modelling whole ecosystems. Interpreting the outputs from such complex models requires an appreciation of the many different types of modelling frameworks being used and their associated limitations and uncertainties. Both users and developers of particular model components will often have little involvement or understanding of other components within such modelling frameworks. Failure to recognise limitations and uncertainties associated with components and how these uncertainties might propagate throughout modelling frameworks can potentially result in poor advice for resource management. Unfortunately, many of the current integrative frameworks do not propagate the uncertainties of their constituent parts. In this review, we outline the major components of a generic whole of ecosystem modelling framework incorporating the external pressures of climate and fishing. We discuss the limitations and uncertainties associated with each component of such a modelling system, along with key research gaps. Major uncertainties in modelling frameworks are broadly categorised into those associated with (i) deficient knowledge in the interactions of climate and ocean dynamics with marine organisms and ecosystems; (ii) lack of observations to assess and advance modelling efforts and (iii) an inability to predict with confidence natural ecosystem variability and longer term changes as a result of external drivers (e.g. greenhouse gases, fishing effort) and the consequences for marine ecosystems. As a result of these uncertainties and intrinsic differences in the structure and parameterisation of models, users are faced with considerable challenges associated with making appropriate choices on which models to use. We suggest research directions required to address these uncertainties, and caution against overconfident predictions. Understanding the full impact of uncertainty makes it clear that full comprehension and robust certainty about the systems themselves are not feasible. A key research direction is the development of management systems that are robust to this unavoidable uncertainty. [ABSTRACT FROM AUTHOR]

Thermodynamic arguments imply that global mean rainfall increases in a warmer atmosphere; however, dynamical effects may result in more significant diversity of regional precipitation change. Here we investigate rainfall changes in the mid-Pliocene Warm Period (~ 3 Ma), a time when temperatures were 2–3ºC warmer than the pre-industrial era, using output from the Pliocene Model Intercomparison Projects phases 1 and 2 and sensitivity climate model experiments. In the Mid-Pliocene simulations, the higher rates of warming in the northern hemisphere create an interhemispheric temperature gradient that enhances the southward cross-equatorial energy flux by up to 48%. This intensified energy flux reorganizes the atmospheric circulation leading to a northward shift of the Inter-Tropical Convergence Zone and a weakened and poleward displaced Southern Hemisphere Subtropical Convergences Zones. These changes result in drier-than-normal Southern Hemisphere tropics and subtropics. The evaluation of the mid-Pliocene adds a constraint to possible future warmer scenarios associated with differing rates of warming between hemispheres. [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) have been observed around the world and are expected to increase in intensity and frequency under anthropogenic climate change. A variety of impacts have been associated with these anomalous events, including shifts in species ranges, local extinctions and economic impacts on seafood industries through declines in important fishery species and impacts on aquaculture. Extreme temperatures are increasingly seen as important influences on biological systems, yet a consistent definition of MHWs does not exist. A clear definition will facilitate retrospective comparisons between MHWs, enabling the synthesis and a mechanistic understanding of the role of MHWs in marine ecosystems. Building on research into atmospheric heatwaves, we propose both a general and specific definition for MHWs, based on a hierarchy of metrics that allow for different data sets to be used in identifying MHWs. We generally define a MHW as a prolonged discrete anomalously warm water event that can be described by its duration, intensity, rate of evolution, and spatial extent. Specifically, we consider an anomalously warm event to be a MHW if it lasts for five or more days, with temperatures warmer than the 90th percentile based on a 30-year historical baseline period. This structure provides flexibility with regard to the description of MHWs and transparency in communicating MHWs to a general audience. The use of these metrics is illustrated for three 21st century MHWs; the northern Mediterranean event in 2003, the Western Australia ‘Ningaloo Niño’ in 2011, and the northwest Atlantic event in 2012. We recommend a specific quantitative definition for MHWs to facilitate global comparisons and to advance our understanding of these phenomena. [ABSTRACT FROM AUTHOR]

In an effort to increase purse seine fishing efficiency for tropical tunas, over 30,000 drifting Fish Aggregating Devices (dFADs) are deployed every year by fishers in the Western and Central Pacific Ocean (WCPO). The use of dFADs also impacts ecosystems, in particular through marine pollution and dFAD beaching. This paper presents the first estimate of dFAD beaching events in the WCPO (>1300 in 2016–2017) and their distribution. Lagrangian simulations of virtual dFADs, released subject to contrasting deployment distributions, help us determine the relative importance of operational versus environmental drivers of dFADs drifting to beaching areas. The highest levels of beaching, occurring on Papua New Guinea and Solomon Islands, are likely a result of the prevailing westward oceanic circulation and subsequent local processes driving dFADs towards land. Similarly, high beaching rates in Tuvalu appear to be due to the general circulation of the WCPO. In contrast, beaching in Kiribati Gilbert Islands appear to be more strongly related to dFAD deployment strategy. These findings indicate that reducing beaching events via changes in deployment locations may be difficult. As such, management approaches combining dFAD deployment limits, the use of biodegradable dFADs, recoveries at-sea close to sensitive areas and/or beached dFAD removal should be considered. [ABSTRACT FROM AUTHOR]