ORCID Profile
0000-0002-4162-3269
Current Organisations
Västra Götalandsregionen
,
University of New South Wales
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Publisher: Springer Science and Business Media LLC
Date: 23-02-2022
DOI: 10.1038/S41586-021-04370-W
Abstract: Warming-induced global water cycle changes pose a significant challenge to global ecosystems and human society. However, quantifying historical water cycle change is difficult owing to a dearth of direct observations, particularly over the ocean, where 77% and 85% of global precipitation and evaporation occur, respectively
Publisher: Wiley
Date: 09-11-2020
Publisher: Authorea, Inc.
Date: 20-07-2023
DOI: 10.22541/ESSOAR.168987142.28631718/V1
Abstract: Greenhouse gases and aerosols play a major role in controlling global climate change. Greenhouse gases drive a radiative imbalance which warms the ocean, while aerosols cool the ocean. Since 1980, the effective radiation felt by the planet due to anthropogenic aerosols has levelled off, global ocean cooling due to aerosols has decelerated, and greenhouse gas-driven ocean warming has accelerated. We explore the deceleration of aerosol-driven ocean cooling by quantifying a time- and spatially-varying ocean heat uptake efficiency, defined as the change in the rate of global ocean heat storage per degree of cooling surface temperature. In aerosol-only simulations, ocean heat uptake efficiency has decreased by 69% since the 1900s. The tropics and sub-tropics have driven this decrease, while the coldest fraction of the ocean continues to sustain cooling and high ocean heat uptake efficiency. Our results identify a growing trend towards less efficient ocean cooling due to aerosols.
Publisher: Authorea, Inc.
Date: 08-07-2023
DOI: 10.22541/ESSOAR.168882017.73914213/V1
Abstract: A holistic review is given of the Southern Ocean dynamic system, in the context of the crucial role it plays in the global climate and the profound changes it is experiencing. The review focuses on connections between different components of the Southern Ocean dynamic system, drawing together contemporary perspectives from different research communities, with the objective of ‘closing loops’ in our understanding of the complex network of feedbacks in the overall system. For the purposes of this review, the Southern Ocean dynamic system is ided into four main components: large-scale circulation cryosphere turbulence and gravity waves. Overviews are given of the key dynamical phenomena for each component, before describing the linkages between the components. The reviews are complemented by an overview of observed Southern Ocean trends and future climate projections. Priority research areas required to improve our understanding of the Southern Ocean system are identified.
Publisher: American Geophysical Union (AGU)
Date: 06-2019
DOI: 10.1029/2018JC014883
Publisher: Authorea, Inc.
Date: 05-08-2023
Publisher: American Meteorological Society
Date: 06-2020
Abstract: Open-ocean convection is a common phenomenon that regulates mixed layer depth and ocean ventilation in the high-latitude oceans. However, many climate model simulations overestimate mixed layer depth during open-ocean convection, resulting in excessive formation of dense water in some regions. The physical processes controlling transient mixed layer depth during open-ocean convection are examined using two different numerical models: a high-resolution, turbulence-resolving nonhydrostatic model and a large-scale hydrostatic ocean model. An isolated destabilizing buoyancy flux is imposed at the surface of both models and a quasi-equilibrium flow is allowed to develop. Mixed layer depth in the turbulence-resolving and large-scale models closely aligns with existing scaling theories. However, the large-scale model has an anomalously deep mixed layer prior to quasi-equilibrium. This transient mixed layer depth bias is a consequence of the lack of resolved turbulent convection in the model, which delays the onset of baroclinic instability. These findings suggest that in order to reduce mixed layer biases in ocean simulations, parameterizations of the connection between baroclinic instability and convection need to be addressed.
Publisher: American Meteorological Society
Date: 03-2022
Abstract: Anthropogenically induced radiative imbalances in the climate system lead to a slow accumulation of heat in the ocean. This warming is often obscured by natural modes of climate variability such as El Niño–Southern Oscillation (ENSO), which drive substantial ocean temperature changes as a function of depth and latitude. The use of watermass coordinates has been proposed to help isolate forced signals and filter out fast adiabatic processes associated with modes of variability. However, how much natural modes of variability project into these different coordinate systems has not been quantified. Here we apply a rigorous framework to quantify ocean temperature variability using both a quasi-Lagrangian, watermass-based temperature coordinate and Eulerian depth and latitude coordinates in a free-running climate model under preindustrial conditions. The temperature-based coordinate removes the adiabatic component of ENSO-dominated interannual variability by definition, but a substantial diabatic signal remains. At slower (decadal to centennial) frequencies, variability in the temperature- and depth-based coordinates is comparable. Spectral analysis of temperature tendencies reveals the dominance of advective processes in latitude and depth coordinates while the variability in temperature coordinates is related closely to the surface forcing. Diabatic mixing processes play an important role at slower frequencies where quasi-steady-state balances emerge between forcing and mixing in temperature, advection and mixing in depth, and forcing and advection in latitude. While watermass-based analyses highlight diabatic effects by removing adiabatic variability, our work shows that natural variability has a strong diabatic component and cannot be ignored in the analysis of long-term trends. Quantifying the ocean warming associated with anthropogenically induced radiative imbalances in the climate system can be challenging due to the superposition with modes of internal climate variability such as El Niño. One method proposed to address this issue is the analysis of temperature changes in fluid-following (or “watermass”) coordinates that filter out fast adiabatic processes associated with these modes of variability. In this study we compare a watermass-based analysis with more traditional analyses of temperature changes at fixed depth and latitude to show that even natural modes of climate variability exhibit a substantial signal in watermass coordinates, particularly at decadal and slower frequencies. This natural variability must be taken into account when analyzing long-term temperature trends in the ocean.
Publisher: American Geophysical Union (AGU)
Date: 09-02-2021
DOI: 10.1029/2020GL089455
Abstract: Using field, numerical, and laboratory studies, we consider the roles of both shear and convection in driving mixing in the interior of the density‐stratified ocean. Shear mixing dominates when the Richardson number Ri 0.25, convective mixing dominates when Ri 1.0, and in the intermediate regime when 0.25 Ri 1.0 both shear and convection can contribute to mixing. For pure shear mixing the mixing efficiency Ri f approaches 0.5, while for pure convective mixing the mixing efficiency Ri f approaches 0.75. The diapycnal diffusivities for the two mechanisms are given by very different expressions. Despite these complexities, a simple mixing length model using the mean flow shear S provides robust estimates of diffusivity across the range 0 Ri 2. To account for the roles of both shear and convection over this range of Ri , we also formulate a modified version of the empirical KPP model for parameterizing ocean mixing in numerical models.
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-1377
Abstract: & & Global water cycle changes induced by anthropogenic climate change pose a growing threat to existing ecosystems and human infrastructure. However, scarce direct observations of precipitation and evaporation means historical water cycle changes remain uncertain. In this work, we apply a novel watermass-based diagnostic framework to the latest observations of ocean salinity to quantify poleward freshwater transport in the earth system since 1970. This observational estimate is not replicated in any model in the current generation of CMIP6 climate models - likely due to the inaccurate representation of surface freshwater flux intensification in such models. These results provide a first-of-its-kind baseline of observed warm-to-cold freshwater transport since 1970, and also underscore the need to further explore surface freshwater fluxes in existing climate models.& &
Publisher: Copernicus GmbH
Date: 10-07-2023
DOI: 10.5194/EGUSPHERE-2023-1220
Abstract: Abstract. The geography of changes in the fluxes of heat, carbon, fresh water and other tracers at the sea surface are highly uncertain and are critical to our understanding of climate change and its impacts. We present a state estimation framework wherein the relative roles of ocean circulation, boundary fluxes and mixing, which describe the evolving state of water masses, can be balanced. In this framework, we define a discrete set of ocean water masses distinguished by their geographical and thermodynamic/chemical properties for specific time periods. Ocean circulation then moves these water masses in geographic space. In phase space, geographically adjacent water masses are able to mix together, representing a convergence, and air-sea property fluxes move the water masses over time. We define an optimisation problem whose solution is constrained by the physically permissible bounds of changes in ocean circulation, air-sea fluxes and mixing. As a proof of concept implementation, we use data from a historical numerical climate model simulation with a closed heat and salinity budget. An inverse model solution is found for the evolution of temperature and salinity consistent with `true' air-sea heat and fresh water fluxes which are introduced as model priors. When a constant bias is introduced to the prior fluxes, the inverse model finds a solution closer to the true fluxes. This framework, which we call the Optimal Transformation Method, represents a modular, relatively computationally cost effective, open source and transparent state estimation tool that complements existing approaches.
Publisher: American Geophysical Union (AGU)
Date: 24-04-2021
DOI: 10.1029/2020GL091439
Abstract: The ocean has absorbed approximately 90% of the accumulated heat in the climate system since 1970. As global warming accelerates, understanding ocean heat content changes and tracing these to surface heat input is increasingly important. We introduce a novel framework by organizing the ocean into temperature‐percentiles from warmest to coldest, allowing us to trace ocean temperature changes to changes in surface fluxes and mixing. Applying this framework to observations and historical CMIP6 simulations, we find that 50 ± 6% of surface heat uptake between 1970 and 2014 is confined to isotherms in the coldest 90% of the ocean volume. These isotherms outcrop over only 23% of the ocean's surface area in the sub‐polar regions, implying a disproportionately large heat input per unit area. Additionally, a cooling bias in the CMIP6 models is traced to inaccurate sea surface temperatures and surface heat fluxes into the warmest 5%–20% of the ocean volume.
Publisher: Wiley
Date: 18-05-2022
Publisher: American Geophysical Union (AGU)
Date: 11-05-2018
DOI: 10.1029/2018GL077711
Publisher: SAGE Publications
Date: 26-10-2021
Abstract: The impact of the COVID-19 pandemic on workload, mental health, and well-being of healthcare workers, and particularly those on the front-line, has received considerable attention. We surveyed hospital employees about their working environment during the pandemic and identified departments which were negatively affected in comparison to the pre-pandemic situation, as well as factors contributing to this. Setting and participants We surveyed all hospital employees at Sahlgrenska University Hospital, Sweden in September 2020 and compared results across departments and to the results of a large employee survey from October 2019. The overall impact of the pandemic on perceived working conditions and possibility for recovery differed among departments. During the pandemic, healthcare workers working with COVID-19 patients reported poorer working environments than other employees. Factors significantly related to perception of work environment and recovery during the pandemic included worries of being infected, departmental transfer, and having insufficient access to personal protective equipment. Men reported better working conditions than women in all, but one item and higher age was related to better perceived working environment. Our results indicate that the pandemic differentially affects hospital departments and underscores the multifactorial nature of this topic. Contributing factors to poor perceived working environment could be addressed at times of high workload, such as during the pandemic, including providing appropriate support to managers, ensuring possibility for recovery during working hours, and acknowledging worries about infection. Young healthcare workers and staff who are relocated due to the pandemic warrant special attention.
Publisher: American Meteorological Society
Date: 05-2023
Abstract: Persistent warming and water cycle change due to anthropogenic climate change modifies the temperature and salinity distribution of the ocean over time. This “forced” signal of temperature and salinity change is often masked by the background internal variability of the climate system. Analyzing temperature and salinity change in water-mass-based coordinate systems has been proposed as an alternative to traditional Eulerian (e.g., fixed-depth, zonally averaged) coordinate systems. The impact of internal variability is thought to be reduced in water-mass coordinates, enabling a cleaner separation of the forced signal from background variability—or a higher “signal-to-noise” ratio. Building on previous analyses comparing Eulerian and water-mass-based one-dimensional coordinates, here we recast two-dimensional coordinate systems—temperature–salinity ( T – S ), latitude–longitude, and latitude–depth—onto a directly comparable equal-volume framework. We compare the internal variability, or “noise” in temperature and salinity between these remapped two-dimensional coordinate systems in a 500-yr preindustrial control run from a CMIP6 climate model. We find that the median internal variability is lowest (and roughly equivalent) in T – S and latitude–depth space, compared with latitude–longitude coordinates. A large proportion of variability in T – S and latitude–depth space can be attributed to processes that operate over a time scale greater than 10 years. Overall, the signal-to-noise ratio in T – S coordinates is roughly comparable to latitude–depth coordinates, but is greater in regions of high historical temperature change. Conversely, latitude–depth coordinates have greater signal-to-noise ratio in regions of historical salinity change. Thus, we conclude that the climatic temperature change signal can be more robustly identified in water-mass coordinates. Changes in ocean temperature and salinity are driven both by human-induced climate change and by modes of natural variability in the climate system, such as El Niño–Southern Oscillation. It can be difficult to isolate the human-induced “signal” of climate change from the natural fluctuations or “noise” in the climate system. Water-mass-based methods, which “follow” a parcel of water around the ocean, have been thought to improve on “Eulerian” (i.e., analyses performed at fixed latitude, longitude, and depth) frames of reference as they are less impacted by the noise. However, it is difficult to cleanly compare between water-mass-based methods and Eulerian methods. Here, we aim to quantify the extent to which water-mass-based frameworks improve on Eulerian frameworks in isolating the climate signal from the noise. We recast water-mass and Eulerian methods onto an equivalent grid, enabling a clean comparison between them, and find that doing so increases the signal-to-noise ratio in water-mass-based coordinates in regions of ocean warming. These results emphasize the utility of water-mass-based methods in analyzing long-term climatic temperature change in the ocean.
No related grants have been discovered for Taimoor Sohail.