Abstract:The Southern Ocean features ventilation pathways that transport surface waters into the subsurface thermocline on time scales from decades to centuries, sequestering anomalies of heat and carbon away from the atmosphere and thereby regulating the rate of surface warming. Despite its importance for climate sensitivity, the factors that control the distribution of heat along these pathways are not well understood. In this study, we use an observationally constrained, physically consistent global ocean model to e… Show more
“…An adjoint model, in this context, is one that starts from a quantity of interest (henceforth referred to as an "objective function")-such as the integrated temperature over a certain region (e.g., Jones et al, 2018Jones et al, , 2019Jones et al, , 2020, the heat or volume transport of a particular current (e.g., Czeschel, et al, 2010Czeschel, et al, , 2012Heimbach et al, 2011;Kostov, et al, 2019;Pillar, et al, 2016;Smith & Heimbach, 2019)-and steps backwards through a linearized version of the model, propagating the sensitivities of the objective function.…”
Section: The Eccov4 Global State Estimate and Its Adjointmentioning
BackgroundThe Southern Ocean (SO) is home to the world's longest and strongest ocean current, the Antarctic Circumpolar Current (ACC), which encircles the globe free of continental barriers. Driven by strong wind and buoyancy forcing, the ACC transports climatically important tracers such as heat, salinity, and carbon between the three major ocean basins. These forcings also create sloping density surfaces (isopycnals) that tilt upwards from north to south, which connect deep waters from around the globe to the surface. At the surface, air-sea interactions modify the properties of water masses. These modified waters then return to depth and into the other ocean basins as dense waters near the Antarctic continental shelf, or as lighter mode and intermediate waters north of the ACC (Lumpkin & Speer, 2007;Marshall & Speer, 2012).The SO is of critical importance to the global oceanic uptake of heat and carbon, due in part to this overturning circulation. It may be responsible for as much as 75% of the global ocean heat uptake and ∼50% of the carbon uptake (Frölicher et al., 2015;Mikaloff Fletcher et al., 2006). Roughly 30% of anthropogenic CO 2 emissions ends up in the ocean (Khatiwala et al., 2013), and around 93% of the excess heat added to the earth system since 1955 has been estimated to be stored in the ocean (Levitus et al., 2012), predominantly in the SO (Roemmich et al., 2015).
“…An adjoint model, in this context, is one that starts from a quantity of interest (henceforth referred to as an "objective function")-such as the integrated temperature over a certain region (e.g., Jones et al, 2018Jones et al, , 2019Jones et al, , 2020, the heat or volume transport of a particular current (e.g., Czeschel, et al, 2010Czeschel, et al, , 2012Heimbach et al, 2011;Kostov, et al, 2019;Pillar, et al, 2016;Smith & Heimbach, 2019)-and steps backwards through a linearized version of the model, propagating the sensitivities of the objective function.…”
Section: The Eccov4 Global State Estimate and Its Adjointmentioning
BackgroundThe Southern Ocean (SO) is home to the world's longest and strongest ocean current, the Antarctic Circumpolar Current (ACC), which encircles the globe free of continental barriers. Driven by strong wind and buoyancy forcing, the ACC transports climatically important tracers such as heat, salinity, and carbon between the three major ocean basins. These forcings also create sloping density surfaces (isopycnals) that tilt upwards from north to south, which connect deep waters from around the globe to the surface. At the surface, air-sea interactions modify the properties of water masses. These modified waters then return to depth and into the other ocean basins as dense waters near the Antarctic continental shelf, or as lighter mode and intermediate waters north of the ACC (Lumpkin & Speer, 2007;Marshall & Speer, 2012).The SO is of critical importance to the global oceanic uptake of heat and carbon, due in part to this overturning circulation. It may be responsible for as much as 75% of the global ocean heat uptake and ∼50% of the carbon uptake (Frölicher et al., 2015;Mikaloff Fletcher et al., 2006). Roughly 30% of anthropogenic CO 2 emissions ends up in the ocean (Khatiwala et al., 2013), and around 93% of the excess heat added to the earth system since 1955 has been estimated to be stored in the ocean (Levitus et al., 2012), predominantly in the SO (Roemmich et al., 2015).
“…The time interval chosen is the annual-average of large-scale PV over year 2011 (Figure 1), as we are interested in seeing how the mode water is forced over several annual cycles. This construction is similar to other adjoint studies investigating the forcing of mode water properties (Jones et al, 2020). Several time intervals were tried in years with differing atmospheric forcing and the results were found to be insensitive to changes in this choice.…”
Subantarctic mode waters have low stratification and are formed through subduction from thick winter mixed layers in the Southern Ocean. To investigate how surface forcing affects the stratification in mode water formation regions in the Southern Ocean, a set of adjoint sensitivity experiments are conducted. The objective function is the annual‐average stratification over the mode water formation region, which is evaluated from potential temperature and salinity adjoint sensitivity experiments. The analysis of impacts, from the product of sensitivities and forcing variability, identifies the separate effects of the wind stress, heat flux, and freshwater flux, revealing that the dominant control on stratification is from surface heat fluxes, as well as a smaller effect from zonal wind stress. The adjoint sensitivities of stratification to surface heat flux reveal a surprising change in sign over 2 years lead time: surface cooling leads to the expected initial local decrease in stratification, but there is a delayed response leading to an increase in stratification. This delayed response in stratification involves effective atmospheric damping of the surface thermal contribution, so that eventually the oppositely‐signed advective haline contribution dominates. This two‐phase response of stratification is found to hold over mode water formation regions in the South Indian and Southeast Pacific sectors of the Southern Ocean, where there are strong advective flows linked to the Antarctic Circumpolar Current.
“…Furthermore, this technique demonstrated that mode water properties responded to gyre advection, wind-driven isopycnal tilt, and even remote coastal waves (figure 5). Consistent with this, Jones et al [49,50] found that the properties of the subducted water in the Southeast Pacific, which ultimately ventilates the subtropical thermocline, are strongly affected by local wind stress over the subtropical gyre.…”
Section: (I) Ship and Aircraft Observationsmentioning
confidence: 80%
“…Consistent with this, Jones et al . [ 49 , 50 ] found that the properties of the subducted water in the Southeast Pacific, which ultimately ventilates the subtropical thermocline, are strongly affected by local wind stress over the subtropical gyre. Figure 5 Schematic illustrating the main kinematic and dynamic sensitivities up to approximately 5 years lag for mode water formation regions (MWFRs) mixed layer properties in all three basins: Indian (yellow), Pacific (cyan) and Atlantic (pink).…”
The 5-year Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA) programme and its 1-year extension ENCORE (ENCORE is the National Capability ORCHESTRA Extension) was an approximately 11-million-pound programme involving seven UK research centres that finished in March 2022. The project sought to radically improve our ability to measure, understand and predict the exchange, storage and export of heat and carbon by the Southern Ocean. It achieved this through a series of milestone observational campaigns in combination with model development and analysis. Twelve cruises in the Weddell Sea and South Atlantic were undertaken, along with mooring, glider and profiler deployments and aircraft missions, all contributing to measurements of internal ocean and air–sea heat and carbon fluxes. Numerous forward and adjoint numerical experiments were developed and supported by the analysis of coupled climate models. The programme has resulted in over 100 peer-reviewed publications to date as well as significant impacts on climate assessments and policy and science coordination groups. Here, we summarize the research highlights of the programme and assess the progress achieved by ORCHESTRA/ENCORE and the questions it raises for the future.
This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.
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