Role of Mixed Layer Depth and Subduction Processes for the Southern Ocean Carbon and Nutrient Cycles

Changes in wind forcing in the Southern Ocean exert a large impact on the dynamics of the surface mixed layer and subduction processes. Over the last two decades, the index of the Southern Annular Mode (SAM) has experienced a trend towards its positive phase, which is characterized by stronger westerly winds. The positive trend in the SAM index results from the complex interaction between the steady increase of atmospheric CO2 concentration due to anthropogenic emissions and the stratospheric ozone depletion. Co-occurring with the wind signal is the global warming effect driven by the increase in atmospheric CO2. Increased wind forcing alone would lead to a deepening of the mixed layer and enhance the supply of carbon and nutrients to the euphotic zone. In contrast, the surface ocean warming alone would lead to more surface stratification, and therefore to a shoaling of the mixed layer. The main objective of this PhD thesis is to answer the question: How did the combined changes in atmospheric forcing affect the surface mixed layer and the carbon and nutrient subduction rates on the timescale of interannual to decadal variability? In the first part of my thesis, I assessed the impact of the recent changes in atmospheric temperature and zonal wind speed on the summer mixed-layer depth (MLD) in the SO (south of 30AAA S) from observations and a set of model sensitivity experiments over the period of 2002-2011. The study showed that summer MLD changes in response to recent atmospheric forcing were zonally asymmetric. Summer MLD increased in the Antarctic Zone of the Atlantic and the Indian Ocean sectors. Overall, the effect of recent changes in wind forcing dominated over temperature-induced changes in summer MLD. In the second part of this thesis, I examined the decadal variability in nutrient and dissolved inorganic carbon (DIC) concentrations in the Antarctic Intermediate Water of the Atlantic sector of the Southern Ocean between 1990 and 2014 using cruise data sampled along the Prime Meridian. The results showed a positive trend in DIC and nitrate concentrations along with a negative trend in temperature and salinity. These observations support a scenario of an increase in the upper-ocean overturning circulation probably linked to the positive trend in the SAM index. The third part of this thesis focused on the SAM impact on the inter-annual variability of carbon and nutrient subduction rates across the base of the winter mixed layer between 1958 and 2016 using a coupled physical-biogeochemical general circulation model. The study showed that the variations in SAM led to large-scale anomalies in carbon and nutrient subduction and obduction rates that are zonally symmetric. More obduction occured south of the Antarctic Polar Front (APF) and more subduction occurred where the MLD gradient is strongest in response to the positive trend in the SAM index. Also, I found that the annual mean carbon and nutrient subduction rates varied by around 10% around the long-term mean on interannual to decadal time scales with a stronger positive trend since 1990 leading to an approximately 20% increase in DIC and nitrate subduction rates between 1990 and 2016. My findings (parts I, II and III) suggest that the positive trend of the SAM index (wind intensification) has profoundly affected the surface mixed layer, and increased upwelling of carbon and nutrient-rich deep water. The increased upwelling is driven by the Ekman divergence and is balanced by the stronger northward Ekman transport across the APF. North of the APF these water masses subduct as mode and intermediate waters. While today changes in the wind forcing play a larger role than atmospheric temperature changes, this might reverse in the future

South Atlantic Interbasin Exchanges of Mass, Heat, Salt and Anthropogenic Carbon

The exchange of mass, heat, salt and anthropogenic carbon (Cant) between the South Atlantic, south of 24°S, and adjacent ocean basins is estimated from hydrographic data obtained during 2008-2009 using an inverse method. Transports of anthropogenic carbon are calculated across the western (Drake Passage), eastern (30°E) and northern (24°S) boundaries. The freshwater overturning transport of 0.09 Sv is southward, consistent with an overturning circulation that exports freshwater from the North Atlantic, and consistent with a bistable Meridional Overturning Circulation (MOC), under conditions of excess freshwater perturbation. At 30°E, net eastward Antarctic Circumpolar Current (ACC) transport, south of the Subtropical Front, is compensated by a 15.9±2.3 Sv westward flow along the Antarctic boundary. The region as a whole is a substantial sink for atmospheric anthropogenic carbon of 0.51±0.37 PgC yr-1, of which 0.18±0.12 PgC yr-1 accumulates and is stored within the water column. At 24°S, a 20.2 Sv meridional overturning is associated with a 0.11 PgC yr-1 Cant overturning. The remainder is transported into the Atlantic Ocean north of 24°S (0.28±0.16 PgC yr-1) and Indian sector of Southern Ocean (1.12±0.43 PgC yr-1), having been enhanced by inflow through Drake Passage (1.07±0.44 PgC yr-1). This underlines the importance of the South Atlantic as a crucial element of the anthropogenic carbon sink in the global oceans

Meridional transport of dissolved inorganic carbon in the South Atlantic Ocean

The meridional oceanic transports of dissolved inorganic carbon and oxygen were calculated using six transoceanic sections occupied in the South Atlantic between 11 degrees S and 30 degrees S. The total dissolved inorganic carbon (TCO2) data were interpolated onto conductivity-temperature-depth data to obtain a high-resolution data set, and Ekman, depth-dependent and depth-independent components of the transport were estimated. Uncertainties in the depth-independent velocity distribution were reduced using an inverse model. The inorganic carbon transport between 11 degrees S and 30 degrees S was southward, decreased slightly toward the south, and was -2150 +/- 200 kmol s(-1) (-0.81 +/- 0.08 Gt C yr(-1)) at 20 degrees S. This estimate includes the contribution of net mass transport required to balance the salt transport through Bering Strait. Anthropogenic CO2 concentrations were estimated for the sections. The meridional transport of anthropogenic CO2 was northward, increased toward the north, and was 430 kmol s(-1) (0.16 Gt C yr(-1)) at 20 degrees S. The calculations imply net southward inorganic carbon transport of 2580 kmol s(-1) (1 Gt C yr(-1)) during preindustrial times. The slight contemporary convergence of inorganic carbon between 10 degrees S and 30 degrees S is balanced by storage of anthropogenic CO2 and a sea-to-air flux implying little local divergence of the organic carbon transport. During the preindustrial era, there was significant regional convergence of both inorganic carbon and oxygen, consistent with a sea-to-air gas flux driven by warming. The northward transport of anthropogenic CO2 carried by the meridional overturning circulation represents an important source for anthropogenic CO2 currently being stored within the North Atlantic Ocean

Spatial and seasonal variability of the air-sea equilibration timescale of carbon dioxide

The exchange of carbon dioxide between the ocean and the atmosphere tends to bring waters within the mixed layer toward equilibrium by reducing the partial pressure gradient across the air-water interface. However, the equilibration process is not instantaneous; in general, there is a lag between forcing and response. The timescale of air-sea equilibration depends on several factors involving the depth of the mixed layer, wind speed, and carbonate chemistry. We use a suite of observational data sets to generate climatological and seasonal composite maps of the air-sea equilibration timescale. The relaxation timescale exhibits considerable spatial and seasonal variations that are largely set by changes in mixed layer depth and wind speed. The net effect is dominated by the mixed layer depth; the gas exchange velocity and carbonate chemistry parameters only provide partial compensation. Broadly speaking, the adjustment timescale tends to increase with latitude. We compare the observationally derived air-sea gas exchange timescale with a model-derived surface residence time and a data-derived horizontal transport timescale, which allows us to define two nondimensional metrics of equilibration efficiency. These parameters highlight the tropics, subtropics, and northern North Atlantic as regions of inefficient air-sea equilibration where carbon anomalies are relatively likely to persist. The efficiency parameters presented here can serve as simple tools for understanding the large-scale persistence of air-sea disequilibrium of CO2 in both observations and models

In situ measurement shows ocean boundary layer physical processes control catastrophic global warming.

The infrared greenhouse gas heat trap at the top of the atmosphere controls anthropogenic global warming (AGW) heat balance. Processes at the top of the ocean similarly control the 93% of AGW in the oceans. The tropics are a global year-round ocean heat source. Heat is transported in the ocean by sinking brine from tropical evaporation and polar freezing. Buoyant freshwater and ice barriers limit heat loss from the surface layer. The almost completely unstudied ocean surface skin is critically important to understanding global warming and climate change processes. Studies to date have concentrated on atmospheric warming mainly from land-air data. In this paper we present the first hourly meridional 3m and surface observations in the equatorial Pacific from Tahiti to Hawaii for direct measurement of evaporation and ocean boundary layer heat trapping. We relate this to poleward heat and freshwater transport and ocean warming moderation by basal icemelt of floating ice explored in a second paper [1]. We show heat sequestration below 3m in the hypersaline (>35.5°) southern hemisphere (SH) is limited to ~6M Jm -2 day-1 but evaporation is 7.3mmm-2day--1, at salinity ~36.4° and temperature >28ºC. In the northern hemisphere (NH) tropics the corresponding figures are ~12 MJm-2day-1 and ~4.5mmm -2day--1. Equatorial upwelling and the 50m deep Bering Strait limit buoyant surface outflow from the North Pacific. We found pairs of counter-rotating vertical meridional tropical cells (MTCs), ~300-1200km wide, ~100m deep form separate SH and NH systems with little cross-equatorial flux. Counter-rotating Lagrangian wind-driven gyres transport heat and freshwater polewards in seasonally and tidally moderated stratified surface waters. The zonal geostrophic balance is maintained by the Equatorial Undercurrent (EUC) with an eastbound core ~140cms-1 and density ~25.0 at 50-150m. Global warming and polar icemelt has been underestimated from wrong assumptions of the processes in the top 3m of oceans. These are the unverified beliefs that ocean evaporation depends on windspeed and relative humidity that the ocean is well mixed to 10m depths, and by neglect of water density determined by both salinity and temperature. Temperature measurement to±0.01ºC is required to account for the 3000x greater volumetric heat capacity of seawater to air (3.9x106: 1.3x103Jm-3°C-1). Most SST data are to atmospheric standards (>±0.5°C). Evaporation depends only on temperature (Clausius-Clapeyron). Heat sequestration depends on the buoyant surface layer processes and underlying density gradient. Eleven interconnected counter-rotating Lagrangian wind-driven surface gyres form a global circulation system that carries buoyant surface water masses at speeds much higher than Eulerian geostrophic currents. Polar ice may erode year-round from basal melting from warm subsurface water.This explains contrasting Arctic/Antarctic warming impacts. We suggest many more in situ 3m timeseries especially meridional ones are needed to confirm our findings. In a second paper on centennial daily surface timeseries we show ocean surface warming trend rate post about 1976-1986 is ~0.037ºCyr-1, i.e. >ºC in 20 years [1]. We suggest global warming research be concentrated on the top of the ocean through multidisciplinary timeseries fieldwork verification, monitoring and modeling. This would best be conducted through a cost-efficient dynamic adaptive scientific management for rapid determination of mitigation and adaptation strategies. Reducing troposphere greenhouse gases can only reduce warming. Mitigation maybe possible through heat energy extraction from geothermal, ocean, tidal and solar sources

The response of the Antarctic Circumpolar Current to recent climate change

Observations show a significant intensification of the Southern Hemisphere westerlies, the prevailing winds between the latitudes of 30° and 60° S, over the past decades. A continuation of this intensification trend is projected by climate scenarios for the twenty-first century. The response of the Antarctic Circumpolar Current and the carbon sink in the Southern Ocean to changes in wind stress and surface buoyancy fluxes is under debate. Here we analyse the Argo network of profiling floats and historical oceanographic data to detect coherent hemispheric-scale warming and freshening trends that extend to depths of more than 1,000 m. The warming and freshening is partly related to changes in the properties of the water masses that make up the Antarctic Circumpolar Current, which are consistent with the anthropogenic changes in heat and freshwater fluxes suggested by climate models. However, we detect no increase in the tilt of the surfaces of equal density across the Antarctic Circumpolar Current, in contrast to coarse-resolution model studies. Our results imply that the transport in the Antarctic Circumpolar Current and meridional overturning in the Southern Ocean are insensitive to decadal changes in wind stress

Meridional ocean carbon transport

The ocean's ability to take up and store CO2 is a key factor for understanding past and future climate variability. However, qualitative and quantitative understanding of surface‐to‐interior pathways, and how the ocean circulation affects the CO2 uptake, is limited. Consequently, how changes in ocean circulation may influence carbon uptake and storage and therefore the future climate remains ambiguous. Here we quantify the roles played by ocean circulation and various water masses in the meridional redistribution of carbon. We do so by calculating streamfunctions defined in dissolved inorganic carbon (DIC) and latitude coordinates, using output from a coupled biogeochemical‐physical model. By further separating DIC into components originating from the solubility pump and a residual including the biological pump, air‐sea disequilibrium, and anthropogenic CO2, we are able to distinguish the dominant pathways of how carbon enters particular water masses. With this new tool, we show that the largest meridional carbon transport occurs in a pole‐to‐equator transport in the subtropical gyres in the upper ocean. We are able to show that this pole‐to‐equator DIC transport and the Atlantic meridional overturning circulation (AMOC)‐related DIC transport are mainly driven by the solubility pump. By contrast, the DIC transport associated with deep circulation, including that in Antarctic bottom water and Pacific deep water, is mostly driven by the biological pump. As these two pumps, as well as ocean circulation, are widely expected to be impacted by anthropogenic changes, these findings have implications for the future role of the ocean as a climate‐buffering carbon reservoir

Heat distribution in the Southeast Pacific is only weakly sensitive to high-latitude heat flux and wind stress.

The Southern Ocean features regionally‐varying ventilation pathways that transport heat and carbon from the surface ocean to the interior thermocline on timescales of decades to centuries, but the factors that control the distribution of heat along these pathways are not well understood. In this study, we use a global ocean state estimate (ECCOv4) to (1) define the recently ventilated interior Pacific (RVP) using numerical passive tracer experiments over a 10‐year period and (2) use an adjoint approach to calculate the sensitivities of the RVP heat content (RVPh) to changes in net heat flux and wind stress. We find that RVPh is most sensitive to local heat flux and wind stress anomalies north of the sea surface height contours that delineate the Antarctic Circumpolar Current, with especially high sensitivities over the South Pacific Gyre. Surprisingly, RVPh is not especially sensitive to changes at higher latitudes. We perform a set of step response experiments over the South Pacific Gyre, the subduction region, and the high‐latitude SO. In consistency with the adjoint sensitivity fields, RVPh is most sensitive to wind stress curl over the subtropical gyre, which alter isopycnal heave, and it is only weakly sensitive to changes at higher latitudes. Our results suggest that despite the localized nature of mode water subduction hotspots, changes in basin‐scale pressure gradients are an important controlling factor on RVPh. Because basin‐scale wind stress is expected to change in the coming decades to centuries, our results may have implications for climate, via the atmosphere/ocean partitioning of heat