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Extensive research has been conducted at Lake Tanganyika to understand its vulnerability to a warming climate and fishing pressure. However, much of this work has been restricted to the more accessible northern basin. Studies from a limited geographic region of the lake are insufficient to explain whole-lake dynamics of the world’s longest lake (~ 670 km N-S). While strong evidence suggests that lake warming has played a critical role in decreasing fish abundance, limnological changes associated with warming have not been investigated lake wide. This study used paleolimnological data from two deep-water sediment cores from southern Lake Tanganyika to determine whether changes observed in the northern and central basins are, in fact, representative of lake-wide phenomena. We infer a decrease in diatom concentration after ~ 3000 cal yrs BP to reflect a decrease in net primary productivity associated with decreasing convective mixing. In contrast, the episodic presence of benthic invertebrates (ostracodes and molluscs) at these deep sites between ~ 1800–500 cal yrs BP, along with high Mn, indicates episodic pulses of much deeper ventilation of the southern basin than has been recognized previously. The presence of periphytic diatom species and benthic invertebrates during periods of strong stratification suggests that the lake bottom was periodically ventilated by descending denser (cooler or more sediment-rich) influent waters along the steep slopes of the coastline. Fish fossil abundance also is correlated with the dominance of heavily silicified diatom taxa, which itself requires stronger wave activity and upwelling of deep, nutrient-rich water. Fossil diatom assemblages show P and Si gradients between the two sites. LT17-TANG17-6A shows a shift in dominance towards lightly silicified taxa (Nitzschia spp.) after ~ 200 cal yrs BP. LT17-TANG17-2A shows a reduction in concentration of mostly the lightly silicified taxa, with few periods having heavily silicified taxa. These results are indicative of a relative reduction in convective lake mixing but are not always coincident with temperature trends, suggesting local windiness may also be important for stratification history. Thus, changes in lake productivity in the southern basin appear to be climate mediated, but in ways not previously documented in the northern and central parts of the lake.

Author(s): Beadling, RL; Russell, JL; Stouffer, RJ; Mazloff, M; Talley, LD; Goodman, PJ; Sallee, JB; Hewitt, HT; Hyder, P; Pandde, Amarjiit | Abstract: AbstractThe air–sea exchange of heat and carbon in the Southern Ocean (SO) plays an important role in mediating the climate state. The dominant role the SO plays in storing anthropogenic heat and carbon is a direct consequence of the unique and complex ocean circulation that exists there. Previous generations of climate models have struggled to accurately represent key SO properties and processes that influence the large-scale ocean circulation. This has resulted in low confidence ascribed to twenty-first-century projections of the state of the SO from previous generations of models. This analysis provides a detailed assessment of the ability of models contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to represent important observationally based SO properties. Additionally, a comprehensive overview of CMIP6 performance relative to CMIP3 and CMIP5 is presented. CMIP6 models show improved performance in the surface wind stress forcing, simulating stronger and less equatorward-biased wind fields, translating into an improved representation of the Ekman upwelling over the Drake Passage latitudes. An increased number of models simulate an Antarctic Circumpolar Current (ACC) transport within observational uncertainty relative to previous generations; however, several models exhibit extremely weak transports. Generally, the upper SO remains biased warm and fresh relative to observations, and Antarctic sea ice extent remains poorly represented. While generational improvement is found in many metrics, persistent systematic biases are highlighted that should be a priority during model development. These biases need to be considered when interpreting projected trends or biogeochemical properties in this region.

The Southern Ocean is central to the global climate and the global carbon cycle, and to the climate's response to increasing levels of atmospheric greenhouse gases, as it ventilates a large fraction of the global ocean volume. Global coupled climate models and earth system models, however, vary widely in their simulations of the Southern Ocean and its role in, and response to, the ongoing anthropogenic trend. Due to the region's complex water-mass structure and dynamics, Southern Ocean carbon and heat uptake depend on a combination of winds, eddies, mixing, buoyancy fluxes, and topography. Observationally based metrics are critical for discerning processes and mechanisms, and for validating and comparing climate and earth system models. New observations and understanding have allowed for progress in the creation of observationally based data/model metrics for the Southern Ocean. Metrics presented here provide a means to assess multiple simulations relative to the best available observations and observational products. Climate models that perform better according to these metrics also better simulate the uptake of heat and carbon by the Southern Ocean. This report is not strictly an intercomparison, but rather a distillation of key metrics that can reliably quantify the "accuracy" of a simulation against observed, or at least observable, quantities. One overall goal is to recommend standardization of observationally based benchmarks that the modeling community should aspire to meet in order to reduce uncertainties in climate projections, and especially uncertainties related to oceanic heat and carbon uptake., National Science Foundation (U.S.) (Award PLR-1246247)

A time-dependent trajectory algorithm is used to determine the sources of the Pacific Ocean Equatorial Undercurrent (EUC) in a global climate model with ¼° (eddy permitting) resolution and forced with realistic winds. The primary sources and pathways are identified, and the transformation of properties in temperature/salinity space is explored. An estimate for the quantity of recirculation, a notoriously difficult property to estimate from observational data, is given. Over two-thirds of the water in the Pacific EUC at 140°W originates south of the equator; 70% of the EUC is ventilated outside of the Tropics (poleward of 13°S or 10°N): three-quarters of these extratropical trajectories travel through the western boundary currents between their subduction and incorporation into the EUC, and one-fifth of the extratropical trajectories enter and leave the tropical band at least once before entering the EUC.

The response of an ocean general circulation model to the onset of deep-water formation in the North Atlantic Ocean is explored. The processes of baroclinic adjustment to the new deep water mass and the advection of the new deep water mass are compared in both space and time. The baroclinic adjustment is gauged by following the anomalies in the 0‐2000 dbar layer thickness and the advection is measured with the aid of idealized passive tracers. Baroclinic adjustment follows the classical boundary layer path and all locations north of the Antarctic Circumpolar Current begin to feel the effects within 20 years. Heat transport in the North Atlantic responds on the adjustment timescale. Advection does not follow the boundary layer path and is much slower: the timescale for NADW to reach the North Pacific Ocean is on the order of 1000 years. While the baroclinic signal is much faster, the initial response is much smaller and probably could not be detected over the random noise in the pressure field outside of the Atlantic basin. Both processes weaken as they move farther from the forcing region.

Simple theoretical arguments are employed to study the dependence of the volume and transport of the Antarctic Bottom Water (AABW) in the Atlantic Ocean on the vertical diffusivity. We have found that while the vertical extent of the AABW cell decreases with the intensification and deepening of the North Atlantic overturning cell, the transport of AABW into the Atlantic increases. The latter fact is explained by the increase in the deep meridional pressure gradient, which drives the flow. An estimate of the AABW transport is then derived from the density balance in the deep western boundary layer.

The global adjustment of the thermocline in response to deepwater formation is studied in a single mode model on a beta-plane. The signal is carried from ocean to ocean by Kelvin waves, which travel equatorward along western boundaries, eastward across the equator, poleward at the eastern boundaries, and then eastward around the southern tip of continents into the next ocean basin. The interior is filled by Rossby waves emanating from eastern boundaries. Stronger (weaker) deepwater formation induces an upward (downward) motion of the main thermocline in the world oceans. The adjustment is completed on centennial time scales.

Two coarse-resolution model experiments are carried out on an OGCM to examine the effects of North Atlantic Deep Water (NADW) formation on the thermohaline circulation (THC) and ventilation timescales of the abyssal ocean. An idealized age tracer is included to gauge the ventilation in the model. One experiment is forced with the present-day climatology, the other has a negative salinity anomaly imposed on the North Atlantic surface to eliminate the formation of NADW. The Atlantic branch of the THC is reversed and the ventilation of the deep Atlantic basin is severely reduced when NADW formation is prevented. The Southern Ocean forms bottom water in both experiments, but downwelling and upwelling in the Southern Ocean are both reduced when NADW is included due to increased stratification of the water column. The Indian and Pacific basins are upwelling regions in both experiments and upper-level upwelling is stronger there when NADW is included; this change leads to cooler temperatures and reduced...

We study the dependence of the volume and transport of the Antarctic Bottom Water (AABW) in the Atlantic Ocean on vertical mixing. Numerical results from a set of OGCM runs show that, while the vertical extent of the AABW cell decreases with intensifying vertical mixing, the cell's transport increases. An analytical model of the deep boundary layers is then used to interpret the results. The decrease in the AABW thickness is explained by the downward expansion of the upper, North Atlantic Deep Water cell. The intensification of the AABW transport is attributed to the increase in the deep meridional pressure gradient, which drives the flow. An estimate of the AABW transport is then derived from the density balance in the deep western boundary layer and compared with the OGCM results.