More Like this

1. Mechanisms for Abrupt Summertime Circumpolar Surface Warming in the Southern Ocean

   https://doi.org/10.1175/JCLI-D-22-0501.1  

   Wilson, Earle A. ; Bonan, David B. ; Thompson, Andrew F. ; Armstrong, Natalie ; Riser, Stephen C. ( October 2023 , Journal of Climate)

   In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss—a stark reversal of the sea ice expansion and surface cooling that prevailed over the preceding decades. Here, we examine the mechanisms that led to the abrupt circumpolar surface warming events that occurred in late 2016 and 2019 and assess the role of internal climate variability. A mixed layer heat budget analysis reveals that these recent circumpolar surface warming events were triggered by a weakening of the circumpolar westerlies, which decreased northward Ekman transport and accelerated the seasonal shoaling of the mixed layer. We emphasize the underappreciated effect of the latter mechanism, which played a dominant role and amplified the warming effect of air–sea heat fluxes during months of peak solar insolation. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. A key insight from this analysis is that the seasonal phasing of springtime mixed layer depth shoaling is an important contributor to summertime SST variability in the Southern Ocean. Thus, future Southern Ocean summertime SST extremes will depend on the coevolution of mixed layer depth and surface wind variability.

   This study examines how reductions in the strength of the circumpolar westerlies can produce abrupt and extreme surface warming across the Southern Ocean. A key insight is that the mixed layer temperature is most sensitive to surface wind perturbations in late austral spring, when the regional mixed layer depth and solar insolation approach their respective seasonal minimum and maximum. This heightened surface temperature response to surface wind variability was realized during the austral spring of 2016 and 2019, when a dramatic weakening of the circumpolar westerlies triggered unprecedented warming across the Southern Ocean. In both cases, the anomalously weak circumpolar winds reduced the northward Ekman transport of cool subpolar waters and caused the mixed layer to shoal more rapidly in the spring, with the latter mechanism being more dominant. Using results from an ensemble of coupled climate simulations, we demonstrate that the 2016 and 2019 Southern Ocean warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM). These results suggest that future Southern Ocean surface warming extremes will depend on both the evolution of regional mixed layer depths and interannual wind variability.

    

   more » « less
2. Carbon Outgassing in the Antarctic Circumpolar Current Is Supported by Ekman Transport From the Sea Ice Zone in an Observation‐Based Seasonal Mixed‐Layer Budget

   https://doi.org/10.1029/2023JC019815  

   Sauvé, Jade ; Gray, Alison R. ; Prend, Channing J. ; Bushinsky, Seth M. ; Riser, Stephen C. ( November 2023 , Journal of Geophysical Research: Oceans)

   Despite its importance for the global cycling of carbon, there are still large gaps in our understanding of the processes driving annual and seasonal carbon fluxes in the high‐latitude Southern Ocean. This is due in part to a historical paucity of observations in this remote, turbulent, and seasonally ice‐covered region. Here, we use autonomous biogeochemical float data spanning 6 full seasonal cycles and with circumpolar coverage of the Southern Ocean, complemented by atmospheric reanalysis, to construct a monthly climatology of the mixed layer budget of dissolved inorganic carbon (DIC). We investigate the processes that determine the annual mean and seasonal cycle of DIC fluxes in two different zones of the Southern Ocean—the Sea Ice Zone (SIZ) and Antarctic Southern Zone (ASZ). We find that, annually, mixing with carbon‐rich waters at the base of the mixed layer supplies DIC which is, in the ASZ, either used for net biological production or outgassed to the atmosphere. In contrast, in the SIZ, where carbon outgassing and the biological pump are weaker, the surplus of DIC is instead advected northward to the ASZ. In other words, carbon outgassing in the southern Antarctic Circumpolar Current (ACC), which has been attributed to remineralized carbon from deep water upwelled in the ACC, is also due to the wind‐driven transport of DIC from the SIZ. These results stem from the first observation‐based carbon budget of the circumpolar Southern Ocean and thus provide a useful benchmark to evaluate climate models, which have significant biases in this region.

    

   more » « less
3. Heavy footprints of upper-ocean eddies on weakened Arctic sea ice in marginal ice zones

   https://doi.org/10.1038/s41467-022-29663-0  

   Manucharyan, Georgy E. ; Thompson, Andrew F. ( April 2022 , Nature Communications)

   Arctic sea ice extent continues to decline at an unprecedented rate that is commonly underestimated by climate projection models. This disagreement may imply biases in the representation of processes that bring heat to the sea ice in these models. Here we reveal interactions between ocean-ice heat fluxes, sea ice cover, and upper-ocean eddies that constitute a positive feedback missing in climate models. Using an eddy-resolving global ocean model, we demonstrate that ocean-ice heat fluxes are predominantly induced by localized and intermittent ocean eddies, filaments, and internal waves that episodically advect warm subsurface waters into the mixed layer where they are in direct contact with sea ice. The energetics of near-surface eddies interacting with sea ice are modulated by frictional dissipation in ice-ocean boundary layers, being dominant under consolidated winter ice but substantially reduced under low-concentrated weak sea ice in marginal ice zones. Our results indicate that Arctic sea ice loss will reduce upper-ocean dissipation, which will produce more energetic eddies and amplified ocean-ice heat exchange. We thus emphasize the need for sea ice-aware parameterizations of eddy-induced ice-ocean heat fluxes in climate models.

    

   more » « less
4. Seasonal Dynamics of Dissolved Iron on the Antarctic Continental Shelf: Late‐Fall Observations From the Terra Nova Bay and Ross Ice Shelf Polynyas

   https://doi.org/10.1029/2022JC018999  

   Sedwick, P. N. ; Sohst, B. M. ; O’Hara, C. ; Stammerjohn, S. E. ; Loose, B. ; Dinniman, M. S. ; Buck, N. J. ; Resing, J. A. ; Ackley, S. F. ( October 2022 , Journal of Geophysical Research: Oceans)

   Over the Ross Sea shelf, annual primary production is limited by dissolved iron (DFe) supply. Here, a major source of DFe to surface waters is thought to be vertical resupply from the benthos, which is assumed most prevalent during winter months when katabatic winds drive sea ice formation and convective overturn in coastal polynyas, although the impact of these processes on water‐column DFe distributions has not been previously documented. We collected hydrographic data and water‐column samples for trace metals analysis in the Terra Nova Bay and Ross Ice Shelf polynyas during April–May 2017 (late austral fall). In the Terra Nova Bay polynya, we observed intense katabatic wind events, and surface mixed layer depths varied from ∼250 to ∼600 m over lateral distances <10 km; there vertical mixing was just starting to excavate the dense, iron‐rich Shelf Waters, and there was also evidence of DFe inputs at shallower depths in the water column. In the Ross Ice Shelf polynya, wind speeds were lower, mixed layers were <300 m deep, and DFe distributions were similar to previous, late‐summer observations, with concentrations elevated near the seafloor. Corresponding measurements of dissolved manganese and zinc, and particulate iron, manganese, and aluminum, suggest that deep DFe maxima and some mid‐depth DFe maxima primarily reflect sedimentary inputs, rather than remineralization. Our data and model simulations imply that vertical resupply of DFe in the Ross Sea occurs mainly during mid‐late winter, and may be particularly sensitive to changes in the timing and extent of sea ice production.

    

   more » « less
5. El Niño–Like Physical and Biogeochemical Ocean Response to Tropical Eruptions

   https://doi.org/10.1175/JCLI-D-18-0458.1  

   Eddebbar, Yassir A. ; Rodgers, Keith B. ; Long, Matthew C. ; Subramanian, Aneesh C. ; Xie, Shang-Ping ; Keeling, Ralph F. ( May 2019 , Journal of Climate)

   The oceanic response to recent tropical eruptions is examined in Large Ensemble (LE) experiments from two fully coupled global climate models, the Community Earth System Model (CESM) and the Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2M), each forced by a distinct volcanic forcing dataset. Following the simulated eruptions of Agung, El Chichón, and Pinatubo, the ocean loses heat and gains oxygen and carbon, in general agreement with available observations. In both models, substantial global surface cooling is accompanied by El Niño–like equatorial Pacific surface warming a year after the volcanic forcing peaks. A mechanistic analysis of the CESM and ESM2M responses to Pinatubo identifies remote wind forcing from the western Pacific as a major driver of this El Niño–like response. Following eruption, faster cooling over the Maritime Continent than adjacent oceans suppresses convection and leads to persistent westerly wind anomalies over the western tropical Pacific. These wind anomalies excite equatorial downwelling Kelvin waves and the upwelling of warm subsurface anomalies in the eastern Pacific, promoting the development of El Niño conditions through Bjerknes feedbacks a year after eruption. This El Niño–like response drives further ocean heat loss through enhanced equatorial cloud albedo, and dominates global carbon uptake as upwelling of carbon-rich waters is suppressed in the tropical Pacific. Oxygen uptake occurs primarily at high latitudes, where surface cooling intensifies the ventilation of subtropical thermocline waters. These volcanically forced ocean responses are large enough to contribute to the observed decadal variability in oceanic heat, carbon, and oxygen.

    

   more » « less