Indian Ocean Subtropical Underwater and the Interannual Variability in its Annual Subduction Rate Associated with the Southern Annular Mode

In this study, the Indian Ocean subtropical underwater (IOSTUW) was investigated as a subsurface salinity maximum using Argo floats (2000–2020) for the first time. It has mean salinity, potential temperature and potential density values of 35.54 ± 0.29 psu, 17.91 ± 1.66 °C, and 25.56 ± 0.35 kg m−3, respectively, and mainly extends between 10°S and 30°S along the isopycnal surface in the subtropical south Indian Ocean. The annual subduction rate of the IOSTUW during the period of 2004-2019 was investigated based on a gridded Argo dataset. The results revealed a mean value of 4.39 Sv (1 Sv=106 m3s−1) with an interannual variability that is closely related to the Southern Annular Mode (SAM). The variation in the annual subduction rate of the IOSTUW is dominated by the lateral induction term, which largely depends on the winter mixed layer depth (MLD) in the sea surface salinity (SSS) maximum region. The anomalies of winter MLD is primarily determined by SAM-related air-sea heat flux and zonal wind anomalies through modulation of the buoyancy. As a result, the annual subduction rate of the IOSTUW generally increased when the SAM index showed negative anomalies and decreased when the SAM index showed positive anomalies. Exceptional cases occurred when the wind anomaly within the SSS maximum region was weak or was dominated by its meridional component.

An Investigation of Extreme Cold Events at the South Pole

From 5 July to 11 September 2012, the Amundsen-Scott South Pole station experienced an unprecedented 78 days in a row with a maximum temperature at or below -50°C. Aircraft and ground-based activity cannot function without risk below this temperature. Lengthy periods of extreme cold temperatures are characterized by a drop in pressure of around 15 hPa over four days, accompanied by winds from grid east. Periodic influxes of warm air from the Weddell Sea raise the temperature as the wind shifts to grid north. The end of the event occurs when the temperature increase is enough to move past the -50°C threshold. This study also examines the length of extreme cold periods. The number of days below -50°C in early winter has been decreasing since 1999, and this trend is statistically significant at the 5% level. Late winter shows an increase in the number of days below -50°C for the same period, but this trend is not statistically significant. Changes in the Southern Annular Mode, El Niño Southern Oscillation, and the Interdecadal Pacific Oscillation/Tripole Index are investigated in relation to the initiation of extreme cold events. None of the correlations are statistically significant. A positive Southern Annular Mode and a La Niña event or a central Pacific El Niño Southern Oscillation pattern would position the upper-level circulation to favor a strong, symmetrical polar vortex with strong westerlies over the Southern Ocean, leading to a cold pattern over the South Pole.

Influence of long-term changes in solar irradiance forcing on the Southern Annular Mode

The Southern Annular Mode (SAM) is the leading mode of climate variability in the extratropical Southern Hemisphere, with major regional climate impacts. Observations, reconstructions, and historical climate simulations all show positive trends in the SAM since the 1960s; however, earlier trends in palaeoclimate SAM reconstructions cannot be reconciled with last millennium simulations. Here we investigate the sensitivity of the SAM to solar irradiance variations using simulations with a range of constant solar forcing values, and last millennium transient simulations with varying amplitude solar forcing scenarios. We find the mean SAM state can be significantly altered by solar irradiance changes, and that transient last millennium simulations using a high-amplitude solar scenario have an improved and significant agreement with proxy-based SAM reconstructions. Our findings suggest that the effects of solar forcing on high-latitude climate may not be adequately incorporated in most last millennium simulations, due to solar irradiance changes that are too small and/or the absence of interactive atmospheric chemistry in global climate models.

Decadal Climate Oscillations, Synoptic Variability, and Ice Core Climate Proxy Records in the Ross Sea Region, Antarctica

<p>This thesis investigates synoptic variability in the Ross Sea region, Antarctica and develops geochemical proxies of this variability from an ice core record in Southern Victoria Land. Particular focus is given to the influence of decadal climate oscillations on synoptic conditions and potential records of these oscillations in ice core proxy records as long-­‐term records of these oscillations are important for understanding future climate change. I present an investigation of the joint influence of the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) on variability in the Amundsen Sea Low (ASL), a dominant climatological feature that strongly influences the weather in the Ross Sea region. It is shown that the positive phase of each oscillation is associated with significant strengthening of the ASL, while negative phases are associated with a weakening. Through regression analysis I show that a simple linear combination of indices representing these oscillations can explain more than 40% of the geopotential height variance in the AS region at a seasonal scale and over 70% of the variance at an annual scale. These results are consistent with the known mechanisms of ENSO and SAM interaction in the region and show that while SAM is dominant hemispherically, ENSO is only influential in the Pacific Sector. Finally it is demonstrated that a simple model of linear reinforcement and interference between the oscillations describes their influence on the variability in the ASL better than models incorporating more complex interactions. Atmospheric back-­‐trajectory modeling and cluster analysis are used to investigate synoptic variability at the Gawn Ice Piedmont (GIP) ice core site in the Ross Sea Region, Antarctica. I identify two dominant air-­‐mass trajectory clusters: oceanic – cyclonic and continental trajectories. My analysis shows that oceanic – cyclonic trajectories peak during April (southern hemisphere winter), while continental trajectories reach their maximum during December (summer). A causal association is demonstrated between ENSO and the frequency of oceanic – cyclonic trajectories originating from the Ross Sea region. In contrast, it is shown that the Southern Annular Mode has little influence on the frequency of cyclonic clusters. I then develop proxy records for the synoptic variability using a shallow firn core from the GIP site containing 8 years of geochemical record. Continental trajectories correlate with concentrations of nitrate (NO3), which is sourced from stratospheric air-­‐masses descending over the Antarctic interior. Oceanic – cyclonic trajectory clusters strongly correlate with deuterium excess at seasonal and inter-­‐annual scales, a proxy sensitive to changes in relative humidity and sea surface temperature (SST) in the in the Ross and Amundsen Seas. Decadal variability in the frequency of oceanic – cyclonic trajectories is discussed with respect to ENSO, SAM, and changes in SST and sea ice extent.</p>

Decadal Climate Oscillations, Synoptic Variability, and Ice Core Climate Proxy Records in the Ross Sea Region, Antarctica

<p>This thesis investigates synoptic variability in the Ross Sea region, Antarctica and develops geochemical proxies of this variability from an ice core record in Southern Victoria Land. Particular focus is given to the influence of decadal climate oscillations on synoptic conditions and potential records of these oscillations in ice core proxy records as long-­‐term records of these oscillations are important for understanding future climate change. I present an investigation of the joint influence of the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) on variability in the Amundsen Sea Low (ASL), a dominant climatological feature that strongly influences the weather in the Ross Sea region. It is shown that the positive phase of each oscillation is associated with significant strengthening of the ASL, while negative phases are associated with a weakening. Through regression analysis I show that a simple linear combination of indices representing these oscillations can explain more than 40% of the geopotential height variance in the AS region at a seasonal scale and over 70% of the variance at an annual scale. These results are consistent with the known mechanisms of ENSO and SAM interaction in the region and show that while SAM is dominant hemispherically, ENSO is only influential in the Pacific Sector. Finally it is demonstrated that a simple model of linear reinforcement and interference between the oscillations describes their influence on the variability in the ASL better than models incorporating more complex interactions. Atmospheric back-­‐trajectory modeling and cluster analysis are used to investigate synoptic variability at the Gawn Ice Piedmont (GIP) ice core site in the Ross Sea Region, Antarctica. I identify two dominant air-­‐mass trajectory clusters: oceanic – cyclonic and continental trajectories. My analysis shows that oceanic – cyclonic trajectories peak during April (southern hemisphere winter), while continental trajectories reach their maximum during December (summer). A causal association is demonstrated between ENSO and the frequency of oceanic – cyclonic trajectories originating from the Ross Sea region. In contrast, it is shown that the Southern Annular Mode has little influence on the frequency of cyclonic clusters. I then develop proxy records for the synoptic variability using a shallow firn core from the GIP site containing 8 years of geochemical record. Continental trajectories correlate with concentrations of nitrate (NO3), which is sourced from stratospheric air-­‐masses descending over the Antarctic interior. Oceanic – cyclonic trajectory clusters strongly correlate with deuterium excess at seasonal and inter-­‐annual scales, a proxy sensitive to changes in relative humidity and sea surface temperature (SST) in the in the Ross and Amundsen Seas. Decadal variability in the frequency of oceanic – cyclonic trajectories is discussed with respect to ENSO, SAM, and changes in SST and sea ice extent.</p>

Assessing the impact of suppressing Southern Ocean SST variability in a coupled climate model

The Southern Ocean exerts a strong influence on global climate, regulating the storage and transport of heat, freshwater and carbon throughout the world’s oceans. While the majority of previous studies focus on how wind changes influence Southern Ocean circulation patterns, here we set out to explore potential feedbacks from the ocean to the atmosphere. To isolate the role of oceanic variability on Southern Hemisphere climate, we perform coupled climate model experiments in which Southern Ocean variability is suppressed by restoring sea surface temperatures (SST) over 40°–65°S to the model’s monthly mean climatology. We find that suppressing Southern Ocean SST variability does not impact the Southern Annular Mode, suggesting air–sea feedbacks do not play an important role in the persistence of the Southern Annular Mode in our model. Suppressing Southern Ocean SST variability does lead to robust mean-state changes in SST and sea ice. Changes in mixed layer processes and convection associated with the SST restoring lead to SST warming and a sea ice decline in southern high latitudes, and SST cooling in midlatitudes. These results highlight the impact non-linear processes can have on a model’s mean state, and the need to consider these when performing simulations of the Southern Ocean.