Asymmetric impacts of El Niño and La Niña on the Pacific–South America teleconnection pattern

El Niño–Southern Oscillation (ENSO) has a huge influence on Antarctic climate variability via Rossby wave trains. In this study, the asymmetry of the ENSO teleconnection in the Southern Hemisphere, as along with the mechanisms involved, is systematically investigated. In four reanalysis datasets, the composite atmospheric circulation anomaly in austral winter over the Amundsen Sea during La Niña is situated more to the west than during El Niño. This asymmetric feature is reproduced by ECHAM5.3.2 forced with both composite and idealized symmetric sea surface temperature anomalies. Utilizing a linear baroclinic model, we find that ENSO-triggered circulation anomalies in the subtropics can readily extract kinetic energy from the climatological mean flow and develop efficiently at the exit of the subtropical jet stream (STJ). The discrepancy in the location of the STJ between El Niño and La Niña causes asymmetric circulation responses by affecting the energy conversion. During El Niño years, anomalous tropical convective precipitation increases the meridional temperature gradient, which in turn leads to the strengthening of the STJ and the eastward movement of the jet core and jet exit in the Pacific. With the movement of the STJ exit, the wave train tends to develop over the eastern region. The opposite is the case during La Niña when the westward shift of the jet exit favors the development of the wave train in the western region. Our findings expand the current understanding regarding ENSO teleconnection.

High-Resolution Water Stable Isotope Ice-Core Record: Roosevelt Island, Antarctica

<p>This thesis presents a water-isotope (δD) record from 1900 to 2009 for the Roosevelt Island Climate Evolution (RICE) ice core, Antarctica. Examination of the RICE isotope record with observation data (using global reanalysis and SST datasets) revealed details of the climate signal that is preserved within the full 763 m isotope record. RICE δD provides a proxy record, which captures the central tropical Pacific ENSO variability, the significant (p < 0.01) central Pacific δD-SST correlation pattern contain the Niño-4 SST region. Central tropical Pacific ENSO variability projects upon the Amundsen Sea region via a Pacific–South American pattern (PSA)-like teleconnection. RICE δD is primarily influenced by Amundsen Sea circulation, which coincides with the leading PSA pattern’s (PSA1) circulation focal point in the Amundsen Sea. Additionally, RICE regional physical setting (sheltered from direct impact from Amundsen Sea cyclones by WA orography) offers a unique setting, where enriched isotopes only are associated with one PSA1 polarity (El Niño, PSA1+, Amundsen Sea anticyclones). In contrast, during La Niña and Amundsen Sea cyclones, δD is depleted. Combined these settings, provides a compelling explanation to why RICE δD preserves PSA1 and ENSO variability. On interannual and seasonal time scales, the RICE δD variability is well-explained by the PSA teleconnections and their interactions over the Pacific sector. The influence from PSA2 on δD is strong during the beginning of the year (December–February, DJF). In contrast, the PSA1 influence is strong during the latter part of the year, peaking in spring (September–November, SON). The isotope record appears to preserve tropical Pacific El Niño-like interdecadal variability, particularly a decadal-signal from the central-Pacific (Niño-4 SST region) and from the Pacific-wide Interdecadal Pacific Oscillation (IPO). On decadal-scales RICE δD is modulated by ENSO and Southern Annular Mode (SAM); when the correlation with SAM is active (during IPO+) δD appears to be in a depleted state and when the correlation with SAM breaks down (during IPO−) δD appears to be in a relatively enriched state. A RICE δD SST proxy reconstruction can potentially provide a record longer than the currently available observational datasets, allowing for examination of intrinsic decadal-scale tropical Pacific climate variability and its extratropical impact.</p>

High-Resolution Water Stable Isotope Ice-Core Record: Roosevelt Island, Antarctica

<p>This thesis presents a water-isotope (δD) record from 1900 to 2009 for the Roosevelt Island Climate Evolution (RICE) ice core, Antarctica. Examination of the RICE isotope record with observation data (using global reanalysis and SST datasets) revealed details of the climate signal that is preserved within the full 763 m isotope record. RICE δD provides a proxy record, which captures the central tropical Pacific ENSO variability, the significant (p < 0.01) central Pacific δD-SST correlation pattern contain the Niño-4 SST region. Central tropical Pacific ENSO variability projects upon the Amundsen Sea region via a Pacific–South American pattern (PSA)-like teleconnection. RICE δD is primarily influenced by Amundsen Sea circulation, which coincides with the leading PSA pattern’s (PSA1) circulation focal point in the Amundsen Sea. Additionally, RICE regional physical setting (sheltered from direct impact from Amundsen Sea cyclones by WA orography) offers a unique setting, where enriched isotopes only are associated with one PSA1 polarity (El Niño, PSA1+, Amundsen Sea anticyclones). In contrast, during La Niña and Amundsen Sea cyclones, δD is depleted. Combined these settings, provides a compelling explanation to why RICE δD preserves PSA1 and ENSO variability. On interannual and seasonal time scales, the RICE δD variability is well-explained by the PSA teleconnections and their interactions over the Pacific sector. The influence from PSA2 on δD is strong during the beginning of the year (December–February, DJF). In contrast, the PSA1 influence is strong during the latter part of the year, peaking in spring (September–November, SON). The isotope record appears to preserve tropical Pacific El Niño-like interdecadal variability, particularly a decadal-signal from the central-Pacific (Niño-4 SST region) and from the Pacific-wide Interdecadal Pacific Oscillation (IPO). On decadal-scales RICE δD is modulated by ENSO and Southern Annular Mode (SAM); when the correlation with SAM is active (during IPO+) δD appears to be in a depleted state and when the correlation with SAM breaks down (during IPO−) δD appears to be in a relatively enriched state. A RICE δD SST proxy reconstruction can potentially provide a record longer than the currently available observational datasets, allowing for examination of intrinsic decadal-scale tropical Pacific climate variability and its extratropical impact.</p>

Variations in and environmental controls of primary productivity in the Amundsen Sea

The Amundsen Sea is one of the regions with the highest primary productivity in the Antarctic. To better understand the role of the Southern Ocean in the global carbon cycle and in climate regulation, a better understanding of the variations in and environmental controls of primary productivity is needed. Using cluster analysis, the Amundsen Sea was divided into nine bioregions. The biophysical differences among bioregions enhanced confidence to identify priorities and regions to study the temporal and spatial variations in primary productivity. Four nearshore bioregions with high net primary productivity or rapidly increasing rates were selected to analyze temporal and spatial variations in primary productivity in the Amundsen Sea. Due to changes in net solar radiation and sea ice, primary production had significant seasonal variation in these four bioregions. The phenology had changed at two bioregions (3 and 5), which has the third and fourth highest primary production, due to changes in the dissolved iron, nitrate, phosphate, and silicate concentrations. Annual primary production showed increasing trends in these four bioregions. The variation in primary production in the bioregion (9), which has the highest primary production, was mainly affected by variations in sea surface temperatures. In the bioregion, which has the second-highest primary production (8), the primary production was significantly positively correlated with sea surface temperature and significantly negatively correlated with sea ice thickness. The long-term changes of primary productivity in bioregions 3 and 5 were thought to be related to changes in the dissolved iron, nitrate, phosphate, and silicate concentrations, and dissolved iron was the limiting factor in these two bioregions. Bioregionalization not only disentangle multiple factors that control the spatial differences, but also disentangle limiting factors that affect the phenology, decadal and long-term changes in primary productivity.

Basal Water Storage Variations beneath Antarctic Ice Sheet Inferred from Multi-source Satellite Data

Antarctic basal water storage variations (BWSV) refer to the mass variations of liquid water beneath Antarctic ice sheet. Identifying these variations is critical to understand the behaviour of ice sheet, yet it is rarely accessible to direct observation. We presented a layered gravity density forward/inversion method for estimating Antarctic BWSV from multi- source satellite observation data, and relevant models. Results reveal spatial variability of BWSV with the mean rate of 43 ± 13 Gt/yr during 2003–2009, which is 21 Gt/yr lower than basal melting rate. This indicates that the basal meltwater beneath Antarctic ice sheet is decreasing with the rate of −21 ± 13 Gt/yr, accounting for 28 % of the mass balance rate (−76 Gt/yr, Shepherd et al. (2018)), and the basal water migrations between basal drainage systems and oceans is non-ignorable in estimating basal mass balance of Antarctic ice sheet. Similar spatial distribution of basal water increases regions and locations of active subglacial lakes indicates that basal water storage in most active subglacial lakes is increasing. In most region of Antarctic ice sheet except Amundsen Sea coast region, the comparison of spatial BWSV and ice velocity displays a positive correlation between considerable basal water increases and rapid/accelerated ice flows, which indicates that BWSV appear to have an important effect on ice flows. Accordingly, we infer that further enhanced flow velocities are expected if basal water continues to increase in these regions.

Validation of a daily satellite-derived Antarctic sea ice velocity product: impacts on ice kinematics

Antarctic sea ice kinematic plays a crucial role in shaping the polar climate and ecosystems. Satellite passive microwave-derived sea ice motion data have been used widely for studying sea ice motion and deformation processes, and provide daily, global coverage at a relatively low spatial-resolution (in the order of 60 × 60 km). In the Arctic, several validated data sets of satellite observations are available and used to study sea ice kinematics, but far fewer validation studies exist for the Antarctic. Here, we compare the widely-used passive microwave-derived Antarctic sea ice motion product by Kimura et al. (2013) with buoy-derived velocities, and interpret the effects of satellite observational configuration on the representation of Antarctic sea ice kinematics. We identify two issues in the Kimura et al. (2013) product: (i) errors in two large triangular areas within the eastern Weddell Sea and western Amundsen Sea relating to an error in the input satellite data composite, and (ii) a more subtle error relating to invalid assumptions for the average sensing time of each pixel. Upon rectification of these, performance of the daily composite sea ice motion product is found to be a function of latitude, relating to the number of satellite swaths incorporated (more swaths further south as tracks converge), and the heterogeneity of the underlying satellite signal (brightness temperature here). Daily sea ice motion vectors calculated using ascending- and descending-only satellite tracks (with a true ~24 h time-scale) are compared with the widely-used combined product (ascending and descending tracks combined together, with an inherent ~39 h time-scale). This comparison reveals that kinematic parameters derived from the shorter time-scale velocity datasets are higher in magnitude than the combined dataset, indicating a high degree of sensitivity to observation time-scale. We conclude that the new generation of “swath-to-swath” (S2S) sea ice velocity datasets, encompassing a range of observational time scales, is necessary to advance future research into sea ice kinematics.

Impacts of Antarctic Sea Ice, AMV and IPO on Extratropical Southern Hemisphere Climate: A Modeling Study

Some studies have discussed potential influences of Antarctic sea ice anomalies, Atlantic Multi-decadal Variability (AMV), and Interdecadal Pacific Oscillation (IPO) on the Southern Hemisphere (SH) climate, individually. However, it is not clear how different combinations of them influence the extratropical SH climate. Here we select three different combinations of strong anomalies in Antarctic sea ice (SI), AMV and/or IPO identified from observations, and investigate their influence on the winter extratropical SH climate using the Community Atmosphere Model. The model results show that atmospheric responses vary with different combinations. When both SI and AMV are in strong positive polarity (SI + AMV), the polar jet shifts equatorward while the subtropical jet shifts poleward, the amplitude of zonal wave number 1 is reduced in high-latitudes with minimal changes in wave number 2, and a north-south circulation dipole occurs in both the Atlantic and Pacific. Different from SI + AMV, when SI is in strong positive polarity and IPO is in strong negative polarity (SI-IPO), the reduction of wave number 1 is dramatically increased, accompanied by remarkably increased wave number 2. The north-south circulation dipole only occurs in the Pacific and is confined to the central and eastern Pacific, whereas the Atlantic is dominated by anomalously anticyclonic circulation. Together, SI + AMV-IPO leads to the largest reduction of wave number 1 in high-latitudes and subtropics, the strongest north-south circulation dipole in the Pacific as well as the Amundsen Sea Low. As a result, the three combinations produce different patterns of surface temperature and precipitation anomalies over Antarctica, Australia and South America.