Atmospheric drivers of a winter-to-spring Lagrangian sea-ice drift in the Eastern Antarctic marginal ice zone

Sea-ice drift in the Antarctic marginal ice zone (MIZ) is discussed using data from a 4-month-long drift of a buoy deployed on a pancake ice floe during the winter sea-ice expansion. We demonstrate increased meandering and drift speeds, and changes in the dynamical regimes of the absolute dispersion during cyclone activity, together with high correlations between drift velocities and wind from atmospheric reanalyses. This indicates a dominant physical control of wind forcing on ice drift and the persistence of free-drift conditions. These conditions occurred despite the buoy remaining largely in >80% ice concentrations and at distances >200 km from the estimated ice edge. The drift is additionally characterised by a strong inertial signature at 13.47 h, which appears initiated by passing cyclones. A wavelet analysis of the buoy's velocity confirms that the momentum transfer from winds at the multi-day frequencies is due to atmospheric forcing, while the initiation of inertial oscillations of sea ice has been identified as the secondary effect. Propagating storm-generated waves may initiate inertial oscillations by increasing the mobility of floes and enhance the drag of the inertial current. This analysis indicates that the Antarctic MIZ in the Indian Ocean sector remains much wider and mobile, during austral winter-to-spring, than defined by sea-ice concentration

A contrast in sea ice drift and deformation between winter and spring of 2019 in the Antarctic marginal ice zone

Two ensembles of buoys, deployed in the marginal ice zone (MIZ) of the north-eastern Weddell Sea region of the Southern Ocean, are analysed to characterise the dynamics driving sea ice drift and deformation during the winter-growth and the spring-retreat seasons of 2019. The results show that although the two buoy arrays were deployed within the same region of ice-covered ocean, their trajectory patterns were vastly different. This indicates a varied response of sea ice in each season to the local winds and currents. Analyses of the winter data showed that the Antarctic Circumpolar Current modulated the drift near the sea ice edge. This led to a highly energetic and mobile ice cover, characterised by free-drift conditions. The resulting drift and deformation were primarily driven by large-scale atmospheric forcing, with negligible contributions due to the wind-forced inertial response. For this highly advective coupled ice–ocean system, ice drift and deformation linearly depended on atmospheric forcing. We also highlight the limits of commercial floating ice velocity profilers in this regime since they may bias the estimates of sea ice drift and the ice type detection. On the other hand, the spring drift was governed by the inertial response as increased air temperatures caused the ice cover to melt and break up, promoting a counterintuitively less wind-driven ice–ocean system that was more dominated by inertial oscillations. In fact, the deformation spectra indicate a strong decoupling to large-scale atmospheric forcing. Further analyses, extended to include the deformation datasets from different regions around Antarctica, indicate that, for similar spatial scales, the magnitude of deformation varies between seasons, regions, and the proximity to the sea ice edge and the coastline. This implies the need to develop rheology descriptions that are aware of the ice types in the different regions and seasons to better represent sea ice dynamics in the MIZ

Physical and morphological properties of first-year Antarctic sea ice in the spring marginal ice zone of the Atlantic-Indian sector

This study presents the first dataset of physical and textural properties of sea ice collected in the South Atlantic and Indian Ocean sector of the Antarctic marginal ice zone (MIZ). Observations of sea ice from this region in the austral spring 2019, including sea-ice core temperature, salinity, crystal size, texture, oxygen isotopes and stratigraphy, were used in conjunction with a Lagrangian back-tracking algorithm and atmospheric reanalyses. This method relates the reconstructed synoptic conditions to sea-ice growth along the transect. A significant difference was found between the stratigraphy of consolidated pack ice samples collected at the same latitude and spanning over 550 km eastwards. The eastward group was found to have more disturbances in their stratigraphy which is attributed to the highly variable atmospheric and sea-ice conditions together with varying wave penetration through the sea-ice pack, notably during the passage of an intense polar cyclone, while the westward group showed no signs of disturbance or deformation. These results indicate that consolidated Antarctic sea-ice floes of similar thickness and from the same latitude in the MIZ have distinct stratigraphic properties, which will influence their physical and biogeochemical features

Atmospheric drivers of ice drift in the Antarctic marginal ice zone

Sea-ice drift in the Antarctic marginal ice zone (MIZ) was investigated using an array of five drifting ice buoys, deployed during the winter sea-ice expansion, in July 2017. An initial 15- day analysis of pancake ice drift is presented, using the cluster of buoys, which shows: (1) exceptionally fast ice drift speeds and increased meandering of the buoys during cyclone activity; (2) high correlation of drift velocities with the surface wind velocities, even at 100% remotely sensed ice concentration, indicating free drift conditions where ice drift is primarily governed by wind; and (3) the presence of a clear energy peak (»13.5 hour period), which is suggested to be excited by the passage of cyclones through the transfer of momentum from wind. Additionally, one of the buoys (buoy U1) drifted for approximately four months from the South Atlantic sector to the Indian Ocean sector of the Southern Ocean. The analysis of this buoy revealed that it remained within the MIZ even during the winter ice expansion, as the mixed pancake-frazil field was maintained. This allowed for a continued assumption of free drift conditions for buoy U1's full drift, where it continued to respond linearly to the momentum transfer from surface winds. The analysis of buoy U1 also indicated a strong inertial signature at a period of 13.47 hours however, the wavelet analysis indicated majority of the power remained within the lower frequencies. This strong influence at the lower (multiday) frequencies has therefore been identified as the primary effect of atmospheric forcing. When these lower frequencies were filtered out using the Butterworth high-pass filter it allowed the inertial oscillations to become more significant within the wavelet power spectrum, where it can be seen that these inertial oscillations were often triggered by the passage of cyclones. The initiation of inertial oscillations of sea ice has therefore been identified as the secondary effect of atmospheric forcing, which dominates ice drift at sub-daily timescales and results in the deviation of ice drift from a straight-line path. This comprehensive analysis suggests that the general concentration-based definition of the MIZ is not enough to describe the sea-ice cover, and that the MIZ, where ice is in free drift and under the influence of cyclone induced inertial motion, and presumably waves, can extend up to »200 km

Contrasting sea ice drift and deformation between winter and spring in the Antarctic marginal ice zone

Two ensembles of buoys, deployed in the north-eastern Weddell Sea region of the Southern Ocean, are analysed to characterise the dynamics driving sea ice drift and deformation during the winter-growth and the spring-retreat seasons of 2019. The results show that although the two buoy arrays were deployed within the same region of ice-covered ocean, their trajectory patterns were vastly different. This indicates a varied response of sea ice in each season to the local winds and currents. Analyses of the winter data showed that the Antarctic Circumpolar Current modulated the drift near the sea ice edge. This led to a highly energetic and mobile ice cover, characterised by free-drift conditions. The resulting drift and deformation were primarily driven by large-scale atmospheric forcing, with negligible contributions due to the wind-forced inertial response. For this highly advective coupled ice-ocean system, ice drift and deformation linearly depends on atmospheric forcing. On the other hand, the drift in spring was governed by the inertial response as increased air temperatures caused the ice cover to melt and break up, within this less advective ice-ocean system. Moreover, the deformation spectra indicate a strong de-coupling to large-scale atmospheric forcing. Analysis, extended to include the datasets of deformation in different regions around Antarctica, indicates that for similar spatial scales the magnitude of deformation may vary between seasons, regions and the proximity to the sea ice edge and the coastline

Effect of Amoxicillin dose and treatment duration on the need for antibiotic re-treatment in children with Community-Acquired Pneumonia: The CAP-IT randomized clinical trial

Importance: The optimal dose and duration of oral amoxicillin for children with community-acquired pneumonia (CAP) are unclear. Objective: To determine whether lower-dose amoxicillin is noninferior to higher dose and whether 3-day treatment is noninferior to 7 days. Design, Setting, and Participants: Multicenter, randomized, 2 × 2 factorial noninferiority trial enrolling 824 children, aged 6 months and older, with clinically diagnosed CAP, treated with amoxicillin on discharge from emergency departments and inpatient wards of 28 hospitals in the UK and 1 in Ireland between February 2017 and April 2019, with last trial visit on May 21, 2019. Interventions: Children were randomized 1:1 to receive oral amoxicillin at a lower dose (35-50 mg/kg/d; n = 410) or higher dose (70-90 mg/kg/d; n = 404), for a shorter duration (3 days; n = 413) or a longer duration (7 days; n = 401). Main Outcomes and Measures: The primary outcome was clinically indicated antibiotic re-treatment for respiratory infection within 28 days after randomization. The noninferiority margin was 8%. Secondary outcomes included severity/duration of 9 parent-reported CAP symptoms, 3 antibiotic-related adverse events, and phenotypic resistance in colonizing Streptococcus pneumoniae isolates. Results: Of 824 participants randomized into 1 of the 4 groups, 814 received at least 1 dose of trial medication (median [IQR] age, 2.5 years [1.6-2.7]; 421 [52%] males and 393 [48%] females), and the primary outcome was available for 789 (97%). For lower vs higher dose, the primary outcome occurred in 12.6% with lower dose vs 12.4% with higher dose (difference, 0.2% [1-sided 95% CI -∞ to 4.0%]), and in 12.5% with 3-day treatment vs 12.5% with 7-day treatment (difference, 0.1% [1-sided 95% CI -∞ to 3.9]). Both groups demonstrated noninferiority with no significant interaction between dose and duration (P =.63). Of the 14 prespecified secondary end points, the only significant differences were 3-day vs 7-day treatment for cough duration (median 12 days vs 10 days; hazard ratio [HR], 1.2 [95% CI, 1.0 to 1.4]; P =.04) and sleep disturbed by cough (median, 4 days vs 4 days; HR, 1.2 [95% CI, 1.0 to 1.4]; P =.03). Among the subgroup of children with severe CAP, the primary end point occurred in 17.3% of lower-dose recipients vs 13.5% of higher-dose recipients (difference, 3.8% [1-sided 95% CI, -∞ to10%]; P value for interaction =.18) and in 16.0% with 3-day treatment vs 14.8% with 7-day treatment (difference, 1.2% [1-sided 95% CI, -∞ to 7.4%]; P value for interaction =.73). Conclusions and Relevance: Among children with CAP discharged from an emergency department or hospital ward (within 48 hours), lower-dose outpatient oral amoxicillin was noninferior to higher dose, and 3-day duration was noninferior to 7 days, with regard to need for antibiotic re-treatment. However, disease severity, treatment setting, prior antibiotics received, and acceptability of the noninferiority margin require consideration when interpreting the findings. Trial Registration: ISRCTN Identifier: ISRCTN76888927