In Austral summer 2016/2017, the sea ice extent (SIE) in the Weddell Sea dropped to a near‐record value in the satellite era (1.88 × 106 km2), a large negative seasonal anomaly that persisted in an unprecedented fashion for the following three summers. Various atmospheric and oceanic factors played a part in the change. Ice loss started in September 2016 when the northern Weddell Sea experienced westerly winds of record strength, advecting multiyear sea ice from the region. In late 2016, a polynya over Maud Rise contributed to low SIE over the eastern Weddell Sea. With extensive areas of open water early in the summer, upper ocean temperatures increased by ~0.5°C, with the anomalies persisting in subsequent years. The reappearance of the Maud Rise polynya in 2017, high ocean temperatures, and storms of record depth kept the summer SIE low.

Antarctic sea ice extent (SIE) has displayed a complex pattern of change over the period for which we have reliable data from passive microwave satellite instruments starting in the late 1970s. Until the mid-1990s there was no significant trend in the annual mean total Antarctic SIE or the extent at the annual minimum (Figure 1a). However, this was followed by an upward trend in both measures, which was accompanied by an increase in the inter-annual variability (Fogt et al., 2022, Figure 1). The overall increase in SIE between the mid-1990s and 2014 masked large regional variations, such as the increase in the Ross Sea and decrease in the Amundsen-Bellingshausen Seas (ABS) (Turner et al., 2015), which was consistent with a deepening of the Amundsen Sea Low (ASL) (Raphael et al., 2015). A number of studies have examined the sea ice increase and suggested it was linked to a range of high latitude and tropical forcing factors (

Satellite observations have shown that the largest and most prolonged Maud Rise open‐ocean polynya since the 1970s appeared on 14 September 2017 (~9.3 × 103 km2) within the seasonal sea‐ice cover which expanded maximum on 1 December 2017 (~298.1 × 103 km2) and existed for 79 days. Record negative anomalies of sea‐ice concentration were observed in and around the polynya. The occurrence of the polynya was associated with a large cyclonic eddy and negative wind stress curl that facilitated melting of sea‐ice. Concurrently, a region of positive sea level pressure anomalies extended over the entire northern Weddell Sea accompanied by record low negative anomalies (deep depressions) over the southwest Weddell Sea and the Maud Rise. The atmospheric circulation anomalies advected moist‐warm air from the midlatitudes, resulted a record atmospheric warming (~11.5 °C) in the Maud Rise that favored this rare event as one of the largest open‐ocean polynyas.

Sea ice extent (SIE) in the Weddell Sea attained exceptionally low levels in April (1.97 million km2) and May (3.06 million km2) 2019, with the values being ~22% below the long-term mean. Using in-situ, satellite and atmospheric reanalysis data, we show the large negative SIE anomalies were driven by the passage of a series of intense and explosive polar cyclones (with record low pressure), also known as atmospheric ‘bombs’, which had atmospheric rivers on their eastern flanks. These storms led to the poleward propagation of record-high swell and wind waves (~9.6 m), resulting in southward ice advection (~50 km). Thermodynamic processes also played a part, including record anomalous atmospheric heat (>138 W m−2) and moisture (>300 kg m−1s−1) fluxes from midlatitudes, along with ocean mixed-layer warming (>2 °C). The atmospheric circulation anomalies were associated with an amplified wave number three pattern leading to enhanced meridional flow between midlatitudes and the Antarctic.

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Recent studies on the diazotrophic cyanobacterium Trichodesmium showed that increasing CO 2 partial pressure (pCO 2 ) enhances N 2 fixation and growth. We studied the in situ and satellite-derived environmental parameters within and outside a Trichodesmium bloom in the western coastal Bay of Bengal (BoB) during the spring intermonsoon 2009. Here we show that the single most important nitrogen fixer in today's ocean, Trichodesmium erythraeum, is strongly abundant in high (≥300 atm) pCO 2 concentrations. N : P ratios almost doubled (∼10) at high pCO 2 region. This could enhance the productivity of N-limited BoB and increase the biological carbon sequestration. We also report presence of an oxygen minimum zone at Thamnapatnam. Earlier studies have been carried out using lab cultures, showing the increase in growth rate of T. erythraeum under elevated pCO 2 conditions, but to our knowledge, this study is the first to report that in natural environment also T. erythraeum prefers blooming in high pCO 2 concentrations. The observed CO 2 sensitivity of T. erythraeum could thereby provide a strong negative feedback to rising atmospheric CO 2 but would also drive towards phosphorus limitation in a future high CO 2 world.

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