The new time series reveals distinctive large-scale patterns in the long-term daily means (for 1979–2020) of both coastal exposure spatial length and timing around Antarctica (Figs. 1 and 2). Average net circum-Antarctic CEL attains its maximum in late February (late austral summer), at ~ 8,950 ± 1,210 km or about 50% of the entire Antarctic coastal perimeter of ~ 17,850 km (Fig. 1a; Extended Data Table 1). The climatological maximum (peak) in CEL sits within a roughly symmetrical annual cycle of 6 months increase in exposure and 6 months retreat. Particularly rapid rates of first increase then decrease in exposure length occur over roughly 2-month phases from mid-December through mid-February (summer) and March through April (autumn), respectively. The remaining 8 months are characterised by a long tail of low CEL (less than ~ 11% or ~ 2000 km) that tapers to a small late-winter climatological minimum CEL (of only ~ 33 km) in late August i.e., late winter (Fig. 1a).
The climatological circum-Antarctic CEL cycle (Fig. 1a) differs substantially from the mean annual cycle of overall sea-ice coverage (extent) in its timing and rates of seasonal change. The net sea-ice cycle is strongly asymmetric around advance and retreat seasons that run for approximately 7.5 months (mid-February through late September or late summer through early spring) and 4.5 months respectively3,21.
The net annual CEL cycle (Fig. 1a) is in fact made up of distinctly-differing regional contributions (Fig. 1b-f), with average annual maximum exposure (expressed as a percentage of total coastline length) ranging from ~ 45% for the Amundsen-Bellingshausen seas (ABS) to ~ 58% for the Ross Sea and Indian Ocean sectors (Extended Data Table 1). Of the five sectors, only the ABS and Weddell Sea have some year-round coastal exposure (on average). Of these, the ABS is notable for both its broad exposure decrease phase extending through winter and its relatively high interannual variability (with one standard deviation at peak annual exposure being > 50% of the average value), while the mean annual cycle in the Weddell Sea resembles the net circum-Antarctic cycle.
In contrast and for the East Antarctic (Indian and W Pacific Ocean) and Ross Sea sectors, the mean annual window of coastal exposure is confined to late November through late April-early May (late austral spring through mid-autumn). Moreover, the mean annual exposure cycle across East Antarctica is characterized by a longer increase compared to decrease phase i.e., by ~ 1 month in the Indian Ocean and ~ 1.5 months in the W Pacific. In the Ross Sea on the other hand, coastal exposure length exhibits a particularly sharp seasonal decline of ~ 70% in only two weeks from late February through early March, after an annual peak that occurs ~ 2 weeks prior to those in the East Antarctic and ABS sectors but near-coincident with that in the Weddell Sea.
Strong regional dependence is also apparent in the mean duration of annual exposure and the timing of the annual exposure window as a function of longitude (Fig. 2) – together with complex intra-regional patterns of variability. Exposure duration covers a wide range, from low (< 2 days) in parts of the W Weddell Sea, eastern Ross Sea and W Pacific Ocean sectors (as well as multiple more localised coastal tracts of the Indian and W Pacific oceans) to very high (~ 290 days) towards the northern tip of the Antarctic Peninsula (Fig. 2a). Elsewhere, mean annual exposure duration is generally ≤ 50 days, with a broader zone of shorter (< 20 day) exposure across the ABS sector that ramps up towards the Antarctic Peninsula. Broadly-speaking, zonally-extensive regions of low and zero exposure duration (Fig. 2a) and frequency of occurrence (Fig. 2b) centred on ~ 150°E, 110°W and 50°W align with well-documented areas of perennial sea-ice coverage offshore21.
Regional contrasts and intra-regional variability are also a feature of the mean timings of annual exposure onset and cessation, the resultant seasonal window of exposure duration and the frequency of occurrence as a function of longitude (Fig. 2b). This again shows the Antarctic Peninsula to be a standout region in terms of the near year-round exposure along much of its western and north-eastern flanks. Elsewhere around Antarctica, the mean timing of annual exposure onset varies by about two months – from mid-November in the East Antarctic sector at ~ 140–150°E (George V Land coast) to mid-January in western parts of the Indian Ocean and the Amundsen Sea. The anomalously-early onset in George V Land is likely linked to the recurrent Mertz Glacier Polynya, which has been shown in previous work22 to play an important role in driving an unusually early and rapid seasonal meltback of sea ice to the coast in that region. By comparison and in autumn, the timing of mean annual cessation of exposure again varies regionally over ~ 2 months, from early March (Ross Sea) to early May (Enderby Land in the western Indian Ocean).
The East and West Antarctic sectors in fact differ substantially in terms of their respective zonal patterns of mean coastal exposure occurrence timings (Fig. 2b). For East Antarctica from ~ 0° to 160°E, mean exposure-window duration is relatively uniform at about 4 ± 0.5 months, but displays a broad wave-like configuration that comprises alternating zones of apparent eastward and westward propagation over time for both exposure onset and subsequent cessation. For example, the timing of the exposure window propagates from west to east in the eastern limb of the Weddell Gyre (0–40°E) over a > 1 month period, but this pattern reverses along much of the adjacent Indian Ocean coast.
The broad wave-like pattern transitions eastwards from the eastern Indian Ocean into a zone of more variable timings and generally lower frequency of occurrence across the W Pacific Ocean. This changes to a narrower (approximately 3-month) and more uniform coastal-exposure window across the central Ross Sea. Moving into the ABS sector, the exposure window is initially fairly narrow (2–3 months), occurs later and is more variable (than the Ross Sea), and also has a substantially lower frequency of occurrence. Exposure duration broadens towards the Antarctic Peninsula, where there is a high frequency of exposure occurrence throughout the year. This is in stark contrast to the adjacent area of zero exposure occurrence in the W/SW Weddell Sea due to the perennial occurrence of sea-ice coverage there. Finally, and from the central Weddell Sea through the Greenwich Meridian, the exposure occurrence window broadens back to ~ 4 to > 5 months, with the timing of exposure onset remaining uniform but cessation timing increasing with distance towards the meridian. These distinct zonal patterns are further depicted in a map of the annual-mean duration of coastal exposure for 1979–2020 for all coastal-adjoining pixels (Fig. 2c).