A dataset of the daily edge of each polynya in the Antarctic

Abstract

Polynyas play a critical role in the formation of Antarctic Bottom Water and the enhancement of polar primary productivity. Accurate and exhaustive identification of Antarctic polynyas is fundamental to advancing in-depth research. However, due to methodological limitations, previous studies paid more attention to frequent polynyas and infrequent polynyas have not been investigated much despite that they could be vulnerable to climate change. Inspired by a cyclone tracking algorithm, we develop a novel method to overcome challenges identifying all types of polynyas satisfying spatiotemporal criteria and tracing their daily evolution, extracting from an extensive amount of sea ice concentration data. Based on it, we establish a dataset called "Daily Edge of Each Polynya in Antarctica" (DEEP-AA). Validation against remote sensing and ship-based observations confirms DEEP-AA's reliability. Compared to existing maps, the DEEP-AA identifies a threefold number of polynyas and reveals the seasonal area recovery of infrequent polynyas is earlier than frequent ones.

Fig. 1

Diagram of the algorithm and the variables contained in the dataset. A detailed diagram of the “open sea removing” algorithm in Step 1 is provided in Supplementary Figure 3. The specifics of the tracing algorithm used in Step 3 are described in Supplementary Text 1.

Fig. 2

(a) The typical area of each polynya, obtained as detailed in the main text (Section: Methods). To facilitate distinction, adjacent polynyas are marked with different colors. (b) Maps of frequent polynyas. A total of 35 frequent polynyas, defined as described in the main text (Section: Overview of the polynyas in the DEEP-AA dataset), are marked in the figure, with their numbers and names written at the bottom of the figure. Their IDs and interannual variation of size and lifespan are listed in Supplementary Figure 10. In both (a,b), we divide the Antarctic into two regions. The locations of these two regions are marked on the upper left side of the subfigures.

Fig. 3

Comparison between our dataset and ship-based observations. (a) Ice and open-water regimes in the course from DEEP-AA and observations. The band along the top comes from DEEP-AA. The light grey blocks indicate the ship was not in a polynya; black ones indicate that it was in the Terra Nova Bay polynya (TNBP); green ones indicate that it was in the Ross Sea polynya (RSP); and blue ones refer to the open sea. The onboard observed ice and polynya regimes are marked below the band; MIZ indicates the marginal ice zone, and FYI indicates first-year ice. All of them come from the PIPERS report in Ackley et al.. The time of entry or exit from the polynya is also marked in (b) with dotted lines. In (b), the blue line shows the ice thickness from the ship-based observations; the blue shaded areas highlight where the ice was thinner than 10 cm (i.e., where the blue line is below the grey dashed line), and the underway wide break is marked by the blue inverted triangle. The red line shows the distance from the ship to the polynyas found by our dataset. The ship trajectory is shown in Supplementary Figure 5.

Fig. 4

The Maud Rise polynya obtained by our dataset in 2017, overlaid on visible light remote sensing images and the SMOS seaice thickness. The red hatched areas denote the extent of the polynya from our dataset, while the yellow areas denote other open waters. In (a–k), the base maps on the left are the visible light remote sensing images from MODIS/Terra, and those on the right indicate the sea ice thickness from SMOS data. Panels (l,m) show the Maud Rise polynya in 2004 and 2016, whose base maps are also visible light images from MODIS/Terra. The location of the study area is marked with a black box in the lower right of the figure.

Fig. 5

Comparison between the traditional integration-based polynya frequency map and the result from our dataset for open-ocean polynyas: (a,b) Maud Rise polynya; (c,d) Cosmonaut polynya; (e,f) Cooperation polynya; (g,h) North Ross Sea polynya; (i,j) halos in the Maud Rise region in 2005 and 2018. All of their locations are marked on the map of Antarctica in the center of the figure. Panels (a), (c), (e), and (g) are the traditional open-water frequency maps integrated from 2003 to 2022. The red borders in (b), (d), (f), and (h) indicate the daily polynya edges obtained by our tracing method in our dataset; the yellow hatched areas indicate the open sea, and the yellow borders without hatching delineate other open waters. The base maps come from MODIS visible light images (b,d,f from MODIS/Terra; h,i,j from MODIS/Aqua). The time is indicated at the bottom of each subfigure.

Fig. 6

Newly found infrequent coastal polynyas. Panel (b) shows the climatic potential position of polynyas in the Ross Sea in Kern’s work. The newly found South Drygalski Ice Tongue polynya and the adjacent polynyas (the Terra Nova Bay polynya and McMurdo polynya) are shown in (c,d). Panels (e,f) illustrate the Peter I Island polynya, and (g,h) the Balleny Islands polynya, which are also marked in (a). Panels (c), (e), and (g) indicate the traditional open-water frequency maps integrated from 2003 to 2022, while the red lines in (d), (f), and (h) indicate the newly found polynyas’ daily edges in our dataset, and the white (yellow) lines indicate the edges of other polynyas (open waters). The base maps come from MODIS/Aqua visible light images at the corresponding times.

Fig. 7

Results of our dataset with infrared remote sensing images during the intrusions of iceberg B15A into the Terra Nova Bay polynya: (a–g) monochrome mosaics of infrared remote sensing images (band 32) from MODIS/Terra, in which the red-hatched regions indicate the extent of the Terra Nova Bay polynya from our dataset, the yellow regions are other open waters, green arrows are the winds, and the edge of iceberg B15A is highlighted by the black border lines; (h) blue mosaic of polynya frequency from our dataset, with the mean wind field from 2003 to 2022 shown by the green arrows.

Fig. 8

Sensitivity of the new method to (a) the threshold for defining open water, (b) the spatial resolution of the input data, (c) the threshold for the number of maintenance days, (d) the parameters for daily-scale tracing, (e) the parameters for yearly-scale tracing, and (f) the masking of landfast ice. The filled bars indicate the numbers of coastal polynyas; the shaded bars indicate the numbers of open-ocean polynyas; the error bars indicate the total area of polynyas plus or minus the standard deviation. Red bars indicate that the input open-water maps come from the SIC, and blue bars that they were obtained by PSSM. The parameters used for the dataset in this paper are formatted in bold typeface.

Fig. 9

Comparison between infrequent and frequent coastal polynyas. (a) Frequency distribution (FD) of annual maintenance days of coastal polynyas (bars), and the corresponding cumulative area (black line). Taking 120 d yr⁻¹ as the boundary (grey dashed line), we classified coastal polynyas into low and high frequency, shown in blue and red respectively in all four subfigures. Their basic characteristics are also written in the figure. (b) Weekly total area of infrequent and frequent polynyas. (c) Weekly numbers of polynyas in the two categories. (d) Weekly mean area per polynya.