Sea ice records over more than a century at an observatory facing the Okhotsk coast of Hokkaido, Japan

<i>Time series of sea ice dates and amounts at Abashiri.</i>

The dataset contains four directories:<br>
<b>raw_data</b>, <b>csv_data</b>, <b>codes</b>, and <b>figs</b>.<br>

<b>raw_data</b> directory contains manually created data in an ascii format:<br>
<b>Si_dates_1892-2025.txt</b>: yearly first and last sea ice dates (FD_sight, FD_shore, LD_sight, and LD_shore) during 1892-2025.<br>
<b>Total_ice_1956-2021.txt</b>: daily total ice amounts during 1956-2021.<br>
<b>Drift_ice_1930-2021.txt</b>: daily drift ice amounts during 1930-2021.<br>
In <b>Total_ice_1956-2021.txt</b> and <b>Drift_ice_1930-2021.txt</b>, six monthly lines with daily values from previous December to May are described for each year; for the daily data, <code>0</code>-<code>9</code> denote the area fraction in quantile, <code>*</code> denotes full ice cover (10), <code>+</code> and <code>-</code> denote 0+ and 10-, respectively.<br>

<b>csv_data</b> directory contains data files in a csv format:<br>
<b>Si_dates_1892-2025.csv</b>, <b>Total_ice_1956-2021.csv</b>, and <b>Drift_ice_1930-2021.csv</b>.<br>
Dates are described in an ISO 8601 format ("YYYY-MM-DD"; "0000-00-00" denotes date not defined; "9999-99-99" denotes missing observation). For total/drift ice amounts, <code>*</code>, <code>-</code>, and <code>+</code> are converted to 10.0, 9.75, and 0.25, respectively.<br>

<b>codes</b> directory contains shared codes:<br>
<b>txt2csv_dates.py</b> converts <b>Si_dates_1892-2025.txt</b> to <b>Si_dates_1982-2025.csv</b>.<br>
<b>txt2csv_ice.py</b> converts ice amount files (.txt to .csv).<br>
<b>plot_fig6a.py</b> and <b>plot_fig7a.py</b> plot ice amount time series (Figs. 6a and 7a) from the csv files.<br>
<b>cumul_dev_si_dates.F90</b> (or <b>cumul_dev_si_dates.py</b>) calculates cumulative deviation from the mean (CDM) values, which are written as <b>cumul_dev_si_dates.dat</b> (<b>cumul_dev_si_dates.csv</b>).<br> <b>cumul_dev_si_dates_plot.gs</b> (<b>plot_cumul_dev_si_dates.py</b>) draws a plot for the CDM time series.<br>

<b>figs</b> directory contains figures.