Antarctic coastal exposure length and index, 1979-2020

Described fully in (https://doi.org/10.21203/rs.3.rs-636839/v1 holder).

Data
The main CEL method, and a subsidiary Coastal Exposure Index or CEI (both described below), are based on daily sea-ice concentration products for the period 1979 through 2020. These products are derived from the multi-satellite passive-microwave brightness temperature time series using the NASA Team algorithm, mapped at 25 km x 25 km resolution and obtained from the NASA National Snow and Ice Data Center (NSIDC) Distributed Active Archive Center (DAAC). Both algorithms are designed to be adaptable for different resolution data. Complete coverage of the entire Antarctic coastal and sea-ice zones is obtained on a daily basis, except for 1979-July 1987 (once every two days). Missing single days during this period are interpolated from the adjoining day's sea-ice concentration values. Averages and climatologies are based on the period 1979-2020, unless otherwise stated. The continental land mask used (gsfc_25s.msk) is also obtained from NSIDC, and includes ice shelves (the seaward extremities of which are taken here to be coastline). Coastline grid points are defined from the continental land mask as any ocean grid point that has land/ice sheet adjacent to it.

Analysis methods
For this study, we developed two new but different algorithms for quantifying and monitoring Antarctic coastal exposure: the Coastal Exposure Index (CEI) and Coastal Exposure Length (CEL) method.

The CEI technique is based on the detection of sea ice presence/absence radially out (northwards) from the coastline along each meridian (at one degree longitudinal spacing), following masking of the ice sheet. The CEI is simply defined as the number of longitudes with no sea ice (threshold set to less than 15% following convention) to the north of the continent, and hence runs from zero to 360. This methodology is trivial and code for this is not included.

CEL is defined as the length (in kms) of the Antarctic coastal perimeter with no adjacent sea ice anywhere offshore (i.e. total exposure of the coast to the open Southern Ocean with no intervening sea ice), but excluding coastal polynyas. By this method, we use the land mask to determine if each coastal grid point has an immediately-adjacent ocean grid point that is ice-free (i.e. has a sea-ice concentration of less than 15%). If this criterion is met, then a nearest (adjoining) neighbour-testing technique is used to determine whether that ocean grid point is exposed in some way to the wider open ocean or is bound by neighbouring sea ice offshore. If any of the neighbouring grid points are classified as “exposed”, or if the total area of neighbouring ice-free grid points exceeds an arbitrary cut-off of 500,000 km2, then that coastal grid point is classified as “exposed”. Otherwise, the grid point and all sea-ice-free neighbouring grid points are deemed to be bounded by sea ice and are classified as a coastal polynya. The length of individual exposed coastal grid points is estimated by taking the square root of the respective pixel area. The length of coastal exposure, either regionally or net circum-Antarctic, is then simply the sum of the length of exposed coastal grid points. The IDL code used for calculating CEL is included here.

完整数据集说明详见文献（https://doi.org/10.21203/rs.3.rs-636839/v1）。

数据
核心的海岸暴露长度（Coastal Exposure Length, CEL）方法与附属的海岸暴露指数（Coastal Exposure Index, CEI，二者详述如下）均基于1979年至2020年的逐日海冰密集度产品。该类产品源自多卫星被动微波亮温时间序列，采用NASA Team算法生成，空间分辨率为25 km×25 km，数据来源于美国国家航空航天局（NASA）国家冰雪数据中心（National Snow and Ice Data Center, NSIDC）分布式主动存档中心（Distributed Active Archive Center, DAAC）。两种算法均设计为可适配不同分辨率的数据集。除1979年至1987年7月期间数据为每两日更新一次外，南极沿岸与海冰区域均可获取逐日完整覆盖数据。该时段内缺失的单日数据通过相邻日期的海冰密集度数值进行插值补全。除非另有说明，所有平均值与气候态均基于1979年至2020年的数据集计算得到。本次研究使用的陆地掩膜（gsfc_25s.msk）同样来源于NSIDC，该掩膜包含冰架（本文将冰架的海侧边缘定义为海岸线）。基于陆地掩膜，海岸线网格点被定义为所有与陆地或冰盖相邻的海洋网格点。

分析方法
本研究开发了两种全新且各具特色的算法，用于量化与监测南极海岸暴露情况：海岸暴露指数（Coastal Exposure Index, CEI）与海岸暴露长度（Coastal Exposure Length, CEL）方法。CEI方法基于冰盖掩膜后，沿每条经线（经度间隔1°）从海岸线向外侧（北向）径向探测海冰的存在与否。CEI的定义极为简便：即大陆北侧海冰密集度低于常规阈值15%的经线数量，其取值范围为0至360。该方法实现逻辑简单，未附带相关代码。

CEL被定义为南极沿岸周边无近海海冰（即海岸完全暴露于南大洋开阔水域且无海冰阻隔）的长度（单位：千米），但不包括沿岸冰间湖（coastal polynya）。基于该方法，我们通过陆地掩膜判断每个沿岸网格点是否存在紧邻的无冰海洋网格点（即海冰密集度低于15%的网格点）。若满足上述判定条件，则采用最近邻（相邻）测试技术判断该海洋网格点是否在一定程度上暴露于开阔大洋，或是被近海相邻海冰所包围。若任意相邻网格点被归类为“暴露”，或相邻无冰网格点的总面积超过任意设定的阈值500,000 km²，则将该沿岸网格点归类为“暴露”。反之，该网格点与所有相邻无冰网格点均被判定为被海冰包围，归类为沿岸冰间湖。单个暴露沿岸网格点的长度通过对应像素面积的平方根进行估算。无论是区域尺度还是全南极环大陆尺度的海岸暴露长度，均为所有暴露沿岸网格点长度的总和。本研究用于计算CEL的交互式数据语言（Interactive Data Language, IDL）代码已随附于本文中。