Constructing a Synoptic Climatology of Greenland Surface Ice-Melt Events using Self-Organizing Maps

Synoptic flow patterns conducive to Greenland surface ice melt on daily to weekly time scales consist of: 1) A strong ridge over Greenland resulting in anomalously warm temperatures and clear skies, and 2) a trough upstream of Greenland resulting in the transport of warm, moist air from middle latitudes to the Arctic. In order to classify synoptic flow patterns that contribute to Greenland surface ice melt, this study utilizes self-organizing maps (SOMs), a type of artificial neural network that clusters data points into various nodes according to the similarity between the synoptic flow patterns. SOMs can be useful for identifying synoptic flow patterns because they allow for objective classification of synoptic flow patterns found in sea level pressure (SLP) and geopotential height fields over a large dataset. The purpose of this study is to use SOMs to identify and analyze the synoptic flow patterns that contribute to anomalous Greenland surface ice melt on daily to weekly time scales.

A synoptic climatology of Greenland surface ice-melt events is constructed using ERA5 reanalysis data at 0.25° resolution for the April–October 1979–2019 time period. The ERA5 reanalysis data are regridded to 1° resolution to render the data compatible with the SOMs algorithm used in this study. Greenland surface ice-melt events are defined as days during the aforementioned time period where the Greenland surface melt extent is at or above the 90^th percentile compared to climatology according to the National Snow and Ice Data Center. 500-hPa geopotential height and SLP data for days where the Greenland surface ice-melt event criterion is met are entered into the SOMs algorithm. The averaged 500-hPa geopotential height and SLP are calculated for each node produced by the SOMs algorithm over a Greenland domain. The SOMs algorithm identifies three main types of synoptic flow patterns during Greenland surface ice-melt events: 1) A strong ridge over Greenland, 2) a positively tilted trough upstream of Greenland, and 3) a strong negatively tilted trough upstream of Greenland. An initial result of this study is that major Greenland summer ice-melt events occur in conjunction with strong ridges over Greenland, while major spring and autumn ice-melt events occur in conjunction with strong negatively tilted troughs upstream of Greenland.