Diagnosing High Sinuosity Regimes Associated with Greenland Ice-Melt Events

Sinuosity, a metric that describes the waviness of the circumpolar flow, is adapted to quantify tropospheric polar vortex modification within longitudinal sectors by localized incursions of warm, moist air from middle latitudes associated with Arctic cyclones (ACs). Sectorial sinuosity is calculated for 90° longitudinal sectors centered on locations of high AC track frequency and is used to quantify flow amplification associated with anomalous Greenland ice-melt events. Characteristic synoptic patterns conducive to these events are diagnosed by computing the forcing terms in the quasi-geostrophic (QG) geopotential height tendency equation.

A sectorial sinuosity climatology is constructed for 1979–2018 using ERA-Interim reanalyses, while AC dates and tracks are extracted from the cyclone climatologies constructed by Sprenger et al. (2017). The sectorial sinuosity climatology is constructed for the equatorward boundary of the tropospheric polar vortex defined by a threshold geopotential height contour on the 300-hPa surface. A climatology of 129 anomalous Greenland ice-melt events is constructed of +1 sigma events that last 3 or more days during April–October 1979–2018 from data taken from the National Snow and Ice Data Center. Characteristic 300-hPa geopotential height patterns associated with anomalous Greenland ice-melt events are identified using self-organizing maps (SOMs) constructed from ERA-Interim reanalysis data. A thermal metric is used to link upper-level tropospheric geopotential height patterns to lower-level temperature patterns. The metric is calculated by taking the area-averaged value of the 700-hPa standardized temperature anomaly for a domain over Greenland for every anomalous Greenland ice-melt day. Events at the 10th percentile and 90th percentile of the standardized anomaly of ice melt are identified. Two events from the lower and upper 10th percentile are selected: the 5–7 June 2009 event and the 9–15 April 2016 event, respectively. Both events are analyzed and interpreted through diagnosis of the 300-hPa QG geopotential height tendency forcing.

Characteristic synoptic patterns identified using SOMs show that flow regimes conducive to anomalous Greenland ice-melt events vary from a strong trough upstream of Greenland to a strong ridge over Greenland. Events at the 10th percentile of the standardized anomaly of ice melt tend to occur in the warmer months (June, July, and August) of the ice-melt season and are associated with low positive values of 700-hPa area-averaged standardized temperature anomaly. Events at the 90th percentile of the standardized anomaly of ice melt tend to occur in the colder months (April, May, September, and October) of the ice-melt season and are associated with high positive values of 700-hPa area-averaged standardized temperature anomaly. A diagnosis of the 300-hPa QG geopotential height tendency forcing for the two selected events show that ridge amplification over Greenland is dominated by geopotential height tendencies forced by the differential advection of temperature by the geostrophic wind. Geopotential height tendencies forced by the advection of geostrophic absolute vorticity by the geostrophic wind play a subordinate role.