Using the self-organizing map (SOM) algorithm, a synoptic climatology was created using daily sea-level pressure anomaly data from the ERA-40 reanalysis for 1961 to 1999 over a North Atlantic domain. Figure 1 shows the SOM with the 35 SLP anomaly patterns, or nodes, identified using the SOM technique. The 35 patterns show different positions and intensities of high (red) and low (blue) pressure systems in the domain. The frequency at which each pattern occurs throughout the time period along with the seasonal frequencies of the patterns, offer information on the climatology of the region. Patterns with cyclones west of Greenland in Baffin Bay were most common in summer, while patterns with intense cyclones south and east of Greenland were most common in winter. The daily evolution of the weather over the North Atlantic corresponds to a shifting to different patterns on the SOM. Three common transitions through the SOM verify three common storm tracks in the North Atlantic region mentioned in previous literature. One transition indicates a blocking of cyclones by the high elevation of the Greenland ice sheet, another suggests the splitting of cyclones, and lastly, the intensification of cyclones in the Icelandic Low region. Several features unique to the Greenland region act together to alter passing cyclones.

The distribution and magnitude of precipitation common to each of the weather patterns in the SOM are shown in Figure 2, once again using ERA-40 data from 1961 to 1999. The forcing for precipitation over the domain is explained by the position of cyclones and their interaction with the topography of the ice sheet. Precipitation over Greenland is isolated in order to further understand the atmospheric input to the mass balance of the Greenland ice sheet. Ideal onshore flow over the southern region produces the highest magnitudes of precipitation over Greenland, for example, node (2,2) in Figure 2. The relatively moist air from over the open North Atlantic Ocean is carried onshore by cyclonic winds, where it is orographically lifted by the steep margins of the southeast portion of the ice sheet. The ice sheet in the southern region reaches as high as 2800 m and the elevation maximum in central Greenland is 3208 m. This orographic lifting of moist air produces the highest precipitation amounts on the ice sheet. Other regions of Greenland also receive precipitation from onshore flow, but in lesser amounts.

The use of daily data allows for a detailed analysis of the synoptic patterns and storm tracks that bring precipitation to the Greenland ice sheet. Even a slight change in cyclone position or intensity changes the location and magnitude of precipitation over Greenland. Therefore, synoptic representations of the region using monthly means or the North Atlantic Oscillation index fail to reveal the day to day forcing responsible for the precipitation distribution over Greenland. The SOM analysis is ideal for distinguishing between these subtle variations in circulation and therefore precipitation.

Current and future work uses these same tools to analyze precipitation patterns over Greenland predicted by IPCC 4th assessment report global climate models. A new SOM will be created to represent the SLP patterns in 15 climate model predictions for the reanalysis period and years 2046 – 2065 and 2081 – 2100. This SOM will be used to evaluate the ability of the various climate models to reproduce the ERA-40 synoptic climatology of the North Atlantic region. The models that best reproduce the reanalysis synoptic climatology over the reanalysis period will be used to analyze predicted future trends in synoptic patterns and precipitation over Greenland between the reanalysis period and 2046 – 2065, then 2081 – 2100. Will certain weather patterns become more common in a warmer atmosphere? Will some weather patterns produce more precipitation over Greenland in the future, making up for mass loss from melt and glacier dynamics? Will the locations of heavy precipitation shift to different Greenland regions in the future? Answers to these questions would provide valuable information on the future state of the mass balance of the Greenland ice sheet and, therefore, our predictions of future sea-level rise and climate stability due to changes in the ice sheet.