We analyzed zonal mean boreal summer (JJA) average temperatures (surface air temperature over land and sea surface temperature over oceans) using each of three alternative datasets: GISTEMP (35) (1880–2015), HadCRUT4 (36) (1894–2015), and Cowtan and Way (37) (1894–2015). These three different datasets make alternative assumptions about how to account for historical data gaps, which is particularly important in the Arctic region that is key to this study, due to the large spatial gaps in this region coupled with the amplified high-latitude warming in the Northern Hemisphere in recent decades. The time period of analysis was constrained by continuous data availability for each latitude band. For example, the HadCRUT4 and Cowtan and Way datasets contain at least one grid cell temperature value in each latitude band across the range of interest (25°N to 75°N) from 1894 to the present. Accordingly, the grid cell coverage is relatively sparse in the early segments of the records and increases toward the present day. As with the model output, we used a simple area-weighted average to create zonal means at a 5° interval (27).

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