Historical Trends Of Seasonal Greenland Blocking Under Different Blocking Metrics

As the Arctic continues to warm, a weakening of upper-tropospheric westerly winds is hypothesized to induce a meandering jet stream and slower propagation of Rossby waves. As such, current hypotheses suggest an increase in Greenland blocking due to increased stationarity of the high amplitude waves. These hypotheses have been supported observationally with the Greenland blocking index (GBI). However, given an expected increase in overall geopotential heights corresponding to increased temperatures in the region, we assess the robustness of trends in Greenland blocking using additional blocking metrics in addition to the GBI, which has largely been the focused blocking metric for this region to date. Our results show sensitivity of the GBI-based increases in blocking to global and zonally averaged 500-hPa geopotential heights, which results in inconsistent increasing trends over the 1979-2018 period when compared with other blocking metrics. Seasonal blocking frequencies of the GBI show a significant increase in blocking for JJA, though no significant trend in JJA blocking occurs for most metrics. Other indices suggest a decrease in blocking frequency in September-November (SON) and December-February (DJF), though these trends are not statistically significant. Yet, when smoothed using a 5-year running mean, these other metrics suggest an increase in both DJF and JJA blocking with a decrease only in SON blocking, which are consistent with findings of significant changes in GBI. We report no best metric for identifying Greenland blocking. Instead, we present some shortcomings of the different metrics used in this study. These results provide insight into selection of Greenland blocking events for future research, as over- or under-estimation of blocking activity can impact estimates of surface mass balance of the ice sheet.