Lower-Latitude Linkages to Two Intense Arctic Cyclones in Early June 2018

Two unusually intense cyclones, AC1 and AC2, occurred over the Arctic Ocean in early June 2018. AC1 achieved a minimum central pressure of ~972 hPa on 4 June, while AC2 reached a minimum central pressure of ~960 hPa on 7 June. Noteworthy aspects of these cyclones included their intensity, linked evolution, and origin. AC1 formed in a frontal trough that extended southwestward from a moderately deep ~988 hPa cyclone located near 63°N and 66°E at 0000 UTC 1 June. By 0000 UTC 2 June, AC1 had intensified to ~994 hPa ahead of a deepening upstream upper-level trough. AC1 continued to deepen and was located near the Kara Sea with an estimated central pressure of ~972 hPa at 0000 UTC 4 June. AC2 formed from the remnants of Tropical Storm (TS) Alberto (2018) near 1200 UTC 31 May ~60 h after TS Alberto made landfall on the Florida panhandle.

A cutoff cyclone (PV anomaly) moving northeastward over the Great Plains on 30 May is instrumental in pulling the TS Alberto remnants northward to a position located just northeast of Lake Superior near 1200 UTC 31 May. Nearly simultaneous downstream ridge building and upper-level jet strengthening to over 80 m s⁻¹ occurs over eastern Canada as the remnants of TS Alberto move northeastward. This ridge building and jet strengthening likely results from a combination of diabatically driven negative PV advection by the irrotational wind from the remnants of TS Alberto and an increase in the potential temperature gradient on the dynamic tropopause as a southeastward-moving tropopause polar vortex (TPV) interacts with the jet. Negative PV advection by the irrotational wind likely supports continued upper-level jet strengthening in the southwesterly flow upstream of an amplifying ridge over eastern Canada.

After AC2 is ingested into the cyclonic circulation of the aforementioned PV anomaly it moves northeastward across Canada and Labrador. AC2, located over the Davis Strait at 0000 UTC 2 June, continues northeastward and subsequently intensifies as a lee cyclone in a poleward jet-exit region on the east coast of central Greenland by 1800 UTC 2 June. AC2 deepens moderately to ~990 hPa as it moves east-southeastward toward northern Finland by 0000 UTC 4 June. AC2 subsequently performs a “Fujiwhara-like” cyclonic interaction with AC1 when this cyclone is located over northern Russia just south of the Kara Sea. AC2 deepens moderately to ~982 hPa as it tracks eastward, then northeastward, and then northward by 1200 UTC 6 June. AC2 deepens further to ~960 hPa in a strong baroclinic zone over the Kara Sea during the next 36 h as it increases in areal extent and absorbs AC1 over the Kara Sea.

The purpose of this presentation is to investigate the concerted role of a series of tropical, midlatitude, and Arctic processes in the culmination of two extreme warm season Arctic cyclones (AC1 and AC2) near the Kara Sea. With regard to Arctic precursors, we investigate how TPVs interacting with the upstream jet over Hudson Bay and Greenland may have contributed to jet intensification and facilitated the downstream development of AC1 and AC2. We then illustrate how diabatic processes associated with the poleward transport of warm, moist air of tropical cyclone origin likely contributed to downstream ridge building, subsequent downstream Rossby wave formation, and deep trough development over northwestern Europe to enable AC1 and AC2 to intensify in a strong baroclinic zone over northern Europe and the Kara Sea. We finally consider the extent to which the continued intensification of AC1 and AC2 over the Kara Sea reflects the presence of frontogenetical forcing along low-level mesoscale baroclinic boundaries between still frozen and ice-free regions over the Arctic Ocean. These continually evolving and changing mesoscale boundaries would likely be very difficult to analyze and initialize in numerical models.