Pack Ice Surface Energy Changes at SHEBA Initiated by Late Summer Synoptic Forcing

Data from recent field programs have suggested that transitions in process regimes, such as those characterizing seasonal mean processes, may happen on the shorter synoptic time scale rather than the longer seasonal time scale. During the time period July 26- Aug. 1, 1998, the SHEBA ice camp on the Arctic pack ice in the Chukchi Sea was affected by a major synoptic event. Before this event, a summertime surface energy regime had been in place for which all surface energy budget terms produced melting of the surface ice. Warm air only a few hundred meters above the surface was even able to produce a positive net longwave radiative surface flux. After the event, the net surface energy budget was only one-third of that prior to the event and some terms, such as the turbulent heat fluxes and net longwave radiation, changed signs to now cool the surface. The net solar fluxes continued to be positive, but were reduced in magnitude due to effects from changes in cloud cover and albedo. This synoptic event drastically changed the role of different processes, and led to significant changes in the thermal characteristics of the atmosphere, pack ice and adjacent leads. It was the first, and arguably largest, step in the transition from a summer melt regime to freeze-up which occurred later in August. This paper will utilize the extensive suite of measurements made at SHEBA, including the hourly data from the SHEBA ASFG tower, to characterize the atmospheric structure just prior to, during, and just after this event, and show how the synoptically-produced changes in the atmosphere led to the changes in the various surface energy budget terms.