Inhomogeneity of the Surface Air Temperature Record from Halley, Antarctica

Commencing in 1956, observations made at Halley Research Station, Antarctica provide one of the longest continuous series of near-surface temperature observations from the Antarctic continent. Since few other records of comparable length are available, the Halley record has been used extensively in studies of long-term Antarctic climate variability and change and has been considered to be representative of a wide surrounding region. The record does not, however, come from a single location but is a composite of observations from a sequence of seven stations, all situated on the Brunt Ice Shelf, that range from around 10 km to 50 km distance from the coast. Until recently, it has generally been assumed that temperature data from all of these stations could be combined into a single homogeneous composite record without adjustment because the surface of the ice shelf is flat and uniform. However, visual inspection of the composite record suggests that the assumption of homogeneity may not be correct. The composite record exhibits an overall cooling trend of around 0.1 °C/decade in annual mean temperature (not statistically significant) which contrasts with the neutral or weak warming trends found across East Antarctica and the larger warming trends seen in West Antarctica and the Antarctic Peninsula. However, when analysed separately, the records from the individual Halley stations that make up the composite record all exhibit weak warming trends.

We have tested the homogeneity of the composite record using a statistical change point detection algorithm. Application of this technique to the composite record detects a sudden cooling of around 0.7 °C in annual mean temperature associated with the move from Halley IV to Halley V station in 1992. This temperature step is consistent with local temperature gradients measured by a network of automatic weather stations. Simulations with a high-resolution atmospheric model reveal that these temperature gradients are strongest during periods when westerly winds drive onshore advection of maritime air. Air masses originating in this sector are warmed at low levels by upward surface heat fluxes over open ocean, thin sea ice and coastal polynyas. Once over the ice shelf, the direction of the surface heat flux reverses and the air cools with increasing distance inland from the coast, generating a significant horizontal near-surface temperature gradient.

For illustrative purposes, we have examined the effect of removing a -0.7 °C jump from the composite record at the time of the Halley IV – Halley V move. The impact of this adjustment is to change the overall trend in the composite record from a weak cooling to a weak warming. Several gridded reconstructions of Antarctic temperature variability that have made use of the Halley composite record all show a large area of cooling stretching inland from Halley that contrasts with warming trends in neighbouring areas. Our analysis suggests that this feature may be an artefact of the inhomogeneity of the Halley composite record. Any future efforts to produce Antarctic temperature reconstructions will need to account for this inhomogeneity.