Beltrami, H., J. E. Smerdon, G. S. Matharoo and N. Nickerson, 2011: Impact of maximum borehole depths on inverted temperature histories in borehole paleoclimatology. Climate of the Past, 7(3): 745-756, doi:10.5194/cp-7-745-2011.
A quantitative assessment is presented for the impact of the maximum depth of a temperature-depth profile on the estimate of the climatic transient and the resultant ground surface temperature (GST) reconstruction used in borehole paleoclimatology. The depth of the profile is important because the downwelling climatic signal must be separated from the quasi-steady state thermal regime established by the energy in the Earth's interior. This component of the signal is estimated as a linear increase in temperature with depth from the lower section of a borehole temperature profile, which is assumed to be unperturbed by recent changes in climate at the surface. The validity of this assumption is dependent on both the subsurface thermophysical properties and the character of the downwelling climatic signal. Such uncertainties can significantly impact the determination of the quasi-steady state thermal regime, and consequently the magnitude of the temperature anomaly interpreted as a climatic signal. The quantitative effects and uncertainties that arise from the analysis of temperature-depth profiles of different depths are presented. Results demonstrate that widely different GST histories can be derived from a single temperature profile truncated at different depths. Borehole temperature measurements approaching 500-600 m depths are shown to provide the most robust GST reconstructions spanning 500 to 1000 yr BP. It is further shown that the bias introduced by a temperature profile of depths shallower than 500-600 m remains even if the time span of the reconstruction target is shortened.
Campanelli, A., S. Massolo, F. Grilli, M. Marini, E. Paschini, P. Rivaro, A. Artegiani and S. S. Jacobs, 2011: Variability of nutrient and thermal structure in surface waters between New Zealand and Antarctica, October 2004-January 2005. Polar Research, 30.
We describe the upper ocean thermal structure and surface nutrient concentrations between New Zealand and Antarctica along five transects that cross the Subantarctic Front, the Polar Front (PF) and the southern Antarctic Circumpolar Current (ACC) front. The surface water thermal structure is coupled with variations in surface nutrient concentrations, making water masses identifiable by both temperature and nutrient ranges. In particular, a strong latitudinal gradient in orthosilicate concentration is centred at the PF. On the earlier sections that extend south-west from the Campbell Plateau, orthosilicate increases sharply southward from 10-15 to 50-55 mu mol l(-1) between 58 degrees S and 60 degrees S, while surface temperature drops from 7 degrees C to 2 degrees C. Nitrate increases more regularly toward the south, with concentrations ranging from 10-12 mu mol l(-1) at 54 degrees S to 25-30 mu mol l(-1) at 66 degrees S. The same features are observed during the later transects between New Zealand and the Ross Sea, but the sharp silica and surface temperature gradients are shifted between 60 degrees S and 64 degrees S. Both temporal and spatial factors may influence the observed variability. The January transect suggests an uptake of silica, orthophosphate and nitrate between 63 degrees S and 70 degrees S over the intervening month, with an average depletion near 37%, 44% and 29%, respectively. An N/P (nitrite+nitrate/orthophosphate) apparent drawdown ratio of 8.8 +/- 4.1 and an Si/N (silicic acid/nitrite+nitrate) apparent drawdown ratio > 1 suggest this depletion results from a seasonal diatom bloom. A southward movement of the oceanic fronts between New Zealand and the Ross Sea relative to prior measurements is consistent with reports of recent warming and changes in the ACC.
Cook, B.I., R. Seager and R.L. Miller, 2011: On the Causes and Dynamics of the Early Twentieth-Century North American Pluvial. J. Climate, 24: 5043, DOI: 10.1175/2011JCLI4201.1.
The early twentieth century North American pluvial (1905-1917) was one of the most ex- treme wet periods of the last five hundred years and directly led to overly generous water allotments in the water-limited American West. Here we examine the causes and dynamics of the pluvial event using a combination of observation-based data sets and general circula- tion model (GCM) experiments. The character of the moisture surpluses during the pluvial differed by region, alternately driven by increased precipitation (the southwest), low evapo- ration from cool temperatures (the central plains), or a combination of the two (the Pacific northwest). Cool temperature anomalies covered much of the west and persisted through most months, part of a globally extensive period of cooler land and sea surface temperatures (SST). Circulation during boreal winter favored increased moisture import and precipita- tion in the southwest, while other regions and seasons were characterized by near normal or reduced precipitation. Anomalies in the mean circulation, precipitation, and SST fields are only partially consistent with the relatively weak El Nin ̃o forcing during the pluvial, suggest- ing a significant role for internal variability or other forcing agents. Differences between the reanalysis dataset, an independent statistical drought model, and GCM simulations high- light some of the remaining uncertainties in understanding the full extent of SST forcing of North American hydroclimatic variability.
Jacobs, S. S., A. Jenkins, C. F. Giulivi and P. Dutrieux, 2011: Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf. Nature Geoscience, 4(8): 519-523.
In 1994, ocean measurements near Antarctica's Pine Island Glacier showed that the ice shelf buttressing the glacier was melting rapidly(1). This melting was attributed to the presence of relatively warm, deep water on the Amundsen Sea continental shelf. Heat, salt and ice budgets along with ocean modelling provided steady-state calving and melting rates(2,3). Subsequent satellite observations and modelling have indicated large system imbalances, including ice-shelf thinning and more intense melting, glacier acceleration and drainage basin drawdown(4-10). Here we combine our earlier data with measurements taken in 2009 to show that the temperature and volume of deep water in Pine Island Bay have increased. Ocean transport and tracer calculations near the ice shelf reveal a rise in meltwater production by about 50% since 1994. The faster melting seems to result mainly from stronger sub-ice-shelf circulation, as thinning ice has increased the gap above an underlying submarine bank on which the glacier was formerly grounded(11). We conclude that the basal melting has exceeded the increase in ice inflow, leading to the formation and enlargement of an inner cavity under the ice shelf within which sea water nearly 4 degrees C above freezing can now more readily access the grounding zone.
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