Revealing the footprint of ENSO in the mass balance of mountain glaciers
Final Report Abstract
Accurate knowledge of the impact of internal atmospheric variability is required for the detection and attribution of climate change and for interpreting glacier records. The project aimed to investigate the impact of the El Niño Southern Oscillation (ENSO) on glacier mass balance at a number of high-mountain sites using a combination of high-elevation observations; reanalysis data; and high-resolution physically based modelling, both of the atmosphere, explicitly resolving the mesoscale processes of air flow modifications by mountains, and of glacier mass balance. The scope of the study was cut to include only one study site, Kilimanjaro, after the proposal funding was reduced and consistent with the suggestion of one reviewer. Our analysis provided two key results. First, partial correlation analysis revealed strong positive associations with both ENSO and the Indian Ocean Zonal Mode (IOZM), but that the correlation was strongest with the latter mode in the air layers near and above the glaciers, due to changes in zonal circulation and moisture transport, emphasizing the importance of the moisture signal from this basin. Second, we found that the most anomalous high-elevation conditions between October and January were simulated during co-occurring positive events. This study provided the first decadal assessment of drivers of interannual atmospheric variability at the glacierized altitudes and laid the foundation for unravelling the contribution of climate modes to observed changes in Kilimanjaro’s glaciers over the last century. However, during this investigation, we noticed that the most positive ENSO event on record was strongly influenced by synoptic rather than climatic atmospheric variability. Specifically, the strong humidity and snowfall anomalies were concentrated during the last 12 days of December 2006, during which time up to four snowfall events whose magnitude exceeded 5% of the average annual total snowfall were measured at the stations. The congruent timing of this strongly positive accumulation with the presence of two consecutive intense tropical cyclones (TCs) passing near the East African coastline motivated us to change the course of the project and pursue a new research direction, investigating the impacts of TCs on Kilimanjaro and the surrounding lowlands. Previous research on TCs in the southwestern Indian Ocean (SWIO) has focused on factors affecting frequency, intensity or trajectory, in particular those impacting landfalls on Madagascar and Mozambique. Our study was the first to assess their influence on atmospheric conditions in the equatorial high mountains. Focusing on the cyclone season from November 2006 to April 2007, we used high-elevation observations; reanalysis data; and highresolution physically based atmospheric modelling to show that TCs can transport moisture to Kilimanjaro, both directly as they approached the East African coastline and indirectly. The indirect moisture transport was produced by a westerly circulation that extended across the African continent past Madagascar and from Kilimanjaro down to 10 – 20°S as lower-level inflow when one or more intense TCs was present in the SWIO. Sensitivity studies suppressing TC development showed that the westerly circulation failed to develop or was greatly weakened and reduced in extent in the absence of the storms, and that there was a correspondingly large decrease in lower tropospheric humidity and in precipitation in the region at Kilimanjaro and in the surrounding lowlands. Our study was also the first to propose TCs as triggers of the westerly circulation and moisture flux, and to support this assertion using physically based atmospheric modelling. Our work also showed that these synoptic-scale phenomena have the potential to influence the signal of atmospheric variability recorded by the glaciers in these regions, which is relevant for interpreting the paleoclimate record from Kilimanjaro ice cores. The success of the project lies in the comprehensive, physically-based explanation of ENSO/IOZM effects on glacierized elevations in the tropics at unprecedented detail. It therefore delivered the promised advance in understanding ENSO impacts at high altitude. In addition, the discovery that seasonal atmospheric anomalies in the lower half of the tropical troposphere might be overwhelmingly caused by synoptic phenomena, and tropical cyclones in particular, opened an exciting direction for future research. We also reported about the project at the beginning through our university’s press channel, https://www.nat.fau.de/2015/10/27/wie-der-pazifik-auf-die-gletscher-wirkt/.
Publications
- (2019) Observed and simulated Indian Ocean Dipole activity since the mid‐19th century and its relation to East African short rains. Int J Climatol (International Journal of Climatology) 39 (11) 4467–4478
Thielke, Anja; Mölg, Thomas
(See online at https://doi.org/10.1002/joc.6085) - Prominent Midlatitude Circulation Signature in High Asia's Surface Climate During Monsoon, Journal of Geophysical Research Atmospheres, 122, 3875–3891, 2017
Mölg, T., Maussion, F., Collier E., Chiang, J.C.H., and Scherer, D.
(See online at https://doi.org/10.1002/2017JD027414) - Recent atmospheric variability at Kibo Summit, Kilimanjaro, and its relation to climate mode activity, Journal of Climate, 31, 3875– 3891, 2018
Collier, E., Mölg, T., and Sauter, T.
(See online at https://doi.org/10.1175/JCLI-D-17-0551.1)
