Atmospheric circulation and ice volume changes for the small and medium glaciers of New Zealand's Southern Alps mountain range 1977–2018

Abstract

New Zealand has a long, continuous record of annual measurements of the altitude of end‐of‐summer‐snowline for a set of 50 Southern Alps “index glaciers.” The record begins for the 1976–1977 glacier year and continues to the present. These are equivalent to equilibrium line altitude (ELA) data. An earlier paper used these to estimate annual mass balance and ice volume for all glaciers up to 2008 and showed a steady decline in ice volume, but with considerable inter‐annual fluctuations. These results are updated to 2018 for the more than 3,000 small and medium‐size glaciers of the Southern Alps. The inclusion of the latter year enabled assessment of the impacts on ice volume of these glaciers from an unprecedented coupled ocean‐atmosphere heatwave in the New Zealand region during the austral summer (DJF) 2017/18. The main purpose of this paper is to examine how these changes in ice volume relate to variability and trends in atmospheric circulation and weather types over four decades. Years with ice volume gains are characterized by below average temperature anomalies, enhanced southwest flow and a tendency for trough weather types over New Zealand. Years with ice volume losses have above average temperature, reduced southwest flow, and increased blocking weather types, with more anticyclones east of New Zealand. Largest annual ice volume loss of 3.6 ± 0.6 km3 (−13.5%) occurred during the 2017/18 summer heatwave for which the main atmospheric anomalies are discussed. Total ice volume of the Southern Alps for the small and medium glaciers has decreased from 26.6 km3 in 1977 to 17.9 km3 in 2018 (a loss of 8.6 km3 or 33%) at a rate of 0.21 km3 a−1. From 1977 to 1997 there was an annual ice gain of +0.30 km3 a−1 but was followed by an accelerating ice loss of −0.67 km3 a−1 for the period 1998–2018. There are significant correlations between ice volume changes and the Trenberth regional circulation indices Z1, Z3 and MZ3. Strongest relationships are with the Southern Annular Mode: a trend towards more positive values coincides with a general loss of ice volume. The Inter‐decadal Pacific Oscillation and El Niño/Southern Oscillation relate to ice volume changes on decadal to inter‐annual time scales. Positive phases of the Inter‐decadal Pacific Oscillation and El Niño years favour ice volume growth as they are cooler, with more precipitation, troughs and southwest flow over New Zealand. Negative phases of the Inter‐decadal Pacific Oscillation and La Niña years favour ice volume loss as they are warmer with less precipitation, more anticyclones and east or northeast flow. The dominant driver for 2017/18 was the positive Southern Annular Mode, and a smaller contribution from La Niña on a background of regional warming. Climate scenarios suggest that such conditions will be rather frequent by the 2,080–2,100 decades, with consequent large reductions in ice volume.