Unparalleled coupled ocean-atmosphere summer heatwaves in the New Zealand region: drivers, mechanisms and impacts
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Unparalleled coupled ocean-atmosphere summer heatwaves in the New Zealand region: drivers, mechanisms and impacts M. James Salinger, et al. [full author details at the end of the article] Received: 27 September 2019 / Accepted: 27 April 2020/ # Springer Nature B.V. 2020
Abstract
During austral summers (DJF) 1934/35, 2017/18 and 2018/19, the New Zealand (NZ) region (approximately 4 million km2) experienced the most intense coupled oceanatmosphere heatwaves on record. Average air temperature anomalies over land were + 1.7 to 2.1 °C while sea surface temperatures (SST) were 1.2 to 1.9 °C above average. All three heatwaves exhibited maximum SST anomalies west of the South Island of NZ. Atmospheric circulation anomalies showed a pattern of blocking centred over the Tasman Sea extending south-east of NZ, accompanied by strongly positive Southern Annular Mode conditions, and reduced trough activity over NZ. Rapid melt of seasonal snow occurred in all three cases. For the two most recent events, combined ice loss in the Southern Alps was estimated at 8.9 km3 (22% of the 2017 volume). Sauvignon blanc and Pinot noir wine grapes had above average berry number and bunch mass in 2018 but were below average in 2019. Summerfruit harvest (cherries and apricots) was 14 and 2 days ahead of normal in 2017/18 and 2018/19 respectively. Spring wheat simulations suggested earlier flowering and lower grain yields compared to average, and below-average yield and tuber quality in potatoes crops occurred. Major species disruption occurred in marine ecosystems. Hindcasts indicate that the heatwaves were either atmospherically driven or arose from combinations of atmospheric surface warming and oceanic heat advection. Keywords Anthropogenic global warming . Marine heatwave . Atmospheric heatwave . Terrestrial ecosystems . Marine ecosystems . Crops
1 Introduction Kidson (1935) described the first documented austral summer (DJF) heatwave covering the New Zealand (NZ) area in 1934/35, with regional temperature anomalies over land averaging + 1.7 °C compared to the 1981–2010 normal. At the time, this event was so unusual, almost 3 °C warmer than other 1930s summers; it was described as ‘remarkably warm’. Salinger et al. (2019a) documented the unprecedented austral summer (DJF) 2017/18 heatwave covering the NZ region. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10584-02002730-5) contains supplementary material, which is available to authorized users.
Climatic Change
Regional average air (over land) and sea surface temperature anomalies were + 2.2 °C and + 1.9 °C, respectively. Numerous terrestrial and marine impacts persisted for the entire austral summer resulting in the (1) largest loss of glacier ice in the Southern Alps since 1962; (2) early Sauvignon blanc wine-grape maturation; and (3) major species disruption in marine ecosystems. Various atmospheric drivers were identified, and the event was associated with very low wind speeds, reduced upper ocean mixing and heat fluxes from the atmo
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