Sixty Years of Widespread Warming in the Southern Middle and High Latitudes (1957–2016)

Abstract

Multiple studies in recent years have documented cooling on the Antarctic Peninsula since the late 1990s, opposing the general longer-term warming trend previously reported for this region. Multi-decadal temperature trends in West Antarctica, in comparison, are more difficult to evaluate, as Byrd is the only long-term station in the area, compared to many long-term staffed stations on the Peninsula. The study presented here aims to puts temperature changes in the Antarctic Peninsula and West Antarctica into a larger spatial and temporal perspective, predominantly focusing on observational data due to the unreliability of reanalyses in the southern high-latitudes prior to 1979. The primary data source comes from monthly stationbased surface temperature observations across the extratropical Southern Hemisphere. Temperature trends are evaluated over various periods, with the longest period from 1957-2016 (1957 is the International Geophysical Year), and then a more recent period of 1979-2016 that is split into 1979-1997 and 1999-2016, to examine the Peninsula cooling. The spatial and temporal aspects of this study, with a maximum duration of 60 years and 59 stations included, and its focus on observational data are what makes it unique. HadISST1 is utilized for sea surface temperatures, and the ERA-Interim reanalysis provides MSLP data for circulation pattern analysis; both of these are used to identify large-scale patterns that correspond with the observational temperatures. Our results confirm the statistically significant cooling in both station observations and sea surface temperature trends throughout the entire Antarctic Peninsula region in the past 17 years (1999-2016), caused by an abnormal pressure pattern driving southerly winds across this area. However, the full 60-year period shows statistically significant, widespread warming across the Southern Hemisphere mid-and high-latitudes, including the iv Antarctic Peninsula and West Antarctica. Positive SST trends broadly reflect these warming trends, especially in the mid-latitudes. Furthermore, we confirm the known prominent influence of the Southern Annular Mode (SAM) on southern mid-and high-latitude climate variability, the positive SAM trend in recent decades, and the related cooling over East Antarctica. Expanding on these results, when we remove the influence of the SAM from the station temperature records, we find statistically significant warming across all of the extratropical Southern Hemisphere, including East Antarctica, revealing strong background warming. If the SAM begins trending less positive in the future (as a result of Antarctic stratospheric ozone recovery, for example), this background warming could become more evident, and could have much greater implications for global climate. v Dedication To my parents: for all your love and support over the years vi Acknowledgements

Recent Near-surface Temperature Trends in the Antarctic Peninsula from Observed, Reanalysis and Regional Climate Model Data

“…This warming and concurrent ice-sheet surface changes observed on the windward coasts are likely to be associated with the recent deepening of the ASL and anomalous northerly warm advection towards the West Antarctic sector and Antarctic Peninsula depicted by the reanalyses. This large-scale forcing mechanism has also been reported in previous studies (e.g., Bromwich et al, 2013a;Raphael et al, 2016;Jones et al, 2019). Jones et al (2019) further highlighted the linkage between the recent ASL deepening and a positive Southern Annular Mode trend.…”

“…This large-scale forcing mechanism has also been reported in previous studies (e.g., Bromwich et al, 2013a;Raphael et al, 2016;Jones et al, 2019). Jones et al (2019) further highlighted the linkage between the recent ASL deepening and a positive Southern Annular Mode trend. On the other hand, the reanalyses show that the summer season is characterized by strengthening of the Weddell Sea low as well as an anticyclonic trend over the Amundsen Sea.…”

“…4a and b). Although Marambio station shows a non-significant annual trend of −0.08°C (10 yr) −1 , a clearer cooling trend exists and autumn warming is replaced by cooling at Marambio station (not shown) when using the same time period for both Larsen Ice Shelf and Marambio on the leeward side (i.e., 1996−2015), which is consistent with the findings of Jones et al (2019). On the windward side, located at a latitude close to the Larsen Ice Shelf station, the San Martin station shows a statistically significant warming trend [+0.52°C (10 yr) −1 ].…”

“…Despite the long-term warming trend, recent studies also show the existence of a regional cooling trend in some parts of Antarctica, including the Antarctic Peninsula, since the late 1990s (Carrasco, 2013;Turner et al, 2016;Oliva et al, 2017). However, this recent cooling period on the peninsula does not yet represent a shift in the overall robust warming trend, and is instead attributed to internal climate variability Gonzalez and Fortuny, 2018;Clem et al, 2019;Jones et al, 2019). The persistent windward warming trend in the autumn season as well as at the annual time scale during the recent cooling period could lead to important implications for the fate of ice sheet surfaces on the windward coasts.…”

“…The steep terrain of the Antarctic Peninsula, therefore, results in harsh conditions that further prevent the deployment of extensive field observations to obtain long-term records. On the one hand, with the aid of few available long-term surface meteorological records, the Antarctic Peninsula is considered to be one of the most vulnerable climate change "hotspots" with a long-term warming trend in near-surface temperature observed over the last 50 years [e.g., Faraday/Vernadsky station, 2.94°C (50 yr) −1 ] (Marshall et al, 2006;Turner et al, 2016, Jones et al, 2019. However, since the late 1990s, a cooling period has been observed in the Antarctic Peninsula that is attributed to atmospheric teleconnections (Car-rasco, 2013;Turner et al, 2016;Oliva et al, 2017).…”