15 NOVEMBER 2020 S T R A S S E T A L . 9863 Multidecadal Warming and Density Loss in the Deep Weddell Sea, Antarctica VOLKER H. STRASS,GERD ROHARDT,TORSTEN KANZOW,MARIO HOPPEMA, AND OLAF BOEBEL Alfred-Wegener-Institut Helmholtz-Zentrum fur€ Polar- und Meeresforschung, Bremerhaven, Germany (Manuscript received 16 April 2020, in final form 10 August 2020) ABSTRACT: The World Ocean is estimated to store more than 90% of the excess energy resulting from man-made greenhouse gas–driven radiative forcing as heat. Uncertainties of this estimate are related to undersampling of the subpolar and polar regions and of the depths below 2000 m. Here we present measurements from the Weddell Sea that cover the whole water column down to the sea floor, taken by the same accurate method at locations revisited every few years since 1989. Our results show widespread warming with similar long-term temperature trends below 700-m depth at all sampling sites. The mean heating rate below 2000 m exceeds that of the global ocean by a factor of about 5. Salinity tends to increase—in contrast to other Southern Ocean regions—at most sites and depths below 700 m, but nowhere strongly enough to fully compensate for the warming effect on seawater density, which hence shows a general decrease. In the top 700 m neither temperature nor salinity shows clear trends. A closer look at the vertical distribution of changes along an ap- proximately zonal and a meridional section across the Weddell Gyre reveals that the strongest vertically coherent warming is observed at the flanks of the gyre over the deep continental slopes and at its northern edge where the gyre connects to the Antarctic Circumpolar Current (ACC). Most likely, the warming of the interior Weddell Sea is driven by changes of the Weddell Gyre strength and its interaction with the ACC. KEYWORDS: Ocean; Southern Ocean; Ocean circulation; Ocean dynamics; In situ oceanic observations 1. Introduction The World Ocean takes a pivotal role for the heat budget of Assessing the heat budget of planet Earth is an essential planet Earth. It features a heat capacity more than 1000 times prerequisite for verifying the concept of anthropogenic global that of the atmosphere (Levitus et al. 2005; Schmitt 2018) and warming and the models describing this (e.g., Hansen et al. the oceans are likely to have taken up more than 90% of the 2011; Palmer 2012; Trenberth and Fasullo 2012; von Schuckmann excess energy from anthropogenic greenhouse gas–driven ra- et al. 2016). Incomplete measurements of the distribution and diative forcing (e.g., Levitus et al. 2012; Balmaseda et al. 2013; temporal changes of heat within the climate system compart- Rhein et al. 2013; von Schuckmann et al. 2016). Robust esti- ments—atmosphere, land, ice, and ocean—bear the risk of mates of the ocean heat budget are thus urgently needed (e.g., leading to incorrect or biased conclusions. When Earth surface Palmer et al. 2011; Gleckler et al. 2016) to better constrain the temperature measurements indicated a stalled rise of global climate model sensitivity to perturbations arising from changes mean temperature, while atmospheric CO2 concentrations in greenhouse gas concentrations (Dessler and Forster 2018; continued to increase during the 15-yr period following the Lewis and Curry 2018) and hence to improve future projections. 1998 El Niño, the paradigm of anthropogenic global warming Existing global estimates of ocean heat content are based on was challenged. Doubt about its existence was cast among the measurements from different instruments such as expendable general public, and used to raise opposition against reduction bathythermographs (XBTs), autonomous profiling floats (Argo), of man-made CO2 emissions (e.g., Lewandowsky et al. 2016; and conductivity–temperature–depth sondes (CTDs) with in- Medhaug et al. 2017). herently different precisions and characteristic profiling depth Since 2012 global mean surface temperatures, however, ranges, namely, 700 m, 2000 m, and full ocean depth, respec- picked up rising and meanwhile have reached new record highs tively (Abraham et al. 2013; Rhein et al. 2013). Data taken (e.g., https://crudata.uea.ac.uk/cru/data/temperature/; GISTEMP by these instruments also cover different periods of time; for Team 2020; Lenssen et al. 2019; NOAA 2020; Zhang et al. instance, XBTs dominated the number of vertical temperature 2019). The 1998–2012 slowdown of surface temperature in- profiles from the 1970s to 1990s until the establishment of crease, often termed the ‘‘global warming hiatus,’’ and its the Argo program, which now delivers more profiles per year ending are now widely explained and mostly attributed to than were available before. However, vertical profiles for the variable distributions of heat between the surface of the ocean deep ocean below the typical 2000-m depth of Argo floats and its deep interior (Kosaka and Xie 2013; Chen and Tung are still sparse, accounting for just a few percent of the 2014; Lee et al. 2015; Fyfe et al. 2016). overall number of temperature profiles (Durack et al. 2018). Undersampled by any kind of temperature records are the Denotes content that is immediately available upon publica- tion as open access. This article is licensed under a Creative Commons Attribution 4.0 license (http://creativecommons.org/ Corresponding author: Volker H. Strass, [email protected] licenses/by/4.0/). DOI: 10.1175/JCLI-D-20-0271.1 Ó 2020 American Meteorological Society Unauthenticated | Downloaded 09/25/21 05:14 PM UTC 9864 JOURNAL OF CLIMATE VOLUME 33 FIG. 1. Study area and location of data records. Map of the Atlantic sector of the Southern Ocean with the generalized circulation of the Weddell Gyre; red curved arrows symbolize the advection and circulation of warm water masses into and within the Weddell Sea; light blue and dark blue curved arrows indicate the formation and outflow of bottom water. Repeated CTD measurements were collected along two transects: the prime meridian (red straight line N: northern Weddell outflow, and blue line E: eastern Weddell inflow) and at the tip of the Antarctic Peninsula across the western outflow (green line W). Further long-term records were obtained from CTD station profiles positioned (location n) on the prime meridian, and (lo- cations e and v) on the Weddell Sea cross section. The area in light gray indicates the deep Weddell Basin analyzed here; it is bounded by the 700-m isobath in the south and west, a straight line running from the South Orkney plateau to 558S, 08 in the north, and the prime meridian section N–E in the east. Also shown are the coastline (thick black) and the shelf ice edge (thin black), and the isobaths of 1000 m (brown), 2500 m (yellow), 3500 m (cyan), 4000 m (light blue), and 5000 m (dark blue). subpolar and polar ocean regions because of the logistic chal- across the equator centered around 1000-m depth (Talley 1996). lenges they bear due to their remoteness and partial ice cover. The denser deep-water fraction is advected southward within Undersampling of the subpolar and polar oceans not only adds the gyres of the Ross Sea and the Weddell Sea (Orsi et al. 1993; uncertainty in global ocean assessments in terms of a possible bias. Fahrbach et al. 1994, 1995). Within the Weddell Gyre the in- Of even more concern is that it is precisely the subpolar/polar flowing heat-advecting Circumpolar Deep Water (CDW) (Reeve regions where most of the exchange of properties between the sea et al. 2016, 2019), locally termed Warm Deep Water (WDW), surface and the deep ocean takes place. A central role in the gains density by heat losses to the atmosphere and surrounding ice meridional overturning circulation (MOC) of the global ocean shelves, and by salinification through brine release during sea ice takes the Southern Ocean (e.g., Marshall and Speer 2012), where formation over the continental shelf (Gill 1973). This density gain water masses derived from the North Atlantic Deep Water results in the formation of Weddell Sea Deep and Bottom Waters (NADW), the Upper and Lower Circumpolar Deep Water (WSDW and WSBW). WSDW and WSBW make a major con- (UCDW, LCDW), are upwelled into the surface layer owing to tribution to the Antarctic Bottom Water (AABW; e.g., Foster and the almost circumpolar, wind-driven Antarctic Divergence. The Carmack 1976; Fahrbach et al. 1995; Orsi et al. 1999, 2002; Jullion lighter fraction of the upwelled deep water returns back north- et al. 2014), which spreads northward to fill most of the deep ward with the upper branch of the MOC, and after modification basins, and thus closes the deepest limb of the MOC. by air–sea interaction subducts at the northern flank of the To contribute to reducing the uncertainties in estimates of Antarctic Circumpolar Current (ACC) to form the Antarctic the global ocean heat budget, our study concentrates on the Intermediate Water (AAIW), which spreads northward and most severely undersampled parts, the subpolar and polar Unauthenticated | Downloaded 09/25/21 05:14 PM UTC 15 NOVEMBER 2020 S T R A S S E T A L . 9865 TABLE 1. Summary of Polarstern cruises during which the database for this study was created. The transect along the prime meridian is marked N and E in Fig. 1, with the distance covered by data calculated from 508S in the southward direction. The distance along the transect across the Weddell Sea from Joinville Island toward Kapp Norvegia (W and dashed green line in Fig. 1) runs from 63.0588S, 55.2668W to 71.2318S, 10.9108W with a break point at 67.7438S, 20.9808W. The transect off the Antarctic Peninsula (W in Fig. 1) coincides partly with the full Weddell Sea cross section but extends in observation time up to year 2019.