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Abstract. With the onset of anthropogenic climate change, it is vital that we understand climate sensitivity and rates of change during periods of warming in the Earth's past to properly inform climate forecasts. To best inform modeling of ongoing and future changes, environmental conditions during past periods of extreme warmth are ideally developed from multiproxy approaches, including the development of novel proxies where traditional approaches fail. This study builds on a proposed sea surface temperature (SST) proxy for the high-latitude Southern Ocean, based on the morphometrics of the ubiquitous Antarctic diatom Fragilariopsis kerguelensis. This species has been shown to display two distinct morphotypes; a low-rectangularity morphotype is interpreted to be more common in warmer waters while a high-rectangularity morphotype is more common in cooler waters. The proportion of the low-rectangularity morphotype (pLR) has been correlated to SST and summer SST (SSST). Here, we examine this proxy by reconstructing SST using sediment samples from the modern seafloor surface in the Amundsen Sea and the Sabrina Coast to test how well two published calibrations of this relationship (Kloster et al., 2018; Glemser et al., 2019) reconstruct SST and SSST in the modern ocean. In the Amundsen Sea surface sediments, we calculate derived SST −1.6 to −1.2 °C and derived SSST 0.6 to 0.7 °C. In the Sabrina Coast surface sediments, we calculate derived SST −0.3 to 0.5 °C and derived SSST 1.4 to 2.5 °C. We discuss the differing population dynamics of F. kerguelensis in our surface samples between the Amundsen Sea and Sabrina Coast because the Amundsen Sea specimens display a lower pLR than Sabrina Coast specimens, although they exist in warmer waters and should display a higher pLR. We also use the two published calibrations to preliminarily reconstruct SST and SSST in the Amundsen Sea over the last interglacial, Marine Isotope Stage 5 (MIS-5). We calculate SSTs that are slightly cooler or within the range of the modern Amundsen Sea for the duration of the last interglacial; we calculate summer SSTs ∼ 1 °C warmer than the modern Amundsen Sea. This suggests MIS-5 SSTs were at most marginally warmer than the modern Amundsen Sea.