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Observed Southern Ocean surface cooling and sea-ice expansion over the past several decades are inconsistent with many historical simulations from climate models. Here we show that natural multidecadal variability involving Southern Ocean convection may have contributed strongly to the observed temperature and sea-ice trends. These observed trends are consistent with a particular phase of natural variability of the Southern Ocean as derived from climate model simulations. Ensembles of simulations are conducted starting from differing phases of this variability. The observed spatial pattern of trends is reproduced in simulations that start from an active phase of Southern Ocean convection. Simulations starting from a neutral phase do not reproduce the observed changes, similarly to the multimodel mean results of CMIP5 models. The long timescales associated with this natural variability show potential for skilful decadal prediction. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
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Articles
https://doi.org/10.1038/s41558-018-0350-3
1Atmospheric and Oceanic Science, Princeton University, Princeton, NJ, USA. 2NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA.
3University Corporation for Atmospheric Research, Boulder, CO, USA. *e-mail: Liping.Zhang@noaa.gov
While Arctic sea ice is rapidly decreasing in association
with increasing surface air temperature1, observations
clearly show an expansion of Southern Ocean (SO) sea-
ice extent (SIE)2 during the satellite era (1979–2012) (Fig. 1a). This
modest increase is consistent with the observed SO cooling trend
(Fig. 1a). The sea surface temperature (SST) and sea-ice concentra-
tion (SIC) trends are not homogeneous in space3,4, with opposing
signs in the Amundsen-Bellingshausen seas versus the Ross and
Weddell seas (Fig. 1b,c). Several mechanisms have been proposed
to explain these trends316. A leading idea involves surface wind
changes314 driven by a number of factors, including a positive trend
of the Southern Annular Mode in response to stratospheric ozone
depletion, or a deepened Amundsen Sea Low driven by remote
tropical Pacific or North Atlantic SST anomalies. In these studies,
the wind-driven surface heat flux, upper ocean dynamics and sea-
ice drift are key drivers for the observed sea-ice and SST dipoles.
Another explanation involves surface freshening15,16 caused by
anthropogenic warming, possibly via global water cycle amplifica-
tion and/or the melting of Antarctic glaciers and ice sheets. The sur-
face freshening enhances stratification and suppresses convective
mixing with the warmer water at depth, producing cold SST anom-
alies and thus inhibiting the melting of Antarctic sea ice. However,
other studies argue that wind anomalies, induced by ozone deple-
tion, favour an overall decrease rather than increase of Antarctic
sea ice17, and that the surface freshening caused by anthropogenic
warming is not large enough to trigger sea-ice expansion18,19.
In this study we examine the possibility that internal variabil-
ity involving deep ocean convection in the SO could be a major
contributor to the observed trends, probably in concert with other
previously identified factors. To explore this, we use simulations
with a newly developed coupled ocean–atmosphere sea-ice model
(SPEAR_AM2—see Methods for model details). When this model
is driven with estimates of changes in past radiative forcing, the
model simulation does not reproduce the observed SST and SIC
trends around the Antarctic. Instead, the model simulates a steady
warming and Antarctic sea-ice loss (Fig. 1d,e), as is commonly seen
in multimodel mean results of Coupled Model Intercomparison
Project Phase 5 (CMIP5) models20. One possible explanation for
the discrepancy between observations and model projections is that
natural variability may play a large role in the observed trends21,22.
Indeed, the observed sea-ice expansion is within the range of natu-
ral variability in the control run2,21,22.
We provide additional evidence supporting a strong role for
natural variability in the observed trends, probably involving
multidecadal modulation of SO convection and deep-water for-
mation. As shown below, various aspects of the observed changes
are consistent with a multidecadal weakening of Antarctic bottom
water (AABW) formation. In the 1970s, the open ocean Weddell
Polynya23,24 (Supplementary Fig. 1) was first seen by satellite. After
1976, no similar Weddell Polynya was observed until 2016. Ship-
based hydrographic observations show that the AABW has warmed
globally between the 1980s and 2000s25,26. Objectively analysed
ocean data also exhibit a warming temperature trend in the SO
subsurface and a cooling trend in the surface (Supplementary Fig. 2).
Both warming trends are consistent with weakened convection.
These observational results suggest a global-scale slowdown of the
bottom, southern limb of the meridional overturning circulation
(MOC) during 1979–201225. Meanwhile, multidecadal variabil-
ity of SO SST27,28 shown in reanalysis (Fig. 1a) and palaeoclimate
records29,30 highlights the low-frequency character of SO climate
and places the recent Antarctic sea-ice trends into a broader context.
SO internal variability in coupled model
In order to compare the observed changes with an estimate of
natural variability, we first examine SO natural climate variability
from an extended simulation of the SPEAR_AM2 model. We see
that the time series of the AABW cell, related to ocean convection
(Methods and Supplementary Figs. 3–5), has pronounced multi-
decadal to centennial-scale fluctuations that begin after an initial
1,000 year spin-up (Fig. 2a). When convection is strong, the mixed
layer depth (MLD) is largest in the open Weddell Sea, near the
Maud Rise (65° S, 0°) (Fig. 2b). This convection location resembles
the observed 1974–1976 Weddell Polynya (Supplementary Fig. 1).
Some MLD changes are also seen over the Antarctic continental
shelves, such as in the west Ross, Weddell seas and East Antarctic,
but the magnitude is much smaller than that in the open ocean
Natural variability of Southern Ocean convection
as a driver of observed climate trends
LipingZhang 1,2*, ThomasL.Delworth1,2, WilliamCooke2,3 and XiaosongYang2,3
Observed Southern Ocean surface cooling and sea-ice expansion over the past several decades are inconsistent with many his-
torical simulations from climate models. Here we show that natural multidecadal variability involving Southern Ocean convec-
tion may have contributed strongly to the observed temperature and sea-ice trends. These observed trends are consistent with
a particular phase of natural variability of the Southern Ocean as derived from climate model simulations. Ensembles of simula-
tions are conducted starting from differing phases of this variability. The observed spatial pattern of trends is reproduced in
simulations that start from an active phase of Southern Ocean convection. Simulations starting from a neutral phase do not
reproduce the observed changes, similarly to the multimodel mean results of CMIP5 models. The long timescales associated
with this natural variability show potential for skilful decadal prediction.
NATURE CLIMATE CHANGE | VOL 9 | JANUARY 2019 | 59–65 | www.nature.com/natureclimatechange 59
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... resulted from anomalous multi-decadal variability or from aspects of the forced climate response not captured by models. Some studies suggest that these difference in Pacific and Southern Ocean trends could have resulted from internal atmosphere-ocean variability (Chung et al., 2019(Chung et al., , 2022Olonscheck et al., 2020;Watanabe et al., 2021;L. Zhang et al., 2019;Zhao & Allen, 2019), while others suggest they result in part from missing forcings or model biases in the pattern of response to forcing (Bintanja et al., 2013;Kohyama et al., 2017;Kostov et al., 2018;Schneider & Deser, 2018;Seager et al., 2019Seager et al., , 2022Suarez-Gutierrez et al., 2021;Wills et al., 2020). It is critical to dist ...
... It has also been suggested that observed trends in the Southern Ocean could result from an anomalous phase of Southern Ocean multi-decadal variability (e.g., L. Zhang et al. (2019)). This remains plausible, though its relevance for lower latitude SST trends depends on an active body of work to quantify the teleconnections from Southern Ocean SST changes (Dong et al., 2022;Hwang et al., 2017;Kang et al., 2019;Kim et al., 2022;X. ...
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... The decades-long overall increasing trend in the Antarctic sea ice has been attributed to internal variability of the climate system. In particularly, the trends have been linked to changes in surface winds (Holland and Kwok, 2012;Holland et al., 2017a) to various ocean processes including Southern Ocean convection (Zhang et al., 2019), ice shelf melt (Bintanja et al., 2013;2015), and ice-ocean feedbacks (Zhang, 2007;Goosse and Zunz, 2014), and to well-known modes of climate system variability (Raphael and Hobbs, 2014) related to ocean-atmosphere interactions including the El Niño-Southern Oscillation (ENSO) (Stammerjohn et al., 2008), the Southern Annular Mode (SAM) (Holland et al., 2017b), the Amundsen Sea Low (ASL) (Fogt et al., 2012;Turner et al., 2015;Holland et al., 2018), the Zonal Wave Three (ZW3) (Raphael, 2007), the Interdecadal Pacific Oscillation (IPO) (Lopez et al., 2016;Meehl et al., 2016), the Atlantic Multidecadal Oscillation (AMO) (Yu et al., 2017), and the South Pacific Oscillation (Yu et al., 2021a). External factors, such as increases in the greenhouse gas emissions and ozone depletion, have also been found to influence Antarctic sea ice trends by modifying high-latitude climate variability modes, particularly the SAM (Thompson and Solomon, 2002;Gillett et al., 2013;Fogt and Zbacnik, 2014;Christidis and Stott, 2015). ...
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In contrast to the declining sea ice extent across most of the Arctic in the past several decades, sea ice extent in the Antarctic has experienced opposite regional trends with regions of sea ice expansion exceeding those of contraction. Various mechanisms have been put forward to explain the Antarctic sea ice trends, but few have been successful at explaining a large portion of the observed trends. Dividing the Southern Ocean into the Pacific, Atlantic, and Indian Ocean sectors and examining the trends over 1979–2018 separately for each sector and for each season of the year, we are able to identify modes of variability that statistically explain more than 42% of the trends in all three sectors and all four seasons. In certain sector and season, up to 94% of the trends can be explained. The leading modes of variability that explain a substantial portion of the trends appear to be related to several known climate variability modes including the Southern Annular Mode (SAM), the Pacific South American (PSA) 1 and 2 and the Zonal Wavenumber 2 to 6 patterns. In the Atlantic and Indian Ocean sectors, the changes in the occurrences of the main contributing modes to sea ice trends are dominated by local sea‐surface temperature (SST) variations, while in the Pacific sector, they are related to changes in global SST. This article is protected by copyright. All rights reserved.
... Some studies argued that the representation of SO convection has far-reaching impacts on the regional climate, including the LLC coverage (Gjermundsen et al., 2021;Winton et al., 2010;Zhang et al., 2019). We speculate that the time scales of cloud feedback acting on GMST increase are different from that of deep ocean circulation in climate model simulations under gradually changing forcing. ...
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Plain Language Summary In December 2015 at Paris, United Nations agreed to hold the increase in the global average temperature to “well below” 2°C above pre‐industrial levels and pursuing efforts to limit 1.5°C above pre‐industrial levels. It naturally leads to a question as to when these targets will reach. However, under a business‐as‐usual scenario, the time to reach these targets varies widely among climate models. Using Coupled Model Intercomparison Project Phase 5 and 6, we show that a 2°C global warming is closely related to the late 19th century condition of Southern Ocean (SO) state such that the initially cold SO climate models actually produced fast warming rate and vice versa. This is because these initially cold SO climate models that mostly accompany more low‐level cloudiness and Antarctic sea ice concentration, could actually absorb more downward shortwave radiation by reducing cloudiness and sea‐ice during a warming progress compared to initially warm SO models. Finally, our results demonstrate that climate models that correctly simulate initial SO state can improve 2°C warming projections with reduced uncertainties.
... This leads to dramatic seasonal change of sea-ice extent from a minimum of nearly 3.0 × 10 6 km 2 in late summer to a maximum of nearly 20.0 × 10 6 km 2 (Fig. 1) in early spring (Gloersen et al. 1992). In addition to the seasonal change, sea-ice area in the Antarctic also changes over a range of timescales, from daily (e.g., Kimura and Wakatsuchi 2011), to intraseasonal (e.g., Baba et al. 2006), interannual (e.g., Comiso et al. 2017), and multi-decadal timescales (e.g., Turner et al. 2016;Goosse and Zunz 2014;Zhang et al. 2019). ...
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