Southern Ocean Sea Surface Temperature Research Articles

Revisiting the reanalysis-model discrepancy in Southern Hemisphere winter storm track trends

Southern Hemisphere (SH) storminess has increased in the satellite era and recent work suggests comprehensive climate models significantly underestimate the trend. Here, we revisit this reanalysis-model trend discrepancy to better understand the mechanisms underlie it. A comprehensive like-for-like analysis shows reanalysis trends exhibit large uncertainty, and coupled climate model simulations exhibit weaker trends than most but not all reanalyses. However, simulations with prescribed sea surface temperature (SST) exhibit significantly greater storminess trends, particularly in the South Pacific, implying SST trend discrepancies in coupled simulations impact storminess trends. Using pacemaker simulations that correct Southern Ocean and tropical east Pacific SST trend discrepancies, we show that storminess trends in coupled simulations are underestimated because they do not capture the enhanced storminess resulting from Southern Ocean cooling and La-Nina-like teleconnection trends. Our findings emphasize large reanalysis uncertainty in SH circulation trends and the impact of regional SST trend discrepancies on circulation trends.

Assessment of the Southern Ocean Sea Surface Temperature Biases in CMIP5 and CMIP6 Models

Assessment of the Southern Ocean Sea Surface Temperature Biases in CMIP5 and CMIP6 Models

Tropical and Antarctic sea ice impacts of observed Southern Ocean warming and cooling trends since 1949

Southern Ocean (SO) sea surface temperatures (SSTs) warmed from approximately 1949–1978 and cooled slightly from 1979–2013. We compare the remote impacts of these SO trends using historical coupled model experiments in which the model’s SO SST anomalies are nudged to observations. Compared to the control (no nudging) ensemble, the nudged ensemble shows enhanced SST warming in the tropical southeast Pacific and Atlantic, and greater Antarctic sea ice loss, during the SO warming period: analogous to the impacts of SO cooling but of opposite sign. The SO-driven response in the tropical Pacific (Atlantic) is statistically significant when considering the trend difference between the two periods, and accounts for 34% (59%) of the observed non-radiatively forced trend. Surface heat budget analysis indicates wind-evaporation-SST feedback dominates over shortwave cloud feedback in amplifying the SO-driven SST trends in the tropics during the SO warming period, opposite to that for the SO cooling period.

Evaluating the Seasonal Responses of Southern Ocean Sea Surface Temperature to Southern Annular Mode in CMIP6 Models

AbstractUsing observations and CMIP6 historical simulations, the seasonal responses of Southern Ocean (50°S–70°S) sea surface temperature (SST) to Southern Annular Mode (SAM) variations are investigated in this study. The results suggest that the averaged Southern Ocean SST in austral spring and summer show a significant cooling in response to a positive SAM, while the responses in austral autumn and winter are negligible. The cooling effect is resulted from the cold water in higher latitudes and deeper oceans brought by the equatorward Ekman transport and the Ekman pumping associated with the positive SAM. Among CMIP6 models, the magnitude of the simulated cooling response connects to the climatological meridional and vertical ocean temperature gradients, and the magnitude of Ekman motion in response to SAM. In addition, the spring and summer SAM plays a more important role in modulating Southern Ocean SST in autumn and winter than the autumn and winter SAM.

A key hub for climate systems: deciphering from Southern Ocean sea surface temperature variability

A key hub for climate systems: deciphering from Southern Ocean sea surface temperature variability

Competing impacts of tropical Pacific and Atlantic on Southern Ocean inter-decadal variability

The observed Southern Ocean sea surface temperature (SST) has experienced prominent inter-decadal variability nearly in phase with the Inter-decadal Pacific Oscillation (IPO), but less associated with the Atlantic Multidecadal Variability (AMV), challenging the prevailing view of Pacific-Atlantic synergistic effects. Yet, the mechanisms of distinct trans-hemispheric connections to the Southern Ocean remain indecisive. Here, by individually constraining the observed cold-polarity and warm-polarity IPO and AMV SSTs in a climate model, we show that the IPO is influential in initiating a basin-wide Southern Ocean response, with the AMV secondary. A tropical Pacific-wide cooling triggers a basin-scale Southern Ocean cold episode through a strong Rossby wave response to the north-to-south cross-equatorial weakened Hadley circulation. By contrast, due to the competing role of tropical Pacific cooling, an Atlantic warming partly cools the Southern Ocean via a weak Rossby wave response to the south-to-north cross-equatorial enhanced Hadley circulation. Conversely, tropical Pacific warming leads to a warm Southern Ocean episode. Our findings highlight that properly accounting for the tropical Pacific SST variability may provide a potential for skillful prediction of Southern Ocean climate change and more reliable estimates of climate sensitivity, currently overestimated by the misrepresented Southern Ocean warming.

A Southern Ocean Sea Surface Temperature compilation for the Last Glacial Cycle

A Southern Ocean Sea Surface Temperature compilation for the Last Glacial Cycle

Winds and Meltwater Together Lead to Southern Ocean Surface Cooling and Sea Ice Expansion

AbstractSouthern Ocean surface cooling and Antarctic sea ice expansion from 1979 through 2015 have been linked both to changing atmospheric circulation and melting of Antarctica's grounded ice and ice shelves. However, climate models have largely been unable to reproduce this behavior. Here we examine the contribution of observed wind variability and Antarctic meltwater to Southern Ocean sea surface temperature (SST) and Antarctic sea ice. The free‐running, CMIP6‐class GISS‐E2.1‐G climate model can simulate regional cooling and neutral sea ice trends due to internal variability, but they are unlikely. Constraining the model to observed winds and meltwater fluxes from 1990 through 2021 gives SST variability and trends consistent with observations. Meltwater and winds contribute a similar amount to the SST trend, and winds contribute more to the sea ice trend than meltwater. However, while the constrained model captures much of the observed sea ice variability, it only partially captures the post‐2015 sea ice reduction.

Recent Multi‐Decadal Southern Ocean Surface Cooling Unlikely Caused by Southern Annular Mode Trends

AbstractOver recent decades, the Southern Ocean (SO) has experienced multi‐decadal surface cooling despite global warming. Earlier studies have proposed that recent SO cooling has been caused by the strengthening of surface westerlies associated with a positive trend of the Southern Annular Mode (SAM) forced by ozone depletion. Here we revisit this hypothesis by examining the relationships between the SAM, zonal winds and SO sea‐surface temperature (SST). Applying a low‐frequency component analysis to observations, we show that while positive SAM anomalies can induce SST cooling as previously found, this seasonal‐to‐interannual modulation makes only a small contribution to the observed long‐term SO cooling. Global climate models well capture the observed interannual SAM‐SST relationship, and yet generally fail to simulate the observed multi‐decadal SO cooling. The forced SAM trend in recent decades is thus unlikely the main cause of the observed SO cooling, pointing to a limited role of the Antarctic ozone hole.

The Largest Ever Recorded Heatwave—Characteristics and Attribution of the Antarctic Heatwave of March 2022

AbstractAn unprecedented heatwave impacted East Antarctica in March 2022, peaking at 39°C above climatology, the largest temperature anomaly ever recorded globally. We investigate the causes of the heatwave, the impact of climate change, and a climate model's ability in simulating such an event. The heatwave, which was skillfully forecast, resulted from a highly anomalous large‐scale circulation pattern that advected an Australian airmass to East Antarctica in 4 days and produced record atmospheric heat fluxes. Southern Ocean sea surface temperatures anomalies had a minimal impact on the heatwave's amplitude. Simulations from a climate model fail to simulate such a large temperature anomaly mostly due to biases in its large‐scale circulation variability, showcasing a pathway for future model improvement in simulating extreme heatwaves. The heatwave was made 2°C warmer by climate change, and end of 21st century heatwaves may be an additional 5–6°C warmer, raising the prospect of near‐melting temperatures over the interior of East Antarctica.

Recent global climate feedback controlled by Southern Ocean cooling

The magnitude of global warming is controlled by climate feedbacks associated with various aspects of the climate system, such as clouds. The global climate feedback is the net effect of these feedbacks, and its temporal evolution is thought to depend on the tropical Pacific sea surface temperature pattern. However, current coupled climate models fail to simulate the pattern observed in the Pacific between 1979 and 2013 and its associated anomalously negative feedback. Here we demonstrate a mechanism whereby the Southern Ocean controls the global climate feedback. Using climate model experiments in which Southern Ocean sea surface temperatures are restored to observations, we show that accounting for recent Southern Ocean cooling—which is absent in coupled climate models—halves the bias in the global climate feedback by removing the cloud component bias. This global impact is mediated by a teleconnection to the Southeast Pacific, where remote sea surface temperature anomalies cause a strong stratocumulus cloud feedback. We propose that this Southern Ocean-driven pattern effect is underestimated in most climate models, owing to an overly weak stratocumulus cloud feedback. Addressing this bias may shift climate sensitivities to higher values than currently simulated as the Southern Ocean undergoes accelerated warming in future projections.

Origins of Southern Ocean warm sea surface temperature bias in CMIP6 models

The warm sea surface temperature (SST) bias in the Southern Ocean (SO) has persisted in several generations of Coupled Model Inter-comparison Project (CMIP) models, yet the origins of such a bias remain controversial. Using the latest CMIP6 models, here we find that the warm SST bias in the SO features a zonally oriented non-uniform pattern mainly located between the northern and southern fronts of the Antarctic Circumpolar Current. This common bias is not likely to be caused by the biases in the surface heat flux or the strength of the Atlantic meridional overturning circulation (AMOC) — the two previously suggested sources of the SO bias based on CMIP5 models. Instead, it is linked to the robust common warm bias in the Northern Atlantic deep ocean through the AMOC transport as an adiabatic process. Our findings indicate that remote oceanic biases that are dynamically connected to the SO should be taken into account to reduce the SO SST bias in climate models.

Global impacts of recent Southern Ocean cooling

Since the beginning of the satellite era, Southern Ocean sea surface temperatures (SSTs) have cooled, despite global warming. While observed Southern Ocean cooling has previously been reported to have minimal impact on the tropical Pacific, the efficiency of this teleconnection has recently shown to be mediated by subtropical cloud feedbacks that are highly model-dependent. Here, we conduct a coupled model intercomparison of paired ensemble simulations under historical radiative forcing: one with freely evolving SSTs and the other with Southern Ocean SST anomalies constrained to follow observations. We reveal a global impact of observed Southern Ocean cooling in the model with stronger (and more realistic) cloud feedbacks, including Antarctic sea-ice expansion, southeastern tropical Pacific cooling, northward-shifted Hadley circulation, Aleutian low weakening, and North Pacific warming. Our results therefore suggest that observed Southern Ocean SST decrease might have contributed to cooler conditions in the eastern tropical Pacific in recent decades.

Southern Ocean Surface Temperatures and Cloud Biases in Climate Models Connected to the Representation of Glacial Deep Ocean Circulation

Abstract Simulating and reproducing the past Atlantic meridional overturning circulation (AMOC) with comprehensive climate models are essential to understanding past climate changes as well as to testing the ability of the models in simulating different climates. At the Last Glacial Maximum (LGM), reconstructions show a shoaling of the AMOC compared to modern climate. However, almost all state-of-the-art climate models simulate a deeper LGM AMOC. Here, it is shown that this paleodata–model discrepancy is partly related to the climate model biases in modern sea surface temperatures (SST) over the Southern Ocean (70°–45°S). Analysis of model outputs from three phases of the Paleoclimate Model Intercomparison Project shows that models with warm Southern Ocean SST biases tend to simulate a deepening of the LGM AMOC, while the opposite is observed in models with cold SST biases. As a result, a positive correlation of 0.41 is found between SST biases and LGM AMOC depth anomalies. Using sensitivity experiments with a climate model, we show, as an example, that changes in parameters associated with the fraction of cloud thermodynamic phase in a climate model reduce the biases in the warm SST over the modern Southern Ocean. The less biased versions of the model then reproduce a colder Southern Ocean at the LGM, which increases formation of Antarctic Bottom Water and causes shoaling of the LGM AMOC, without affecting the LGM climate in other regions. The results highlight the importance of sea surface conditions and clouds over the Southern Ocean in simulating past and future global climates. Significance Statement To test the ability of comprehensive climate models, simulations of the Last Glacial Maximum (LGM) have been conducted. However, most models simulated a deeper Atlantic meridional overturning circulation (AMOC) in the LGM, which contradicts paleodata suggesting a shallower AMOC. Here, using multimodel analysis and sensitivity experiments with a climate model, we show that this paleodata–model discrepancy is partly related to model biases in the modern Southern Ocean. Improvements in Southern Ocean surface temperatures and clouds reproduce a colder climate over the Southern Ocean at the LGM, which causes an intense shoaling of the AMOC due to increased formation of Antarctic Bottom Water. These results demonstrate the important effect of model biases over the Southern Ocean on simulating past climates.

Model‐Data Comparison of Antarctic Winter Sea‐Ice Extent and Southern Ocean Sea‐Surface Temperatures During Marine Isotope Stage 5e

AbstractMarine Isotope Stage (MIS) 5e (130–116 ka) represents a “laboratory” for evaluating climate model performance under warmer‐than‐present conditions. Climate model simulations for MIS 5e have previously failed to produce Southern Ocean (SO) sea‐surface temperatures (SSTs) and sea‐ice extent reconstructed from marine sediment core proxy records. Here we compare state of the art HadGEM3 and HadCM3 simulations of Peak MIS 5e SO summer SSTs and September sea‐ice concentrations with the latest marine sediment core proxy data. The model outputs and proxy records show the least consistency in the regions located near the present‐day SO gyre boundaries, implying the possibility that model simulations are currently unable to fully realize changes in gyre extent and position during MIS 5e. Including Heinrich 11 meltwater forcing in Peak MIS 5e climate simulations improves the likeness to proxy data but it is clear that longer (3–4 ka) run times are required to fully test the consistency between models and data.

Thermal trait variation may buffer Southern Ocean phytoplankton from anthropogenic warming.

Despite the potential of standing genetic variation to rescue communities and shape future adaptation to climate change, high levels of uncertainty are associated with intraspecific trait variation in marine phytoplankton. Recent model intercomparisons have pointed to an urgent need to reduce uncertainty in the projected responses of marine ecosystems to climate change, including Southern Ocean (SO) surface waters, which are among the most rapidly warming habitats on Earth. Because SO phytoplankton growth responses to warming sea surface temperature (SST) are poorly constrained, we developed a high-throughput growth assay to simultaneously examine inter- and intra-specific thermal trait variation in a group of 43 taxonomically diverse and biogeochemically important SO phytoplankton called diatoms. We found significant differential growth performance among species across thermal traits, including optimum and maximum tolerated growth temperatures. Within species, coefficients of variation ranged from 3% to 48% among strains for those same key thermal traits. Using SO SST projections for 2100, we predicted biogeographic ranges that differed by up to 97% between the least and most tolerant strains for each species, illustrating the role that strain-specific differences in temperature response can play in shaping predictions of future phytoplankton biogeography. Our findings revealed the presence and scale of thermal trait variation in SO phytoplankton and suggest these communities may already harbour the thermal trait diversity required to withstand projected 21st-century SST change in the SO even under severe climate forcing scenarios.

Roles of Meridional Overturning in Subpolar Southern Ocean SST Trends: Insights from Ensemble Simulations

Abstract One of the most puzzling observed features of recent climate has been a multidecadal surface cooling trend over the subpolar Southern Ocean (SO). In this study we use large ensembles of simulations with multiple climate models to study the role of the SO meridional overturning circulation (MOC) in these sea surface temperature (SST) trends. We find that multiple competing processes play prominent roles, consistent with multiple mechanisms proposed in the literature for the observed cooling. Early in the simulations (twentieth century and early twenty-first century) internal variability of the MOC can have a large impact, in part due to substantial simulated multidecadal variability of the MOC. Ensemble members with initially strong convection (and related surface warming due to convective mixing of subsurface warmth to the surface) tend to subsequently cool at the surface as convection associated with internal variability weakens. A second process occurs in the late-twentieth and twenty-first centuries, as weakening of oceanic convection associated with global warming and high-latitude freshening can contribute to the surface cooling trend by suppressing convection and associated vertical mixing of subsurface heat. As the simulations progress, the multidecadal SO variability is suppressed due to forced changes in the mean state and increased oceanic stratification. As a third process, the shallower mixed layers can then rapidly warm due to increasing forcing from greenhouse gas warming. Also, during this period the ensemble spread of SO SST trend partly arises from the spread of the wind-driven Deacon cell strength. Thus, different processes could conceivably have led to the observed cooling trend, consistent with the range of possibilities presented in the literature. To better understand the causes of the observed trend, it is important to better understand the characteristics of internal low-frequency variability in the SO and the response of that variability to global warming.

Assessing the impact of suppressing Southern Ocean SST variability in a coupled climate model

The Southern Ocean exerts a strong influence on global climate, regulating the storage and transport of heat, freshwater and carbon throughout the world’s oceans. While the majority of previous studies focus on how wind changes influence Southern Ocean circulation patterns, here we set out to explore potential feedbacks from the ocean to the atmosphere. To isolate the role of oceanic variability on Southern Hemisphere climate, we perform coupled climate model experiments in which Southern Ocean variability is suppressed by restoring sea surface temperatures (SST) over 40°–65°S to the model’s monthly mean climatology. We find that suppressing Southern Ocean SST variability does not impact the Southern Annular Mode, suggesting air–sea feedbacks do not play an important role in the persistence of the Southern Annular Mode in our model. Suppressing Southern Ocean SST variability does lead to robust mean-state changes in SST and sea ice. Changes in mixed layer processes and convection associated with the SST restoring lead to SST warming and a sea ice decline in southern high latitudes, and SST cooling in midlatitudes. These results highlight the impact non-linear processes can have on a model’s mean state, and the need to consider these when performing simulations of the Southern Ocean.

Southern Ocean sea surface temperature synthesis: Part 2. Penultimate glacial and last interglacial

The last interglacial (LIG: ∼130 to 115 thousand years before present) is often used as an analogue for near-future climate warming. Antarctic Ice Sheet response to LIG warming is of particular interest, because of its implications for sea level rise. Comparison between LIG climate simulations and proxy-based reconstructions of Southern Ocean sea surface temperature (SST) remains challenging, due to high uncertainties in both reconstructions and simulations. In this two-part study, the accompanying paper (Part 1) addressed uncertainties in the SST reconstructions by evaluating proxies relevant to Southern Ocean SST, and made recommendations for which proxies and respective calibrations are most reliable on glacial-interglacial time scales in this region. In the second part (this paper), we now apply these recommendations to a synthesis of Southern Ocean SST over the penultimate glacial and LIG. Similar to previous studies, we find that LIG warming at 40°S to 60°S reached 1.6 ± 1.1 °C (annual mean) or 1.9 ± 1.3 °C (austral summer: JFM) relative to present. Annual/summer cooling in the penultimate glacial maximum reached −3.6 ± 1.0 °C/−4.0 ± 1.2 °C, similar to the last glacial maximum. Compared with the previous LIG SST syntheses, our reported uncertainties more strongly reflect geographic variability and dating errors, as we have reduced errors in the individual temperature reconstructions and do not date records by aligning peaks in their SST. However, the reconstruction errors are still important, and we do not recommend detailed interpretation of temperature records from small numbers of sites. Instead, comparisons of our new synthesis with model simulations should focus only on the regional average.

Southern Ocean sea surface temperature synthesis: Part 1. Evaluation of temperature proxies at glacial-interglacial time scales

Quaternary interglacial climates are often used as analogues for how the Antarctic Ice Sheet will respond to future climate warming. Southern Ocean marine sediments provide an important paleoclimate archive in this respect. Sea surface temperature (SST) reconstructions in the Southern Ocean depend exclusively on the fossils or geochemical signatures of planktic organisms, but the strengths of these SST proxies remain poorly quantified in this region. To improve confidence in paleoclimate reconstructions, Part 1 of this two-part study evaluates the reliability of Southern Ocean SST proxies employed at Quaternary glacial-interglacial time scales, focusing on three key potential problems: advection/dispersion, seasonality, and non-thermal influences. We find that foraminifera assemblages and long-chain alkenones likely provide the most reliable SST reconstructions in this region. Diatom assemblages and the Globigerina bulloides Mg/Ca ratio are considered to be ‘moderately’ reliable. Both are subject to potentially significant non-thermal influences, and diatom assemblages are likely modified by species-dependent advection as they sink to the sea floor. Nevertheless, diatoms are valuable at higher latitudes, since alkenones and foraminifera assemblages lose sensitivity below ∼1 to 2 °C. Dinocyst assemblages, radiolarian assemblages, GDGTs and Neogloboquadrina pachyderma Mg/Ca are considered the least reliable in the Southern Ocean, due to weak calibrations, poorly-constrained non-thermal influences, and/or strong advection bias. We note that the seasonality of all proxies remains poorly constrained. Overall, Southern Ocean SST reconstructions using the recommended proxies and calibrations should be robust when averaging across multiple sites and proxy types, but should be treated with caution when analysing spatial variability, a small number of sites, or a single proxy type. Quantifying the effect of advection should be a priority for all planktic groups employed in Southern Ocean paleoclimate reconstructions.