Tropical forcing of increased Southern Ocean climate variability revealed by a 140-year subantarctic temperature reconstruction

Occupying about 14% of the world's surface, the Southern Ocean plays a fundamental role in ocean and atmosphere circulation, carbon cycling and Antarctic ice-sheet dynamics. Unfortunately, high interannual variability and a dearth of instrumental observations before the 1950s limits our understanding of how marine-atmosphere-ice domains interact on multi-decadal timescales and the impact of anthropogenic forcing. Here we integrate climate-sensitive tree growth with ocean and atmospheric observations on southwest Pacific subantarctic islands that lie at the boundary of polar and subtropical climates (52-54°S). Our annually resolved temperature reconstruction captures regional change since the 1870s and demonstrates a significant increase in variability from the 1940s, a phenomenon predating the observational record. Climate reanalysis and modelling show a parallel change in tropical Pacific sea surface temperatures that generate an atmospheric Rossby wave train which propagates across a large part of the Southern Hemisphere during the austral spring and summer. Our results suggest that modern observed high interannual variability was established across the mid-twentieth century, and that the influence of contemporary equatorial Pacific temperatures may now be a permanent feature across the mid- to high latitudes

Challenges and opportunities for improved understanding of regional climate dynamics

Dynamical processes in the atmosphere and ocean are central to determining the large-scale drivers of regional climate change, yet their predictive understanding is poor. Here, we identify three frontline challenges in climate dynamics where significant progress can be made to inform adaptation: response of storms, blocks and jet streams to external forcing; basin-to-basin and tropical–extratropical teleconnections; and the development of non-linear predictive theory. We highlight opportunities and techniques for making immediate progress in these areas, which critically involve the development of high-resolution coupled model simulations, partial coupling or pacemaker experiments, as well as the development and use of dynamical metrics and exploitation of hierarchies of models

Observed ENSO teleconnections to Southern Ocean SST anomalies diagnosed from a surface mixed layer heat budget

This study examines the mechanisms that govern the El Niño Southern Oscillation (ENSO) teleconnections to variability in extratropical Southern Hemisphere sea surface temperature (SST) anomalies using several observational and reanalysis products. During the warm season, turbulent heat fluxes and heat advection by Ekman currents contribute equally to ENSO-related SST variability throughout the Southern Ocean, while shortwave radiation has a larger contribution in the subtropics. A comparison of the ocean mixed layer heat budget between multiple re-analysis products reveals that the relative contributions of the turbulent, radiative and Ekman heat fluxes to ENSO-related SST anomalies is consistent between the NCEP, ERA-40 and OAFlux/ISCCP products, but discrepancies exist with regard to the structure and amplitude of the turbulent heat fluxes and, to a lesser extent, the shortwave radiation. However, a comparison between the tendencies in SST and flux-derived temperatures reveals minimal residuals, suggesting that the ENSO-signal in the SST field can be resolved by turbulent, Ekman, and radiative heat fluxes, particularly in the ERA-40 products

Observed variations in multi-decadal Antarctic sea ice trends during 1979-2012

The spatiotemporal sensitivity of Antarctic sea iceseason length trends are examined using satellite-derivedobservations over 1979¿2012. While the large-scale spatialstructure of multidecadal trends has varied little duringthe satellite record, the magnitude of trends has undergonesubstantial weakening over the past decade. This weakeningis particularly evident in the Ross and Bellingshausen Seas,where a 25¿50% reduction is observed when comparingtrends calculated over 1979¿2012 and 1979¿1999.Multidecadal trends in the Bellingshausen Sea are found tobe dominated by variability over subdecadal time scales,particularly the rapid decline in season length observedbetween 1979 and 1989. In fact, virtually no trend isdetectable when the first decade is excluded from trendcalculations. In contrast, the sea ice expansion in the RossSea is less influenced by shorter-term variability, withtrends shown to be more consistent at decadal time scalesand beyond. Understanding these contrasting characteristicshave implications for sea ice trend attribution. Citation:Simpkins, G. R., L. M. Ciasto, and M. H. England (2013),Observed variations in multidecadal Antarctic sea ice trendsduring 1979¿2012, Geophys. Res. Lett., 40, 3643¿3648,doi:10.1002/grl.50715

On the persistence of cold-season SST anomalies associated with the Annular Modes

In this study, a simple stochastic climate model is used to examine the impact of the ocean mixed layer depth, surface turbulent energy fluxes, and Ekman currents on the persistence of cold-season extratropical sea surface temperature (SST) anomalies associated with variability in the annular modes of atmospheric circulation in both hemispheres. Observational analysis reveals that during the cold season, SST anomalies associated with the southern annular mode (SSTSAM) persist considerably longer than those associated with the northern annular mode (SSTNAM). Using the simple model, it is shown that the persistence of the cold-season SSTSAM is consistent with the simple stochastic climate paradigm in which the atmospheric forcing is approximated as white noise, and the persistence of SST anomalies can be largely determined by the thermal inertia of the ocean mixed layer. In the North Atlantic, however, the simple climate model overestimates the persistence of the cold-season SSTNAM. It is thought that this overestimate occurs because the NAM-related heat flux forcing cannot be described purely as white noise but must also include a feedback from the underlying SST anomalies

Seasonal relationships between large-scale climate variability and Antarctic Sea ice concentration

The observed relationships between anomalous Antarctic sea ice concentration (SIC) and the leading patterns of Southern Hemisphere (SH) large-scale climate variability are examined as a function of season over 1980–2008. Particular emphasis is placed on 1) the interactions between SIC, the southern annular mode (SAM), and El Niño–Southern Oscillation (ENSO); and 2) the contribution of these two leading modes to the 29-yr trends in sea ice. Regression, composite, and principal component analyses highlight a seasonality in SH sea ice–atmosphere interactions, whereby Antarctic sea ice variability exhibits the strongest linkages to the SAM and ENSO during the austral cold season months. As noted in previous work, a dipole in SIC anomalies emerges in relation to the SAM, characterized by centers of action located near the Bellingshausen/Weddell and Amundsen/eastern Ross Seas. The structure and magnitude of this SIC dipole is found to vary considerably as a function of season, consistent with the seasonality of the overlying atmospheric circulation anomalies. Relative to the SAM, the pattern of sea ice anomalies linked to ENSO exhibits a similar seasonality but tends to be weaker in amplitude and more diffuse in structure. The relationships between ENSO and sea ice also exhibit a substantial nonlinear component, highlighting the need to consider both season and phase of the ENSO cycle when diagnosing ENSO–SIC linkages. Trends in SIC over 1980–2008 are not significantly related to trends in either the SAM or ENSO during any season, including austral summer when the trend in the SAM is most pronounced

Control of Mode and Intermediate Water Mass Properties in Drake Passage by the Amundsen Sea Low

The evolution of the physical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage region is examined on time scales down to intraseasonal, within the 1969–2009 period. Both SAMW and AAIW experience substantial interannual to interdecadal variability, significantly linked to the action of the Amundsen Sea low (ASL) in their formation areas. Observations suggest that the interdecadal freshening tendency evident in SAMW over the past three decades has recently abated, while AAIW has warmed significantly since the early 2000s. The two water masses have also experienced a substantial lightening since the start of the record. Examination of the mechanisms underpinning water mass property variability shows that SAMW characteristics are controlled predominantly by a combination of air–sea turbulent heat fluxes, cross-frontal Ekman transport of Antarctic surface waters, and the evaporation–precipitation balance in the Subantarctic zone of the southeast Pacific and Drake Passage, while AAIW properties reflect air–sea turbulent heat fluxes and sea ice formation in the Bellingshausen Sea. The recent interdecadal evolution of the ASL is consistent with both the dominance of the processes described here and the response of SAMW and AAIW on that time scale