The sensitivity of southeast pacific heat distribution to local and remote changes in ocean properties

AbstractThe Southern Ocean features ventilation pathways that transport surface waters into the subsurface thermocline on time scales from decades to centuries, sequestering anomalies of heat and carbon away from the atmosphere and thereby regulating the rate of surface warming. Despite its importance for climate sensitivity, the factors that control the distribution of heat along these pathways are not well understood. In this study, we use an observationally constrained, physically consistent global ocean model to examine the sensitivity of heat distribution in the recently ventilated subsurface Pacific (RVP) sector of the Southern Ocean to changes in ocean temperature and salinity. First, we define the RVP using numerical passive tracer release experiments that highlight the ventilation pathways. Next, we use an ensemble of adjoint sensitivity experiments to quantify the sensitivity of the RVP heat content to changes in ocean temperature and salinity. In terms of sensitivities to surface ocean properties, we find that RVP heat content is most sensitive to anomalies along the Antarctic Circumpolar Current (ACC), upstream of the subduction hotspots. In terms of sensitivities to subsurface ocean properties, we find that RVP heat content is most sensitive to basin-scale changes in the subtropical Pacific Ocean, around the same latitudes as the RVP. Despite the localized nature of mode water subduction hotspots, changes in basin-scale density gradients are an important controlling factor on heat distribution in the southeast Pacific.</jats:p

Stabilization of dense Antarctic water supply to the Atlantic Ocean overturning circulation

The lower limb of the Atlantic overturning circulation is resupplied by the sinking of dense Antarctic Bottom Water (AABW) that forms via intense air–sea–ice interactions next to Antarctica, especially in the Weddell Sea. In the last three decades, AABW has warmed, freshened and declined in volume across the Atlantic Ocean and elsewhere, suggesting an ongoing major reorganization of oceanic overturning. However, the future contributions of AABW to the Atlantic overturning circulation are unclear. Here, using observations of AABW in the Scotia Sea, the most direct pathway from the Weddell Sea to the Atlantic Ocean, we show a recent cessation in the decline of the AABW supply to the Atlantic overturning circulation. The strongest decline was observed in the volume of the densest layers in the AABW throughflow from the early 1990s to 2014; since then, it has stabilized and partially recovered. We link these changes to variability in the densest classes of abyssal waters upstream. Our findings indicate that the previously observed decline in the supply of dense water to the Atlantic Ocean abyss may be stabilizing or reversing and thus call for a reassessment of Antarctic influences on overturning circulation, sea level, planetary-scale heat distribution and global climate

Observing the four dimensional structure and variability of the Southern Ocean using satellite altimetry

We present a gravest empirical mode (GEM) projection, of temperature and salinity fields in the Southern Ocean that, combined with satellite altimetry, produces time evolving temperature, salinity and velocity fields, and use these to observe the mean and synoptically varying properties of the Southern Ocean from 1992-2006. Historical hydrography from 1920-2006 is used to produce GEM projections of the circumpolar temperature and‚Äö salinity fields in longitude/dynamic height space between 25-5400 dbar. Combining these fields with altimefric SSH creates synoptic temperature and salinity fields (satGEM fields) at seven-day time intervals on a 1/3¬¨‚àû grid. The satGEM fields resolve front and eddy features significantly more accurately than climatologies and can reproduce the time evolution of the T-S fields. These are used to create baroclinic velocities that produce realistic ACC volume transports and correlate well with ARGO velocities (u and v coefficients of 0.60 and 0.53). Although these fields accurately estimate a pointwise mean southward eddy heat transport of -37.7 KWm\^{-2}\ in the SAF, the global mean northward eddy heat and freshwater transports of ‚ÄövÑvÆ0.08 ¬¨¬± 0.01 PW and 0.025 ¬¨¬± 0.01 Sv are small due to the satGEM's inability to resolve eddy tilt. An explicit eddy tracking method produces similar transports, and as the two methods work at different length scales we combine them for a minimum bound on the eddy transport across the SAF of 0.14 ¬¨¬± 0.03 PW of heat southward and 0.04 ¬¨¬± 0.03 Sv of freshwater northward. There is a warming (1.219¬¨¬±0.089 Wm\^{-2}\) and weak freshening (5.85¬¨¬±0.16 mmy\^{-1}\ m\^{-2}\) of the ACC induced by adiabatic Water mass movement between 1992-2006. The diabatic contribution due to heat and freshwater fluxes drives a cooling (-0.628 ¬¨¬± 0.129 Wtp\^{-2}\) and freshening (30.27 ¬¨¬± 0.70 mmy\^{-1}\ m\^{-2}\), with a net trend of 0.591 ¬¨¬± 0.093 Wm\^{-2}\ and 36.12 ¬¨¬± 0.68 mmy\^{-1}\ m\^{-2}\. Although there is no trend in zonal ACC mass transport at any longitude, nor a change in eddy kinetic energy, eddy number or eddy meridional property transport, there is substantial variability driven by the Southern Annular Mode (SAM) and the El Ni‚âàv†o Southern Oscillation (ENSO). The SAM influences the Southern Ocean at frequencies of less than three months, while ENSO operates on interannual timescales. The ENSO driven trend in the Pacific dominates the total adiabatic heat and freshwater content change during strong El Ni‚âàv†o and La Ni‚âàv†a years (1997-2002), while outside this period the SAM is a greater contributor, but with a lag of 4-5 years. An increased SAM leads to a roughly circumpolar 5% increase in the zonal volume transport of the ACC. Additionally, there is a clear lag of 1.6-3.2 years between an increase in the SAM and an increase in the EKE and number of eddies across the whole ACC. The response of eddy heat and freshwater transport is less clear, but at a similar lag there is an increase of 0.01-0.1 PW of southward heat transport and 0.01-0.1 Sv of northward freshwater transport across the SAF. The weak transport response to wind stress change and lagged eddy transport indicates that the ACC is in an eddy saturated state

Unsupervised Clustering of Southern Ocean Argo Float Temperature Profiles

The Southern Ocean has complex spatial variability, characterized by sharp fronts, steeply tilted isopycnals, and deep seasonal mixed layers. Methods of defining Southern Ocean spatial structures traditionally rely on somewhat ad hoc combinations of physical, chemical, and dynamic properties. As a step toward an alternative approach for describing spatial variability in temperature, here we apply an unsupervised classification technique (i.e., Gaussian mixture modeling or GMM) to Southern Ocean Argo float temperature profiles. GMM, without using any latitude or longitude information, automatically identifies several spatially coherent circumpolar classes influenced by the Antarctic Circumpolar Current. In addition, GMM identifies classes that bear the imprint of mode/intermediate water formation and export, large-scale gyre circulation, and the Agulhas Current, among others. Because GMM is robust, standardized, and automated, it can potentially be used to identify structures (such as fronts) in both observational and model data sets, possibly making it a useful complement to existing classification techniques.Natural Environment Research Council (NERC). Grant Numbers: NE/N018028/1, NE/N018095/1, NE/L002434/

The Role of Ocean Dynamics in King Penguin Range Estimation

In a recent Article, Cristofari et al.1 discuss the impact that movements of the Antarctic Polar Front have had on historical king penguin populations, and make future projections based on potential climate change scenarios. They predict that 70% of the world’s breeding population will be severely impacted as the Polar Front, with its nutrient-rich upwelling and high productivity, moves polewards beyond the foraging range of many penguin colonies. We highlight here, however, that a detailed analysis of the models used does not support the projection of a poleward shift in the Polar Front’s future position, and that recent analyses show no evidence for such a shift having occurred in the last several decades

Estimating the Four-Dimensional Structure of the Southern Ocean Using Satellite Altimetry

A gravest empirical mode (GEM) projection of temperature and salinity fields over the circumpolar Southern Ocean is presented and is used in combination with satellite altimetry to produce gridded, full-depth, time-evolving temperature, salinity, and velocity fields. Optimal interpolation of historical hydrography, including Argo floats, is used to produce GEM projections of the circumpolar temperature and salinity fields. Parameterizing these fields by dynamic height allows the use of altimetric SSH values from 1992-2006 to create synoptic temperature and salinity fields at weekly intervals on a ⅓° grid at 36 depth levels. The satellite-derived temperature and salinity fields generally capture over 90% of the property variance below the thermocline, with RMS residuals of 1.16°C and 0.132 in salinity at the surface, decreasing to less than 0.45°C and 0.05 below 500 dbar. The combination of altimetry with the GEM fields allows the resolution of the subsurface structure of the filamentary fronts and eddy features. Velocity fields derived from the time-evolving temperature and salinity fields reproduce the Antarctic Circumpolar Current (ACC) velocity structure well, and are strongly correlated (r > 0.7) with in situ measurements from current meters and drifters, with RMS velocity residuals of 4.8-14.8 cm-1 in the Subantarctic Front