In comparison to the deep ocean, the upper mixed layer is a region typically characterized by substantial vertical gradients in water properties. Within the Tropics, the rich variability in the vertical shapes and forms that these structures can assume through variation in the atmospheric forcing results in a differential effect in terms of the temperature and salinity stratification. Rather than focusing on the strong halocline above the thermocline, commonly referred to as the salinity barrier layer, the present study takes into account the respective thermal and saline dependencies in the Brunt-Väisälä frequency (N²) in order to isolate the specific role of the salinity stratification in the layers above the main pycnocline. We examine daily vertical profiles of temperature and salinity from an ocean reanalysis over the period 2001–2007. We find significant seasonal variations in the Brunt-Väisälä frequency profiles are limited to the upper 300 m depth. Based on this, we determine the ocean salinity stratification (OSS) to be defined as the stabilizing effect (positive values) due to the haline part of N² averaged over the upper 300 m. In many regions of the tropics, the OSS contributes 40–50% to N² as compared to the thermal stratification and, in some specific regions, exceeds it for a few months of the seasonal cycle. Away from the tropics, for example, near the centers of action of the subtropical gyres, there are regions characterized by the permanent absence of OSS. In other regions previously characterized with salinity barrier layers, the OSS obviously shares some common variations; however, we show that where temperature and salinity are mixed over the same depth, the salinity stratification can be significant. In addition, relationships between the OSS and the sea surface salinity are shown to be well defined and quasilinear in the tropics, providing some indication that in the future, analyses that consider both satellite surface salinity measurements at the surface and vertical profiles at depth will result in a better determination of the role of the salinity stratification in climate prediction systems.

Changes in the metastability of the Southern Hemisphere 500-hPa circulation are examined using both cluster analysis techniques and split-flow blocking indices. The cluster methodology is a purely data-driven approach for parameterization whereby a multiscale approximation to nonstationary dynamical processes is achieved through optimal sequences of locally stationary fast vector autoregressive factor (VARX) processes and some slow (or persistent) hidden process switching between them. Comparison is made with blocking indices commonly used in weather forecasting and climate analysis to identify dynamically relevant metastable regimes in the 500-hPa circulation in both reanalysis and Atmospheric Model Intercomparison Project (AMIP) datasets. The analysis characterizes the metastable regime in both reanalysis and model datasets prior to 1978 as positive and negative phases of a hemispheric midlatitude blocking state with the southern annular mode (SAM) associated with a transition state. Post-1978, the SAM emerges as a true metastable state replacing the negative phase of the hemispheric blocking pattern. The hidden state frequency of occurrences exhibits strong trends. The blocking pattern dominates in the early 1980s, and then gradually decreases. There is a corresponding increase in the SAM frequency of occurrence. This trend is largely evident in the reanalysis summer and spring but was not evident in the AMIP dataset. Further comparison with the split-flow blocking indices reveals a superficial correspondence between the cluster hidden state frequency of occurrences and split-flow indices. Examination of composite states shows that the blocking indices capture splitting of the zonal flow whereas the cluster composites reflect coherent block formation. Differences in blocking climatologies from the respective methods are discussed.

Across the long-term (∼43-year) satellite record, Antarctic sea ice extent shows a small overall circumpolar increase, resulting from opposing regional sea ice concentration anomalies. Running short-term samples of the same sea ice concentration data, however, show that the long-term trend pattern is dominated by the earliest years of the satellite record. Compensating regional anomalies diminish over time, and in the most recent decade, these tend towards spatial homogeneity instead. Running 30-year trends show the regional pattern of sea ice behaviour reversing over time; while in some regions, trend patterns abruptly shift in line with the record anomalous sea ice behaviour of recent years, in other regions a steady change predates these record anomalies. The shifting trend patterns in many regions are co-located with enhanced north–south flow due to an increasingly wave-3-like structure of the Southern Annular Mode. Sea surface temperature anomalies also shift from a circumpolar cooling to a regional pattern that resembles the increasingly asymmetric structure of the Southern Annular Mode, with warming in regions of previously increasing sea ice such as the Ross Sea.

In this study, we compare optimally interpolated monthly time series Tasman Sea XBT data and a comprehensive set of ocean data assimilation models forced by atmospheric reanalysis to investigate the stability of the Tasman Sea thermocline and the transport variability of the East Australian Current (EAC), the Tasman Front, and EAC‐extension. We find that anomalously weaker EAC transport at 25°S corresponds to an anomalously weaker Tasman Front and anomalously stronger EAC‐extension. We further show that, post about 1980 and relative to the previous 30 years, the anomalously weaker EAC transport at 25°S is associated with large‐scale changes in the Tasman Sea; specifically stronger stratification above the thermocline, larger thermocline temperature gradients, and enhanced energy conversion. Significant correlations are found between the Maria Island station Sea Surface Temperature (SST) variability and stratification, thermocline temperature gradient, and baroclinic energy conversion suggesting that nonlinear dynamical responses to variability in the basin‐scale wind stress curl are important drivers of decadal variability in the Tasman Sea. We further show that the stability of the EAC is linked, via the South Caledonian Jet, to the stability of the pan‐basin subtropical South Pacific Ocean “storm track.”