Helen E. Phillips

Eddy Variability and Energetics from Direct Current Measurements in the Antarctic Circumpolar Current South of Australia

Two-year time series measurements of current velocity and temperature in the Subantarctic Front (SAF) south of Australia from 1993 to 1995 provide estimates of eddy fluxes of heat and momentum across the Antarctic Circumpolar Current (ACC) and further insight into the variability of the ACC. The SAF was found to be an energetic, meandering jet with vertically coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm s21 at 1150 dbar. A daily varying coordinate frame that follows the direction of flow allowed mesoscale variability of the SAF to be isolated from variability due to meandering of the front and proved very successful for examining eddy fluxes. Vertically averaged cross-stream eddy heat flux was 11.3 kW m 22 poleward and significantly different from zero at 95% for fluctuations in the 2‐90 day band. Zonally integrated, this eddy heat flux ( 50.9 PW) is more than large enough to balance heat lost south of the Polar Front and is as large as cross-SAF fluxes found in Drake Passage. Cross-stream eddy momentum fluxes were small, not significantly different from zero, and of indeterminate sign, but tending to decelerate the mean flow. A relationship between vertical motion and meander phase identified in the Gulf Stream was found to hold for the ACC. Eddy kinetic energy levels were similar to those in Drake Passage and southeast of New Zealand. Eddy potential energy was up to an order of magnitude larger than at other ACC sites, most likely because meandering of the front is more common here. Baroclinic conversion was found to be the dominant mechanism by which eddies grow south of Australia. Typical eddy growth time is estimated to be 30 days, approximately twice as fast as in Drake Passage, consistent with eddy energy growing rapidly downstream.

Space and time scales for optimal interpolation of temperature - Tropical Pacific Ocean

Abstract Autocorrelation functions of sea surface temperature (SST) and depth of the 20°C isotherm (D 20 ) are estimated at 95 locations along the tropical Pacific expendable bathythermograph (XBT) ship-of-opportunity tracks, and used to determine statistical parameters required for optimal interpolation. The parameters are variances for signal and noise, and spatial/temporal decorrelation scales. Estimates were made for two periods of time: June 1979 to May 1982 proceding the 1982/83 El Nino, and June 1979 to May 1983 including it. Parameters for the first period indicate smaller scales and weaker signals than the ones for the second. The difference results from El Nino, whose large temperature signal dominates the statistical structure. Understanding the dynamics of the smaller interannual signals and in particular the precursors to El Nino is as important as understanding El Nino. The smaller scales are therefore recommended for optimal interpolation and design of the XBT network. Three summaries of the scales are presented: 1) averages in three degree latitude bands; 2) values in dynamic regions representing the major zonal currents; and 3) median values for 95 estimates in the tropical Pacific 18°N to 18°S. The median values are recommended for design of the tropical XBT network. The median scales for the depth of the 20°C isotherm are: 3° latitude, 15° longitude and 2 months; and the median signal-to-noise (amplitude) ratio is 0.75. Scales for SST are greater than or equal to scales for D 20 , thus subsurface temperature is the limiting factor in designing the network. Analysis of mapping errors suggests that optimal sampling requires two to three samples per decorrelation scale. Two modes of XBT sampling are recommended for TOGA. Broadscale sampling for horizontal mapping of the temperature field is recommended at a density of two samples per scale, which requires an XBT station every 1.5° latitude and 7.5° longitude, monthly. The broadscale mode should be implemented in as large an area as possible with available shipping. Sampling on repeated transequatorial sections for time series studies is recommended at a density of three samples per scale, which requires 18 sections per year, with stations every 1° latitude. The time series mode is useful for more accurate studies of thermal structure and currents, and should be implemented on a few routes in each ocean which transect major thermal features.

Interannual variability in the freshwater content of the Indonesian‐Australian Basin

An average freshening of 0.2 psu, extending from 100 degrees E to Australia, 25 degrees S to Indonesia and down to 180 m depth, persisted for more than 3 years from 1999 to 2002. We map the anomaly using CTD profiles from Argo floats and suggest that the dominant forcing for the anomaly is surface freshwater flux over the Indonesian seas that is advected into the region. Using historical CTD data and surface freshwater flux reanalysis products we show that the Indonesian Australian Basin experiences strong interannual variability in upper ocean freshwater content and that the recent fresh event, a result of a long-lasting La Nina, is unprecedented during the last 25 years.

A Mean Synoptic View of the Subantarctic Front South of Australia

Abstract A mean synoptic view of the subantarctic front (SAF) is obtained from current meter and hydrographic data by averaging absolute and baroclinic velocity measurements in bins defined by a cross-stream coordinate that moves with the current. The cross-stream coordinate is derived from current meter measurements of vertical shear of the horizontal velocity and is expressed in terms of specific volume anomaly at 780 dbar (δ780). By averaging absolute and baroclinic velocity measurements in stream coordinates, the spatial smoothing that results from Eulerian averaging of measurements of a meandering current is avoided. The mean SAF velocity profile is composed of one central peak and several smaller peaks. In the central peak, the 2-yr mean absolute velocity from current meters reaches 52 cm s−1 at 420 dbar; the mean baroclinic velocity from six CTD sections reaches 34 cm s−1. The SAF flow is coherent at all levels, reaches the seafloor, and is at least 220 km wide. The cross-stream structure of barocl...

Ocean Response and Feedback to the SST Dipole in the Tropical Atlantic

The equatorial SST dipole represents a mode of climate variability in the tropical Atlantic Ocean that is closely tied to cross-equatorial flow in the atmosphere, from the cold to the warm hemisphere. It has been suggested that this mode is sustained by a positive feedback of the tropical winds on the cross-equatorial SST gradient. The role, if any, of the tropical ocean is the focus of this investigation, which shows that at the latitudes of the SST signal (centered on 108N/S) there is a weak positive feedback suggested in data from the last half century, that the cross-equatorial wind stress is closely coupled to this SST gradient on monthly time scales with no discernable lag, and that the period from January to June is the most active period for coupling. Northward (southward) anomalies of cross-equatorial wind stress are associated with a substantial negative (positive) wind stress curl. This wind system can thus drive a cross-equatorial Sverdrup transport in the ocean from the warm to the cold side of the equator (opposite the winds) with a temporal lag of only a few months. The oceanic observations of subsurface temperature and a numerical model hindcast also indicate a clear relationship between this mode of wind-driven variability and changes in the zonal transport of the North Equatorial Countercurrent. It is estimated that the time-dependent oceanic flow is capable of providing a significant contribution to the damping of the SST dipole but that external forcing is essential to sustaining the coupled variability.

Bermuda’s tale of two time series : Hydrostation S and BATS

Abstract This paper describes the oceanic variability at Bermuda between 1989 and 1999, recorded in two overlapping hydrographic time series. Station S and Bermuda Atlantic Time Series Study (BATS), which are 60 km apart, both show that a multidecadal trend of deep warming has reversed, likely as a result of the increased production of Labrador Sea Water since the early 1980s. In addition to recording similar changes in watermass properties, the two time series show similar mean vertical structure and variance as a function of pressure for temperature, salinity, and density above 1500 dbar. The seasonal cycles of these water properties at the two sites are statistically indistinguishable. The time series differ in the individual eddy events they record and in their variability below 1500 dbar. The two time series are used to investigate the propagation of eddy features. Coherence and phase calculated from the low-mode variability of density show westward propagation at ∼3 cm s−1 of wavelengths around 300–...

Aquarius sea surface salinity in the South Indian Ocean: revealing annual-period planetary waves

A new milestone has been reached with the launch of two dedicated satellite missions to routinely measure the sea surface salinity (SSS) fields from space at global and regional scales. In the present work, a thorough analysis of the first two years of Aquarius SSS data in the South Indian Ocean is performed. This analysis is focused on three questions: How accurate is Aquarius SSS related to in situ data from the fresh Indonesian Throughflow and salty subtropical waters? Can Aquarius give a spatial context for the data measured by the RAMA mooring system? Are westward propagating annual-period signals described in recent model simulations reproduced by Aquarius-derived SSS? We find Aquarius observations to be highly correlated with those of Argo floats, with small disagreements occurring near oceanic fronts. Aquarius gives fresher SSS than in-situ data in the tropical region due to rainfall effects, except in the eastern basin where the freshening seems to be related to sharp localized leakages of very fresh waters from the Indonesian seas that the Aquarius product is not able to properly resolve. Aquarius data is shown to reproduce quite well the annual cycle obtained from RAMA and Argo gridded datasets. The annual cycle in Aquarius is characterized by SSS propagating features with different characteristics west and east of the Ninety East Ridge. These features are strikingly different from sea surface height waves. Our results suggest that SSS annual propagation might be reflecting coupled ocean-atmosphere dynamics and surface-subsurface processes operating over the entire South Indian Ocean.

On the nonequivalent barotropic structure of the Antarctic Circumpolar Current: An observational perspective

We examine the vertical structure of the horizontal flow and diagnose vertical velocities in the Antarctic Circumpolar Current (ACC) near the Kerguelen Plateau using EM-APEX profiling floats. Eight floats measured horizontal velocity, temperature, and salinity profiles to 1600 dbar, with a vertical spacing of 3–5 dbar four times per day over a period of approximately 3 months. Horizontal velocity profiles show a complex vertical structure with strong rotation of the velocity vector through the water column. The distribution of rotation angles from 1247 profiles is approximately Gaussian and rotations of either sign are equally likely. Forty percent of profiles with speed greater than 5 cm s−1 have a depth-integrated rotation of less than 15 degrees over 1300 dbar, while the other 60% demonstrate significantly stronger rotation. Consequently, most profiles do not conform to the equivalent barotropic model (deep flow parallel and proportional to the surface flow) used in simplified dynamical models and in Gravest Empirical Mode climatologies of the ACC. Nevertheless, since we find the mean rotation to be zero, an equivalent barotropic assumption is valid to first order. Vertical velocities inferred using conservation of mass and a gradient wind balance in natural coordinates have magnitudes on the order of 100 m/day. We find robust patterns of upwelling and downwelling phase-locked to meanders in the flow, as found in earlier studies. With the advent of high-resolution observations such as those presented here, and high-resolution models, we can advance to a more complete understanding of the rich variability in ACC structure that is neglected in the equivalent barotropic model.

Salinity dominance on the Indian Ocean Eastern Gyral current

15 i S, opposite to the direction predicted by classical theories of wind-driven circulation and is a source of water for the Leeuwin Current. In the upper ocean, a strong salinity front exists between fresh water from the Indonesian Through- flow (ITF) in the South Equatorial Current (SEC) and salty subtropical waters. In that region, salinity overwhelms the temperature contribution to density gradients, generat- ing eastward geostrophic shear and establishing the EGC. Without the salinity front the EGC cannot be maintained: If the salinity contribution is neglected in the calculation of geostrophic currents, the EGC vanishes. Our observa- tional analysis associated with the fact that both Sverdrup and Ekman theories produce westward flows in the region strongly supports the idea that the EGC is a salinity-driven current. Citation: Menezes, V. V., H. E. Phillips, A. Schiller, C. M. Domingues, and N. L. Bindoff (2013), Salinity dominance on the Indian Ocean Eastern Gyral current, Geophys. Res. Lett., 40, 5716-5721, doi:10.1002/2013GL057887.

South Indian Countercurrent and associated fronts

A striking feature of the South Indian Ocean circulation is the presence of the eastward South Indian Countercurrent (SICC) that flows in a direction opposite to that predicted by the classical theories of wind-driven circulation. Several authors suggest that the SICC resembles the subtropical countercurrents (STCCs) observed in other oceans, which are defined as narrow eastward jets on the equatorward side of subtropical gyres, where the depth-integrated flow is westward. These jets are associated with subsurface thermal fronts at thermocline depths by the thermal wind relation. However, the subsurface thermal front associated with the SICC has not been described to date. Other studies conjecture an important role for salinity in controlling the SICC. In the present work, we analyze three Argo-based atlases and data from six hydrographic cruises to investigate whether the SICC is accompanied by permanent thermal and density fronts including salinity effects. The seasonal cycle of these fronts in relation to the SICC strength is also investigated. We find that the SICC is better described as composed of three distinct jets, which we name the northern, central, and southern SICC. We find that the southern SICC around 26°S has an associated thermal front at subsurface depths around 100–200 m with salinity being of secondary importance. The southern branch strength is related to mode waters poleward of the front, similar to a STCC-like current. However, the SICC multiple jet structure seems to be better described as resulting from PV staircases.