Christine Provost

Sustained monitoring of the southern ocean at Drake Passage: Past achievements and future priorities

Drake Passage is the narrowest constriction of the Antarctic Circumpolar Current (ACC) in the Southern Ocean, with implications for global ocean circulation and climate. We review the long-term sustained monitoring programs that have been conducted at Drake Passage, dating back to the early part of the twentieth century. Attention is drawn to numerous breakthroughs that have been made from these programs, including (1) the first determinations of the complex ACC structure and early quantifications of its transport; (2) realization that the ACC transport is remarkably steady over interannual and longer periods, and a growing understanding of the processes responsible for this; (3) recognition of the role of coupled climate modes in dictating the horizontal transport and the role of anthropogenic processes in this; and (4) understanding of mechanisms driving changes in both the upper and lower limbs of the Southern Ocean overturning circulation and their impacts. It is argued that monitoring of this passage remains a high priority for oceanographic and climate research but that strategic improvements could be made concerning how this is conducted. In particular, long-term programs should concentrate on delivering quantifications of key variables of direct relevance to large-scale environmental issues: In this context, the time-varying overturning circulation is, if anything, even more compelling a target than the ACC flow. Further, there is a need for better international resource sharing and improved spatiotemporal coordination of the measurements. If achieved, the improvements in understanding of important climatic issues deriving from Drake Passage monitoring can be sustained into the future.

Direct velocity measurements in the Malvinas Current

We present a set of current meter measurements collected in the southwest Atlantic from December 1993 to June 1995. Four moorings were set out to monitor the Malvinas (Falkland) Current at 40°–41°S, and one mooring was located within the Brazil-Malvinas Front area at 38°S. The mean flow of the Malvinas Current has an equivalent barotropic structure, whereas the vertical structure of the variable part of the flow is dominated by a barotropic-like empirical mode for regions shallower than 2500 m, surface intensified, which appears as a linear combination of barotropic and baroclinic modes, suggesting mode coupling due to the steep topography. This mode appears to gradually become nearly exactly barotropic toward shallower depths. Higher empirical modes, bottom intensified and associated with higher frequencies, suggest waves trapped by the conjugated effects of topography and stratification. The time variability is discussed in terms of along- and cross-isobath flows. The along-shelf flow features substantial energy in the 50 to 70-day band. There is no evidence of an annual cycle nor of a distinct semiannual cycle, but there is a suggestion of significant energy at periods a little shorter (135 days). The cross-shelf variability is important, with a quite clear annual cycle, associated with excursions of the subantarctic front. Its phase coincides with that of the Brazil Current transport, suggesting that annual migrations of the front are predominately determined by the strength of the latter, whereas the intensity of the Malvinas Current would only have marginal impact, a conclusion that may not hold at other timescales.

Analysis of satellite sea surface temperature time series in the Brazil-Malvinas Current Confluence region: Dominance of the annual and semiannual periods

We study the dominant periodic variations of sea surface temperature (SST) in the Brazil-Malvinas Confluence region from a satellite-derived data set compiled by Olson et al. (1988). This data set is composed of 202 sea surface temperature images with a 4 × 4 km resolution and extends over 3 years (from July 1984 to July 1987). Each image is a 5-day composite. The dominant signal, as already observed by Podesta et al. (1991), has a 1-year period. We first fit a single-frequency sinusoidal model of the annual cycle in order to estimate mean temperature, amplitude, and phase at 159 points uniformly distributed over the region. The residuals are generally small (less than 2°C). The largest departures from this cycle are located either in the Brazil-Malvinas frontal region or in the southeastern part of the region. Other periods in SST variations are identified by means of periodograms of the 159 residual time series in which the annual cycle has been substracted. The periodograms show that a semiannual frequency signal is present at almost every location. The ratio of the semiannual amplitude to the annual amplitude increases southward from 0% at 30°S to reach up to 45% at 50°S. In the south the semiannual signal creates an asymmetry, and the resulting (total) annual cycle has a cold period (winter) longer than the warm one (summer). In the frontal region the annual and semiannual signals have an important interannual variation. This semiannual frequency is associated with the semiannual wave present in the atmospheric forcing of the southern hemisphere. Differential heating over the mid-latitude oceans and the high-latitude ice-covered Antarctic Continent has been suggested as the cause of this semiannual wave (Van Loon, 1967).

Leads in Arctic pack ice enable early phytoplankton blooms below snow-covered sea ice

The Arctic icescape is rapidly transforming from a thicker multiyear ice cover to a thinner and largely seasonal first-year ice cover with significant consequences for Arctic primary production. One critical challenge is to understand how productivity will change within the next decades. Recent studies have reported extensive phytoplankton blooms beneath ponded sea ice during summer, indicating that satellite-based Arctic annual primary production estimates may be significantly underestimated. Here we present a unique time-series of a phytoplankton spring bloom observed beneath snow-covered Arctic pack ice. The bloom, dominated by the haptophyte algae Phaeocystis pouchetii, caused near depletion of the surface nitrate inventory and a decline in dissolved inorganic carbon by 16 ± 6 g C m−2. Ocean circulation characteristics in the area indicated that the bloom developed in situ despite the snow-covered sea ice. Leads in the dynamic ice cover provided added sunlight necessary to initiate and sustain the bloom. Phytoplankton blooms beneath snow-covered ice might become more common and widespread in the future Arctic Ocean with frequent lead formation due to thinner and more dynamic sea ice despite projected increases in high-Arctic snowfall. This could alter productivity, marine food webs and carbon sequestration in the Arctic Ocean.

Sea surface velocities from sea surface temperature image sequences: 1. Method and validation using primitive equation model output

An inverse variational finite element model is developed for the purpose of estimating ocean surface velocity fields from sequences of temperature fields. The cross-isotherm component of the velocity is controlled by a mixed layer integrated formulation of the heat balance. The aperture problem imposes additional constraints on the flow field: we control the divergence and the vorticity of the horizontal velocity. The method is then applied to sequences of sea surface temperature (SST) fields from a fine-mesh numerical simulation over the Brazil-Malvinas Confluence region. The difference between the actual velocity and the SST inverted velocity is 11% rms in magnitude and 17° rms in direction. These differences are analyzed; divergence and vorticity are computed. The hypothesis of neglecting the source terms made in the formulation of the mixed layer integrated heat balance is verified. The sensitivity of the solution to the influence of the constraints is examined. Perturbations are performed yielding fields of ellipses of covariance. We obtain on average 15% uncertainties in magnitude and 25° in direction of the velocity. Differences between the actual and SST inverted velocities fall into those covariance ellipses, except over regions where temperature does not change from one image to the other.

Subtropical mode waters in the South Atlantic Ocean

This systematic study of the subtropical mode waters in the South Atlantic is based on hydrological data (the basin-wide South Atlantic Ventilation Experiment, Oceanus, and Ajax sections, and the World Ocean Circulation Experiment (WOCE) sections in the Brazil-Malvinas Confluence region), the WOCE expendable bathythermograph data set, and the atmospheric forcing produced by the National Centers for Environmental Prediction reanalysis. The main ventilation window of the subtropical gyre lies in the southwest and corresponds to the southern part of the recirculation region and to the overshoot of the Brazil Current (BC). Some ventilation occurs along the southern branch of the gyre through deep mixed layers in harsh winters and in the east through the rings shed by the Agulhas Current. However, the ventilation from the eastern border concerns a small volume of water with highly variable characteristics. Three main types of subtropical mode water are observed: the lightest around the 26.2 density surface and the second and third around the 26.5 and 26.7 density surfaces, respectively. The 26.2 subtropical mode water (STMW1) corresponds to potential temperatures between 16° and 18°C and salinities between 35.9 and 36.2. The STMW1 thickness does not exceed 150 m. The volume of the 26.5 mode water (STMW2) is larger. It is produced through the formation of deep mixed layers (reaching 300 m or more) in the active ventilation window over the BC overshoot. Its potential temperature ranges between 14° and 16°C, and its salinity ranges between 35.5 and 35.9. Its thickness can be above 300 m. The last component observed around the 26.7 density surface (STMW3) has a potential temperature between 12° and 14°C and a salinity of about 35.2 to 35.5. It is formed on the eastern flank of the BC overshoot and, during harsh winters, in the center of the southern branch of the gyre. Within the main ventilation window the STMWs exhibit quite a significant variability in characteristics and volume. There is a large interannual variability in the atmospheric forcing over the BC recirculation and BC extension, and the observed variations of the subtropical mode waters are qualitatively correlated to the atmospheric forcing variations.

Brazil-Malvinas Confluence: Water mass composition

A quantitative analysis of water masses in the Brazil-Malvinas Confluence zone is performed with a least squares multiple tracer analysis using data from the confluence winter 1989 cruise. The purpose is to find the mixture of source water types that best describes the composition of a given water sample. This method is valuable in regions involving strong mixing among various source water types, as is the Brazil-Malvinas Confluence zone. Seven main core layers are identified in this region, and all are retained for the analysis: the Thermocline Water (TW), the Subantarctic Surface Water (SASW), the Antarctic Intermediate Water (AAIW), the Upper Circumpolar Deep Water (UCDW), the North Atlantic Deep Water (NADW), the Lower Circumpolar Deep Water (LCDW), and the Weddell Sea Deep Water (WSDW). Tracers selected are temperature, salinity, dissolved nutrients, and oxygen. The results show the proportion of each source water type along four east-west sections (35.4°S, 36.5°S, 37.9°S, 39°S). They are accurate to within 20% for all sources. The solution presents evidence of local recirculation of AAIW largely influenced by the two strong currents, Brazil and Malvinas. Southward TW and NADW separate from the coast, NADW turning eastward at a higher latitude than TW.

Winter to summer oceanographic observations in the Arctic Ocean north of Svalbard

Oceanographic observations from the Eurasian Basin north of Svalbard collected between January and June 2015 from the N-ICE2015 drifting expedition are presented. The unique winter observations are a key contribution to existing climatologies of the Arctic Ocean, and show a ∼100m deep winter mixed layer likely due to high sea ice growth rates in local leads. Current observations for the upper ∼200m show mostly a barotropic flow, enhanced over the shallow Yermak Plateau. The two branches of inflowing Atlantic Water are partly captured, confirming that the outer Yermak Branch follows the perimeter of the plateau, and the inner Svalbard Branch the coast. Atlantic Water observed to be warmer and shallower than in the climatology, is found directly below the mixed layer down to 800m depth, and is warmest along the slope, while properties inside the basin are quite homogeneous. From late May onwards, the drift was continually close to the ice edge and a thinner surface mixed layer and shallower Atlantic Water coincided with significant sea ice melt being observed. This article is protected by copyright. All rights reserved.

Volume transport of the Malvinas Current: Can the flow be monitored by TOPEX/POSEIDON?

Current meter measurements were collected between 40° and 41°S in the Malvinas (Falkland) Current from December 1993 to June 1995. Owing to the premature failure of a mooring, a reliable volume transport time series could only be calculated for 254 days, leading to a mean transport of about 41.5 Sv with a standard deviation of 12.2 Sv. This time series is tentatively extended to 386 days. It is also shown that the TOPEX/POSEIDON altimeter, combined with the statistical information on the vertical structure of the current provided by the current meters, can be used to sensitively monitor the flow. A 3-year-long time series of transport is derived, which is well correlated (0.8) to the transport estimated from current meters, including about 60% of the variance of the flow at periods beyond 20 days; this series makes it possible to consider a relatively broad spectral range. Dominant periods are 50–80 days and close to 180 days. Interannual variations are large. Comparatively, little energy is found at the annual period, suggesting that the Malvinas Current has only little impact on the annual migrations of the confluence. The predominance of a semiannual cycle is compatible with a remote forcing of the flow suggested by previous numerical studies.

Hydrographic conditions in the surface layers over the slope-open ocean transition area near the Brazil-Malvinas confluence during austral summer 1990

Abstract The Confluence 3 cruise during February 1990 provides the first hydrographic survey of the Brazil-Malvinas Confluence region with such a refined spatial resolution over the continental slope. It is also the first survey in the region ever obtained in an austral summer. The austral summer 1990 atmospheric conditions were anomalously warm over the region. The surface waters were separated from the underlying waters by a thin and sharp seasonal thermocline which hid the real location of the Brazil-Malvinas thermocline front on infrared satellite imagery. The surface salinity was influenced by the large Rio de La Plata runoff which was anomalously high by a factor of 3. Salinity values less than 32 psu were encountered. These low salinity waters were entrained by the surface east-west front away from the coast. Four types of fronts were observed in the surface layer: (i) an east-west front associated with the structure of the seasonal thermocline separating cold, low salinity, oxygen supersaturated and productive waters to the south from warm, salty, weakly undersaturated and poor chlorophyll a waters to the north, (ii) a north-south front in salinity due to the outflow of the Rio de La Plata, (iii) a shelf break front between the shelf water and the Malvinas Water characterized by higher Chl a values, and (iv) a front between the Malvinas Water and the Brazil-Malvinas return flow.