Juan Brown

The production of North Atlantic Deep Water: Sources, rates, and pathways

Updating an earlier account by Dickson et al., (1990), this paper reviews the initial development phase of North Atlantic Deep Water (NADW) production from the points where the dense inflows from Nordic seas cross the Greenland-Scotland Ridge to the point off south Greenland where the buildup of new production appears almost complete. In particular, three long-term current meter arrays totaling 91 instruments and set at ∼160 km intervals south from the Denmark Strait sill are used to validate earlier short-term arrays by others and, in combination with these earlier arrays, to describe the downstream evolution of mean speed, depth and entrainment, the variability of the overflow current in space and time, and the likely contribution of the other three main constituents of NADW production at densities greater than σθ = 27.8. From the points of overflow (5.6 Sv) the transport within this range increases by entrainment and confluence with other contributory streams to around 13.3 Sv at Cape Farewell. While recirculating elements prevent us from determining the net southgoing transport, a NADW transport of this order appears consistent with recent estimates of net abyssal flow passing south through the North and South Atlantic.

Seasonal evolution of the cold pool gyre, in the western Irish Sea

An extensive cruise program during 1994, 1995 and 1996 provided observations that describe the seasonal evolution of the three-dimensional density field in the western Irish Sea. A cold, dense pool flanked by strong nearbed density gradients was present from May until October. In spring, salinity had a dominant influence on the density structure but from June until October temperature controlled the density stratification. The trajectories of 55 satellite-tracked Argos drifters demonstrated the existence of the cyclonic circulation pattern that constitutes the western Irish Sea gyre and defined the gyres spatial extent. Several distinct recirculation paths were observed and the implications for planktonic organisms of the seasonal variability in the recirculation are discussed. Drifter speeds were in good agreement with geostrophic calculations based on the observed density field and with de-tided acoustic Doppler current profiler (ADCP) measurements. The dynamical significance of strong nearbed gradients (bottom fronts) is highlighted. The location of the dynamically significant baroclinicity below the level of wind mixing explains the persistence of the cold pool and cyclonic circulation until late October. The dataset described here is valuable for environmental management purposes and it facilitates testing of the prognostic capabilities of the present generation of density-advecting numerical models.

Thermohaline circulation of shallow tidal seas

The mechanisms controlling the temperature and salinity structure of shallow continental shelf seas have been understood for over thirty years, yet knowledge of what drives their large-scale circulation has remained relatively unknown. Here we describe a decade long programme of measurements, using satellite-tracked drifting buoys on the northwest European shelf, to draw attention to a striking picture of highly organised thermohaline circulation consisting of narrow, near surface, fast flowing jets. These are ubiquitous above sharp horizontal gradients in bottom temperatures and/or salinities. The circulation phenomena we describe are likely to be prevalent on all similar, wide, tidally energetic continental shelves including those off north-eastern China, Argentina and parts of the Arctic. The robust, repeatable observation of the key role of jets above bottom fronts results in a fundamental reassessment of how we view the dynamics of shelf seas.

Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough, Antarctica

Hydrographic, current meter and ADCP data collected during two recent cruises in the South Indian Ocean (RRS Discovery cruise 200 in February 1993 and RRS Discovery cruise 207 in February 1994) are used to investigate the current structure within the Princess Elizabeth Trough (PET), near the Antarctic continent at 85°E, 63–66°S. This gap in topography between the Kerguelen Plateau and the Antarctic continent, with sill depth 3750 m, provides a route for the exchange of Antarctic Bottom Water between the Australian–Antarctic Basin and the Weddell–Enderby Basin. Shears derived from ADCP and hydrographic data are used to deduce the barotropic component of the velocity field, and thus the volume transports of the water masses. Both the Southern Antarctic Circumpolar Current Front (SACCF) and the Southern Boundary of the Antarctic Circumpolar Current (SB) pass through the northern PET (latitudes 63 to 64.5°S) associated with eastward transports. These are deep-reaching fronts with associated bottom velocities of several cm s-1. Antarctic Bottom water (AABW) from the Weddell–Enderby Basin is transported eastwards in the jets associated with these fronts. The transport of water with potential temperatures less than 0°C is 3 (±1) Sv. The SB is shown to meander in the PET, caused by the cyclonic gyre immediately west of the PET in Prydz Bay. The AABW therefore also meanders before continuing eastwards. In the southern PET (latitudes 64.5 to 66°S) a bottom intensified flow of AABW is observed flowing west. This AABW has most likely formed not far from the PET, along the Antarctic continental shelf and slope to the east. Current meters show that speeds in this flow have an annual scalar mean of 10 cm s-1. The transport of water with potential temperatures less than 0°C is 20 (±3) Sv. The southern PET features westward flow throughout the water column, since the shallower depths are dominated by the flow associated with the Antarctic Slope Front. Including the westward flow of bottom water, the total westward transport of the whole water column in the southern PET is 45 (±6) Sv.

Comment on "Use of local tidal records to identify relative sea level change: accuracy and error for decision makers" by Powell VA, McGlashan DJ, Duck RW (2012) J Coast Conserv

In this Comment we refer to our strong reservations concerning the paper by Powell et al. J Coast Conserv, (2012) recently published on the Online First web site of the Journal of Coastal Conservation. The paper makes a number of comments on data obtained from the Permanent Service for Mean Sea Level (PSMSL) and British Oceanographic Data Centre (BODC) which are incorrect or misleading. In addition, some of their comments on sea-level science in general need to be challenged and corrected.