Description: Labrador Sea convection was most intense and reached the greatest depths in the early 1990s, followed by weaker, shallower, and more variable convection after 1995. The Simple Ocean Data Assimilation (SODA) version 2.0.2/2.0.4 assimilation model is used to explore convective activity in the North Atlantic Ocean for the period from 1992 to 2007. Hydrographic conditions, which are relatively well observed during this period, are used to compare modeled and observed winter mixed-layer depths and water mass anomalies in relation to Deep Western Boundary Current transports and meridional overturning circulation (MOC) changes at the exit of the subpolar basin. The assimilation differs markedly from local observations in the March mixed-layer depth, which represents deep convection and water mass transformation. However, mean MOC rates at the exit of the subpolar gyre, forced by stratification in the mid-latitudes, are similar to estimates based on observations and show no significant decrease during the 1992–2007 period. SODA reproduces the deep Labrador Sea Water formation in the western North Atlantic without any clear indication of significant formation in the Irminger Sea while the lighter upper Labrador Sea Water density range is reached in the Irminger Sea in the 1990s, in agreement with existing assumptions of deep convection in the Irminger Sea and also supported by computed lag correlations with the Labrador Sea. Deep Water transformation mainly takes place in the eastern North Atlantic. The introduction of CFC-11 into the SODA model as a tracer reproduces the mean and multiyear variations of observed distributions.

Description: Observations in the central tropical Atlantic are used to investigate the circulation, the variability, and the near-equatorial meridional flow in this oceanic region. Meridional sections confirm that the southern band of the South Equatorial Current is a broad sluggish flow transporting subtropical water northwestward toward the western boundary. Variability in the South Equatorial Current is weak with an annual signal of about 2 cm/s. Recent equatorial flow observations agree with the previously proposed mean flow field, indicating that a permanent tropical circulation exists that is composed of several zonal current and countercurrent bands of small vertical and meridional extent compared to the subtropical gyres. However, wave phenomena superimpose on the mean flow field. On seasonal time scales the variability in the zonal flow field near the equator is dominated by the semiannual cycle in the central and eastern part while the annual cycle dominates in the western part. This seasonal variability is caused by the propagation of equatorial Rossby and Kelvin waves generated mainly by the zonal wind anomaly at the equator. Despite the observations of instantaneous cross-equatorial velocities and of floats crossing the equator it remains unclear whether there is a net cross-equatorial flow in the central tropical Atlantic in addition to cross-equatorial exchanges via thermocline convergence, upwelling and Ekman divergence. Three floats deployed at 200 m and 400 m depth either leave their deployment region at the equator to join the North Equatorial Undercurrent and progress further northward or in two cases have been deployed in the southern hemisphere and drift towards the equator.

Description: An analysis of TOPEX/POSEIDON altimeter data and in situ current and temperature data obtained between April 1995 and October 1996 from a moored array shows strong intraseasonal fluctuations in the southwestern Arabian Sea, an oceanic region where the Great Whirl (GW), a predominantly wind-generated, very energetic anticyclone, is present during the Southwest Monsoon. Fluctuation periods between 30 and 50 days, up to 100 days during some years, are observed in the 8-year altimetric dataset, mostly during late summer and fall. These fluctuations are largest in a 1000 km-wide region off the Somali, Omani and Yemeni coasts north of 5°N, suggesting a local generation mechanism. The in situ data at different moorings show strong and coherent fluctuations that are characterized by southwestward phase propagation and northward energy propagation. Their periods range from 30 to 60 days and increase steadily from July 1995 to January 1996. In the first stage, these periods are at and below the cut-off period of freely propagating, first baroclinic mode Rossby waves, but approach this theoretical limit later in the year. Instabilities of the flow in the transition region between the Southern Gyre and the GW are likely sources of these fluctuations.

Description: In this paper, we review observations, theory and model results on the monsoon circulation of the Indian Ocean. We begin with a general overview, discussing wind-stress forcing fields and their anomalies, climatological distributions of stratification, mixed-layer depths, altimetric sea-level distributions, and seasonal circulation patterns (Section 2). The three main monsoon circulation sections deal with the equatorial regime (Section 3), the Somali Current and western Arabian Sea (Section 4), and the Bay of Bengal, seasonally reversing monsoon currents south of India and Sri Lanka, and the eastern and central Arabian Sea (Section 5). For the equatorial regime, we discuss equatorial jets and undercurrents, their interactions with the eastern and western boundaries, and intraseasonal and vertically propagating signals. In the Somali Current section, we describe the ocean's responses to the summer and winter monsoon winds, and outline the modelling efforts that have been carried out to understand them. In the Bay of Bengal section, we present observational and modeling evidence showing the importance of remote forcing from the east, which to a large extent originates along the equator. In the following three sections, we review the southern-hemi sphere subtropical regime and its associated boundary currents (Section 6), the Indonesian Throughflow (Section 7), the Red Sea and Persian Gulf circulations (Section 8), and discuss aspects of their interactions with other Indian-Ocean circulations. Next, we describe the Indian Ocean's deep and shallow meridional overturning cells (Section 9). Model results show large seasonal variability of the meridional overturning streamfunction and heat flux, and we discuss possible physical mechanisms behind this variability. While the monsoon-driven variability of the deep cell is mostly a sloshing motion affecting heat storage, interesting water-mass transformations and monsoonal reversals occur in the shallow cross-equatorial cell. In the mean, the shallow cell connects the subduction areas in the southern subtropics and parts of the Indonesian Throughflow waters with the upwelling areas of the northern hemisphere via the cross-equatorial Somali Current. Its near-surface branch includes a shallow equatorial roll that is seasonally reversing. We close by looking at coupled ocean-climate anomalies, in particular the large events that were observed in the tropical and subtropical Indian Ocean in 1993/94 and 1997/98. These events have been interpreted as an independent Indian-Ocean climate mode by some investigators and as an ENSO-forced anomaly by others.

Description: The Indian Ocean differs from the other two large oceans in not possessing an annual-mean equatorial upwelling regime. While the subtropical cells (STCs) of the Atlantic and Pacific Oceans connect subtropical subduction regimes with tropical upwelling via equatorward thermocline flows and coastal undercurrents, much of the upwelling in the Indian Ocean occurs in the coastal regimes of the northern hemisphere. Consequently, the counterpart of the STCs of the other oceans has to be a cross-equatorial cell connecting the southern subtropical subduction zone via the Somali Current with the upwelling areas off Somalia and Oman. The southward return flow is by interior Ekman transports. This annual-mean picture is accomplished by a dominance of the summer monsoon, during which only northern upwelling occurs, over the winter monsoon. Pathways of the thermocline flows related to the shallow overturning circulations are investigated here and estimates of subduction and upwelling are presented. From the observed mean northward flow of thermocline waters within the Somali Current and the interior southward cross-equatorial return flow the magnitude of the cross-equatorial cell is estimated at 6 Sv, with part of the thermocline waters being supplied by the Indonesian Throughflow. From observations we estimate that the northern upwelling occurs dominantly through the offshore outflows of the Somali Current by the Southern Gyre and Great Whirl and to a lesser degree off Oman. However, we also present model results suggesting a much lower role of Somali upwelling and a significant contribution from open-ocean upwelling in cyclonic domes around India and Sri Lanka. An interesting aspect of the Indian Ocean cross-equatorial cell is the mechanism by which the Ekman transport crosses the equator. Typically, Ekman transports during the summer (winter) monsoon are southward (northward) on both sides of the equator, while mean meridional winds on the equator are in the respective opposite direction. Earlier model evidence had suggested that this type of forcing should lead to an equatorial roll with northward surface flow and southward subsurface flow during the summer monsoon and reverse orientation during the winter monsoon. Observational evidence is presented here, based on shipboard ADCP sections, moored stations and surface drifters, confirming the existence of the equatorial roll. It is strongly developed in the western Indian Ocean during the SW monsoon where the wind conditions for the roll are best met. While in the central Indian Ocean and during the winter monsoon the roll appears to be a more transient phenomenon, superimposed by equatorial-wave currents. The evidence further suggests that the roll is mostly confined to the surface-mixed layer and is, therefore, of little consequence for the meridional heat transport.

Description: Repeated hydrographic observations between 1996 and 2001 of the deep water mass distribution on four sections in the western Labrador Sea and northwestern North Atlantic at about 56°N, 53°N, 48°N and 43°N show significant changes in the water mass characteristics. These changes are spreading southward mainly with the Deep Western Boundary Current (DWBC). Shallower convection forms a convective water mass known as upper Labrador Sea Water (ULSW). During periods of deep convection in the Labrador Sea, ULSW was described to be formed in the western boundary current region. In the post deep convection period 1996 to 2001 ULSW was formed in the western and central Labrador Sea and spreads mainly westward towards and along the western boundary. At 53°N ULSW moves southward as a part of the deep Labrador Current, also constituting the upper part of the DWBC. In the early 1990s the deep convection produced a large volume of deep Labrador Sea Water (LSW) which filled intermediate layers of the central region of the Labrador Sea. After these years the convection became weaker, with no apparent LSW renewal in 1996, partial mixing down to 1500 m in 1997 and no notable LSW formation between 1998 and 2001. At the southwestern exit of the Labrador Sea at 53°N the deep LSW in 2001 was least in thickness and highest in salinity and temperature compared to the years since 1996. This reflects restratification which resulted in an increase in the density stratification between 1000 and 2000 m in the central Labrador Sea as well as year-to-year transformation of the LSW core. LSW passes 43°N off the Grand Banks about 1 to 2 years after it was first seen at 56°N. At the 48°N and 43°N sections the northward flowing North Atlantic Current (NAC), farther offshore than the DWBC, complicates the property distributions. Saltier and warmer LSW recirculates northward with the NAC at 43°N. Between 1996 and 2001 the Gibbs Fracture Zone Water (GFZW) turned colder and fresher. The Denmark Strait Overflow Water (DSOW) showed two periods of cooling and freshening, separated by an abrupt (rapid) increase in temperature and salinity within a year. The arrival time of this increase at the different locations implies a DSOW spreading time that is no more than two years from 56°N to 43°N near the western boundary, or four years from the sill of the Denmark Strait to the Grand Banks.

Description: Recent results from hydrographic, chlorofluoromethane (CFM) and current measurements during an R.V. Meteor cruise in February/March 1994 underscore the importance of the Vema Fracture Zone (VFZ), located near 11°N on the Mid-Atlantic Ridge, for the transport of bottom water from the deep western basin of the equatorial Atlantic into the eastern abyss. The eastward transport in the bottom water range, of 1.8-2.0 Sv below 2.0°C, and of 2.1–2.4 Sv below the level of no motion at 3640 m, was determined by a combination of geostrophic calculations and direct current observations by a lowered ADCP. The comparison to former results indicates that the eastward flow in the VFZ is rather persistent. The water mass properties (Θ, S and CFMs) in the VFZ were compared to stations in the Guiana Basin, in the equatorial channel, and in the Brazil Basin at 10°S suggesting a significant contribution of North Atlantic Deep Water to the entire bottom water layer in the VFZ.

Description: This paper is an observational study of small-scale coherent eddies in the Labrador Sea, a region of dense water formation thought to be of considerable importance to the North Atlantic overturning circulation. Numerical studies of deep convection emphasize coherent eddies as a mechanism for the lateral transport of heat, yet their small size has hindered observational progress. A large part of this paper is therefore devoted to developing new methods for identifying and describing coherent eddies in two observational platforms, current meter moorings and satellite altimetry. Details of the current and water mass structure of individual eddy events, as they are swept past by an advecting flow, can then be extracted from the mooring data. A transition is seen during mid-1997, with long-lived boundary current eddies dominating the central Labrador Sea year-round after this time, and convectively formed eddies similar to those seen in deep convection modeling studies apparent prior to this time. The TOPEX / Poseidon altimeter covers the Labrador Sea with a loose “net” of observations, through which coherent eddies can seem to appear and disappear. By concentrating on locating and describing anomalous events in individual altimeter tracks, a portrait of the spatial and temporal variability of the underlying eddy field can be constructed. The altimeter results reveal an annual “pulsation” of energy and of coherent eddies originating during the late fall at a particular location in the boundary current, pinpointing the time and place of the boundary current-type eddy formation. The interannual variability seen at the mooring is reproduced, but the mooring site is found to be within a localized region of greatly enhanced eddy activity. Notably lacking in both the annual cycle and interannual variability is a clear relationship between the eddies or eddy energy and the intensity of wintertime cooling. These eddy observations, as well as hydrographic evidence, suggest an active role for boundary current dynamics in shaping the energetics and water mass properties of the interior region.