Background and FAQ

Western Boundary Time Series

Climate models have shown that variations of the transport of the Meridional Overturning Cell (MOC) in the Atlantic Ocean have significant impacts on the climate at both the national and global level. In the subtropical North Atlantic, the meridional overturning circulation consists primarily of two western boundary components: the northward flowing Gulf Stream and the southward flowing Deep Western Boundary Current.

The Gulf Stream is the strong surface intensified flow along the east coast of the United States that brings warms waters of tropical origin along the eastern seaboard of the United States. The Gulf Stream also brings with it carbon, nutrients and tropical fish. It supplies warm waters along the coast that impact a multitude of important climate phenomena including hurricane intensification, winter storm formation and moderate European weather. The Gulf Stream includes the bulk of what we call the upper limb of the thermohaline circulation in the subtropical Atlantic, in addition to a strong wind-driven flow. As the Gulf Stream flows northward, it loses heat to the atmosphere until eventually in the subpolar North Atlantic some of it becomes cold enough to sink to the bottom of the ocean. This cold deep water then returns southward along the continental slope of the eastern United States as the Deep Western Boundary Current, completing the circuit of the overturning circulation.

Off the coast of Florida, the Gulf Stream is referred to as the Florida Current and is fortuitously confined within the limited geographic channel between Florida and the Bahamas Islands, thus making a long-term observing system cost effective and sustainable. Similarly, the Deep Western Boundary Current is located within several hundred miles to the east of the Abaco Island, Grand Bahamas. The convenient geometry of the Bahamas Island chain thus allows an effective choke point for establishing a long term monitoring program of this deep limb of the overturning circulation.

Background and FAQ:

Background: Ocean Circulation and Climate Variability

Numerical modeling, paleoclimatic and observational studies indicate that both the wind- and thermohaline driven components of the total ocean circulation can play important roles in driving climate variability at time-scales greater than interannual. Thus to meet NOAA Climate Goal objectives it is critical to have in place a global observing system that can be used to forecast, nowcast, hindcast, attribute, and detect oceanic climate change at these time scales as well as to provide data to initialize, calibrate and validate oceanic forecast models. NOAA's Office of Climate Observations (OCO) recognizes this need with the goal "to build and sustain the ocean component of a global climate observing system". NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML) is working together with OCO partners to achieve this goal. Specifically, funded by OAR and OGP we are generating an integrated suite of ocean observations that will characterize the present state of the ocean, provide early warning signs for potential climate change and be used for model initialization and validation.

Currently, the AOML observations discussed here fall into four categories: Atlantic meridional heat transport, global surface currents, global heat storage and Atlantic boundary currents variability. Oceanic meridional heat flux is a critical element of the earth's climate system as it contributes to balancing the global air-sea energy budget. Each category can stand alone as an indicator of the state of the ocean, but more importantly, they complement each other in such a way as to provide critical information on the role of the ocean in climate change. For instance, estimates of the rate of heat storage change between meridional heat transport sections can serve to validate the latter values while providing a measure of the energy transfer between the air and sea. Wind-driven Ekman transports obtained from global drifter observations are an important component of meridional heat flux in many portions of the ocean. Boundary currents play an important role in closing the transports balance. In addition, tracking the properties of Ekman transports and boundary currents, while not providing a measure of the total oceanic meridional heat transport at a particular, will offer alerts to major changes in these features and potential feedbacks to the atmosphere. Finally, many of the observations being provided have been collected for many decades. Thus, they allow for calculation of such properties as the mean and annual cycle of western boundary current transports and heat storage changes. This information will be invaluable in evaluating the ability of coupled forecast models to simulate the present oceanic climate and ultimately forecast future ocean variability.

Oceanographers frequently decompose the total oceanic circulation into two components: a wind-driven component existing primarily in the horizontal planeand a thermohaline-driven component (definition link) existing primarily in the vertical plane (also called the meridional overturning circulation, MOC. Although this decomposition is a simplification of the dynamics of the motion in the ocean (the two components are not separable in the complete equations of motion), it provides a framework for describing responses of the ocean to different surface forcing functions.

Numerical modeling, paleoclimate and observational studies indicate that both the wind-driven and thermohaline circulation can play an important role in longer-term (greater than decadal) climate variability. The U.S. National Oceanic and Atmospheric Administration addresses both components to satisfy its missions of detecting, attributing and forecasting long-term climate change. We contribute to NOAA's mission by developing and providing observational benchmarks (i.e., indices) for various components of the wind-driven circulation (hereinafter WDC) and MOC in the Atlantic Ocean.

Many early NOAA programs (e.g. STACS, ACCP) were searching for indices of critical North Atlantic WDC and MOC features to monitor. Although not originally NOAA programs, other studies have considered the contribution of southern hemisphere features to the MOC. For continuity of the upper layer limb of the MOC, exchanges are required: from the Indian Ocean to the South Atlantic; across the South Atlantic; and across the equator. The inter-ocean exchange takes place through the Benguela/Agulhas system, south of South Africa. The Agulhas Current at its retroflection sheds energetic rings that carry salt and warm water into the South Atlantic. Satellite altimetric measurements have been calibrated to provide estimates of the transport of the Agulhas Current and the separated rings. The extension of the Benguela Current brings the Indian Ocean waters to the central South Atlantic as it flow northwestward in the South Atlantic subtropical gyre.

The pathways of the upper limb MOC transport are then complicated by the wind-driven circulation features along the western boundary and the interior tropical Atlantic (i.e., equatorial upwelling, off-equatorial down welling, zonal currents), that provide obstacles for this limb to move from the South Atlantic to the North Atlantic. Currently, there is insufficient understanding and data to identify precisely these pathways. However numerical models do provide some initial guidance. Using an eddy-resolving numerical circulation model, (Fratantoni et al., 2000) concluded that 14 Sv of upper limb MOC flow is partitioned among three pathways connecting the equatorial and tropical wind-driven gyre: a frictional western boundary current accounting for 6.8 Sv; a diapycnal pathway involving wind- forced equatorial upwelling and interior Ekman transport, 4.2 Sv; and North Brazil Current (NBC) rings shed at the NBC retroflection, 3 Sv. The results of an AOML, university observational program indicate that previous estimates both in the numbers of rings per year and in their contribution to hemispheric exchanges were low. Based on the results of this work, a monitoring strategy is being developed to monitor ring formation and propagation.

Both the intensity of the subtropical gyre and a component of the warm upper level poleward flow in the North Atlantic are being monitored by submarine cable observations in the Straits of Florida. Similarly, the characteristics of the cold deep return flow are being tracked by research vessel transects across the DWBC east of the Bahamas. In the North Atlantic Ocean, time-series of both the upper layer temperature structure within the subtropical gyre and total water column changes across the basin are being maintained.

The recent history of these and other components of the MOC and WDC motions are characterized by data collected over the past 10 to 50 years. These benchmarks are designed to serve several purposes. Independently these benchmarks serve as indices for (1) the intensity of various components of the MOC and WDC, thereby providing alerts for dramatic changes in these features and (2) verification of the ability of GCM's to simulate the ocean's role in climate variability. Collectively, when assimilated into GCM's they will provide global benchmarks for detection and attribution of climate change. All the benchmarks presently available are shown in this Figure.


What is the MOC (Meridional Overturning Circulation)?

"The thermohaline circulation is a global ocean circulation. It is driven by differences in the density of the sea water which is controlled by temperature (thermal) and salinity (haline). In the North Atlantic it transports warm and salty water to the North. There the water is cooled and sinks into the deep ocean. This newly formed deep water is subsequently exported southward. This slow (~0.1 m/s), but giant circulation has a flow equal to about 100 Amazon Rivers. Together with the Gulfstream it contributes (2/3 and 1/3) to the comparatively warm sea surface temperature along the coast of western Europe and to the relative mild European winters. Once the water are in the deep, they remain from the atmosphere for up to 1000 years." - Broecker, W., Chaotic Climate, Scientific American, November, 62-68, 1995.

Here is an animation that illustrates the global MOC.

For further reading:

The Thermohaline Circulation
Tim Osborn, University of East Anglia Climate Research Unit


What is a performance measurement?

A performance measure is a structured statement describing how progress in a scientific program will be evaluated. Performance measures consist of four parts: indicator, unit of measure, baseline and target. An indicator defines the attribute or characteristic to be measured. The unit of measure describes what is to be measured. A baseline establishes the basis for comparison through an initial collection and analysis of data. A baseline should include both a starting date and level. A target establishes the desired level to be reached in a defined period, usually stated as an improvement over the baseline. Targets are based on research and a thorough understanding of the goal/program and are challenging, worthwhile and achievable (NOAA definition). A metric is any type of measurement used to gauge some quantifiable component of an agency's performance. NOAA/OGPs Office of Climate Observations (OCO) currently has several performance measures in place.