The Atlantic meridional overturning circulation (MOC), which provides one-quarter of the global meridional heat transport, is composed of a number of separate flow components. How changes in the ...strength of each of those components may affect that of the others has been unclear because of a lack of adequate data. We continuously observed the MOC at 26.5 degrees N for 1 year using end-point measurements of density, bottom pressure, and ocean currents; cable measurements across the Straits of Florida; and wind stress. The different transport components largely compensate for each other, thus confirming the validity of our monitoring approach. The MOC varied over the period of observation by +/-5.7 x 10(6) cubic meters per second, with density-inferred and wind-driven transports contributing equally to it. We find evidence for depth-independent compensation for the wind-driven surface flow.
The advective transit time of temperature-salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, because it ...has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult, because most observational and high-resolution model data are too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of "supertrajectories" of any required length using a Monte Carlo simulation. These supertrajectories allow analysis of the circulation patterns and time scales based on Lagrangian data: in this case, observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OGCM for the Earth Simulator (OFES). The observational supertrajectories can only be used to study the two-dimensional (2D) surface flow, whereas the numerical supertrajectories can be used to study the full three-dimensional circulation. Results for the surface circulation indicate that the supertrajectories starting in the Agulhas Current and ending in the North Atlantic take at least 4 yr and most complete the journey in 30-40 yr. This time scale is, largely because of convergence and subduction in the subtropical gyres, longer than the 10-25 yr it takes the 3D numerical supertrajectories to complete the journey. PUBLICATION ABSTRACT
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The three‐dimensional circulation of the Red Sea is studied using a set of Miami Isopycnic Coordinate Ocean Model (MICOM) simulations. The model performance is tested against the few available ...observations in the basin and shows generally good agreement with the main observed features of the circulation. The main findings of this analysis include an intensification of the along‐axis flow toward the coasts, with a transition from western intensified boundary flow in the south to eastern intensified flow in the north, and a series of strong seasonal or permanent eddy‐like features. Model experiments conducted with different forcing fields (wind‐stress forcing only, surface buoyancy forcing only, or both forcings combined) showed that the circulation produced by the buoyancy forcing is stronger overall and dominates the wind‐driven part of the circulation. The main circulation pattern is related to the seasonal buoyancy flux (mostly due to the evaporation), which causes the density to increase northward in the basin and produces a northward surface pressure gradient associated with the downward sloping of the sea surface. The response of the eastern boundary to the associated mean cross‐basin geostrophic current depends on the stratification and β‐effect. In the northern part of the basin this results in an eastward intensification of the northward surface flow associated with the presence of Kelvin waves while in the south the traditional westward intensification due to Rossby waves takes place. The most prominent gyre circulation pattern occurs in the north where a permanent cyclonic gyre is present that is involved in the formation of Red Sea Outflow Water (RSOW). Beneath the surface boundary currents are similarly intensified southward undercurrents that carry the RSOW to the sill to flow out of the basin into the Indian Ocean.
Results from two 1/12° eddy‐resolving simulations, together with data‐based transport estimates at 26.5°N and 41°N, are used to investigate the temporal variability of the Atlantic meridional ...overturning circulation (AMOC) during 2004–2012. There is a good agreement between the model and the observation for all components of the AMOC at 26.5°N, whereas the agreement at 41°N is primarily due to the Ekman transport. We found that (1) both observations and model results exhibit higher AMOC variability on seasonal and shorter time scales than on interannual and longer time scales; (2) on intraseasonal and interannual time scales, the AMOC variability is often coherent over a wide latitudinal range, but lacks an overall consistent coherent pattern over the entire North Atlantic; and (3) on seasonal time scales, the AMOC variability exhibits two distinct coherent regimes north and south of 20°N, due to different wind stress variability in the tropics and subtropics. The high AMOC variability south of 20°N in the tropical Atlantic comes primarily from the Ekman transport of the near‐surface water, and is modulated to some extent by the transport of the Antarctic Intermediate water below the thermocline. These results highlight the importance of the surface wind in driving the AMOC variability.
Key Points
The AMOC exhibits high variability on seasonal and shorter time scales
The intraseasonal and interannual AMOC variability lack a coherent pattern
The seasonal AMOC variability exhibits two distinct coherent regimes
The Atlantic meridional overturning circulation (MOC), which provides one-quarter of the global meridional heat transport, is composed of a number of separate flow components. How changes in the ...strength of each of those components may affect that of the others has been unclear because of a lack of adequate data. We continuously observed the MOC at 26.5°N for 1 year using end-point measurements of density, bottom pressure, and ocean currents; cable measurements across the Straits of Florida; and wind stress. The different transport components largely compensate for each other, thus confirming the validity of our monitoring approach. The MOC varied over the period of observation by ±5.7 x 10⁶ cubic meters per second, with density-inferred and wind-driven transports contributing equally to it. We find evidence for depth-independent compensation for the wind-driven surface flow.
The structure and variability of the Florida Current between 25° and 26°N are investigated using HF radar ocean current measurements to provide the most detailed view of the surface jet to date. A ...2‐D jet coordinate analysis is performed to define lateral displacements of the jet in time (meandering), and associated structural variations over a 2 year period (2005–2006). In the jet coordinate frame, core speed has a median value of ∼160 cm s−1 at the central latitude of the array (25.4°N), with a standard deviation (STD) of 35 cm s−1. The jet meanders at timescales of 3–30 days, with a STD of 8 km, and a downstream phase speed of ∼80 km d−1. Meandering accounts for ∼45% of eddy kinetic energy computed in a fixed (geographical) reference frame. Core speed, width, and shear undergo the same dominant 3–30 day variability, plus an annual cycle that matches seasonality of alongshore wind stress. Jet transport at 25.4°N exhibits a different seasonality to volume transport at 27°N, most likely driven by input from the Northwest Providence Channel. Core speed correlates inversely with Miami sea level fluctuations such that a 40 cm s−1 deceleration is associated with a ∼10 cm elevation in sea level, although there is no correlation of sea level to jet meandering or width. Such accurate quantification of the Florida Current's variability is critical to understand and forecast future changes in the climate system of the North Atlantic, as well as local impacts on coastal circulation and sea level variability along south Florida's coastline.
Key Points
The Florida Current surface jet exhibits dominant variability from 3 to 30 days in meandering, core speed, width, and shear
The jet structure undergoes a robust annual cycle in core speed, width, and shear, corresponding to local wind stress
The annual cycle of jet transport is very different between 25.5°N and 27°N, likely driven by input from the Northwest Providence Channel
Observations are used to develop metrics for interannual tropical instability wave (TIW) variability in the Atlantic and to relate that variability to larger scale processes. The analysis is ...partitioned into different latitude bands to distinguish between off‐equatorial (5°S, 2°N, and 5°N) and near‐equatorial (2°S and 0°) TIWs. TIW metrics based on sea surface temperature (SST) and sea level anomaly (SLA) fluctuations are compared against interannual anomalies of SST in the cold tongue region. To examine the role of barotropic shear instabilities in modulating the intensity of a TIW season, wind stress and near‐surface current indices are developed in regions where the shear between the Equatorial Undercurrent (EUC) and the northern branch of the South Equatorial Current (nSEC) and between the nSEC and the North Equatorial Countercurrent (NECC) are expected to be largest. Good agreement is found between the SST and SLA TIW metrics along the off‐equatorial latitude bands, and interannual variations of both metrics can largely be attributed to barotropic shear instabilities. In particular, years with low (high) TIW variance along the off‐equatorial latitude bands are associated with anomalously warm (cold) SSTs in the cold tongue region, weak (strong) wind stress divergence and curl in the EUC‐nSEC region, and weak (strong) zonal current shear in the nSEC‐NECC region. In contrast, in the near‐equatorial latitude bands, poor agreement is found between interannual TIW activity based on the SST and SLA metrics, and near‐equatorial TIW variability cannot be explained by the large‐scale SST, wind stress divergence and curl, and current shear indices.
Key Points
SST and SLA metrics are developed for interannual Atlantic TIW variability
For off‐equatorial TIWs, SLA and SST metrics agree well with one another
Off‐equatorial interannual TIW variance attributed to barotropic instabilities
On the Atlantic inflow to the Caribbean Sea Johns, William E.; Townsend, Tamara L.; Fratantoni, David M. ...
Deep-sea research. Part I, Oceanographic research papers,
02/2002, Letnik:
49, Številka:
2
Journal Article
Recenzirano
New observations are summarized that lead to the first comprehensive description of the mean inflow distribution in the passages connecting the Atlantic Ocean with the Caribbean Sea. The total ...Caribbean inflow of
28
Sv
is shown to be partitioned approximately equally between the Windward Islands Passages (∼
10
Sv
), Leeward Islands Passages (∼
8
Sv)
, and the Greater Antilles Passages (∼
10
Sv)
. These results are compared to a numerical model study using a 6-layer, 1/4° resolution Atlantic Basin version of the NRL Layered Ocean Model. Results from two simulations are described, including a purely wind-forced model driven by Hellerman and Rosenstein (J. Phys. Oceanogr. 13 (1983) 1093) monthly winds, and a model with an additional 14 Sv meridional overturning cell driven by inflow/outflow ports at the northern (65°N) and southern (20°S) model boundaries. The purely wind-driven version of the model exhibits a total Caribbean inflow of
17
Sv
, consistent with expectations from steady, non-topographic Sverdrup theory. Nearly all of the wind-driven inflow occurs north of Martinique at latitude ∼15°N. The net transport through the Lesser Antilles passages south of 15°N (Grenada, St. Vincent, and St. Lucia passages) is nearly zero when the model is forced by winds alone. The addition of a
14
Sv
meridional cell in the model increases the net Caribbean inflow to
28
Sv
, with nearly all of the additional 11 Sv of inflow entering through the southern Lesser Antilles passages. The modeled inflow distribution resulting from the combined wind and overturning forced experiment is found to compare favorably with the observations.
The seasonal cycle of the total inflow in the combined forcing experiment has a mixed annual/semiannual character with maximum in spring and summer and minimum in fall, with a total range of about
4
Sv
. The seasonal cycle of the Florida Current resulting from this inflow variation is in good qualitative agreement with observations. Most of the seasonal inflow variation occurs through the Windward Islands passages in the far southern Caribbean, whose annual cycle slightly leads that of the Florida and Yucatan Currents. Variability of the modeled inflow on shorter time scales shows a dramatic change in character moving northward along the Antilles arc. The southern passages exhibit large fluctuations on 30–80 day time scales, which decay to very small amplitudes north of Dominica. Much of this variability is caused by North Brazil Current Rings that propagate northwestward from the equatorial Atlantic and interact with the abrupt island arc topography. The total range of transport variability in individual passages predicted by the model is consistent with observations. However, observations are presently too limited to confirm the seasonal cycles or variability spectra in the Caribbean passages.
This study aims to explore the relationship between air–sea density flux and isopycnal meridional overturning circulation (MOC), using the Intergovernmental Panel on Climate Change (IPCC) Fourth ...Assessment Report (AR4) model projections of the twenty-first-century climate. The focus is on the semiadiabatic component of MOC beneath the mixed layer; this component is described using the concept of the push–pull mode, which represents the combined effects of the adiabatic push into the deep ocean in the Northern Hemisphere and the pull out of the deep ocean in the Southern Hemisphere. The analysis based on the GFDL Climate Model version 2.1 (CM2.1) simulation demonstrates that the push–pull mode and the actual isopycnal MOC at the equator evolve similarly in the deep layers, with their maximum transports decreasing by 4–5 Sv (1 Sv ≡ 10⁶ m³ s−1) during years 2001–2100. In particular, the push–pull mode and actual isopycnal MOC are within approximately 10% of each other at the density layers heavier than 27.55 kg m−3, where the reduction in the MOC strength is the strongest. The decrease in the push–pull mode is caused by the direct contribution of the anomalous heat, rather than freshwater, surface fluxes. The agreement between the deep push–pull mode and MOC in the values of linear trend and variability on time scales longer than a decade suggests a largely adiabatic pole-to-pole mechanism for these changes. The robustness of the main conclusions is further explored in additional model simulations.
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Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
A pilot experiment using an array of 45 drifters to explore the circulation in the north and central Aegean Sea is described. The global positioning system drifters with holey-sock drogues ...provide positions every hour with data recovery through the Argos system. The drifters were launched in four separate deployments over a 1-yr period. The resulting trajectories confirm the existence of a current around the rim of the basin consistent with a buoyancy plume created by the outflow of Black Sea waters through the Dardanelles (Strait of Çanakkale in Turkish). The degree to which this is augmented by an Ekman response to the dominant northerly winds is not obvious in the dataset owing to mesoscale dynamics that obscure the existence of any westward Ekman flow. The mesoscale eddy field involves anticylonic eddies in the current around the rim of the basin consistent with eddies with low-salinity-water cores. Cyclones are also seen, with the most prominent forming over deep regions in the basin topography. The array also documents the interaction of the currents with the straits through the Sporades and Cyclades island groups. These interactions are complicated by the nature of the mesoscale flow and in some trajectories suggest a Bernouilli acceleration in straits; in others the flow through the island groups appears to be more diffusive and involves deceleration and eddy motions. The rapid sampling by the drifters reveals an extremely nonlinear submesoscale eddy field in the basin with length scales less than 4 km and Rossby numbers of order 1. A better understanding of the dynamics of these features is of importance for understanding the circulation of the basin.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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