scholarly journals Improvements in LICOM2. Part I: Vertical Mixing

2014 ◽  
Vol 31 (2) ◽  
pp. 531-544 ◽  
Author(s):  
Wen-Yu Huang ◽  
Bin Wang ◽  
Yong-Qiang Yu ◽  
Li-Juan Li
Keyword(s):  
Pacific Ocean ◽  
North Atlantic Ocean ◽  
Vertical Mixing ◽  
Mesoscale Eddy ◽  
Warm Pool ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Computational Stability ◽  
Atmospheric Sciences ◽  
The North

Abstract Better computational stability is achieved in an improved version of the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG)/Institute of Atmospheric Physics (IAP) Climate Ocean Model, version 2 (LICOM2, the standard version), after improvements to the implementations of the vertical mixing, mesoscale eddy parameterization, and bottom drag schemes. The large warm biases of LICOM2 in the western Pacific Ocean and eastern Indian Ocean warm pool and on the east coast of the Pacific Ocean are significantly improved. The salinity bias in the tropical Pacific Ocean related to the warm bias of the warm pool is also alleviated. The simulation of the Atlantic meridional overturning circulation is improved because of enhanced vertical mixing in the high latitudes of the North Atlantic Ocean. The new version also presents a stronger Deacon cell, and thus a more powerful Antarctic Circumpolar Current that is closer to the observation, due to weaker southward mesoscale eddy transport in the Southern Ocean.

AMOC response to changing resolution in the Finite-volumE Sea ice–Ocean Model

2020 ◽  
Author(s):  
Dmitry Sidorenko ◽  
Sergey Danilov ◽  
Nikolay Koldunov ◽  
Patrick Scholz
Keyword(s):  
Sea Ice ◽  
Finite Volume ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Ocean Model ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Meridional Transport ◽  
The North ◽  
The North Atlantic

<p>The Atlantic meridional overturning circulation (AMOC) is the most common diagnostics of numerical simulations. Generally it is computed as a streamfunction of zonally averaged flow along the constant depth. More rarely it is computed as zonally averaged along constant isopycnals. The latter computation, however, allows one to better distinguish between water masses and physical processes contributing to the meridional transport. We analyze the AMOC in global simulations based on the Finite-volumE Sea ice–Ocean Model (FESOM 2.0) using eddy permitting to eddy resolving configurations in the North Atlantic. We (1) split the AMOC computed in density space into the constitutes induced by surface buoyancy fluxes and cross isopycnal transformations, (2) identify the water masses which contribute to the formation of the North Atlantic Deep Water and (3) study the AMOC response to the permitting or resolving eddies in the North Atlantic ocean.</p>

Temporal Changes in Ocean Eddy Transports

2006 ◽  
Vol 36 (3) ◽  
pp. 543-550 ◽  
Author(s):  
Detlef Stammer ◽  
Carl Wunsch ◽  
Kyozo Ueyoshi
Keyword(s):  
Pacific Ocean ◽  
Kinetic Energy ◽  
Ocean Circulation ◽  
North Pacific ◽  
North Atlantic Ocean ◽  
North Pacific Ocean ◽  
Mean Value ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Decadal Scale

Abstract New estimates from 11 yr of altimetric data are made of the global time-average variability kinetic energy and its decadal-scale variability. Making the approximation that the variability reflects primarily eddy motions, a time-mean, but spatially varying, eddy mixing coefficient is then estimated along with its changes over the last decade. With a record length more than 2 times that previously available, the time-mean variability kinetic energy KE is statistically more reliable and smoother in its spatial pattern. Minimum values of KE are present in the subpolar North Pacific Ocean and in the eastern South Pacific (both less than 100 cm2 s−2). In contrast to the North Pacific, the subpolar North Atlantic Ocean shows relatively enhanced KE. Eddy kinetic energy and eddy mixing appear to have declined during the last decade over large parts of the western Pacific Ocean, in some regions by as much as 50% of the time-mean value. Increased eddy variability can be found in the Kuroshio and Gulf Stream regions, as well as in the Agulhas region, east of Australia, and at several locations along the Antarctic Circumpolar Current. Somewhat enhanced eddy variability and eddy mixing are also apparent in the eastern tropical Pacific. A numerical simulation of the ocean circulation at 1° spatial resolution over a 10-yr period suggests that variations in eddy mixing of this order of magnitude measurably affect the deep temperature field in the vicinity of permanent frontal structures on a time scale of less than 4 yr. The meridional overturning circulation also reacts on these time scales. If persistent over longer periods in the ocean, these effects would be important for climate simulations.

scholarly journals Fast Northward Energy Transfer in the Atlantic due to Agulhas Rings

2007 ◽  
Vol 37 (9) ◽  
pp. 2305-2315 ◽  
Author(s):  
Erik van Sebille ◽  
Peter Jan van Leeuwen
Keyword(s):  
Atlantic Ocean ◽  
Transit Time ◽  
Wave Energy ◽  
North Atlantic Ocean ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Western Coast ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning

Abstract The adiabatic transit time of wave energy radiated by an Agulhas ring released in the South Atlantic Ocean to the North Atlantic Ocean is investigated in a two-layer ocean model. Of particular interest is the arrival time of baroclinic energy in the northern part of the Atlantic, because it is related to variations in the meridional overturning circulation. The influence of the Mid-Atlantic Ridge is also studied, because it allows for the conversion from barotropic to baroclinic wave energy and the generation of topographic waves. Barotropic energy from the ring is present in the northern part of the model basin within 10 days. From that time, the barotropic energy keeps rising to attain a maximum 500 days after initiation. This is independent of the presence or absence of a ridge in the model basin. Without a ridge in the model, the travel time of the baroclinic signal is 1300 days. This time is similar to the transit time of the ring from the eastern to the western coast of the model basin. In the presence of the ridge, the baroclinic signal arrives in the northern part of the model basin after approximately 10 days, which is the same time scale as that of the barotropic signal. It is apparent that the ridge can facilitate the energy conversion from barotropic to baroclinic waves and the slow baroclinic adjustment can be bypassed. The meridional overturning circulation, parameterized in two ways as either a purely barotropic or a purely baroclinic phenomenon, also responds after 1300 days. The ring temporarily increases the overturning strength. The presence of the ridge does not alter the time scales.

scholarly journals Wind-driven transport of fresh shelf water into the upper 30 m of the Labrador Sea

Ocean Science ◽  
2018 ◽  
Vol 14 (5) ◽  
pp. 1247-1264 ◽  
Author(s):  
Lena M. Schulze Chretien ◽  
Eleanor Frajka-Williams
Keyword(s):  
North Atlantic ◽  
Surface Waters ◽  
Deep Convection ◽  
Greenland Ice Sheet ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Labrador Sea ◽  
The North ◽  
The Impact

Abstract. The Labrador Sea is one of a small number of deep convection sites in the North Atlantic that contribute to the meridional overturning circulation. Buoyancy is lost from surface waters during winter, allowing the formation of dense deep water. During the last few decades, mass loss from the Greenland ice sheet has accelerated, releasing freshwater into the high-latitude North Atlantic. This and the enhanced Arctic freshwater export in recent years have the potential to add buoyancy to surface waters, slowing or suppressing convection in the Labrador Sea. However, the impact of freshwater on convection is dependent on whether or not it can escape the shallow, topographically trapped boundary currents encircling the Labrador Sea. Previous studies have estimated the transport of freshwater into the central Labrador Sea by focusing on the role of eddies. Here, we use a Lagrangian approach by tracking particles in a global, eddy-permitting (1/12∘) ocean model to examine where and when freshwater in the surface 30 m enters the Labrador Sea basin. We find that 60 % of the total freshwater in the top 100 m enters the basin in the top 30 m along the eastern side. The year-to-year variability in freshwater transport from the shelves to the central Labrador Sea, as found by the model trajectories in the top 30 m, is dominated by wind-driven Ekman transport rather than eddies transporting freshwater into the basin along the northeast.

scholarly journals Investigating the impact of Lake Agassiz drainage routes on the 8.2 ka cold event with a climate model

2009 ◽  
Vol 5 (3) ◽  
pp. 471-480 ◽  
Author(s):  
Y.-X. Li ◽  
H. Renssen ◽  
A. P. Wiersma ◽  
T. E. Törnqvist
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Climate Model ◽  
North Atlantic Ocean ◽  
Sensitivity Study ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Cold Event ◽  
The North ◽  
The Impact

Abstract. The 8.2 ka event is the most prominent abrupt climate change in the Holocene and is often believed to result from catastrophic drainage of proglacial lakes Agassiz and Ojibway (LAO) that routed through the Hudson Bay and the Labrador Sea into the North Atlantic Ocean, and perturbed Atlantic meridional overturning circulation (MOC). One key assumption of this triggering mechanism is that the LAO freshwater drainage was dispersed over the Labrador Sea. Recent data, however, show no evidence of lowered δ18O values, indicative of low salinity, from the open Labrador Sea around 8.2 ka. Instead, negative δ18O anomalies are found close to the east coast of North America, extending as far south as Cape Hatteras, North Carolina, suggesting that the freshwater drainage may have been confined to a long stretch of continental shelf before fully mixing with North Atlantic Ocean water. Here we conduct a sensitivity study that examines the effects of a southerly drainage route on the 8.2 ka event with the ECBilt-CLIO-VECODE model. Hosing experiments of four routing scenarios, where freshwater was introduced to the Labrador Sea in the northerly route and to three different locations along the southerly route, were performed to investigate the routing effects on model responses. The modeling results show that a southerly drainage route is possible but generally yields reduced climatic consequences in comparison to those of a northerly route. This finding implies that more freshwater would be required for a southerly route than for a northerly route to produce the same climate anomaly. The implicated large amount of LAO drainage for a southerly routing scenario is in line with a recent geophysical modelling study of gravitational effects on sea-level change associated with the 8.2 ka event, which suggests that the volume of drainage might be larger than previously estimated.

scholarly journals Hurricane Katrina (2005). Part II: Evolution and Hemispheric Impacts of a Diabatically Generated Warm Pool

2007 ◽  
Vol 135 (12) ◽  
pp. 3927-3949 ◽  
Author(s):  
Ron McTaggart-Cowan ◽  
Lance F. Bosart ◽  
John R. Gyakum ◽  
Eyad H. Atallah
Keyword(s):  
United States ◽  
Hurricane Katrina ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Flow Analysis ◽  
Warm Pool ◽  
The United States ◽  
The North ◽  
The North Atlantic ◽  
Upper Level

Abstract The landfall of Hurricane Katrina (2005) near New Orleans, Louisiana, on 29 August 2005 will be remembered as one of the worst natural disasters in the history of the United States. By comparison, the extratropical transition (ET) of the system as it accelerates poleward over the following days is innocuous and the system weakens until its eventual demise off the coast of Greenland. The extent of Katrina’s perturbation of the midlatitude flow would appear to be limited given the lack of reintensification or downstream development during ET. However, the slow progression of a strong upper-tropospheric warm pool across the North Atlantic Ocean in the week following Katrina’s landfall prompts the question of whether even a nonreintensifying ET event can lead to significant modification of the midlatitude flow. Analysis of Hurricane Katrina’s outflow layer after landfall suggests that it does not itself make up the long-lived midlatitude warm pool. However, the interaction between Katrina’s anticyclonic outflow and an approaching baroclinic trough is shown to establish an anomalous southwesterly conduit or “freeway” that injects a preexisting tropospheric warm pool over the southwestern United States into the midlatitudes. This warm pool reduces predictability in medium-range forecasts over the North Atlantic and Europe while simultaneously aiding in the development of Hurricanes Maria and Nate. The origin of the warm pool is shown to be the combination of anticyclonic upper-level features generated by eastern Pacific Hurricane Hilary and the south Asian anticyclone (SAA). The hemispheric nature of the connections involved with the development of the warm pool and its injection into the extratropics has an impact on forecasting, since the predictability issue associated with ET in this case involves far more than the potential reintensification of the transitioning system itself.

scholarly journals Changes in extreme regional sea surface height due to an abrupt weakening of the Atlantic meridional overturning circulation

Ocean Science ◽  
2014 ◽  
Vol 10 (6) ◽  
pp. 881-891 ◽  
Author(s):  
S.-E. Brunnabend ◽  
H. A. Dijkstra ◽  
M. A. Kliphuis ◽  
B. van Werkhoven ◽  
H. E. Bal ◽  
...  
Keyword(s):  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Sea Surface Height ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Sea Level Changes ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning

Abstract. As an extreme scenario of dynamical sea level changes, regional sea surface height (SSH) changes that occur in the North Atlantic due to an abrupt weakening of the Atlantic meridional overturning circulation (AMOC) are simulated. Two versions of the same ocean-only model are used to study the effect of ocean model resolution on these SSH changes: a high-resolution (HR) strongly eddying version and a low-resolution (LR) version in which the effect of eddies is parameterised. The weakening of the AMOC is induced in both model versions by applying strong freshwater perturbations around Greenland. A rapid decrease of the AMOC in the HR version induces much shorter return times of several specific regional and coastal extremes in North Atlantic SSH than in the LR version. This effect is caused by a change in main eddy pathways associated with a change in separation latitude of the Gulf Stream.

scholarly journals Millennial Variability in an Idealized Ocean Model: Predicting the AMOC Regime Shifts

2014 ◽  
Vol 27 (10) ◽  
pp. 3551-3564 ◽  
Author(s):  
Florian Sévellec ◽  
Alexey V. Fedorov
Keyword(s):  
Ocean Circulation ◽  
Salient Feature ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Small Perturbations ◽  
Ocean Stratification ◽  
The North ◽  
Chaotic Model ◽  
Meridional Overturning

Abstract A salient feature of paleorecords of the last glacial interval in the North Atlantic is pronounced millennial variability, commonly known as Dansgaard–Oeschger events. It is believed that these events are related to variations in the Atlantic meridional overturning circulation and heat transport. Here, the authors formulate a new low-order model, based on the Howard–Malkus loop representation of ocean circulation, capable of reproducing millennial variability and its chaotic dynamics realistically. It is shown that even in this chaotic model changes in the state of the meridional overturning circulation are predictable. Accordingly, the authors define two predictive indices which give accurate predictions for the time the circulation should remain in the on phase and then stay in the subsequent off phase. These indices depend mainly on ocean stratification and describe the linear growth of small perturbations in the system. Thus, monitoring particular indices of the ocean state could help predict a potential shutdown of the overturning circulation.

scholarly journals Predictability and Decadal Variability of the North Atlantic Ocean State Evaluated from a Realistic Ocean Model

2017 ◽  
Vol 30 (2) ◽  
pp. 477-498 ◽  
Author(s):  
Florian Sévellec ◽  
Alexey V. Fedorov
Keyword(s):  
Atlantic Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Decadal Variability ◽  
North Atlantic Ocean ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
The North ◽  
The North Atlantic

This study investigates the excitation of decadal variability and predictability of the ocean climate state in the North Atlantic. Specifically, initial linear optimal perturbations (LOPs) in temperature and salinity that vary with depth, longitude, and latitude are computed, and the maximum impact on the ocean of these perturbations is evaluated in a realistic ocean general circulation model. The computations of the LOPs involve a maximization procedure based on Lagrange multipliers in a nonautonomous context. To assess the impact of these perturbations four different measures of the North Atlantic Ocean state are used: meridional volume and heat transports (MVT and MHT) and spatially averaged sea surface temperature (SST) and ocean heat content (OHC). It is shown that these metrics are dramatically different with regard to predictability. Whereas OHC and SST can be efficiently modified only by basin-scale anomalies, MVT and MHT are also strongly affected by smaller-scale perturbations. This suggests that instantaneous or even annual-mean values of MVT and MHT are less predictable than SST and OHC. Only when averaged over several decades do the former two metrics have predictability comparable to the latter two, which highlights the need for long-term observations of the Atlantic meridional overturning circulation in order to accumulate climatically relevant data. This study also suggests that initial errors in ocean temperature of a few millikelvins, encompassing both the upper and deep ocean, can lead to ~0.1-K errors in the predictions of North Atlantic sea surface temperature on interannual time scales. This transient error growth peaks for SST and OHC after about 6 and 10 years, respectively, implying a potential predictability barrier.

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