The Atlantic Meridional Mode and Its Coupled Variability with the Guinea Dome

2010 ◽  
Vol 23 (2) ◽  
pp. 455-475 ◽  
Author(s):  
Takeshi Doi ◽  
Tomoki Tozuka ◽  
Toshio Yamagata
Keyword(s):  
Mixed Layer ◽  
General Circulation ◽  
Surface Wind ◽  
Circulation Model ◽  
Late Fall ◽  
Sst Anomaly ◽  
Meridional Mode ◽  
Coupled Variability ◽  
Ekman Upwelling ◽  
Atlantic Meridional Mode

Abstract Using an ocean–atmosphere coupled general circulation model, air–sea interaction processes associated with the Atlantic meridional mode are investigated from a new viewpoint of its link with the Guinea Dome in the northern tropical Atlantic. The subsurface thermal oceanic dome develops off Dakar from late spring to late fall owing to wind-induced Ekman upwelling. Its seasonal evolution is due to surface wind variations associated with the northward migration of the intertropical convergence zone (ITCZ). Since the upwelling cools the mixed layer in the Guinea Dome region during summer, it is very important to reproduce its variability in order to simulate the sea surface temperature (SST) there. During the preconditioning phase of the positive (negative) Atlantic meridional mode, the dome is anomalously weak (strong) and the mixed layer is anomalously deep (shallow) there in late fall. This condition reduces (enhances) the sensitivity of the mixed layer temperature to the climatological atmospheric cooling. As a result, the positive (negative) SST anomaly appears there in early winter. Then, it develops in the following spring through the wind–evaporation–SST (WES) positive feedback associated with the anomalous northward (southward) migration of the ITCZ. This, in turn, leads to the stronger (weaker) Ekman upwelling and colder (warmer) subsurface temperature in the dome region during summer. It plays an important role on the decay of the warm (cold) SST anomaly through entrainment as a negative feedback. Therefore, simulating this interesting air–sea interaction in the Guinea Dome region is critical in improving prediction skill for the Atlantic meridional mode.

scholarly journals Generation Mechanism of the South Pacific Subtropical Dipole

2013 ◽  
Vol 26 (16) ◽  
pp. 6033-6045 ◽  
Author(s):  
Yushi Morioka ◽  
J. V. Ratnam ◽  
Wataru Sasaki ◽  
Yukio Masumoto
Keyword(s):  
Mixed Layer ◽  
General Circulation ◽  
Surface Wind ◽  
Circulation Model ◽  
South Pacific ◽  
Shortwave Radiation ◽  
Central Basin ◽  
The South ◽  
Sst Anomaly ◽  
Sst Anomalies

Abstract Distinct pattern of interannual variability in sea surface temperature (SST) in the South Pacific [i.e., the South Pacific subtropical dipole (SPSD)] is examined using outputs from a coupled general circulation model. The SPSD appears as the second empirical orthogonal function (EOF) mode of the SST anomalies in the South Pacific and is associated with a northeast–southwest-oriented dipole of positive and negative SST anomalies in the central basin. The positive and negative SST anomaly poles start to develop during austral spring, reach their peak during austral summer, and gradually decay afterward. Close examination of mixed-layer heat balance yields that the SST anomaly poles develop mainly because warming of the mixed layer by shortwave radiation is modulated by the anomalous mixed-layer thickness. Over the positive (negative) pole, the mixed layer becomes thinner (thicker) than normal and acts to enhance (reduce) the warming of the mixed layer by climatological shortwave radiation. This thinner (thicker) mixed layer may be related to the suppressed (enhanced) evaporation associated with the overlying sea level pressure (SLP) anomalies. Weaker-than-normal surface wind also contributes to the thinner mixed layer in the case of the positive pole. Furthermore, the SLP anomalies are linked with the geopotential height anomalies in the upper troposphere and are associated with a stationary Rossby wave pattern along the westerly jet in the midlatitudes. This suggests that the SLP anomalies that generate the SPSD are not locally excited but remotely induced signals.

scholarly journals The Annular Response to Tropical Pacific SST Forcing

2006 ◽  
Vol 19 (9) ◽  
pp. 1802-1819 ◽  
Author(s):  
Shuanglin Li ◽  
Martin P. Hoerling ◽  
Shiling Peng ◽  
Klaus M. Weickmann
Keyword(s):  
General Circulation ◽  
Storm Track ◽  
Circulation Model ◽  
Tropical Pacific ◽  
Transient Eddy ◽  
West Pacific ◽  
Core Energy ◽  
The Pacific ◽  
Sst Forcing ◽  
Sst Anomaly

Abstract The leading pattern of Northern Hemisphere winter height variability exhibits an annular structure, one related to tropical west Pacific heating. To explore whether this pattern can be excited by tropical Pacific SST variations, an atmospheric general circulation model coupled to a slab mixed layer ocean is employed. Ensemble experiments with an idealized SST anomaly centered at different longitudes on the equator are conducted. The results reveal two different response patterns—a hemispheric pattern projecting on the annular mode and a meridionally arched pattern confined to the Pacific–North American sector, induced by the SST anomaly in the west and the east Pacific, respectively. Extratropical air–sea coupling enhances the annular component of response to the tropical west Pacific SST anomalies. A diagnosis based on linear dynamical models suggests that the two responses are primarily maintained by transient eddy forcing. In both cases, the model transient eddy forcing response has a maximum near the exit of the Pacific jet, but with a different meridional position relative to the upper-level jet. The emergence of an annular response is found to be very sensitive to whether transient eddy forcing anomalies occur within the axis of the jet core. For forcing within the jet core, energy propagates poleward and downstream, inducing an annular response. For forcing away from the jet core, energy propagates equatorward and downstream, inducing a trapped regional response. The selection of an annular versus a regionally confined tropospheric response is thus postulated to depend on how the storm tracks respond. Tropical west Pacific SST forcing is particularly effective in exciting the required storm-track response from which a hemisphere-wide teleconnection structure emerges.

scholarly journals Diagnosing Natural Variability of North Atlantic Water Masses in HadCM3

2005 ◽  
Vol 18 (12) ◽  
pp. 1925-1941 ◽  
Author(s):  
Keith Haines ◽  
Chris Old
Keyword(s):  
Mixed Layer ◽  
General Circulation ◽  
Circulation Model ◽  
Surface Heat ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Mode Water ◽  
Surface Heat Fluxes ◽  
Mode Waters ◽  
Winter Mixed Layer

Abstract A study of thermally driven water mass transformations over 100 yr in the ocean component of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) is presented. The processes of surface-forced transformations, subduction and mixing, both above and below the winter mixed layer base, are quantified. Subtropical Mode Waters are formed by surface heat fluxes and subducted at more or less the same rate. However, Labrador Seawater and Nordic Seawater classes (the other main subduction classes) are primarily formed by mixing within the mixed layer with very little formation directly from surface heat fluxes. The Subpolar Mode Water classes are dominated by net obduction of water back into the mixed layer from below. Subtropical Mode Water (18°C) variability shows a cycle of formation by surface fluxes, subduction ∼2 yr later, followed by mixing with warmer waters below the winter mixed layer base during the next 3 yr, and finally obduction back into the mixed layer at 21°C, ∼5 yr after the original formation. Surface transformation of Subpolar Mode Waters, ∼12°C, are led by surface transformations of warmer waters by up to 5 yr as water is transferred from the subtropical gyre. They are also led by obduction variability from below the mixed layer, by ∼2 yr. The variability of obduction in Subpolar Mode Waters also appears to be preceded, by 3–5 yr, by variability in subduction of Labrador Sea Waters at ∼6°C. This supports a mechanism in which southward-propagating Labrador seawater anomalies below the subpolar gyre can influence the upper water circulation and obduction into the mixed layer.

scholarly journals Ocean Eddy Energetics in the Spectral Space as Revealed by High-Resolution General Circulation Models

2019 ◽  
Vol 49 (11) ◽  
pp. 2815-2827
Author(s):  
Shengpeng Wang ◽  
Zhao Jing ◽  
Qiuying Zhang ◽  
Ping Chang ◽  
Zhaohui Chen ◽  
...  
Keyword(s):  
Energy Conversion ◽  
General Circulation ◽  
Surface Wind ◽  
Circulation Model ◽  
General Circulation Models ◽  
Ocean General Circulation Model ◽  
Energy Cascade ◽  
Spectral Space ◽  
Inverse Energy Cascade ◽  
Barotropic Energy Conversion

AbstractIn this study, the global eddy kinetic energy (EKE) budget in horizontal wavenumber space is analyzed based on 1/10° ocean general circulation model simulations. In both the tropical and midlatitude regions, the barotropic energy conversion from background flow to eddies is positive throughout the wavenumber space and generally peaks at the scale (Le) where EKE reaches its maximum. The baroclinic energy conversion is more pronounced at midlatitudes. It exhibits a dipolar structure with positive and negative values at scales smaller and larger than Le, respectively. Surface wind power on geostrophic flow results in a significant EKE loss around Le but deposits energy at larger scales. The interior viscous dissipation and bottom drag inferred from the pressure flux convergence act as EKE sink terms. The latter is most efficient at Le while the former is more dominant at smaller scales. There is an evident mismatch between EKE generation and dissipation in the spectral space especially at the midlatitudes. This is reconciled by a dominant forward energy cascade on the equator and a dominant inverse energy cascade at the midlatitudes.

scholarly journals Numerical investigation of the Arctic ice–ocean boundary layer and implications for air–sea gas fluxes

Ocean Science ◽  
2017 ◽  
Vol 13 (1) ◽  
pp. 61-75 ◽  
Author(s):  
Arash Bigdeli ◽  
Brice Loose ◽  
An T. Nguyen ◽  
Sylvia T. Cole
Keyword(s):  
Gas Exchange ◽  
Sea Ice ◽  
Mixed Layer ◽  
General Circulation ◽  
Mixed Layer Depth ◽  
Circulation Model ◽  
Horizontal Resolution ◽  
The Arctic ◽  
Massachusetts Institute Of Technology ◽  
Institute Of Technology

Abstract. In ice-covered regions it is challenging to determine constituent budgets – for heat and momentum, but also for biologically and climatically active gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we sought to evaluate if numerical model output helps us to better estimate the physical forcing that drives the air–sea gas exchange rate (k) in sea ice zones. We used the budget of radioactive 222Rn in the mixed layer to illustrate the effect that sea ice forcing has on gas budgets and air–sea gas exchange. Appropriate constraint of the 222Rn budget requires estimates of sea ice velocity, concentration, mixed-layer depth, and water velocities, as well as their evolution in time and space along the Lagrangian drift track of a mixed-layer water parcel. We used 36, 9 and 2 km horizontal resolution of regional Massachusetts Institute of Technology general circulation model (MITgcm) configuration with fine vertical spacing to evaluate the capability of the model to reproduce these parameters. We then compared the model results to existing field data including satellite, moorings and ice-tethered profilers. We found that mode sea ice coverage agrees with satellite-derived observation 88 to 98 % of the time when averaged over the Beaufort Gyre, and model sea ice speeds have 82 % correlation with observations. The model demonstrated the capacity to capture the broad trends in the mixed layer, although with a significant bias. Model water velocities showed only 29 % correlation with point-wise in situ data. This correlation remained low in all three model resolution simulations and we argued that is largely due to the quality of the input atmospheric forcing. Overall, we found that even the coarse-resolution model can make a modest contribution to gas exchange parameterization, by resolving the time variation of parameters that drive the 222Rn budget, including rate of mixed-layer change and sea ice forcings.

scholarly journals Tropical Atlantic Mixed Layer Buoyancy Seasonality: Atmospheric and Oceanic Physical Processes Contributions

Atmosphere ◽  
2020 ◽  
Vol 11 (6) ◽  
pp. 649
Author(s):  
Ibrahima Camara ◽  
Juliette Mignot ◽  
Nicolas Kolodziejczyk ◽  
Teresa Losada ◽  
Alban Lazar
Keyword(s):  
Heat Flux ◽  
Mixed Layer ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Brazilian Coast ◽  
Freshwater Flux ◽  
Physical Processes ◽  
Inter Tropical Convergence Zone ◽  
Near Surface

This study investigates the physical processes controlling the mixed layer buoyancy using a regional configuration of an ocean general circulation model. Processes are quantified by using a linearized equation of state, a mixed-layer heat, and a salt budget. Model results correctly reproduce the observed seasonal near-surface density tendencies. The results indicate that the heat flux is located poleward of 10° of latitude, which is at least three times greater than the freshwater flux that mainly controls mixed layer buoyancy. During boreal spring-summer of each hemisphere, the freshwater flux partly compensates the heat flux in terms of buoyancy loss while, during the fall-winter, they act together. Under the seasonal march of the Inter-tropical Convergence Zone and in coastal areas affected by the river, the contribution of ocean processes on the upper density becomes important. Along the north Brazilian coast and the Gulf of Guinea, horizontal and vertical processes involving salinity are the main contributors to an upper water change with a contribution of at least twice as much the temperature. At the equator and along the Senegal-Mauritanian coast, vertical processes are the major oceanic contributors. This is mainly due to the vertical gradient of temperature at the mixed layer base in the equator while the salinity one dominates along the Senegal-Mauritania coast.

Precipitation Response to the Gulf Stream in an Atmospheric GCM*

2010 ◽  
Vol 23 (13) ◽  
pp. 3676-3698 ◽  
Author(s):  
Akira Kuwano-Yoshida ◽  
Shoshiro Minobe ◽  
Shang-Ping Xie
Keyword(s):  
Gulf Stream ◽  
General Circulation ◽  
Narrow Band ◽  
Deep Convection ◽  
Surface Wind ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Convective Precipitation ◽  
Precipitation Response ◽  
Upward Velocity

Abstract The precipitation response to sea surface temperature (SST) gradients associated with the Gulf Stream is investigated using an atmospheric general circulation model. Forced by observed SST, the model simulates a narrow band of precipitation, surface convergence, and evaporation that closely follows the Gulf Stream, much like satellite observations. Such a Gulf Stream rainband disappears in the model when the SST front is removed by horizontally smoothing SST. The analysis herein shows that it is convective precipitation that is sensitive to SST gradients. The Gulf Stream anchors a convective rainband by creating surface wind convergence and intensifying surface evaporation on the warmer flank. Deep convection develops near the Gulf Stream in summer when the atmosphere is conditionally unstable. As a result, a narrow band of upward velocity develops above the Gulf Stream throughout the troposphere in summer, while it is limited to the lower troposphere in other seasons.

Mechanisms of Northern Tropical Atlantic Variability and Response to CO2 Doubling

2007 ◽  
Vol 20 (11) ◽  
pp. 2691-2705 ◽  
Author(s):  
Wim-Paul Breugem ◽  
Wilco Hazeleger ◽  
Reindert J. Haarsma
Keyword(s):  
Moisture Transport ◽  
Seasonal Cycle ◽  
General Circulation ◽  
Circulation Model ◽  
Atmospheric Co2 ◽  
Boreal Winter ◽  
Tropical Atlantic ◽  
Wes Feedback ◽  
Meridional Mode ◽  
Northern Tropical Atlantic

Abstract A model study has been made of the mechanisms of the meridional mode in the northern tropical Atlantic (NTA) and the response to a doubling of atmospheric CO2. The numerical model consists of an atmospheric general circulation model (GCM) coupled to a passive mixed layer model for the ocean. Results from two simulations are shown: a control run with present-day atmospheric CO2 and a run with a doubled CO2 concentration. The results from the control run show that the wind–evaporation–SST (WES) feedback is confined to the deep NTA. Furthermore, the temporal evolution of the meridional mode is phase locked with the seasonal cycle of the climatological intertropical convergence zone (CITCZ). The WES feedback is positive in boreal winter and spring when the CITCZ is located close to the equator but negative in summer and fall when the CITCZ shifts toward the north of the deep NTA. Similarly, the damping of the SST anomalies in the deep NTA by moisture-induced evaporation anomalies is much stronger in summer and fall than in winter and spring, related to a change in anomalous moisture transport. The results from the double-CO2 run show a substantial northward shift of the CITCZ in boreal winter and spring but little change in summer and fall. The change in the CITCZ can be explained by strong warming at the high northern latitudes in combination with a seasonally dependent WES feedback with accompanying changes in moisture transport in the deep NTA. The latter indicates that the change in the CITCZ is subject to phase locking with the seasonal cycle of the CITCZ itself. The meridional mode in the double-CO2 run weakens by 10%–20%. This originates from the weakening of the positive WES feedback in the deep NTA, which in turn is attributed to the northward shift of the CITCZ; because in the double-CO2 run the CITCZ stays south of the deep NTA for a shorter time period, the positive WES feedback in the deep NTA acts less long, and damping by moisture-induced evaporation anomalies starts earlier than in the control run.

scholarly journals Frontogenesis in the Agulhas Return Current Region Simulated by a High-Resolution CGCM

2017 ◽  
Vol 47 (11) ◽  
pp. 2691-2710 ◽  
Author(s):  
Shun Ohishi ◽  
Tomoki Tozuka ◽  
Meghan F. Cronin
Keyword(s):  
High Resolution ◽  
Mixed Layer ◽  
General Circulation ◽  
Circulation Model ◽  
Frontal Region ◽  
Return Current ◽  
Sst Front ◽  
Vorticity Advection ◽  
Southern Side ◽  
The Mean

AbstractDetailed mechanisms for frontogenesis/frontolysis of the sea surface temperature (SST) front in the Agulhas Return Current (ARC) region are investigated using outputs from a high-resolution coupled general circulation model. The SST front is maintained throughout the year through an approximate balance between frontolysis by surface heat flux and frontogenesis by horizontal advection. Although a southward (northward) cross-isotherm flow on the northern (southern) side of the front is weaker than a strong eastward along-isotherm current in the frontal region, this cross-isotherm confluent flow advects warmer (cooler) temperature toward the SST front north (south) of the front and acts as the dominant frontogenesis mechanism. In addition, stronger (weaker) frontogenesis in austral summer (winter) is attributed to the stronger (weaker) cross-isotherm confluence, which may be linked to seasonal variations of the Agulhas Current, ARC, and Antarctic Circumpolar Current. On the other hand, the contribution from entrainment is relatively small, because frontolysis by larger (smaller) entrainment velocity on the northern (southern) side opposes frontogenesis by less (more) effective cooling associated with a thicker (thinner) mixed layer and smaller (larger) temperature difference between the mixed layer and entrained water in the northern (southern) region. To gain further insight into the time-mean cross-isotherm confluent flow in the frontal region, the vorticity balance is examined. It is shown that anticyclonic (cyclonic) vorticity advection north (south) of the front by the mean cross-isotherm confluence is in balance with the sum of cyclonic (anticyclonic) vorticity advection by the mean along-isotherm flow and cross-isotherm eddy–mean interaction.

Simulation of the Diurnal Cycle in Tropical Rainfall and Circulation during Boreal Summer with a High-Resolution GCM

2010 ◽  
Vol 138 (9) ◽  
pp. 3434-3453 ◽  
Author(s):  
Jeffrey J. Ploshay ◽  
Ngar-Cheung Lau
Keyword(s):  
Diurnal Cycle ◽  
General Circulation ◽  
Large Scale ◽  
Surface Wind ◽  
Circulation Model ◽  
Horizontal Resolution ◽  
Boreal Summer ◽  
Tropical Rainfall ◽  
Early Afternoon ◽  
Seaward Migration

Abstract The simulation of the diurnal cycle (DC) of precipitation and surface wind pattern by a general circulation model (GCM) with a uniform horizontal resolution of 50 km over the global domain is evaluated. The model output is compared with observational counterparts based on datasets provided by the Tropical Rainfall Measuring Mission and reanalysis products of the European Centre for Medium-Range Weather Forecasts. The summertime diurnal characteristics over tropical regions in Asia, the Americas, and Africa are portrayed using the amplitude and phase of the first harmonic of the 24-h cycle, departures of data fields during selected hours from the daily mean, and differences between extreme phases of the DC. There is general agreement between the model and observations with respect to the large-scale land–sea contrasts in the DC. Maximum land precipitation, onshore flows, and landward migration of rainfall signals from the coasts occur in the afternoon, whereas peak maritime rainfall and offshore flows prevail in the morning. Seaward migration of precipitation is discernible over the western Bay of Bengal and South China Sea during nocturnal and morning hours. The evolution from low-intensity rainfall in the morning/early afternoon to heavier precipitation several hours later is also evident over selected continental sites. However, the observed incidence of rainfall with very high intensity in midafternoon is not reproduced in the model atmosphere. Although the model provides an adequate simulation of the daytime upslope and nighttime downslope winds in the vicinity of mountain ranges, valleys, and basins, there are notable discrepancies between model and observations in the DC of precipitation near some of these orographic features. The model does not reproduce the observed seaward migration of precipitation from the western coasts of Myanmar (Burma) and India, and from individual islands of the Indonesian Archipelago at nighttime.

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