scholarly journals Causes of Strengthening and Weakening of ENSO Amplitude under Global Warming in Four CMIP5 Models*

2015 ◽  
Vol 28 (8) ◽  
pp. 3250-3274 ◽  
Author(s):  
Lin Chen ◽  
Tim Li ◽  
Yongqiang Yu
Keyword(s):  
Global Warming ◽  
Wind Stress ◽  
Zonal Wind ◽  
Southern Oscillation ◽  
Heat Budget ◽  
Meridional Overturning Circulation ◽  
Cmip5 Models ◽  
Zonal Wind Stress ◽  
Equatorial Thermocline ◽  
Change In Mean

Abstract The mechanisms for El Niño–Southern Oscillation (ENSO) amplitude change under global warming are investigated through quantitative assessment of air–sea feedback processes in present-day and future climate simulations of four models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Two models (MPI-ESM-MR and MRI-CGCM3) project strengthened ENSO amplitude, whereas the other two models (CCSM4 and FGOALS-g2) project weakened ENSO amplitude. A mixed layer heat budget diagnosis shows that the major cause of the projected ENSO amplitude difference between the two groups is attributed to the changes of the thermocline and zonal advective feedbacks. A weaker (stronger) equatorial thermocline response to a unit anomalous zonal wind stress forcing in the Niño-4 region is found in CCSM4 and FGOALS-g2 (MPI-ESM-MR and MRI-CGCM3). The cause of the different response arises from the change in the meridional scale of ENSO. A narrower (wider) meridional width of sea surface temperature (SST) and zonal wind stress anomalies causes a strengthening (weakening) of the equatorial thermocline response and thus stronger Bjerknes and zonal advective feedbacks, as the subsurface temperature and zonal current anomalies depend on the thermocline response; consequently, the ENSO amplitude increases (decreases). The change of ENSO meridional width is caused by the change in mean meridional overturning circulation in the equatorial Pacific Ocean, which depends on change of mean wind stress and SST warming patterns under global warming.

scholarly journals The Effect of Wind Stress Anomalies and Location in Driving Pacific Subtropical Cells and Tropical Climate

2019 ◽  
Vol 32 (5) ◽  
pp. 1641-1660 ◽  
Author(s):  
Giorgio Graffino ◽  
Riccardo Farneti ◽  
Fred Kucharski ◽  
Franco Molteni
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Tropical Climate ◽  
Global Ocean ◽  
Subsurface Water ◽  
Zonal Wind Stress ◽  
Wind Stress Anomaly ◽  
Equatorial Thermocline ◽  
The Tropics ◽  
Sst Anomalies

Abstract The importance of subtropical and extratropical zonal wind stress anomalies on Pacific subtropical cell (STC) strength is assessed through several idealized and realistic numerical experiments with a global ocean model. Different zonal wind stress anomalies are employed, and their intensity is strengthened or weakened with respect to the climatological value throughout a suite of simulations. Subtropical strengthened (weakened) zonal wind stress anomalies result in increased (decreased) STC meridional mass and energy transport. When upwelling of subsurface water into the tropics is intensified (reduced), a distinct cold (warm) anomaly appears in the equatorial thermocline and up to the surface, resulting in significant tropical sea surface temperature (SST) anomalies. The use of realistic wind stress anomalies also suggests a potential impact of midlatitude atmospheric modes of variability on tropical climate through STC dynamics. The remotely driven response is compared with a set of simulations where an equatorial zonal wind stress anomaly is imposed. A dynamically distinct response is achieved, whereby the equatorial thermocline adjusts to the wind stress anomaly, resulting in significant equatorial SST anomalies as in the remotely forced simulations but with no role for STCs. Significant anomalies in Indonesian Throughflow transport are generated only when equatorial wind stress anomalies are applied, leading to remarkable heat content anomalies in the Indian Ocean. Equatorial wind stress anomalies do not involve modifications of STC transport but could set up the appropriate initial conditions for a tropical–extratropical teleconnection involving Hadley cells, exciting an STC anomalous transport, which ultimately feeds back on the tropics.

Climate Change over the Equatorial Indo-Pacific in Global Warming*

2009 ◽  
Vol 22 (10) ◽  
pp. 2678-2693 ◽  
Author(s):  
Chie Ihara ◽  
Yochanan Kushnir ◽  
Mark A. Cane ◽  
Victor H. de la Peña
Keyword(s):  
Climate Change ◽  
Global Warming ◽  
Indian Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Climate Models ◽  
Emission Scenario ◽  
Equatorial Indian Ocean ◽  
Zonal Wind Stress ◽  
Sst Gradient

Abstract The response of the equatorial Indian Ocean climate to global warming is investigated using model outputs submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. In all of the analyzed climate models, the SSTs in the western equatorial Indian Ocean warm more than the SSTs in the eastern equatorial Indian Ocean under global warming; the mean SST gradient across the equatorial Indian Ocean is anomalously positive to the west in a warmer twenty-first-century climate compared to the twentieth-century climate, and it is dynamically consistent with the anomalous westward zonal wind stress and anomalous positive zonal sea level pressure (SLP) gradient to the east at the equator. This change in the zonal SST gradient in the equatorial Indian Ocean is detected even in the lowest-emission scenario, and the size of the change is not necessarily larger in the higher-emission scenario. With respect to the change over the equatorial Pacific in climate projections, the subsurface central Pacific displays the strongest cooling or weakest warming around the thermocline depth compared to that above and below in all of the climate models, whereas changes in the zonal SST gradient and zonal wind stress around the equator are model dependent and not straightforward.

scholarly journals Reconstructing the Meridional Overturning Circulation from Boundary Densities and the Zonal Wind Stress

2007 ◽  
Vol 37 (3) ◽  
pp. 743-763 ◽  
Author(s):  
J. Hirschi ◽  
J. Marotzke
Keyword(s):  
Time Scales ◽  
Wind Stress ◽  
Zonal Wind ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Temperature Conductivity ◽  
Boundary Density ◽  
Zonal Wind Stress ◽  
Meridional Overturning ◽  
Temporal And Spatial

Abstract Numerical models are used to test whether the meridional overturning circulation (MOC) can be reconstructed from boundary densities and the wind stress. In idealized model setups without topography the strength as well as the temporal and spatial variability of the MOC cell can largely be reproduced from boundary densities and the zonal wind stress. With added slopes along the meridional boundaries, most of the depth-averaged flow is missed and neither strength nor spatial structure of the MOC is well reproduced. However, the temporal evolution of both MOC and its estimate are similar. In an eddy-permitting model with realistic bottom topography the contribution of the depth-averaged meridional flow to the MOC is captured at some places while it is missed at others. Nevertheless, boundary densities and the zonal wind stress allow the leading modes of the temporal and spatial MOC variability to be reproduced. On seasonal time scales most of the MOC variability is due to the wind stress but changes in the boundary density affect the MOC as well. On interannual time scales the MOC variability largely reflects changes in the boundary density. Generally, the MOC reconstructions are accurate when bottom velocities are small, an assumption made in the reconstruction approach. The results are relevant for estimates of both the modern and the past MOC. In the real ocean, boundary densities can be obtained from measurements of temperature, conductivity, and pressure in the water column, whereas past seawater densities have left their imprint in sea sediments.

scholarly journals Impact of Equatorial Wind Stress on Ekman Transport During the Mature Phase of the Indian Ocean Dipole

2021 ◽  
Author(s):  
Linfang Zhang ◽  
Yaokun Li ◽  
Jianping Li
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Indian Ocean Dipole ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Zonal Wind Stress ◽  
Mature Phase ◽  
The Indian Ocean ◽  
The Impact

Abstract This paper investigates the impact of equatorial wind stress on the equatorial Ekman transport during the Indian Ocean dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper levels. In normal years, the zonal wind stress south of the equator is easterly and that north of the equator is westerly, which contributes to southward Ekman transport at the upper levels to form the climatological Indian Ocean shallow meridional overturning circulation. During the years of positive IOD events, abnormal easterly winds near the equator bring southward Ekman transport south of the equator while they bring northward Ekman transport north of the equator. This causes the seawater to move away from the equator and hence induces upwelling near the equator, which forms a pair of small circulation cells that are symmetric about the equator at the upper levels (approximately 100 m deep). The abnormal circulation cell south (north) of the equator strengthens (weakens) the southward (southward) motion south (north) of the equator. During years with negative IOD events, the opposite occurs. In addition, during the mature period of IOD, the remote sea surface temperature anomaly (SSTA) such as El Niño–Southern Oscillation (ENSO) may exert some influence on equatorial wind stress and Ekman transport anomaly but the influence is weak.

scholarly journals Southern Ocean Overturning Compensation in an Eddy-Resolving Climate Simulation

2016 ◽  
Vol 46 (5) ◽  
pp. 1575-1592 ◽  
Author(s):  
Stuart P. Bishop ◽  
Peter R. Gent ◽  
Frank O. Bryan ◽  
Andrew F. Thompson ◽  
Matthew C. Long ◽  
...  
Keyword(s):  
Sea Ice ◽  
Wind Stress ◽  
Zonal Wind ◽  
Large Scale ◽  
Meridional Overturning Circulation ◽  
Climate Simulation ◽  
Transient Eddy ◽  
Zonal Wind Stress ◽  
Perturbation Experiment ◽  
The Mean

AbstractThe Southern Ocean’s Antarctic Circumpolar Current (ACC) and meridional overturning circulation (MOC) response to increasing zonal wind stress is, for the first time, analyzed in a high-resolution (0.1° ocean and 0.25° atmosphere), fully coupled global climate simulation using the Community Earth System Model. Results from a 20-yr wind perturbation experiment, where the Southern Hemisphere zonal wind stress is increased by 50% south of 30°S, show only marginal changes in the mean ACC transport through Drake Passage—an increase of 6% [136–144 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1)] in the perturbation experiment compared with the control. However, the upper and lower circulation cells of the MOC do change. The lower cell is more affected than the upper cell with a maximum increase of 64% versus 39%, respectively. Changes in the MOC are directly linked to changes in water mass transformation from shifting surface isopycnals and sea ice melt, giving rise to changes in surface buoyancy forcing. The increase in transport of the lower cell leads to upwelling of warm and salty Circumpolar Deep Water and subsequent melting of sea ice surrounding Antarctica. The MOC is commonly supposed to be the sum of two opposing components: a wind- and transient-eddy overturning cell. Here, the transient-eddy overturning is virtually unchanged and consistent with a large-scale cancellation of localized regions of both enhancement and suppression of eddy kinetic energy along the mean path of the ACC. However, decomposing the time-mean overturning into a time- and zonal-mean component and a standing-eddy component reveals partial compensation between wind-driven and standing-eddy components of the circulation.

scholarly journals Reassessing Conceptual Models of ENSO

2015 ◽  
Vol 28 (23) ◽  
pp. 9121-9142 ◽  
Author(s):  
Felicity S. Graham ◽  
Jaclyn N. Brown ◽  
Andrew T. Wittenberg ◽  
Neil J. Holbrook
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Conceptual Models ◽  
Southern Oscillation ◽  
Oscillator Model ◽  
Sustained Oscillation ◽  
Complex Nature ◽  
Enso Cycle ◽  
Thermocline Depth ◽  
Zonal Wind Stress

Abstract The complex nature of the El Niño–Southern Oscillation (ENSO) is often simplified through the use of conceptual models, each of which offers a different perspective on the ocean–atmosphere feedbacks underpinning the ENSO cycle. One theory, the unified oscillator, combines a variety of conceptual frameworks in the form of a coupled system of delay differential equations. The system produces a self-sustained oscillation on interannual time scales. While the unified oscillator is assumed to provide a more complete conceptual framework of ENSO behaviors than the models it incorporates, its formulation and performance have not been systematically assessed. This paper investigates the accuracy of the unified oscillator through its ability to replicate the ENSO cycle modeled by flux-forced output from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM). The anomalous sea surface temperature equation reproduces the main features of the corresponding tendency modeled by ACCESS-OM reasonably well. However, the remaining equations for the thermocline depth anomaly and zonal wind stress anomalies are unable to accurately replicate the corresponding tendencies in ACCESS-OM. Modifications to the unified oscillator, including a diagnostic form of the zonal wind stress anomaly equations, improve its ability to emulate simulated ENSO tendencies. Despite these improvements, the unified oscillator model is less adept than the delayed oscillator model it incorporates in capturing ENSO behavior in ACCESS-OM, bringing into question its usefulness as a unifying ENSO framework.

scholarly journals On the fast response of the Southern Ocean to changes in the zonal wind

Ocean Science ◽  
2007 ◽  
Vol 3 (3) ◽  
pp. 417-427 ◽  
Author(s):  
D. J. Webb ◽  
B. A. de Cuevas
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Surface Wind ◽  
Drake Passage ◽  
Fast Response ◽  
Momentum Balance ◽  
Zonal Wind Stress ◽  
Model Studies ◽  
Barotropic Flow

Abstract. Model studies of the Southern Ocean, reported here, show that the Antarctic Circumpolar Current responds within two days to changes in the zonal wind stress at the latitudes of Drake Passage. Further investigation shows that the response is primarily barotropic and that, as one might expect, it is controlled by topography. Analysis of the results show that the changes in the barotropic flow are sufficient to transfer the changed surface wind stress to the underlying topography and that during this initial phase baroclinic processes are not involved. The model results also show that the Deacon Cell responds to changes in the wind stress on the same rapid time scale. It is shown that the changes in the Deacon Cell can also be explained by the change in the barotropic velocity field, an increase in the zonal wind stress producing an increased northward flow in shallow regions and southward flow where the ocean is deep. This new explanation is unexpected as previously the Deacon Cell has been thought of as a baroclinic feature of the ocean. The results imply that where baroclinic processes do appear to be involved in either the zonal momentum balance of the Southern Ocean or the formation of the Deacon Cell, they are part of the long term baroclinic response of the ocean's density field to the changes in the barotropic flow.

Southern Ocean Circulation and Eddy Compensation in CMIP5 Models

2013 ◽  
Vol 26 (18) ◽  
pp. 7198-7220 ◽  
Author(s):  
Stephanie M. Downes ◽  
Andrew McC. Hogg
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Surface Wind ◽  
Surface Heat ◽  
Meridional Overturning Circulation ◽  
Cmip5 Models ◽  
Strong Increase ◽  
Coupled Models ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract Thirteen state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to evaluate the response of the Antarctic Circumpolar Current (ACC) transport and Southern Ocean meridional overturning circulation to surface wind stress and buoyancy changes. Understanding how these flows—fundamental players in the global distribution of heat, gases, and nutrients—respond to climate change is currently a widely debated issue among oceanographers. Here, the authors analyze the circulation responses of these coarse-resolution coupled models to surface fluxes. Under a future CMIP5 climate pathway where the equivalent atmospheric CO2 reaches 1370 ppm by 2100, the models robustly project reduced Southern Ocean density in the upper 2000 m accompanied by strengthened stratification. Despite an overall increase in overlying wind stress (~20%), the projected ACC transports lie within ±15% of their historical state, and no significant relationship with changes in the magnitude or position of the wind stress is identified. The models indicate that a weakening of ACC transport at the end of the twenty-first century is correlated with a strong increase in the surface heat and freshwater fluxes in the ACC region. In contrast, the surface heat gain across the ACC region and the wind-driven surface transports are significantly correlated with an increased upper and decreased lower Eulerian-mean meridional overturning circulation. The change in the eddy-induced overturning in both the depth and density spaces is quantified, and it is found that the CMIP5 models project partial eddy compensation of the upper and lower overturning cells.

scholarly journals Using Initialized Hindcasts to Assess Simulations of 1970–2009 Equatorial Pacific SST, Zonal Wind Stress, and Surface Flux Trends

2014 ◽  
Vol 27 (19) ◽  
pp. 7385-7393 ◽  
Author(s):  
Amy Solomon
Keyword(s):  
Time Scales ◽  
Wind Stress ◽  
Zonal Wind ◽  
Surface Flux ◽  
Warming Trend ◽  
Warm Pool ◽  
Equatorial Pacific ◽  
External Forcing ◽  
Cold Tongue ◽  
Zonal Wind Stress

Abstract Initialized decadal hindcasts are used to assess simulations of 1970–2009 equatorial Pacific SST, zonal wind stress, and surface flux trends. Initialized hindcasts are useful to assess how well the models simulate observed trends, as well as how simulations of observed trends (due primarily to natural variability) differ from ensemble-mean forecasted trends (due to the response to an increase in external forcing). All models forecast a statistically significant warming trend in both the warm-pool and cold-tongue regions. However, while the warm-pool warming trend is within the observed estimates, the cold-tongue warming trend is an order of magnitude larger than an ENSO residual estimated using SST instrumental reconstructions. Multimodel ensemble means formed using forecasts 6–10 years from initialization with 40 ensemble members do not produce an unambiguous zonal SST gradient response to an increase in external forcing. Systematic biases are identified in forecasts of surface fluxes. For example, in the warm-pool region all year-1 forecasts produce SST trends similar to observations but ocean mixed layer and net surface heat flux trends with an opposite sign to air–sea datasets. In addition, year-1 forecasts produce positive shortwave feedbacks on decadal time scales, whereas 6–10-yr forecasts produce negative or statistically insignificant shortwave flux feedbacks on decadal time scales, suggesting sensitivity to circulations forced by the initialized ocean state. In the cold-tongue region initialized ensembles forecast positive net radiative flux trends even though shortwave flux trends are negative (i.e., for increasing cloudiness). This is inconsistent with air–sea datasets, which uniformly show that the net surface radiative flux feedback is a damping of the underlying SSTs.

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