scholarly journals Free and Forced Variability of the Tropical Atlantic Ocean: Role of the Wind–Evaporation–Sea Surface Temperature Feedback

2010 ◽  
Vol 23 (22) ◽  
pp. 5958-5977 ◽  
Author(s):  
Salil Mahajan ◽  
R. Saravanan ◽  
Ping Chang
Keyword(s):  
Atlantic Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Climate Model ◽  
Coupled Model ◽  
Sea Surface ◽  
Tropical Atlantic ◽  
Wes Feedback ◽  
Meridional Mode

Abstract The role of the wind–evaporation–sea surface temperature (WES) feedback in the low-frequency natural variability of the tropical Atlantic is studied using an atmospheric global climate model—the NCAR Community Climate Model, version 3 (CCM3)—thermodynamically coupled to a slab ocean model (SOM). The coupled model is modified to suppress the WES feedback and is compared to a control run. Singular value decomposition (SVD) analysis over the tropical Atlantic reveals that the coupled meridional mode of the Atlantic Ocean is amplified in the presence of the WES feedback. In its absence, the meridional mode still exists, but with a weaker amplitude. A feedback mechanism that involves the near-surface specific humidity is proposed to sustain the weaker Atlantic meridional mode in the absence of the WES feedback. Similar analysis of coupled model integrations when forced with an artificial El Niño–Southern Oscillation (ENSO)-like SST cycle in the Pacific reveals that in the presence of the WES feedback, the meridional mode is the preferred mode of response of the tropical Atlantic to ENSO forcing. In the absence of the WES feedback, the tropical Atlantic response is unlike the meridional mode and the effects of tropospheric warming and subsidence dominate. Regression analysis over the tropical Atlantic reveals that the meridional mode response to ENSO peaks in the spring and begins to decay in the fall in the coupled model in the presence of the WES feedback. The WES feedback also appears to be responsible for the northward migration of the ITCZ during ENSO events.

scholarly journals An Analytical Framework for Understanding Tropical Meridional Modes

2017 ◽  
Vol 30 (9) ◽  
pp. 3303-3323 ◽  
Author(s):  
Cristian Martinez-Villalobos ◽  
Daniel J. Vimont
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Ocean Model ◽  
Sea Surface ◽  
Transient Growth ◽  
Kelvin Wave ◽  
Wes Feedback ◽  
Meridional Mode ◽  
The Tropics

A theoretical framework is developed for understanding the transient growth and propagation characteristics of thermodynamically coupled, meridional mode–like structures in the tropics. The model consists of a Gill–Matsuno-type steady atmosphere under the long-wave approximation coupled via a wind–evaporation–sea surface temperature (WES) feedback to a “slab” ocean model. When projected onto meridional basis functions for the atmosphere the system simplifies to a nonnormal set of equations that describes the evolution of individual sea surface temperature (SST) modes, with clean separation between equatorially symmetric and antisymmetric modes. The following major findings result from analysis of the system: 1) a transient growth process exists whereby specific SST modes propagate toward lower-order modes at the expense of the higher-order modes; 2) the same dynamical mechanisms govern the evolution of symmetric and antisymmetric SST modes except for the lowest-order wavenumber, where for symmetric structures the atmospheric Kelvin wave plays a critically different role in enhancing decay; and 3) the WES feedback is positive for all modes (with a maximum for the most equatorially confined antisymmetric structure) except for the most equatorially confined symmetric mode where the Kelvin wave generates a negative WES feedback. Taken together, these findings explain why equatorially antisymmetric “dipole”-like structures may dominate thermodynamically coupled ocean–atmosphere variability in the tropics. The role of nonnormality and the role of realistic mean states in meridional mode variability are discussed.

Local and remote causes of the equatorial Pacific cold sea surface temperature bias in the Kiel Climate Model

2020 ◽  
Author(s):  
Yuming Zhang ◽  
Tobias Bayr ◽  
Mojib Latif ◽  
Zhaoyang Song ◽  
Wonsun Park ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Ocean Circulation ◽  
Climate Model ◽  
Coupled Model ◽  
Equatorial Pacific ◽  
Ocean Dynamics ◽  
Sea Surface ◽  
Sst Bias ◽  
Atmosphere Model

<p>We investigate the origin of the equatorial Pacific cold sea surface temperature (SST) bias and its link to wind biases, local and remote, in the Kiel Climate Model (KCM) with dedicated coupled and stand-alone atmosphere model experiments. In the coupled experiments, the National Centers for Environmental Prediction Climate Forecast System Reanalysis (NCEP/CFSR) wind stress is prescribed over four different spatial domains: globally, over the equatorial Pacific (EP), the northern Pacific (NP) and southern Pacific (SP). The corresponding cold SST bias over the equatorial Pacific is reduced by 94%, 48%, 11% and 22%, respectively. Thus, the equatorial Pacific SST bias is mainly attributed to the wind bias over the EP region, with small but not negligible contributions from the SP and NP regions. Biases in the ocean dynamics cause the EP SST bias, while the atmospheric thermodynamics counteract it.</p><p>To examine the origin of wind biases, we force the atmospheric component of the KCM in stand-alone mode by observed SSTs and simulated SSTs from the coupled experiments with the KCM. The results show that wind biases over the EP, NP and SP regions are initially generated in the atmosphere model and further enhanced by the biased SST in the coupled model.</p><p>We conclude that the cold SST bias over the equatorial Pacific originates from biases in the ocean circulation that are forced by the biased surface winds over the EP, NP and SP regions. On the other hand, the cold equatorial Pacific SST bias amplifies the wind biases over the EP, NP and SP regions, which in turn enhances the cold SST bias by ocean-atmosphere coupling.</p>

Tropical Pacific and Atlantic Climate Variability in CCSM3

2006 ◽  
Vol 19 (11) ◽  
pp. 2451-2481 ◽  
Author(s):  
Clara Deser ◽  
Antonietta Capotondi ◽  
R. Saravanan ◽  
Adam S. Phillips
Keyword(s):  
Climate Variability ◽  
Surface Temperature ◽  
Spatial Pattern ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
El Nino ◽  
Coupled Model ◽  
Sea Surface ◽  
Thermocline Depth ◽  
Tropical Atlantic

Abstract Simulations of the El Niño–Southern Oscillation (ENSO) phenomenon and tropical Atlantic climate variability in the newest version of the Community Climate System Model [version 3 (CCSM3)] are examined in comparison with observations and previous versions of the model. The analyses are based upon multicentury control integrations of CCSM3 at two different horizontal resolutions (T42 and T85) under present-day CO2 concentrations. Complementary uncoupled integrations with the atmosphere and ocean component models forced by observed time-varying boundary conditions allow an assessment of the impact of air–sea coupling upon the simulated characteristics of ENSO and tropical Atlantic variability. The amplitude and zonal extent of equatorial Pacific sea surface temperature variability associated with ENSO is well simulated in CCSM3 at both resolutions and represents an improvement relative to previous versions of the model. However, the period of ENSO remains too short (2–2.5 yr in CCSM3 compared to 2.5–8 yr in observations), and the sea surface temperature, wind stress, precipitation, and thermocline depth responses are too narrowly confined about the equator. The latter shortcoming is partially overcome in the atmosphere-only and ocean-only simulations, indicating that coupling between the two model components is a contributing cause. The relationships among sea surface temperature, thermocline depth, and zonal wind stress anomalies are consistent with the delayed/recharge oscillator paradigms for ENSO. We speculate that the overly narrow meridional scale of CCSM3's ENSO simulation may contribute to its excessively high frequency. The amplitude and spatial pattern of the extratropical atmospheric circulation response to ENSO is generally well simulated in the T85 version of CCSM3, with realistic impacts upon surface air temperature and precipitation; the simulation is not as good at T42. CCSM3's simulation of interannual climate variability in the tropical Atlantic sector, including variability intrinsic to the basin and that associated with the remote influence of ENSO, exhibits similarities and differences with observations. Specifically, the observed counterpart of El Niño in the equatorial Atlantic is absent from the coupled model at both horizontal resolutions (as it was in earlier versions of the coupled model), but there are realistic (although weaker than observed) SST anomalies in the northern and southern tropical Atlantic that affect the position of the local intertropical convergence zone, and the remote influence of ENSO is similar in strength to observations, although the spatial pattern is somewhat different.

scholarly journals Origin of Weakened Interannual Sea Surface Temperature Variability in the Southeastern Tropical Atlantic Ocean

2020 ◽  
Vol 47 (20) ◽  
Author(s):  
Arthur Prigent ◽  
Rodrigue Anicet Imbol Koungue ◽  
Joke F. Lübbecke ◽  
Peter Brandt ◽  
Mojib Latif
Keyword(s):  
Atlantic Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Temperature Variability ◽  
Sea Surface ◽  
Tropical Atlantic ◽  
Tropical Atlantic Ocean

Weakening Satellite Era and Future Tropical Atlantic Sea Surface Temperature Variability

2021 ◽  
Author(s):  
Arthur Prigent ◽  
Rodrigue Anicet Imbol Koungue ◽  
Joke Lübbecke ◽  
Peter Brandt ◽  
Jan Harlaß ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Zonal Wind ◽  
Coupled Model ◽  
Sea Surface ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Sst Variability ◽  
Remote Forcing ◽  
Local Forcing

<p>Since 2000, a substantial weakening in the equatorial and southeastern tropical Atlantic sea surface temperature (SST) variability is observed. Observations and reanalysis products reveal, for example, that relative to 1982–1999, the March‐April‐May SST variability in the Angola‐Benguela area (ABA) has decreased by more than 30%. Both equatorial remote forcing and local forcing are known to play an important role in driving SST variability in the ABA. Here we show that compared to 1982–1999, since 2000, equatorial remote forcing had less influence on ABA SSTs, whereas local forcing has become more important. In particular, the robust correlation between the equatorial zonal wind stress and the ABA SSTs has substantially weakened, suggesting less influence of Kelvin waves on ABA SSTs. Moreover, the strong correlation linking the South Atlantic Anticyclone and the ABA SSTs has reduced. Multidecadal surface warming of the ABA could also have played a role in weakening the interannual SST variability.</p><p>To investigate future changes in tropical Atlantic SST variability, an ensemble of nested high-resolution coupled model simulations under the global warming scenario RCP8.5 is analyzed. SST variability in both the ABA and equatorial cold tongue is found to decrease along with reduced western equatorial Atlantic zonal wind variability.  </p>

scholarly journals Coupled Ocean–Atmosphere Interaction and Variability in the Tropical Atlantic Ocean with and without an Annual Cycle

2008 ◽  
Vol 21 (21) ◽  
pp. 5501-5523 ◽  
Author(s):  
Susan C. Bates
Keyword(s):  
Atlantic Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Annual Cycle ◽  
Sea Surface ◽  
Tropical Atlantic ◽  
Feedback Mechanisms ◽  
Tropical Atlantic Variability ◽  
Ocean Atmosphere ◽  
Mean State

Abstract Many previous studies point to a connection between the annual cycle and interannual variability in the tropical Atlantic Ocean. To investigate the importance of the annual cycle in the generation of tropical Atlantic variability (TAV) as well as its associated coupled feedback mechanisms, a set of controlled experiments is conducted using a global coupled ocean–atmosphere general circulation model (GCM) in which the climatological annual cycle is modified. An anomaly coupling strategy was developed to improve the model-simulated annual cycle and mean sea surface temperature (SST), which is critical to the experiments. Experiments include a control simulation in which the annual cycle is present and a fixed annual cycle simulation in which the coupled model is forced to remain in a perpetual annual mean state. Results reveal that the patterns of TAV, defined as the leading three rotated EOFs, and their relationship to coupled feedback mechanisms are present even in the absence of the annual cycle, suggesting that the generation of TAV is not dependent on the annual cycle. Each pattern of variability arises from an alteration of the easterly trade winds. Results suggest that it is the presence of these winds in the mean state that is the determining factor for the structure of the coupled ocean–atmosphere variability. Additionally, the patterns of variability persist longer in the simulation with no annual cycle. Most remarkable is the doubling of the decay phase related to the north tropical Atlantic variability, which is attributed to the persistence of the local wind–evaporation–sea surface temperature (WES) feedback mechanism. The author concludes that the annual cycle acts to cut off or interrupt conditions favorable for feedback mechanisms to operate, therefore putting a limit on the length of the event life cycle.

Sign in / Sign up

Export Citation Format

Share Document