Reexamining the tropical Atlantic influence on ENSO using perfect model predictability experiments

2021 ◽  
Author(s):  
Ingo Richter ◽  
Yu Kosaka ◽  
Hiroki Tokinaga ◽  
Shoichiro Kido
Keyword(s):  
Initial Conditions ◽  
Boreal Summer ◽  
Similar Amount ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Boreal Spring ◽  
Modes Of Variability ◽  
Control Simulation ◽  
Starting Point ◽  
Perfect Model

<p>The potential influence of the tropical Atlantic on the development of ENSO has received increased attention over recent years. In particular equatorial Atlantic variability (also known as the Atlantic zonal mode or AZM) has been shown to be anticorrelated with ENSO, i.e. cold AZM events in boreal summer (JJA) tend to be followed by El Niño in winter (DJF), and vice versa for warm AZM events. One problem with disentangling the two-way interaction between the equatorial Atlantic and Pacific is that both ENSO and the AZM tend to develop in boreal spring (MAM).</p><p>Here we use a set of GCM sensitivity experiments to quantify the strength of the Atlantic-Pacific link. The starting point is a 1000-year free-running control simulation with the GFDL CM 2.1 model. From this control simulation, we pick years in which a cold AZM event in JJA is followed by an El Niño in DJF. These years serve as initial conditions for “perfect model” prediction experiments with 10 ensemble members each. In the control experiments, the predictions evolve freely for 12 months from January 1 of each selected year. In the second set of predictions, SSTs are gradually relaxed to climatology in the tropical Atlantic, so that the cold AZM event is suppressed. In the third set of predictions, we restore the tropical Pacific SSTs to climatology, so that the El Niño event is suppressed.</p><p>The results suggest that, on average, the tropical Atlantic SST anomalies increase the strength of El Niño in the following winter by about 10-20%. If, on the other hand, El Niño development is suppressed, the amplitude of the cold AZM event also reduces by a similar amount. The results suggest that, in the context of this GCM, the influence of AZM events on ENSO development is relatively weak but not negligible. The fact that ENSO also influences the AZM in boreal spring highlights the complex two-way interaction between these two modes of variability.</p>

scholarly journals Tropical Atlantic SST Prediction with Coupled Ocean–Atmosphere GCMs

2006 ◽  
Vol 19 (23) ◽  
pp. 6047-6061 ◽  
Author(s):  
Timothy N. Stockdale ◽  
Magdalena A. Balmaseda ◽  
Arthur Vidard
Keyword(s):  
Initial Conditions ◽  
Coupled Model ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Seasonal Forecasts ◽  
Coupled Models ◽  
Equatorial Ocean ◽  
Sst Anomalies

Abstract Variations in tropical Atlantic SST are an important factor in seasonal forecasts in the region and beyond. An analysis is given of the capabilities of the latest generation of coupled GCM seasonal forecast systems to predict tropical Atlantic SST anomalies. Skill above that of persistence is demonstrated in both the northern tropical and equatorial Atlantic, but not farther south. The inability of the coupled models to correctly represent the mean seasonal cycle is a major problem in attempts to forecast equatorial SST anomalies in the boreal summer. Even when forced with observed SST, atmosphere models have significant failings in this area. The quality of ocean initial conditions for coupled model forecasts is also a cause for concern, and the adequacy of the near-equatorial ocean observing system is in doubt. A multimodel approach improves forecast skill only modestly, and large errors remain in the southern tropical Atlantic. There is still much scope for improving forecasts of tropical Atlantic SST.

scholarly journals On the Inconsistent Relationship between Pacific and Atlantic Niños*

2012 ◽  
Vol 25 (12) ◽  
pp. 4294-4303 ◽  
Author(s):  
Joke F. Lübbecke ◽  
Michael J. McPhaden
Keyword(s):  
Wind Stress ◽  
El Niño ◽  
El Nino ◽  
Boreal Summer ◽  
Western Boundary ◽  
Tropical Atlantic ◽  
Cold Tongue ◽  
Trade Winds ◽  
Boreal Spring ◽  
Northern Tropical Atlantic

Abstract The tropical Atlantic wind response to El Niño forcing is robust, with weakened northeast trade winds north of the equator and strengthened southeast trade winds along and south of the equator. However, the relationship between sea surface temperature (SST) anomalies in the eastern equatorial Pacific and Atlantic is inconsistent, with El Niño events followed sometimes by warm and other times by cold boreal summer anomalies in the Atlantic cold tongue region. Using observational data and a hindcast simulation of the Nucleus for European Modeling of the Ocean (NEMO) global model at 0.5° resolution (NEMO-ORCA05), this inconsistent SST relationship is shown to be at least partly attributable to a delayed negative feedback in the tropical Atlantic that is active in years with a warm or neutral response in the eastern equatorial Atlantic. In these years, the boreal spring warming in the northern tropical Atlantic that is a typical response to El Niño is pronounced, setting up a strong meridional SST gradient. This leads to a negative wind stress curl anomaly to the north of the equator that generates downwelling Rossby waves. When these waves reach the western boundary, they are reflected into downwelling equatorial Kelvin waves that reach the cold tongue region in late boreal summer to counteract the initial cooling that is due to the boreal winter wind stress response to El Niño. In contrast, this initial cooling persists or is amplified in years in which the boreal spring northern tropical Atlantic warming is weak or absent either because of a positive North Atlantic Oscillation (NAO) phase or an early termination of the Pacific El Niño event.

Evaluating the performance of CMIP6 models in the tropical Atlantic: mean state, variability, and remote impacts

2020 ◽  
Author(s):  
Ingo Richter ◽  
Hiroki Tokinaga
Keyword(s):  
General Circulation ◽  
General Circulation Models ◽  
Tropical Pacific ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Westerly Wind ◽  
Equatorial Atlantic ◽  
Wide Range ◽  
Mean State ◽  
The Tropical Pacific

<p>General circulation models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are examined with respect to their ability to simulate the mean state and variability of the tropical Atlantic, as well as its linkage to the tropical Pacific. While, on average, mean state biases have improved little relative to the previous intercomparison (CMIP5), there are now a few models with very small biases. In particular the equatorial Atlantic warm SST and westerly wind biases are mostly eliminated in these models. Furthermore, interannual variability in the equatorial and subtropical Atlantic is quite realistic in a number of CMIP6 models, which suggests that they should be useful tools for understanding and predicting variability patterns. The evolution of equatorial Atlantic biases follows the same pattern as in previous model generations, with westerly wind biases during boreal spring preceding warm sea-surface temperature (SST) biases in the east during boreal summer. A substantial portion of the westerly wind bias exists already in atmosphere-only simulations forced with observed SST, suggesting an atmospheric origin. While variability is relatively realistic in many models, SSTs seem less responsive to wind forcing than observed, both on the equator and in the subtropics, possibly due to an excessively deep mixed layer originating in the oceanic component. Thus models with realistic SST amplitude tend to have excessive wind amplitude. The models with the smallest mean state biases all have relatively high resolution but there are also a few low-resolution models that perform similarly well, indicating that resolution is not the only way toward reducing tropical Atlantic biases. The results also show a relatively weak link between mean state biases and the quality of the simulated variability. The linkage to the tropical Pacific shows a wide range of behaviors across models, indicating the need for further model improvement.</p>

scholarly journals Weakened Interannual Variability in the Tropical Atlantic

2021 ◽  
Author(s):  
Laura Sobral Verona ◽  
Paulo Silva ◽  
Ilana Wainer ◽  
Myriam Khodri
Keyword(s):  
Interannual Variability ◽  
Warming Trend ◽  
Precipitation Trend ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Bjerknes Feedback ◽  
Trade Winds ◽  
Modes Of Variability ◽  
Sst Variability ◽  
Wes Feedback

Abstract Climate variability in the Tropical Atlantic is complex with strong ocean-atmosphere coupling, where the sea surface temperature (SST) variability impacts the hydroclimate of the surrounding continents. We observe a decrease in the variability of the Tropical Atlantic after 1970 in both CMIP6 models and observations. Most of the Tropical Atlantic interannual variability is explained by its equatorial (Atlantic Zonal Mode, AZM) and meridional (Atlantic Meridional Mode, AMM) modes of variability. The observed wind relaxation after 1970 in both the equatorial and Tropical North Atlantic (TNA) plays a role in the decreased variability. Concerning the AZM, a widespread warming trend is observed in the equatorial Atlantic accompanied by a weakening trend of the trade winds. This drives a weakening in the Bjerknes Feedback by deepening the thermocline in the eastern equatorial Atlantic and increasing the thermal damping. Even though individually the TNA and Tropical South Atlantic (TSA) show increased variability, the observed asymmetric warming in the Tropical Atlantic and relaxed northeast trade winds after the 70s play a role in decreasing the AMM variability. This configuration leads to positive Wind-Evaporation-SST (WES) feedback, increasing further the TNA SST, preventing AMM from changing phases as before 1970. Associated with it, the African Sahel shows a positive precipitation trend and the Intertropical Convergence Zone tends to shift northward, which acts on maintaining the increased precipitation.

scholarly journals Control Simulation Experiment with the Lorenz’s Butterfly Attractor

2021 ◽  
Author(s):  
Takemasa Miyoshi ◽  
Qiwen Sun
Keyword(s):  
Simulation Experiment ◽  
Initial Conditions ◽  
Weather Prediction ◽  
Effective Control ◽  
Observation Error ◽  
Small Perturbations ◽  
Control Simulation ◽  
Starting Point ◽  
Nwp Model ◽  
Systems Simulation

Abstract. In numerical weather prediction (NWP), the sensitivity to initial conditions brings chaotic behaviors and an intrinsic limit to predictability, but it also implies an effective control in which a small control signal grows rapidly to make a substantial difference. The Observing Systems Simulation Experiment (OSSE) is a well-known approach to study predictability, where “the nature” is synthesized by an independent NWP model run. In this study, we extend the OSSE and design the control simulation experiment (CSE) where we apply a small signal to control “the nature”. Idealized experiments with the Lorenz-63 three-variable system show that we can control “the nature” to stay in a chosen regime without shifting to the other, i.e., in a chosen wing of the Lorenz’s butterfly attractor, by adding small perturbations to “the nature”. Using longer-lead-time forecasts, we achieve more effective control with a perturbation size less than only 3 % of the observation error. We anticipate our idealized CSE to be a starting point for realistic CSE using the real-world NWP systems, toward possible future applications to reduce weather disaster risks. The CSE may be applied to other chaotic systems beyond NWP.

Weakened impact of the Atlantic Niño on the future Guinean coast rainfall

2021 ◽  
Author(s):  
Koffi Worou ◽  
Hugues Goosse ◽  
Thierry Fichefet
Keyword(s):  
Climate Models ◽  
Climate Model ◽  
Rainfall Variability ◽  
Coupled Model ◽  
Sea Surface ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Sea Level Pressure ◽  
Guinean Coast

<p>Much of the rainfall variability in the Guinean coast area during the boreal summer is driven by the sea surface temperature (SST) variations in the eastern equatorial Atlantic, amplified by land-atmosphere interactions. This oceanic region corresponds to the center of action of the Atlantic Equatorial mode, also termed Atlantic Niño (ATL3), which is the leading SST mode of variability in the tropical Atlantic basin. In years of positive ATL3, above normal SST conditions in the ATL3 area weaken the sea level pressure gradient between the West African lands and the ocean, which in turn reduces the monsoon flow penetration into Sahel. Subsequently, the rainfall increases over the Guinean coast area. According to observations and climate models, the relation between the Atlantic Niño and the rainfall in coastal Guinea is stationary over the 20<sup>th</sup> century. While this relation remains unchanged over the 21<sup>st</sup> century in climate model projections, the strength of the teleconnection is reduced in a warmer climate. The weakened ATL3 effect on the rainfall over the tropical Atlantic (in years of positive ATL3) has been attributed to the stabilization of the atmosphere column above the tropical Atlantic. Analysis of historical and high anthropogenic emission scenario (the Shared Socioeconomic Pathways 5-8.5) simulations from 31 models participating in the sixth phase of the Coupled Model Intercomparison Project suggests an additional role of the Bjerkness feedback. A weakened SST amplitude related to ATL3 positive phases reduces the anomalous westerlies, which in turn increases the upwelling cooling effect on the sea surface. Both the Guinean coast region and the equatorial Atlantic experiment the projected rainfall reduction associated with ATL3, with a higher confidence over the ocean than over the coastal lands.</p>

scholarly journals Analysis of the Atlantic Meridional Mode Using Linear Inverse Modeling: Seasonality and Regional Influences

2012 ◽  
Vol 25 (4) ◽  
pp. 1194-1212 ◽  
Author(s):  
Daniel J. Vimont
Keyword(s):  
High Latitude ◽  
Inverse Modeling ◽  
Southern Oscillation ◽  
Tropical Pacific ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Boreal Spring ◽  
Meridional Mode ◽  
Atlantic Meridional Mode ◽  
Sst Anomalies

Abstract Predictability and variability of the tropical Atlantic Meridional Mode (AMM) is investigated using linear inverse modeling (LIM). Analysis of the LIM using an “energy” norm identifies two optimal structures that experience some transient growth, one related to El Niño–Southern Oscillation (ENSO) and the other to the Atlantic multidecadal oscillation (AMO)/AMM patterns. Analysis of the LIM using an AMM-norm identifies an “AMM optimal” with similar structure to the second energy optima (OPT2). Both the AMM-optimal and OPT2 exhibit two bands of SST anomalies in the mid- to high-latitude Atlantic. The AMM-optimal also contains some elements of the first energy optimal (ENSO), indicating that the LIM captures the well-known relationship between ENSO and the AMM. Seasonal correlations of LIM predictions with the observed AMM show enhanced AMM predictability during boreal spring and for long-lead (around 11–15 months) forecasts initialized around September. Regional LIMs were constructed to determine the influence of tropical Pacific and mid- to high-latitude Atlantic SST on the AMM. Analysis of the regional LIMs indicates that the tropical Pacific is responsible for the AMM predictability during boreal spring. Mid- to high-latitude SST anomalies contribute to boreal summer and fall AMM predictability, and are responsible for the enhanced predictability from September initial conditions. Analysis of the empirical normal modes of the full LIM confirms these physical relationships. Results indicate a potentially important role for mid- to high-latitude Atlantic SST anomalies in generating AMM (and tropical Atlantic SST) variations, though it is not clear whether those anomalies provide any societally useful predictive skill.

scholarly journals Estimating the Role of SST in Atmospheric Surface Wind Variability over the Tropical Atlantic and Pacific

2019 ◽  
Vol 32 (13) ◽  
pp. 3899-3915 ◽  
Author(s):  
Ingo Richter ◽  
Takeshi Doi
Keyword(s):  
Surface Wind ◽  
Internal Variability ◽  
Early Summer ◽  
Boreal Summer ◽  
Minor Role ◽  
Boreal Winter ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Wind Variability ◽  
A Minor

Abstract The influence of sea surface temperature (SST) on interannual surface wind variability in the tropical Atlantic and Pacific is estimated using sensitivity experiments with the SINTEX-F GCM and the ensemble spread in a nine-member control simulation. Two additional estimates are derived for both SINTEX-F and the ERA-Interim reanalysis using regression analysis and singular value decomposition. All methods yield quite consistent estimates of the fraction of surface wind variability that is determined by SST and therefore potentially predictable. In the equatorial Atlantic, analysis suggests that for the period 1982–2014 approximately 2/3 of surface zonal wind variability in boreal spring and early summer is potentially predictable, while 1/3 is due to noise. Of the predictable component, up to about 35% may be driven from outside the tropical Atlantic, suggesting an important role for remote forcing and a diminished one for local feedbacks. In the northern tropical Atlantic, only 30% of boreal winter variability is predictable, most of which is forced from the Pacific. This suggests a minor role for local coupled air–sea feedbacks. For the equatorial Pacific, the results suggest high predictability throughout the year, most of which is due to local SST, with the tropical Atlantic only playing a minor role in boreal summer. In the tropical Atlantic, atmospheric internal variability is strongly dependent on the presence of deep convection, which, in turn, is related to mean SST. A similar, but weaker, state dependence of internal variability is evident in the tropical Pacific.

Atlantic Zonal Mode-Monsoon Teleconnection in a Warming Scenario

2021 ◽  
Author(s):  
Sabeerali C. T ◽  
Ajaya Mohan Ravindran ◽  
Praveen V
Keyword(s):  
Indian Subcontinent ◽  
Coupled Model ◽  
Summer Monsoon Rainfall ◽  
Convincing Evidence ◽  
External Boundary ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Warming Scenario ◽  
Sst Variability

Abstract The dominant interannual SST variability in the eastern equatorial Atlantic referred to as the Atlantic Zonal Mode (AZM), which peaks in boreal summer impacts global weather patterns. The cold (warm) phase of this ocean-atmospheric coupled phenomenon enhances (weakens) the intensity of the Indian summer monsoon rainfall (ISMR). Observational studies show a strengthening relationship between AZM and ISMR in recent decades, providing a predictive signal for the ISMR. However, a suite of Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulations in the highest emission scenario (SSP58.5) show a weakening relationship between ISMR and AZM in the future (2050-2099). The strengthening of atmospheric thermal stability over the tropical Atlantic in the warming scenario weakens the associated convection over the eastern equatorial Atlantic in response to the warm phase of AZM. This leads to weakening velocity potential response over the Indian subcontinent, resulting in a weak AZM-ISMR relationship. There is no convincing evidence to indicate that either the tropical Atlantic SST bias or the AZM-ISMR teleconnection bias plays a crucial role in the potential weakening of this relationship. These results imply that ISMR prediction will become more challenging in a warming scenario as one of the major external boundary forces that influence monsoon weakens.

scholarly journals Some Overlooked Features of Tropical Atlantic Climate Leading to a New Niño-Like Phenomenon*

2006 ◽  
Vol 19 (22) ◽  
pp. 5859-5874 ◽  
Author(s):  
Yuko Okumura ◽  
Shang-Ping Xie
Keyword(s):  
Southern Oscillation ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Gulf Of Guinea ◽  
Equatorial Atlantic ◽  
Atlantic Sector ◽  
The Pacific ◽  
Interannual Rainfall Variability ◽  
Ocean Atmosphere ◽  
Atlantic Niño

Abstract The Atlantic Niño, an equatorial zonal mode akin to the Pacific El Niño–Southern Oscillation (ENSO), is phase-locked to boreal summer when the equatorial easterly winds intensify and the thermocline shoals in the Gulf of Guinea. A suite of satellite and in situ observations reveals a new mode of tropical Atlantic variability that displays many characteristics of the zonal mode but instead peaks in November–December (ND). This new mode is found to be statistically independent from both the Atlantic Niño in the preceding summer and the Pacific ENSO. The origin of this ND zonal mode lies in an overlooked aspect of the seasonal cycle in the equatorial Atlantic. In November the equatorial easterly winds intensify for the second time, increasing upwelling and lifting the thermocline in the Gulf of Guinea. An analysis of high-resolution climatological data shows that these dynamical changes induce a noticeable SST cooling in the central equatorial Atlantic. The shoaling thermocline and increased upwelling enhance the SST sensitivity to surface wind changes, reinvigorating equatorial ocean–atmosphere interaction. The resultant ocean–atmospheric anomalies are organized into patterns that give rise to positive mutual feedback as Bjerknes envisioned for the Pacific ENSO. This ND zonal mode significantly affects interannual rainfall variability in coastal Congo–Angola during its early rainy season. It tends to further evolve into a meridional mode in the following March–April, affecting precipitation in northeast Brazil. Thus it offers potential predictability for climate over the Atlantic sector in early boreal winter, a season for which local ocean–atmosphere variability was otherwise poorly understood.

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