scholarly journals Diabatic heating governs the seasonality of the Atlantic Niño

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Hyacinth C. Nnamchi ◽  
Mojib Latif ◽  
Noel S. Keenlyside ◽  
Joakim Kjellsson ◽  
Ingo Richter
Keyword(s):  
Feedback Loop ◽  
Diabatic Heating ◽  
Seasonal Migration ◽  
Equatorial Atlantic ◽  
Bjerknes Feedback ◽  
Atlantic Sector ◽  
Inter Tropical Convergence Zone ◽  
Sst Variability ◽  
Leading Mode ◽  
Atlantic Niño

AbstractThe Atlantic Niño is the leading mode of interannual sea-surface temperature (SST) variability in the equatorial Atlantic and assumed to be largely governed by coupled ocean-atmosphere dynamics described by the Bjerknes-feedback loop. However, the role of the atmospheric diabatic heating, which can be either an indicator of the atmosphere’s response to, or its influence on the SST, is poorly understood. Here, using satellite-era observations from 1982–2015, we show that diabatic heating variability associated with the seasonal migration of the Inter-Tropical Convergence Zone controls the seasonality of the Atlantic Niño. The variability in precipitation, a measure of vertically integrated diabatic heating, leads that in SST, whereas the atmospheric response to SST variability is relatively weak. Our findings imply that the oceanic impact on the atmosphere is smaller than previously thought, questioning the relevance of the classical Bjerknes-feedback loop for the Atlantic Niño and limiting climate predictability over the equatorial Atlantic sector.

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 The Influence of Cloud Feedbacks on Equatorial Atlantic Variability

2015 ◽  
Vol 28 (7) ◽  
pp. 2725-2744 ◽  
Author(s):  
Katinka Bellomo ◽  
Amy C. Clement ◽  
Thorsten Mauritsen ◽  
Gaby Rädel ◽  
Bjorn Stevens
Keyword(s):  
Climate Variability ◽  
Cloud Cover ◽  
South Atlantic ◽  
Cloud Feedback ◽  
Equatorial Atlantic ◽  
Cloud Feedbacks ◽  
Sst Variability ◽  
Slab Ocean ◽  
Atlantic Niño ◽  
Sst Anomalies

Abstract Observations show that cloud feedback over the Namibian stratocumulus region is positive because cloud cover is anticorrelated with local sea surface temperature (SST) anomalies. Moreover, regressions of observed atmospheric fields on equatorial Atlantic SST anomalies indicate that cloud feedbacks over the Namibian stratocumulus region covary with Atlantic Niño. However, from observations alone, it is not possible to quantify the influence of regional cloud feedbacks on equatorial climate variability. To address this question, a set of sensitivity experiments are conducted using an atmospheric general circulation model (ECHAM6) coupled to a slab ocean in which the strength of positive cloud feedback is enhanced over several regions in the South Atlantic basin. Enhanced positive cloud feedback over the Namibian stratocumulus region increases local as well as equatorial SST variability, whereas enhanced cloud feedback over other regions in the South Atlantic increases local SST variability but exhibits negligible responses at the equator. The authors’ results indicate that the Namibian region plays a central role in enhancing equatorial SST variability because it is located where the SST anomalies associated with the simulated Atlantic Niño in the slab-ocean model develop. These results highlight the important role of the regional coupling of cloud cover over the Namibian region with local SSTs and its effects on equatorial Atlantic climate variability.

scholarly journals ENSO Influence on the Atlantic Niño, Revisited: Multi-Year versus Single-Year ENSO Events

2019 ◽  
Vol 32 (14) ◽  
pp. 4585-4600 ◽  
Author(s):  
Hiroki Tokinaga ◽  
Ingo Richter ◽  
Yu Kosaka
Keyword(s):  
Southern Oscillation ◽  
Walker Circulation ◽  
Equatorial Atlantic ◽  
Bjerknes Feedback ◽  
Central Pacific ◽  
Boreal Spring ◽  
Enso Events ◽  
The Past ◽  
Convergence Zones ◽  
Atlantic Niño

Abstract The influence of El Niño–Southern Oscillation (ENSO) on the Atlantic Niño over the past 113 years is investigated by comparing multi-year and single-year ENSO events. Multi-year ENSO events sustain an anomalous zonal gradient of sea surface temperature (SST) in the equatorial western to central Pacific even during boreal spring and summer. This SST gradient is coupled with an anomalous Walker circulation and atmospheric deep convection through the Bjerknes feedback. During multi-year La Niñas, for example, a strengthened Pacific Walker circulation extends into the tropical Atlantic in boreal spring, a season when both the Pacific and Atlantic intertropical convergence zones become more symmetric about the equator. As a result, surface westerly wind anomalies appear over the equatorial Atlantic, triggering an Atlantic Niño. By contrast, such a teleconnection is not found in the spring following the peak of single-year ENSO events. A Pacific pacemaker model experiment reproduces the observed atmospheric response and its impact on the Atlantic Niño, further supporting the importance of prolonged ENSO forcing. The contrasting influence of multi-year and single-year events explains the fragile relationship between ENSO and the Atlantic Niño. An empirical orthogonal function (EOF) analysis shows that the leading EOF mode (EOF-1) for the spring tropical western to central Pacific SST anomalies captures the characteristics of multi-year ENSO events. EOF-1 is highly correlated with the summer Atlantic Niño over the past 113 years while the Niño-3 SST is not. These correlations indicate that ocean–atmosphere coupling in the equatorial western to central Pacific plays a major role in shaping ENSO teleconnections in boreal spring.

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.

scholarly journals Causes of Enhanced SST Variability over the Equatorial Atlantic and Its Relationship to the Atlantic Zonal Mode in CMIP5

2017 ◽  
Vol 30 (16) ◽  
pp. 6171-6182 ◽  
Author(s):  
Yun Yang ◽  
Shang-Ping Xie ◽  
Lixin Wu ◽  
Yu Kosaka ◽  
Jianping Li
Keyword(s):  
Climate Model ◽  
Coupled Model ◽  
Cmip5 Models ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Equatorial Atlantic ◽  
Bjerknes Feedback ◽  
Thermal Forcing ◽  
Sst Variability ◽  
Tropical North Atlantic ◽  
Intercomparison Project

A spurious band of enhanced sea surface temperature (SST) variance (SBEV) is identified over the northern equatorial Atlantic in the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model, version 2.1. The SBEV is especially pronounced in boreal spring owing to the combined effect of both anomalous atmospheric thermal forcing and oceanic vertical upwelling. The SBEV is a common bias in phase 5 of the Coupled Model Intercomparison Project (CMIP5), found in 14 out of 23 models. The SBEV in CMIP5 is associated with the atmospheric thermal forcing and the oceanic vertical upwelling, similar to GFDL CM2.1. While the tropical North Atlantic variability is only weakly correlated with the Atlantic zonal mode (AZM) in observations, the SBEV in CMIP5 produces conditions that drive and intensify the AZM variability via triggering the Bjerknes feedback. This partially explains why AZM is strong in some CMIP5 models even though the equatorial cold tongue and easterly trades are biased low.

scholarly journals Is There Evidence of Changes in Tropical Atlantic Variability Modes under AMO Phases in the Observational Record?

2018 ◽  
Vol 31 (2) ◽  
pp. 515-536 ◽  
Author(s):  
Marta Martín-Rey ◽  
Irene Polo ◽  
Belén Rodríguez-Fonseca ◽  
Teresa Losada ◽  
Alban Lazar
Keyword(s):  
Southern Oscillation ◽  
Walker Circulation ◽  
Atlantic Multidecadal Oscillation ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
The North ◽  
Sst Variability ◽  
Tropical Atlantic Variability ◽  
Atlantic Niño

The Atlantic multidecadal oscillation (AMO) is the leading mode of Atlantic sea surface temperature (SST) variability at multidecadal time scales. Previous studies have shown that the AMO could modulate El Niño–Southern Oscillation (ENSO) variance. However, the role played by the AMO in the tropical Atlantic variability (TAV) is still uncertain. Here, it is demonstrated that during negative AMO phases, associated with a shallower thermocline, the eastern equatorial Atlantic SST variability is enhanced by more than 150% in boreal summer. Consequently, the interannual TAV modes are modified. During negative AMO, the Atlantic Niño displays larger amplitude and a westward extension and it is preceded by a simultaneous weakening of both subtropical highs in winter and spring. In contrast, a meridional seesaw SLP pattern evolving into a zonal gradient leads the Atlantic Niño during positive AMO. The north tropical Atlantic (NTA) mode is related to a Scandinavian blocking pattern during winter and spring in negative AMO, while under positive AMO it is part of the SST tripole associated with the North Atlantic Oscillation. Interestingly, the emergence of an overlooked variability mode, here called the horseshoe (HS) pattern on account of its shape, is favored during negative AMO. This anomalous warm (cool) HS surrounding an eastern equatorial cooling (warming) is remotely forced by an ENSO phenomenon. During negative AMO, the tropical–extratropical teleconnections are enhanced and the Walker circulation is altered. This, together with the increased equatorial SST variability, could promote the ENSO impacts on TAV. The results herein give a step forward in the better understanding of TAV, which is essential to improving its modeling, impacts, and predictability.

scholarly journals Atlantic Niño/Niña Prediction Skills in NMME Models

Atmosphere ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 803
Author(s):  
Ran Wang ◽  
Lin Chen ◽  
Tim Li ◽  
Jing-Jia Luo
Keyword(s):  
Sea Surface ◽  
Prediction Skill ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Equatorial Atlantic ◽  
Multimodel Ensemble ◽  
Spring Predictability Barrier ◽  
Anomaly Correlation Coefficient ◽  
Earth’S Climate ◽  
Atlantic Niño

The Atlantic Niño/Niña, one of the dominant interannual variability in the equatorial Atlantic, exerts prominent influence on the Earth’s climate, but its prediction skill shown previously was unsatisfactory and limited to two to three months. By diagnosing the recently released North American Multimodel Ensemble (NMME) models, we find that the Atlantic Niño/Niña prediction skills are improved, with the multi-model ensemble (MME) reaching five months. The prediction skills are season-dependent. Specifically, they show a marked dip in boreal spring, suggesting that the Atlantic Niño/Niña prediction suffers a “spring predictability barrier” like ENSO. The prediction skill is higher for Atlantic Niña than for Atlantic Niño, and better in the developing phase than in the decaying phase. The amplitude bias of the Atlantic Niño/Niña is primarily attributed to the amplitude bias in the annual cycle of the equatorial sea surface temperature (SST). The anomaly correlation coefficient scores of the Atlantic Niño/Niña, to a large extent, depend on the prediction skill of the Niño3.4 index in the preceding boreal winter, implying that the precedent ENSO may greatly affect the development of Atlantic Niño/Niña in the following boreal summer.

scholarly journals Leading Modes of the Upper-Ocean Temperature Interannual Variability along the Equatorial Atlantic Ocean in NCEP GODAS

2013 ◽  
Vol 26 (13) ◽  
pp. 4649-4663 ◽  
Author(s):  
Zeng-Zhen Hu ◽  
Arun Kumar ◽  
Bohua Huang ◽  
Jieshun Zhu
Keyword(s):  
Interannual Variability ◽  
Zonal Wind ◽  
Thermocline Depth ◽  
Ocean Temperature ◽  
Equatorial Atlantic ◽  
Equatorial Atlantic Ocean ◽  
Leading Mode ◽  
Equatorial Ocean ◽  
Meridional Mode ◽  
Atlantic Meridional Mode

Abstract In this work, the authors analyze the physical mechanisms of interannual variability of the upper-ocean temperature anomaly (OTA) in the equatorial Atlantic Ocean, using ocean reanalysis from the National Centers for Environmental Prediction (NCEP) Global Ocean Data Assimilation System. The variability of equatorial Atlantic OTA is dominated by two leading modes. The first mode is characterized by same-sign variation along the thermocline with pronounced amplitude in the central and eastern equatorial Atlantic. This mode represents the modulation of the overall thermocline depth at the equator generated by net heat convergence in the equatorial ocean, with heat content first accumulated mainly in the off-equatorial northwestern Atlantic in response to anomalous wind curl associated with Atlantic meridional mode. The second leading mode shows an opposite variation between the western and eastern Atlantic. This mode is mainly driven by the zonal wind stress fluctuation confined in the southwestern tropical and equatorial Atlantic and reflects the equatorial balanced response between the zonal slope of the equatorial thermocline depth and the atmospheric zonal wind variations with pronounced surface wind and ocean anomalies in the southwestern and equatorial ocean. The different characteristics of these two modes suggest that they may occur independently. In fact, evolution of the two leading modes is approximately in quadrature, and they may also occur in sequence on interannual time scales. The two leading mode-associated air–sea interaction processes suggest that the Atlantic meridional mode and zonal mode are statistically and physically connected in their evolution.

scholarly journals Relationship between the Hadley Circulation and Different Tropical Meridional SST Structures during Boreal Summer

2018 ◽  
Vol 31 (16) ◽  
pp. 6575-6590 ◽  
Author(s):  
Juan Feng ◽  
Jianping Li ◽  
Feifei Jin ◽  
Sen Zhao ◽  
Jianlei Zhu
Keyword(s):  
Asian Summer Monsoon ◽  
Hadley Circulation ◽  
Dominant Mode ◽  
Boreal Summer ◽  
Sst Variability ◽  
Warming Pattern ◽  
Leading Mode ◽  
Asian Summer ◽  
Tropical Sea ◽  
Relationship Of

Abstract The relationship of the Hadley circulation (HC) to different tropical sea surface temperature (SST) meridional structures during boreal summer is investigated over the period of 1979–2016. After decomposing the variations of the HC into the equatorially asymmetric HC (HEA), zonal-mean equatorially asymmetric SST (SEA), equatorially symmetric HC (HES), and equatorially symmetric SST (SES) components, the ratio of the HEA associated with SEA with respect to the HES associated with SES is around 2 across multiple reanalyses, which is a smaller ratio than in the annual and seasonal cycle. The reduced ratio of the HC to SST is due to the regional SST variation in the Asian summer monsoon (ASM) domain. The first leading mode (EOF1) of the regional SST variability in the ASM domain is dominated by a homogeneous warming pattern. This pattern is associated with an equatorially asymmetric HC, but it has an opposite direction to the climatological HEA and so weakens the HEA. The second dominant mode has an El Niño–like pattern, which resembles the distribution of the principal mode of the SST in the non-ASM region. Both modes are responsible for the variation of HES. However, the SST EOF1 in the ASM domain displays a significant upward trend, favoring a suppressed HEA, and leading to the smaller ratio of the HC to SST during boreal summer. Moreover, the variation of the SST EOF1 is closely linked with the intensity of the ASM, highlighting the potential modulation by the ASM of the relation between the HC and SST during boreal summer.

scholarly journals Indian Ocean Dipole leads to Atlantic Niño

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Lei Zhang ◽  
Weiqing Han
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Southern Oscillation ◽  
Global Climate ◽  
Tropical Indian Ocean ◽  
Equatorial Atlantic ◽  
Trade Winds ◽  
Interannual Climate Variability ◽  
Numerical Model Experiments ◽  
Atlantic Niño

AbstractAtlantic Niño is the Atlantic equivalent of El Niño-Southern Oscillation (ENSO), and it has prominent impacts on regional and global climate. Existing studies suggest that the Atlantic Niño may arise from local atmosphere-ocean interaction and is sometimes triggered by the Atlantic Meridional Mode (AMM), with overall weak ENSO contribution. By analyzing observational datasets and performing numerical model experiments, here we show that the Atlantic Niño can be induced by the Indian Ocean Dipole (IOD). We find that the enhanced rainfall in the western tropical Indian Ocean during positive IOD weakens the easterly trade winds over the tropical Atlantic, causing warm anomalies in the central and eastern equatorial Atlantic basin and therefore triggering the Atlantic Niño. Our finding suggests that the cross-basin impact from the tropical Indian Ocean plays a more important role in affecting interannual climate variability than previously thought.

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