WES feedback and the Atlantic Meridional Mode: observations and CMIP5 comparisons

2016 ◽  
Vol 49 (5-6) ◽  
pp. 1665-1679 ◽  
Author(s):  
Dillon J. Amaya ◽  
Michael J. DeFlorio ◽  
Arthur J. Miller ◽  
Shang-Ping Xie
Keyword(s):  
Wes Feedback ◽  
Meridional Mode ◽  
Atlantic Meridional Mode

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 Impact of the Atlantic Meridional Mode on Gulf Stream North Wall Position

2018 ◽  
Vol 31 (21) ◽  
pp. 8875-8894 ◽  
Author(s):  
Sultan Hameed ◽  
Christopher L. P. Wolfe ◽  
Lequan Chi
Keyword(s):  
Gulf Stream ◽  
Cold Water ◽  
Meridional Overturning Circulation ◽  
Major Influence ◽  
Tropical Atlantic ◽  
The North ◽  
Meridional Mode ◽  
North Wall ◽  
Atlantic Meridional Mode ◽  
Wall Position

The path of the Gulf Stream as it leaves the continental shelf near Cape Hatteras is marked by a sharp gradient in ocean temperature known as the North Wall. Previous work in the literature has considered processes related to the North Atlantic Oscillation (NAO) in triggering latitudinal displacements of the North Wall position. This paper presents evidence that the Atlantic meridional mode (AMM) also impacts interannual variations of the North Wall position. The AMM signal from the tropics propagates to the Gulf Stream near the 200-m depth, and there are two time scales for this interaction. Anomalous Ekman suction induced by AMM cools the tropical Atlantic. The cold water in the Caribbean Sea is entrained into the currents feeding the Gulf Stream, and this cooling signal reaches the North Wall within a year. A second mechanism involves cold anomalies in the western tropical Atlantic, which initially propagate westward as baroclinic planetary waves, reaching the Gulf Stream and resulting in a southward shift in the North Wall position after a delay of about one year. In an analysis for the period 1961–2015, AMM’s signal dominates North Wall fluctuations in the upper 300 m, while NAO is the major influence below ~500 m; the influence of both the teleconnections is seen between 300 and 500 m. The relationship between the Atlantic meridional overturning circulation (AMOC) and the North Wall is investigated for the 2005–15 period and found to be statistically significant only at the sea surface in one of the three North Wall indices used.

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 Variability of the Atlantic Meridional Mode during the Atlantic Hurricane Season

2011 ◽  
Vol 24 (5) ◽  
pp. 1409-1424 ◽  
Author(s):  
Dimitry Smirnov ◽  
Daniel J. Vimont
Keyword(s):  
Tropical Cyclone ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Ocean Model ◽  
Atlantic Hurricane ◽  
Hurricane Season ◽  
Meridional Mode ◽  
Upper Level ◽  
Atlantic Meridional Mode ◽  
Sst Anomalies

Abstract An observational and modeling study is conducted to investigate the structure of the Atlantic Meridional Mode (AMM) during the Atlantic hurricane season, and the relationship between AMM-related SST anomalies and environmental conditions that influence seasonal tropical cyclone activity. The observational analysis shows that during the Atlantic hurricane season the AMM exhibits a similar SST and low-level wind structure as during boreal spring (when the AMM is most active). Observed AMM SST variations are accompanied by air temperature and moisture anomalies that are limited to the boundary layer and an anomalous baroclinic circulation structure in the northern subtropical Atlantic with an anomalous lower-level cyclonic circulation residing under an anomalous upper-level anticyclone during a warm phase. This baroclinic structure contributes to a reduction in vertical wind shear over the tropical Atlantic that is dominated by changes in the upper-level flow. Two sets of model experiments were conducted, in which the NCAR Community Atmospheric Model version 3.1 (CAM3.1) was coupled to a slab ocean model or a data ocean model. In each experiment, the model was either initialized with or forced by AMM-like SST anomalies during boreal summer. The simulations yielded a similar spatial structure to that in the observations, including the baroclinic atmospheric circulation and associated reduction in vertical wind shear. The similarity between the modeled and observed AMM structures strongly suggests a causal relationship in which the AMM-like SST anomalies are responsible for generating environmental conditions that can strongly influence seasonal tropical cyclone variability.

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.

scholarly journals Multidecadal Cycles of the Climatic Index Atlantic Meridional Mode: Sunspots that Affect North and Northeast of Brazil

2020 ◽  
Author(s):  
Cleber Souza Correa ◽  
Roberto Lage Guedes ◽  
André Muniz Marinho da Rocha ◽  
Karlmer Abel Bueno Corrêa
Keyword(s):  
Surface Temperature ◽  
Statistical Correlation ◽  
Sunspot Activity ◽  
Climatic Index ◽  
Temperature Anomalies ◽  
Monthly Average ◽  
Historical Series ◽  
Fernando De Noronha ◽  
Meridional Mode ◽  
Atlantic Meridional Mode

Using the 1951-2017 historical series of the Atlantic Meridional Mode (AMM) index and the monthly number of sunspots, it was possible to observe a significant association between them. The use of wavelet and cross-wavelet analysis showed the presence of multidecadal cycles pronounced in eleven years, as well as cycles of 2.66 and 5.33. AMM index showed, in the part of the Sea Surface Temperature (SST), the presence of a weak signal of 21.33 years. Influence and association of sunspot variability on surface temperature in Northern and Northeastern regions of Brazil were investigated. Using a non-parametric statistical correlation test, the historical series of surface temperature anomalies in five locations (Belém, São Luiz, Fortaleza, Fernando de Noronha, and Natal) were compared with the monthly solar-series anomalies. The temperature series used were the minimum monthly average, the monthly average, and maximum monthly average temperatures, with their respective anomalies in relation to the mean. However, among all the series (except for São Luiz), the analyzed minimum temperature anomalies showed a negative correlation with sunspots. As a preliminary result, the analyzed climatic indexes present an apparent degree of memory associated with the variability of sunspot activity.

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.

Climatological Variations in North Atlantic Tropical Cyclone Tracks

2012 ◽  
Vol 25 (2) ◽  
pp. 657-673 ◽  
Author(s):  
Angela J. Colbert ◽  
Brian J. Soden
Keyword(s):  
Tropical Cyclone ◽  
El Niño ◽  
Large Scale ◽  
El Nino ◽  
Environmental Parameters ◽  
Steering Flow ◽  
Systematic Shift ◽  
Meridional Mode ◽  
The Relationship ◽  
Atlantic Meridional Mode

Abstract This study investigates the relationship between tropical cyclone (TC) tracks and climatological variations in large-scale environmental parameters associated with the TC steering flow. By using the Atlantic Ocean hurricane database for 1950–2010, TCs that form in the main development region (MDR) are categorized into one of three track types: straight moving, recurving landfall, or recurving ocean. As expected, the straight-moving storms are associated with a westward extension and strengthening of the subtropical high, whereas the recurving ocean storms are associated with a weakening of the high. The presence of El Niño conditions in the tropical Pacific Ocean is shown to be associated with a weakening of the high, an increase in the percentage of recurving ocean TCs, and a decrease in the percentage of recurving landfall TCs. Positive phases of the Atlantic Meridional Mode are associated with an increase in the percentage of recurving ocean TCs and a decrease in the percentage of straight-moving TCs. Synthetic tracks are simulated for each storm using a beta and advection model. Sensitivity experiments using both observed and uniformly seeded genesis locations indicate that the path of straight-moving TCs is largely a reflection of their tendency to form in the southwestern portion of the MDR rather than of differences in steering flow. These experiments also suggest that the shift in TC tracks associated with El Niño/La Niña conditions is largely attributable to changes in the steering flow, whereas the track changes associated with variations in the Atlantic Meridional Mode are due to a systematic shift in genesis location.

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