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  • Atlantic Meridional Mode (AMM) / Atlantic Multidecadal Oscillation (AMO) and Variability (AMV)


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    Here are the current Papers & Articles under the research topics

    • Atlantic Meridional Mode (AMM )
    • Atlantic Multidecadal Oscillation (AMO )
    • Atlantic Multidecadal Variability (AMV).

    Click on the title of a paper you are interested in to go straight to the full paper. Papers and articles covering the basics (ideal for learning) are shown in Green.

    About the Atlantic Multidecadal Oscillation (AMO)
    Continually updated FAQs from NOAA

    What is the Atlantic AMO (Atlantic Multidecadal Oscillation)?
    NOAA description/explanation of what the AMO is.

    Atlantic Multidecadal Oscillation (AMO) - NOAA Data
    AMO Index updated monthly by NOAA

    Climate impacts of the Atlantic Multidecadal Oscillation simulated in the CMIP5 models: A re‐evaluation based on a revised index
    Published April 2017.
    Abstract:
    The Atlantic Multidecadal Oscillation (AMO) has pronounced influences on weather and climateacross the globe. This study provides a direct comparison of the observed AMO-related surface temperature and precipitation anomalies to those simulated in the Coupled Model Intercomparison Project Phase 5(CMIP5) models. It is found that the model-simulated AMO-related features are obscured by the global signal in some key regions if the North Atlantic sea surface temperature (SST) itself is used to represent the AMO as in previous studies. After the global mean SST is removed from the North Atlantic SST, the CMIP5 models show substantially better agreement with the observations in terms of the AMO-related worldwide impacts,such as the Pacific SST and the rainfall over the United States and India. These results suggest the removal of the global signal or signals originating in other ocean basins is a necessary procedure to uncover the AMO features in climate model simulations.

    Emerging negative Atlantic Multidecadal Oscillation index in spite of warm subtropics
    Published July 2017.
    Abstract:
    Sea surface temperatures in the northern North Atlantic have shown a marked decrease over the past several years. The sea surface in the subpolar gyre is now as cold as it was during the last cold phase of the Atlantic Multidecadal Oscillation index in the 1990s. This climate index is associated with shifts in hurricane activity, rainfall patterns and intensity, and changes in fish populations. However, unlike the last cold period in the Atlantic, the spatial pattern of sea surface temperature anomalies in the Atlantic is not uniformly cool, but instead has anomalously cold temperatures in the subpolar gyre, warm temperatures in the subtropics and cool anomalies over the tropics. The tripole pattern of anomalies has increased the subpolar to subtropical meridional gradient in SSTs, which are not represented by the AMO index value, but which may lead to increased atmospheric baroclinicity and storminess. Here we show that the recent Atlantic cooling is likely to persist, as predicted by a statistical forecast of subsurface ocean temperatures and consistent with the irreversible nature of watermass changes involved in the recent cooling of the subpolar gyre.

    ENSO Amplitude Modulation Associated with the Mean SST Changes in the Tropical Central Pacific Induced by Atlantic Multidecadal Oscillation
    Published July 2014.
    Abstract:
    The mechanism associated with the modulation of the El Niño–Southern Oscillation (ENSO) amplitude caused by the Atlantic multidecadal oscillation (AMO) is investigated by using long-term historical observational data and various types of models. The observational data for the period 1900–2013 show that the ENSO variability weakened during the positive phase of the AMO and strengthened in the negative phase.Such a relationship between the AMO and ENSO amplitude has been reported by a number of previous studies. In the present study the authors demonstrate that the weakening of the ENSO amplitude during the positive phase of the AMO is related to changes of the SST cooling in the eastern and central Pacific accompanied by the easterly wind stress anomalies in the equatorial central Pacific, which were reproducedreasonably well by coupled general circulation model (CGCM) simulations performed with the Atlantic Ocean SST nudged perpetually with the observed SST representing the positive phase of the AMO and the free integration in the other ocean basins. Using a hybrid coupled model, it was determined that the mechanism associated with the weakening of the ENSO amplitude is related to the westward shift and weakening of the ENSO zonal wind stress anomalies accompanied by the westward shift of precipitation anomalies associated with the relatively cold background mean SST over the central Pacific.

    Gulf Stream Excursions and Sectional Detachments Generate the Decadal Pulses in the Atlantic Multidecadal Oscillation
    Published Jan 2018.
    Abstract:
    Decadal pulses within the lower-frequency Atlantic Multidecadal Oscillation (AMO) are a prominent but underappreciated AMO feature, representing decadal variability of the subpolar gyre (e.g., the 1970s Great Salinity Anomaly) and wielding notable influence on the hydroclimate of the African and American continents. Here we seek clues into their origin in the spatio temporal developmentof the Gulf Stream’s (GS) meridional excursions and sectional detachments evident in the 1954-2012 record of ocean surface and subsurface salinity and temperature observations.

    Impact of the Atlantic Multidecadal Oscillation (AMO) on deriving anthropogenic warming rates from the instrumental temperature record
    Published Oct 2014.
    Abstract:
    The instrumental surface air temperature record has been used in several statistical studies to assess the relative role of natural and anthropogenic drivers of climate change. The results of those studies varied considerably, with anthropogenic temperature trends over the past 25–30 years suggested to range from 0.07to 0.20◦C decade−1. In this short communication, we assess the origin of these differences and highlight the inverse relation between the temperature trend of the past 30 years and the weight given to the Atlantic Multidecadal Oscillation (AMO) as an explanatory factor in the multiple linear regression (MLR) tool that is usually employed. We highlight that robust MLR outcomes require a better understanding of the AMO in general and,more specifically, of its characterization. Our results indicate that both the high and the low end of the anthropogenic trend over the past 30 years found in previous studies are unlikely and that a transient climate response of 1.6 (1.0–3.3)◦C best captures the historic instrumental temperature record.

    Impacts of the Atlantic Multidecadal Variability on North American Summer Climate and Heat Waves
    Published Feb 2018.
    Abstract:
    The impacts of the Atlantic Multidecadal Variability (AMV) on the summer North American climate are investigated using three Coupled Global Climate Models (CGCMs) in which the North Atlantic sea surface temperatures (SSTs) are restored to observed AMV anomalies. Large ensemble simulations are performed in order to estimate how AMV can modulate the occurrence of extreme weather events like heat waves. We show that, in response to an AMV warming, all models simulate a precipitation deficit and a warming over Northern Mexico and Southern US that lead to an increased number of heat wave days by about 30% compared to an AMV cooling. The physical mechanisms associated with these impacts are discussed. The positive tropical Atlantic SST anomalies associated with the warm AMV drive a Matsuno-Gill-like atmospheric response that favors subsidence over Northern Mexico and Southern US. This leads to a warming of the whole tropospheric column, and to a decrease in relative humidity, cloud cover, and precipitation. Soil moisture also plays a role in the modulation of heat wave occurrence by AMV. An AMV warming favors dry soil conditions over Northern Mexico and Southern US by driving year-round precipitation deficit through atmospheric teleconnections coming both directly from the North Atlantic SST forcing and indirectly from the Pacific. The indirect AMV teleconnections highlight the importance of using CGCMs to fully assess the AMV impacts on North America. Given the potential predictability of the AMV, the teleconnections discussed here imply a source of predictability for the North American climate variability and in particular for the occurrence of heat waves at multi-year timescale.

    Insights into Atlantic multidecadal variability using the Last Millennium Reanalysis framework
    Published 2018.
    Abstract:
    The Last Millennium Reanalysis (LMR) employs a data assimilation approach to reconstruct climate fields from annually resolved proxy data over years 0–2000 CE.We use the LMR to examine Atlantic multidecadal variability (AMV) over the last 2 millennia and find several robust thermodynamic features associated with a positive Atlantic Multidecadal Oscillation (AMO) index that reveal a dynamically consistent pattern of variability: the Atlantic and most continents warm; sea ice thins over the Arctic and re-treats over the Greenland, Iceland, and Norwegian seas; and equatorial precipitation shifts northward. The latter is consistent with anomalous southward energy transport mediated by the atmosphere. Net downward shortwave radiation in-creases at both the top of the atmosphere and the surface,indicating a decrease in planetary albedo, likely due to a de-crease in low clouds. Heat is absorbed by the climate system and the oceans warm. Wavelet analysis of the AMO time series shows a reddening of the frequency spectrum on the 50-to 100-year timescale, but no evidence of a distinct multi-decadal or centennial spectral peak. This latter result is in-sensitive to both the choice of prior model and the calibration dataset used in the data assimilation algorithm, suggesting that the lack of a distinct multidecadal spectral peak is a robust result.

    Interdecadal variability in pan-Pacific and global SST, revisited
    Published May 2018.
    Abstract:
    Interest in the “Interdecadal Pacific Oscillation (IPO)” in the global SST has surged recently on suggestions that the Pacific may be the source of prominent interdecadal variations observed in the global-mean surface temperature possibly through the mechanism of low-frequency modulation of the interannual El Nino-Southern Oscillation (ENSO) phenomenon. IPO was defined by performing empirical orthogonal function (EOF) analysis of low-pass filtered SST. The low-pass filtering creates its unique set of mathematical problems—in particular, mode mixing—and has led to some questions, many unanswered. To understand what these EOFs are, we express them first in terms of the recently developed pairwise rotated EOFs of the unfiltered SST, which can largely separate the high and low frequency bands without resorting to filtering. As reported else-where, the leading rotated dynamical modes (after the global warming trend) of the unfiltered global SST are: ENSO, Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). IPO is not among them. The leading principal component (PC) of the low-pass filtered global SST is usually defined as IPO and it is seen to comprise of ENSO, PDO and AMO in various proportions depending on the filter threshold. With decadal filtering, the contribution of the interannual ENSO is understandably negligible. The leading dynamical mode of the filtered global SST is mostly AMO, and therefore should not have been called the Interdecadal “Pacific” Oscillation. The leading dynamical mode of the filtered pan-Pacific SST is mostly PDO. This and other low-frequency variability that have the action center in the Pacific, from either the pan-Pacific or global SST, have near zero global mean.

    Is There Evidence of Changes in Tropical Atlantic Variability Modes under AMO Phases in the Observational Record?
    Published Jan 2018.
    Abstract:
    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 ElNiño–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 stepforward in the better understanding of TAV, which is essential to improving its modeling, impacts, and predictability.

    Linking Emergence of the Central Pacific El Niño to the Atlantic Multidecadal Oscillation
    Published May 2014.
    Abstract:
    The ocean–atmosphere coupling in the northeastern subtropical Pacific is dominated by a Pacific meridi-onal mode (PMM), which spans between the extratropical and tropical Pacific and plays an important role inconnecting extratropical climate variability to the occurrence of El Niño. Analyses of observational data and numerical model experiments were conducted to demonstrate that the PMM (and the subtropical Pacific coupling) experienced a rapid strengthening in the early 1990s and that this strengthening is related to an intensification of the subtropical Pacific high caused by a phase change of the Atlantic multidecadal oscillation (AMO). This PMM strengthening favored the development of more central Pacific (CP)-type El Niño events.The recent shift from more conventional eastern Pacific (EP) to more CP-type El Niño events can thus be at least partly understood as a Pacific Ocean response to a phase change in the AMO.

    Oceanic forcing of the interhemispheric SST dipole associated with the Atlantic Multidecadal Oscillation
    Published May 2018.
    Abstract:
    In this study, the interhemispheric sea surface temperature (SST) signature of the Atlantic Multidecadal Oscillation (AMO) is analyzed and compared between observations and slab ocean model (SOM) simulations. Observational analysis suggests a robust inter hemispheric SST dipole across the Atlantic associated with the AMO, manifested by a strong inverse relationship between the AMO and subpolar South Atlantic decadal SST anomalies. None of the SOMs analyzed could reproduce the observed interhemispheric dipole of the AMO; instead,they consistently simulate an interhemispheric coherent SST pattern. In the SOMs, the North Atlantic decadal SST anomalies synchronize the variations of South Atlantic SST through a cross-hemispheric atmosphere teleconnection and thermodynamic processes. This discrepancy between the SOM simulations and the observation is possibly due to deficiencies in representing ocean dynamical processes. Further analyses of the fully coupled versions of the SOMs suggest that the observed interhemispheric dipole of the AMO can be reproduced only by including ocean dynamics related to the Atlantic meridional overturning circulation. Our findings highlight that the ocean dynamics play a non-negligible role and should be taken into consideration in better understanding the observed feature of the AMO.

    Relationship of Multidecadal Global Temperatures to Multidecadal Oceanic Oscillations
    Published 2011.
    An interesting paper published by Department of Geology, Western Washington University, Bellingham. No abstract but the paper looks at the interaction and impact of various teleconnections including the AMO, SOI, PDO, ENSO and NAO.

    The Atlantic Meridional Mode and hurricane activity
    Published 2007.
    Abstract:
    Connections between the Atlantic Meridional Mode (AMM) and seasonal hurricane activity are investigated. The AMM, a dynamical “mode” of variability intrinsic to the tropical coupled ocean‐atmosphere system, is strongly related to seasonal hurricane activity on both decadal and interannual time scales. The connection arises due to the AMM's relationship with a number of local climatic conditions that all cooperate in their influence on hurricane activity. Further analysis indicates that the Atlantic Multi‐decadal Oscillation (AMO) can excite the AMM on decadal time scales. As such, it is suggested that the AMO's influence on seasonal hurricane activity manifests itself through the AMM. This relationship between the AMM, AMO, and seasonal hurricane activity refocuses our understanding of how climate variations relate to seasonal hurricane activity in the Atlantic, and offers an improved framework beyond purely thermodynamic arguments that relates hurricanes to large‐scale climate variations.

    The Dynamical Influence of the Atlantic Multidecadal Oscillation on Continental Climate
    Published Aug 2017.
    Abstract:
    The Atlantic multidecadal oscillation (AMO) in sea surface temperature (SST) has been shown to influence the climate of the surrounding continents. However, it is unclear to what extent the observed impact of the AMO is related to the thermodynamical influence of the SST variability or the changes in large-scale atmospheric circulation. Here, an analog method is used to decompose the observed impact of the AMO into dynamical and residual components of surface air temperature (SAT) and precipitation over the adjacent continents. Over Europe the influence of the AMO is clearest during the summer, when the warm SAT anomalies are interpreted to be primarily thermodynamically driven by warm upstream SST anomalies but also amplified by the anomalous atmospheric circulation. The overall precipitation response to the AMO in summer is generally less significant than the SAT but is mostly dynamically driven. The decomposition is also applied to the North American summer and the Sahel rainy season. Both dynamical and residual influences on the anomalous precipitation over the Sahel are substantial, with the former dominating over the western Sahel region and the latter being largest over the eastern Sahel region. The results have potential implications for understanding the spread in AMO variability in coupled climate models and decadal prediction systems.

    The Influence of El Niño–Southern Oscillation and the Atlantic Multidecadal Oscillation on Caribbean Tropical Cyclone Activity
    Published June 2010.
    Abstract:
    Caribbean basin tropical cyclone activity shows significant variability on interannual as well as multidecadal time scales. Comprehensive statistics for Caribbean hurricane activity are tabulated, and then large-scale climate features are examined for their impacts on this activity. The primary interannual driver of variability isfound to be El Nin ̃o–Southern Oscillation, which alters levels of activity due to changes in levels of vertical wind shear as well as through column stability. Much more activity occurs in the Caribbean with La Nin ̃a conditions than with El Nin ̃o conditions. On the multidecadal time scale, the Atlantic multidecadal oscillationis shown to play a significant role in Caribbean hurricane activity, likely linked to its close relationship with multidecadal alterations in the size of the Atlantic warm pool and the phase of the Atlantic meridional mode.When El Nin ̃o–Southern Oscillation and the Atlantic multidecadal oscillation are examined in combination,even stronger relationships are found due to a combination of either favorable or unfavorable dynamic and thermodynamic factors. For example, 29 hurricanes tracked into the Caribbean in the 10 strongest La Nin ̃a years in a positive Atlantic multidecadal oscillation period compared with only two hurricanes tracking through the Caribbean in the 10 strongest El Nin ̃o years in a negative Atlantic multidecadal oscillation period.

    The impact of the AMO on multidecadal ENSO variability
    Published April 2017.
    Abstract:
    Multidecadal shifts in El Niño–Southern Oscillation (ENSO) variability have been observed,but it is unclear if this variability is just a random variation in the ENSO cycle or whether it is forced by othermodes of climate variability. Here we show a strong influence of the Atlantic on the multidecadal variability of ENSO. The Atlantic Multidecadal Oscillation (AMO) is the dominant mode of multidecadal sea surface temperature (SST) variability in the Atlantic Ocean. Changes in AMO-related tropical Atlantic SSTs are known to force changes in the Walker circulation in the tropical Pacific Ocean. Using conceptual and coupled model experiments, we show that these changes to the Walker circulation modify ENSO stability on both annual and multidecadal time scales leading to a distinctive pattern of multidecadal ENSO variability that we find inobservations and ocean reanalyses

    The Central Role of Ocean Dynamics in Connecting the NAO to the Extratropical Component of the AMO
    Published Jan 2017.
    Abstract:
    The relationship between the North Atlantic Oscillation (NAO) and Atlantic sea surface temperature(SST) variability is investigated using models and observations. Coupled climate models are used in which the ocean component is either a fully dynamic ocean or a slab ocean with no resolved ocean heat transport. Ontime scales less than 10 yr, NAO variations drive a tripole pattern of SST anomalies in both observations and models. This SST pattern is a direct response of the ocean mixed layer to turbulent surface heat flux anomalies associated with the NAO. On time scales longer than 10 yr, a similar relationship exists between the NAO and the tripole pattern of SST anomalies in models with a slab ocean. A different relationship exists both for the observations and for models with a dynamic ocean. In these models, a positive (negative) NAO anomaly leads, after a decadal-scale lag, to a monopole pattern of warming (cooling) that resembles the Atlantic multidecadal oscillation (AMO), although with smaller-than-observed amplitudes of tropical SST anomalies.Ocean dynamics are critical to this decadal-scale response in the models. The simulated Atlantic meridional overturning circulation (AMOC) strengthens (weakens) in response to a prolonged positive (negative) phase of the NAO, thereby enhancing (decreasing) poleward heat transport, leading to broad-scale warming(cooling). Additional simulations are used in which heat flux anomalies derived from observed NAO variations from 1901 to 2014 are applied to the ocean component of coupled models. It is shown that ocean dynamics allow models to reproduce important aspects of the observed AMO, mainly in the Subpolar Gyre.

    The role of historical forcings in simulating the observed Atlantic multidecadal oscillation
    Published Feb 2017.
    Abstract:
    We analyze the Atlantic multidecadal oscillation (AMO) in the preindustrial (PI) and historical (HIST) simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) to assess the drivers of the observed AMO from 1865 to 2005. We draw 141 year samples from the 41 CMIP5 model's PI runs and compare the correlation and variance between the observed AMO and the simulated PI and HIST AMO. The correlation coefficients in 38 forced (HIST) models are above the 90% confidence level and explain up to 56% of the observed variance. The probability that any of the unforced (PI) models do as well is less than 3% in 31 models. Multidecadal variability is larger in 39 CMIP5 HIST simulations and in all HIST members of the Community Earth System Model Large Ensemble than their corresponding PI. We conclude that there is an essential role for external forcing in driving the observed AMO.

     

     

     

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