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  • Atmospheric Angular Momentum (AAM)

    Here are the current Papers & Articles under the research topic Atmospheric Angular Momentum (AAM). Click on the title of a paper you are interested in to go straight to the full paper.

    Angular momentum in the global atmospheric circulation
    Published Dec 2007.
    Angular momentum is a variable of central importance to the dynamics of the atmosphere both regionally and globally. Moreover, the angular momentum equations yield a precise description of the dynamic interaction of the atmosphere with the oceans and the solid Earth via various torques as exerted by friction, pressure against the mountains and the nonspherical shape of the Earth, and by gravity. This review presents recent work with respect to observations and the theory of atmospheric angular momentum of large‐scale motions. It is mainly the recent availability of consistent global data sets spanning decades that sparked renewed interest in angular momentum. In particular, relatively reliable estimates of the torques are now available. In addition, a fairly wide range of theoretical aspects of the role of angular momentum in atmospheric large‐scale dynamics is covered.

    A Synoptic–Dynamic Model of Subseasonal Atmospheric Variability
    Published May 2006.
    A global synoptic–dynamic model (GSDM) of subseasonal variability is proposed to provide a framework for real-time weather–climate monitoring and to assist with the preparation of medium-range (e.g., week1–3) predictions. The GSDM is used with a regional focus over North America during northern winter. A case study introduces the time scales of the GSDM and illustrates two circulation transitions related to eastward-moving wave energy signals and their connection to remote tropical forcing. Global and zonal atmospheric angular momentum (AAM) is used to help define the synoptic evolution of the GSDM components and to link regional synoptic variations with physical processes like the global mountain and frictional torque. The core of the GSDM consists of four stages based on the Madden–Julian oscillation (MJO) recurrence time. Additionally, extratropical behaviors including teleconnection patterns, baroclinic life cycles, and monthly oscillations provide intermediate and fast time scales that are combined with the quasi-oscillatory (30–70 day) MJO to define multiple time-/space-scale linear relationships. A unique feature of the GSDM is its focus on global and regional circulation transitions and the related extreme weather events during periods of large global AAM tendency.

    Atmospheric torques and Earth’s rotation: what drove the millisecond-level length-of-day response to the 2015–2016 El Niño?
    Published July 2017.
    El Niño–Southern Oscillation (ENSO) events are classically associated with a significant increase inthe length of day (LOD), with positive mountain torques arising from an east–west pressure dipole in the Pacific driving a rise of atmospheric angular momentum (AAM) and consequent slowing of the Earth’s rotation. The large 1982–1983 event produced a lengthening of the day of about 0.9 ms, while a major ENSO event during the 2015–2016 winter season produced an LOD excursion reaching 0.81 ms in January 2016. By evaluating the anomaly in mountain and friction torques, we found that (i) as a mixed eastern–central Pacific event, the 2015–2016 mountain torque was smaller than for the 1982–1983 and 1997–1998 events, which were pure eastern Pacific events, and (ii) the smaller mountain torque was compensated for by positive friction torques arisingfrom an enhanced Hadley-type circulation in the eastern Pacific, leading to similar AAM–LOD signatures for all three extreme ENSO events. The 2015–2016 event thus contradicts the existing paradigm that mountain torques cause the Earth rotation response for extreme El Niño events.

    Axial Angular Momentum: Vertical Fluxes and Response to Torques
    Published Nov 2003.
    The horizontally averaged global angular momentum u at a certain height reacts only to the vertical divergence of the angular momentum flux at least above the crest height of the earth’s orography. The flux is tied to the torques at the surface. Data are used to evaluate the flux and the response ofmto the torques. It is shown that the accuracy of the data is sufficient for an investigation of this interaction. It is found that the horizontally averaged angular momentum in the upper troposphere and lower stratosphere tends to be negative before an event of positive friction torque. Downward transports of negative angular momentum from these layers allow the angular momentum to further decrease near the ground, even shortly before the event although the friction torque is positive at that time. The impact of the mountains during this process is demonstrated. The ensuing positive response to the friction torque is felt throughout the troposphere.The final decay of this reaction involves downward transports of m with typical velocities of;1–2 km day21. The angular momentum in the lower troposphere tends to be negative before an event of positive mountain torque. There is a short burst of rapid upward transport of positive angular momentum during the event itself,which reaches the stratosphere within 1–2 days. A phase of decay follows with slow downward transport of positive angular momentum.

    Centennial Trend and Decadal-to-Interdecadal Variability of Atmospheric Angular Momentum in CMIP3 and CMIP5 Simulation.
    Published Nov 2012.
    The climatology and trend of atmospheric angular momentum from the phase 3 and the phase 5 ClimateModel Intercomparison Project (CMIP3 and CMIP5, respectively) simulations are diagnosed and validated with the Twentieth Century Reanalysis (20CR). It is found that CMIP5 models produced a significantlysmaller bias in the twentieth-century climatology of the relative MR and omega MV angular momentum compared to CMIP3. The CMIP5 models also produced a narrower ensemble spread of the climatology and trend of MR and MV. Both CMIP3 and CMIP5 simulations consistently produced a positive trend in MR and MV for the twentieth and twenty-first centuries. The trend for the twenty-first century is much greater, reflecting the role of greenhouse gas (GHG) forcing in inducing the trend. The simulated increase in MR for the twentieth century is consistent with reanalysis. Both CMIP3 and CMIP5 models produced a wide range of magnitudes of decadal and interdecadal variability of MR compared to 20CR. The ratio of the simulated standard deviation of decadal or interdecadal variability to its observed counterpart ranges from 0.5 to over 2.0 for individual models. Nevertheless, the bias is largely random and ensemble averaging brings the ratio to within 18% of the reanalysis for decadal and interdecadal variability for both CMIP3 and CMIP5. The twenty-first-century simulations from both CMIP3 and CMIP5 produced only a small trend in the amplitude of decadal or interdecadal variability, which is not statistically significant. Thus, while GHG forcing induces a significant increase in the climatological mean of angular momentum, it does not significantly affect its decadal-to-interdecadal variability in the twenty-first century.

    Isentropic Pressure and Mountain Torques
    Published March 2009.
    The relation of pressure torques and mountain torques is investigated on the basis of observations for the polarcaps, two midlatitude and two subtropical belts, and a tropical belt by evaluating the lagged covariances of these torques for various isentropic surfaces. It is only in the polar domains and the northern midlatitude belts that the transfer of angular momentum to and from the earth at the mountains is associated with pressure torques acting in the same sense. The situation is more complicated in all other belts. The covariances decline with increasing potential temperature (height). The role of both torques in the angular momentum budget of a belt is discussed

    Latitude–Height Structure of the Atmospheric Angular Momentum Cycle Associated with the Madden–Julian Oscillation
    Published June 2006.
    The angular momentum cycle of the Madden–Julian oscillation is analyzed by regressing the zonally averaged axial angular momentum (AAM) budget including fluxes and torques against the first two principal components P1 and P2 of the empirical orthogonal functions (EOFs) of outgoing longwave radiation (OLR). The maximum of P1coincides with an OLR minimum near 150°E and a shift from anomalously negative AAM to positive AAM in the equatorial troposphere. AAM anomalies of one sign develop first in the upper-equatorial troposphere and then move downward and poleward to the surface of the subtropics within two weeks. During the same time the opposite sign AAM anomaly develops in the upper-equatorial troposphere. The tropical troposphere is warming when P1 approaches its maximum while the stratosphere is cooling. The torques are largest in the subtropics and are linked with the downward and poleward movement of AAM anomalies. The evolution is conveniently summarized using a time–height depiction of the global mean AAM and vertical flux anomaly.

    Mountains, the Global Frictional Torque, and the Circulation over the Pacific–North American Region
    Published April 2003.
    The global mountain (tM) and frictional (tF) torques are lag correlated within the intraseasonal band, with tF leading tM. The correlation accounts for 20%–45% of their variance. Two basic feedbacks contribute to the relationship. First, the mountain torque forces global atmospheric angular momentum (AAM) anomalies and the frictional torque damps them; thus,dtF/dt}2tM. Second, frictional torque anomalies are associated with high-latitude sea level pressure (SLP) anomalies, which contribute to subsequent mountain torque anomalies; thus,dtM/dt}tF. These feedbacks help determine the growth and decay of global AAM anomalies on intraseasonal timescales.The low-frequency intraseasonal aspect of the relationship is studied for northern winter through lag regressions on tF. The linear Madden–Julian oscillation signal is first removed from tF to focus the analysis on midlatitude dynamical processes. The decorrelation timescale of tF is similar to that of teleconnection patterns and zonal index cycles, and these familiar circulation features play a prominent role in the regressed circulation anomalies.The results show that an episode of interaction between the torques is initiated by an amplified transport of zonal mean–zonal momentum across 358N. This drives a dipole pattern of zonal mean–zonal wind anomalies near 258and 508N, and associated SLP anomalies. The SLP anomalies at higher latitudes play an important role in the subsequent evolution. Regionally, the momentum transport is linked with large-scale eddies over the east Pacific and Atlantic Oceans that have an equivalent barotropic vertical structure. As these eddies persist/amplify, baroclinic wave trains disperse downstream over North American and east Asian topography. The wave trains interact with the preexisting, high-latitude SLP anomalies and drive them southward, east of the mountains.This initiates a large monopole mountain torque anomaly in the 208–508N latitude band. The wave trains associated with the mountain torque produce additional momentum flux convergence anomalies that 1) maintain the zonal wind anomalies forced by the original momentum transport anomalies and 2) help drive a global frictional torque anomaly that counteracts the mountain torque. Global AAM anomalies grow and decay over a 2-week period, on average.Over the Pacific–North American region, the wave trains evolve into the Pacific–North American (PNA) pattern whose surface wind anomalies produce a large portion of the compensating frictional torque anomaly. Case studies from two recent northern winters illustrate the interaction.

    Regional Sources of Mountain Torque Variability and High-Frequency Fluctuations in Atmospheric Angular Momentum
    Published Oct 1997.
    The sources of high-frequency (#14 day) fluctuations in global atmospheric angular momentum (AAM) are investigated using several years of surface torque and AAM data. The midlatitude mountain torque associated with the Rockies, Himalayas, and Andes is found to be responsible for much of the high-frequency fluctuationsin AAM, whereas the mountain torque in the Tropics and polar regions as well as the friction torque play amuch lesser role on these timescales. A maximum in the high-frequency mountain torque variance occurs during the cool season of each hemisphere, though the Northern Hemisphere maximum substantially exceeds that of the Southern. This relationship indicates the seasonal role played by each hemisphere in the high-frequency fluctuations of global AAM. A case study reveals that surface pressure changes near the Rockies and Himalayas produced by mobile synoptic-scale systems as they traversed these mountains contributed to a large fluctuation in mountain torque and a notable high-frequency change in global AAM in mid-March 1996. This event was also marked by a rapid fluctuation in length of day (LOD), independently verifying the direct transfer of angular momentum from the atmosphere to solid earth below. A composite study of the surface pressure patterns present during episodes of high-frequency fluctuations in AAM reveals considerable meridional elongation of the surface pressure systems along the mountain ranges, thus establishing an extensive cross-mountain surface pressure gradient and producing a large torque. The considerable along-mountain extent of these surface pressure systems may help to explain the ability of individual synoptic-scale systems to affect the global AAM. Furthermore, midlatitude synoptic-scale systems tend to be most frequent in the cool season of each hemisphere, consistent with the contemporary maximum in hemispheric high-frequency mountain torque variance.

    Relationship between Tropical Pacific SST and global atmospheric angular momentum in coupled models
    Published Jan 2004.
    The sensitivity parameter S1=∆AAM/∆SST, where ∆AAMand∆SST represent the anomalies of global atmospheric angular momentum (AAM) and tropical Pacific sea surface temperature (SST) in the NINO3.4 region, is compared for the CMIP2+ coupled models.The parameter quantifies the strength of atmospheric zonal mean zonal wind response to SST anomaly in the equatorial Pacific, an important process for the climate system.Although the simulated ∆AAMand∆SST are found to exhibit great disparity, their ratios agree better among the coupled models (and with observation) with no significant outliers.This indicates that the processes that connect the AAM anomaly to tropical SST anomaly are not sensitive to the base SST and the detail of convective heating and are relatively easy to reproduce by the coupled models. Through this robust ∆SST−∆AAM relationship, the model bias in tropical Pacific SST manifests itself in the bias in atmospheric angular momentum.The value of S1 for an atmospheric model forced by observed SST is close to that for a coupled model with a similar atmospheric component, suggesting that the ∆SST−∆AAM relationship is dominated by a one−way influence of the former forcing the latter.The physical basis for the ∆SST−∆AAM relationship is explored using a statistical equilibrium argument that links ∆SST to the anomaly of tropical tropospheric temperature.The resulting meridional gradient of tropospheric temperature is then linked to the change in zonal wind in the subtropical jets, the main contributor to ∆AAM, by thermal wind balance.

    Stochastic and oscillatory forcing of global atmospheric angular momentum
    Published June 2000.
    The temporal variability and forcing of global atmospheric angular momentum (AAM)  is studied using a three-component Markov model derived from observed statistics of global AAM  and the global torques. The model consists of stochastic forcing by the
    mountain (rM) and friction (r•)  torque plus a pervasive negative feedback on AAM by the friction torque. AAM  anomalies are damped at a 30-day timescale and forced by torques having 1.5-day (rM) and 6-day (r•)  decorrelation timescales. A large portion of
    the intraseasonal variance and covariance of AAM,  r•,  and rF is accounted for by the Markov  model.  Differences  between  the  modeled  and  the  observed  covariances  are maximized in the 10- to 90-day band and account for 10-30%  of the variance when using data not stratified by season. An especially prominent deviation from the Markov model is the oscillatory forcing of AAM  by the frictional torque at 30- to 60-day periods. Additionally, there is greater coherent variance between rF and r•  across the entire 10-
    to 90-day band, with the frictional torque leading the mountain torque. This "feedback" between the global torques results from physical processes not represented in the Markov model. The synoptic characteristics of the stochastic mountain and frictional torques and of the oscillatory Madden-Julian Oscillation are described.

    Studies of atmospheric angular momentum
    This is a section from a 2001 NOAA scientific paper. No abstract available.

    The atmospheric angular momentum cycle during the tropical Madden-Julian Oscillation
    Published 1994.

    The Dynamics of Intraseasonal Atmospheric Angular Momentum Oscillations
    Published Oct 1996.
    The global and zonal atmospheric angular momentum (AAM) budget is computed from seven years of National Centers for Environmental Prediction data and a composite budget of intraseasonal (30–70 day) variations during northern winter is constructed. Regressions on the global AAM tendency are used to produce maps of outgoing longwave radiation, 200-hPa wind, surface stress, and sea level pressure during the composite AAM cycle. The primary synoptic features and surface torques that contribute to the AAM changes are described.In the global budget, the friction and mountain torques contribute about equally to the AAM tendency. The friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres. The mountain torque peaks when anomalies in the midlatitude Northern Hemisphere and subtropical Southern Hemisphere are weak but of the same sign.The picture is different for the zonal mean budget, in which the meridional convergence of the northward relative angular momentum transport and the friction torque are the dominant terms. During the global AAM cycle, zonal AAM anomalies move poleward from the equator to the subtropics primarily in response to momentum transports. These transports are associated with the spatial covariance of the filtered (30–70 day)perturbations with the climatological upper-tropospheric flow. The zonally asymmetric portion of these perturbations develop when convection begins over the Indian Ocean and maximize when convection weakens over the western Pacific Ocean. The 30–70-day zonal mean friction torque results from 1) the surface winds induced by the upper-tropospheric momentum sources and sinks and 2) the direct surface wind response to warm pool convection anomalies.The signal in relative AAM is complemented by one in ‘‘Earth’’ AAM associated with meridional redistributions of atmospheric mass. This meridional redistribution occurs preferentially over the Asian land mass and is linked with the 30–70-day eastward moving convective signal. It is preceded by a surface Kelvin-like wave in the equatorial Pacific atmosphere that propagates eastward from the western Pacific region to the South American topography and then moves poleward as an edge wave along the Andes. This produces a mountain torque on the Andes, which also causes the regional and global AAM to change

    The intraseasonal atmospheric angular momentum associated with MJO convective initiations
    Published Jan 2016.
    The first part of this study examines the driving mechanisms of the equatorial intraseasonal relative atmospheric angular momentum (AAM ) and its dynamical relationship to the upper‐tropospheric zonal wind over the Western Hemisphere (WH ) during the convective initiation of the Madden–Julian Oscillation (MJO) over the Indian Ocean. The budget analysis shows that the main driver of the equatorial intraseasonal AAM anomaly is the meridional transport of momentum induced by the modulation of the background subtropical eddies by the intraseasonal eddies. While the subtropical eddies over the central Pacific basin partly drive the equatorial AAM by meridionally transporting the momentum, the equatorial zonal wind associated with the same subtropical eddies is zonally advected and locally amplified over the east Pacific and Atlantic basins. The common source phenomena that transport momentum result in simultaneous evolution of the WH upper‐tropospheric zonal wind and the AAM on intraseasonal time‐scales, but their main driving mechanisms are different.

    The second part of the study investigates the influence of the equatorial intraseasonal AAM state on the subsequent development of initiating MJO convection over the Indian Ocean. In the presence of the WH upper‐tropospheric easterly wind, MJO convection tends to develop a stronger enhanced convective envelope when the initiation occurs during the negative intraseasonal AAM state, which strengthens and extends the upper‐tropospheric easterly wind in the WH . When the AAM anomaly is positive, it tends to induce stronger mid‐tropospheric convergence above the region of convective initiation, thereby suppressing the lower‐tropospheric updraught and suppressing the further growth of convection. The results show that the combined effects of the WH circumnavigating circulation and the AAM can influence the subsequent development of MJO convection over the Indian Ocean.

    Torques and the Related Meridional and Vertical Fluxes of Axial Angular Momentum
    Published March 2005.
    The budget equation of the zonally averaged angular momentum is analyzed by introducing belts of 1000-km width to cover the meridional plane from pole to pole up to an altitude of 28 km. Using ECMWF Re-Analysis (ERA) data the fluxes of angular momentum are evaluated as well as the mountain and friction torques per belt. Generalized stream functions and velocity potentials are introduced to better depict the fluxes related to the angular momentum transferred at the ground during an event of mountain or friction torque.

    The variance of the total flux divergence per belt is one order of magnitude larger than those of the torques. All variances peak at midlatitudes. As a rule, the structure of the generalized stream functions changes little during an event; that is, the structure of the nondivergent part of the fluxes is stable. That of the divergent part, as represented by the velocity potential, undergoes a rapid change near the peak of a torque event. Positive friction torque events in midlatitude belts are preceded by a divergence of angular momentum fluxes in that belt, which is linked to the anticyclonic mass circulation needed to induce the positive torque. The divergence in the belt breaks down shortly before the torque is strongest. Angular momentum is transported upward from the ground after that. Much of the angular momentum generated in a midlatitude belt by positive mountain torques is transported out of the domain, but there is also a short burst of upward transports. Angular momentum anomalies linked to torque events near the equator tend to be symmetric with respect to the equator. Related fluxes affect the midlatitudes of both hemispheres.

    Unusual Behavior in Atmospheric Angular Momentum during the 1965 and 1972 El Niños
    Published Feb 2003.
    The global atmospheric angular momentum (AAM) is known to increase with tropical eastern Pacific seasurface temperature (SST) anomalies during El Nino events. Using a reanalysis dataset, the ratio of the monthly AAM anomaly to El Nin ̃ o SST anomaly (based on the Nin ̃ o-3.4 index) is found to be approximately 1 angular momentum unit (51025kg m2s21) per degree Celsius for most post-1975 El Nin ̃ os. This ratio is much smaller,however, during the 1965/66 and 1972/73 El Nin ̃ os, raising the possibilities that either the early reanalysis data are in error due to sparse observations, or the atmospheric response to the two early El Nin ̃ os was unusual. The possibility of a severe data problem in the reanalysis is ruled out by cross-validating the AAM time series with independent measurements of length of day. The latitudinal structures of the zonal wind anomalies in 1965/66and 1972/73 are examined for both the reanalysis and a set of general circulation model (GCM) simulations.Multiple GCM runs with specified SST produce a more positive ensemble-mean AAM anomaly in 1965 thanits counterpart in the reanalysis. The GCM-simulated ensemble-mean zonal wind anomaly resembles the canonical El Nin ̃ o response with accelerations of subtropical zonal jets in both hemispheres, a pattern that is almost absent in the reanalysis. On the other hand, a large spread exists among the individual ensemble members in the 1965/66 GCM simulations. Although the majority of the individual ensemble members shows the canonical El Nin ̃oresponse, two outliers (out of 12 runs) exhibit very small zonal wind responses in the Northern Hemisphere similar to the reanalysis. Thus, the observed AAM anomaly during 1965/66 is interpreted as an outlier with atmospheric noise being strong enough to overwhelm the canonical El Nin ̃ o response. The low AAM in the1972/73 event is related in the reanalysis to a significantly negative zonal wind response on the equator. This signal is robustly reproduced, although with a slightly smaller amplitude, in the ensemble mean and all individual ensemble members in the GCM simulations. The small ensemble standard deviation and large ensemble-mean response on the equator indicate that the negative response is due to the lower-boundary forcing related to the SST anomaly. The fact that the AAM anomaly in 1972/73 is not well correlated with the Nin ̃ o-3.4 index, then,indicates that SST anomalies outside the conventional El Nin ̃ o region may be responsible for the low AAM.The uncharacteristically low values of global AAM in 1965/66 and 1972/73 contribute to a low mean for the decade before 1975, which, combined with high AAM in the post-1980 era, produces a significant upward trend in AAM in the second half of the twentieth century. If the weak AAM anomalies during the two pre-1975 ElNin ̃ os are due to random noise or incidental non-El Nin ̃ o influences, taking them at face value would result inan overestimate of about 15%–20% in the multidecadal trend of AAM due to boundary forcing alone. Notably,a multidecadal trend in AAM is also simulated in the ensemble mean of the multiple GCM runs, but its magnitude is smaller than the observed counterpart and more consistent with the multidecadal trend of the Nin ̃ o-3.4 index.The implications of these findings for climate change detection are discussed.

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