Eddy–Zonal Flow Interactions and the Persistence of the Zonal Index

2007 ◽  
Vol 64 (9) ◽  
pp. 3296-3311 ◽  
Author(s):  
Edwin P. Gerber ◽  
Geoffrey K. Vallis
Keyword(s):  
Time Scales ◽  
Time Scale ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Model Parameters ◽  
Mean Flow ◽  
Annular Modes ◽  
Zonal Index ◽  
Flow Interactions

Abstract An idealized atmospheric general circulation model is used to investigate the factors controlling the time scale of intraseasonal (10–100 day) variability of the extratropical atmosphere. Persistence on these time scales is found in patterns of variability that characterize meridional vacillations of the extratropical jet. Depending on the degree of asymmetry in the model forcing, patterns take on similar properties to the zonal index, annular modes, and North Atlantic Oscillation. It is found that the time scale of jet meandering is distinct from the obvious internal model time scales, suggesting that interaction between synoptic eddies and the large-scale flow establish a separate, intraseasonal time scale. A mechanism is presented by which eddy heat and momentum transport couple to retard motion of the jet, slowing its meridional variation and thereby extending the persistence of zonal index and annular mode anomalies. The feedback is strong and quite sensitive to model parameters when the model forcing is zonally uniform. However, the time scale of jet variation drops and nearly all sensitivity to parameters is lost when zonal asymmetries, in the form of topography and thermal perturbations that approximate land–sea contrast, are introduced. A diagnostic on the zonal structure of the zonal index provides intuition on the physical nature of the index and annular modes and hints at why zonal asymmetries limit the eddy–mean flow interactions.

Low-Frequency Variability in a Turbulent Baroclinic Jet: Eddy–Mean Flow Interactions in a Two-Level Model

2010 ◽  
Vol 67 (2) ◽  
pp. 452-467 ◽  
Author(s):  
Joseph Bernstein ◽  
Brian Farrell
Keyword(s):  
Time Scale ◽  
General Circulation ◽  
Channel Model ◽  
Low Frequency ◽  
Circulation Model ◽  
Coupled System ◽  
Mean Flow ◽  
Low Frequency Variability ◽  
Flow Interactions ◽  
The Waves

Abstract The origin of low-frequency variability in the midlatitude jet is investigated using a two-level baroclinic channel model. The model state fields are separated into slow and fast components using intermediate time- scale averaging. In the equation for the fast variables the nonlinear wave–wave interactions are parameterized as a stochastic excitation. The slowly varying ensemble mean eddy fluxes obtained from the resulting stochastic turbulence model are coupled with the slowly varying mean flow dynamics. This forms a coupled set of deterministic equations on the slow time scale that governs the dynamics of the eddy–mean flow interaction. The equilibria of this coupled system are found as a function of the excitation strength, which controls the level of turbulence. At low levels of turbulence the equilibrated flow with zonally symmetric mean forcing remains zonally symmetric, but as excitation increases it undergoes zonal symmetry-breaking bifurcations. Time-dependent flows arising from these bifurcations take the form of westward-propagating wavelike structures resembling blocking patterns. Features of these waves are characteristic of blocking in both observations and atmospheric general circulation model simulations including retrogression, eddy variance concentrated upstream of the waves, and eddy momentum flux forcing the waves.

scholarly journals Wave–Mean-Flow Interactions in Moist Baroclinic Life Cycles

2017 ◽  
Vol 74 (7) ◽  
pp. 2143-2162 ◽  
Author(s):  
Ray Yamada ◽  
Olivier Pauluis
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Moisture Flux ◽  
Life Cycles ◽  
Heat Fluxes ◽  
Latent Heating ◽  
Mean Flow ◽  
Momentum Exchange ◽  
Flow Interactions

Abstract Previous studies show that the moist Eliassen–Palm (EP) flux captures a greater eddy momentum exchange through form drag than the dry EP flux in the midlatitude climate. This suggests that the eddy moisture flux acts to decrease the baroclinicity of the zonal jet. This study investigates such a mechanism in moist baroclinic life cycles, which are simulated in an idealized general circulation model with large-scale condensation as the only moist process. The runs are analyzed using a linear diagnostic based on the Kuo–Eliassen equation to decompose the jet change into parts driven by individual forcing terms. It is shown that the wave-induced latent heating drives an indirect Eulerian-mean cell on the equatorward flank of the jet, which acts to reduce the baroclinicity in that region. The eddy sensible heat fluxes act to reduce the baroclinicity near the center of the jet. The moist baroclinic forcing strengthens as the amount of initially available moisture increases. The effect of the eddy moisture flux on the transformed Eulerian-mean (TEM) and isentropic dynamics is also considered. It is shown that the circulation and EP flux on moist isentropes is around 4 times as strong and extends farther equatorward than on dry isentropes. The equatorward extension of the moist EP flux coincides with the region where the baroclinic forcing is driven by latent heating. The moist EP flux successfully captures the moisture-driven component of the baroclinic forcing that is not seen in the dry EP flux.

scholarly journals Low-Latitude Freshwater Influence on Centennial Variability of the Atlantic Thermohaline Circulation

2004 ◽  
Vol 17 (23) ◽  
pp. 4498-4511 ◽  
Author(s):  
Michael Vellinga ◽  
Peili Wu
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
General Circulation ◽  
Large Scale ◽  
Thermohaline Circulation ◽  
Process Analysis ◽  
Circulation Model ◽  
Freshwater Flux ◽  
Impact Surface ◽  
Hadley Centre

Abstract Variability of the thermohaline circulation (THC) has been analyzed in a long control simulation by the Met Office's Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It is shown that internal THC variability in the coupled climate system is concentrated at interannual and centennial time scales, with the centennial mode being dominant. Centennial oscillations of the THC can impact surface climate via an interhemispheric SST contrast of 0.1°C in the Tropics and more than 0.5°C in mid- and high latitudes. A mechanism is proposed based on detailed process analysis involving large-scale air–sea interaction on multidecadal time scales. Anomalous northward ocean heat transport associated with a strong phase of the Atlantic THC generates a cross-equatorial SST gradient. This causes the ITCZ to move to a more northerly position with increased strength. The extra rainfall resulting from the anomalous ITCZ imposes a freshwater flux and produces a salinity anomaly in the tropical North Atlantic. Such sustained salinity anomalies slowly propagate toward the subpolar North Atlantic at a lag of 5–6 decades. The accumulated low-salinity water lowers upper-ocean density, which causes the THC to slow down. The oscillation then enters the opposite phase.

scholarly journals The Impact of GCM Dynamical Cores on Idealized Sudden Stratospheric Warmings and Their QBO Interactions

2016 ◽  
Vol 73 (9) ◽  
pp. 3397-3421 ◽  
Author(s):  
Weiye Yao ◽  
Christiane Jablonowski
Keyword(s):  
General Circulation ◽  
Flow Characteristics ◽  
Circulation Model ◽  
Mean Flow ◽  
Single Vortex ◽  
Tem Analysis ◽  
Quasi Biennial Oscillation ◽  
Spectral Transform ◽  
Flow Interactions ◽  
The Impact

Abstract The paper demonstrates that sudden stratospheric warmings (SSWs) can be simulated in an ensemble of dry dynamical cores that miss the typical SSW forcing mechanisms like moist processes, land–sea contrasts, or topography. These idealized general circulation model (GCM) simulations are driven by a simple Held–Suarez–Williamson (HSW) temperature relaxation and low-level Rayleigh friction. In particular, the four dynamical cores of NCAR’s Community Atmosphere Model, version 5 (CAM5), are used, which are the semi-Lagrangian (SLD) and Eulerian (EUL) spectral-transform models and the finite-volume (FV) and the spectral element (SE) models. Three research themes are discussed. First, it is shown that SSW events in such idealized simulations have very realistic flow characteristics that are analyzed via the SLD model. A single vortex-split event is highlighted that is driven by wavenumber-1 and -2 wave–mean flow interactions. Second, the SLD simulations are compared to the EUL, FV, and SE dynamical cores, which sheds light on the impact of the numerical schemes on the circulation. Only SLD produces major SSWs, while others only exhibit minor stratospheric warmings. These differences are caused by SLD’s more vigorous wave–mean flow interactions in addition to a warm pole bias, which leads to relatively weak polar jets in SLD. Third, it is shown that tropical quasi-biennial oscillation (QBO)–like oscillations and SSWs can coexist in such idealized HSW simulations. They are present in the SLD dynamical core that is used to analyze the QBO–SSW interactions via a transformed Eulerian-mean (TEM) analysis. The TEM results provide support for the Holton–Tan effect.

scholarly journals A Multivariate Quantile-Matching Bias Correction Approach with Auto- and Cross-Dependence across Multiple Time Scales: Implications for Downscaling

2016 ◽  
Vol 29 (10) ◽  
pp. 3519-3539 ◽  
Author(s):  
Rajeshwar Mehrotra ◽  
Ashish Sharma
Keyword(s):  
Water Resources ◽  
Time Scales ◽  
Bias Correction ◽  
Time Scale ◽  
General Circulation ◽  
Circulation Model ◽  
Multiple Time Scales ◽  
Multiple Time ◽  
Multiple Variables ◽  
Multivariate Quantile

Abstract A novel multivariate quantile-matching nesting bias correction approach is developed to remove systematic biases in general circulation model (GCM) outputs over multiple time scales. This is a significant advancement over typical quantile-matching alternatives available for bias correction, as they implicitly assume that correction of individual variable attributes will lead to correction of dependence biases between multiple variables. Furthermore, existing approaches perform bias correction at a given time scale (e.g., daily), whereas applications often require biases to be addressed at more than one time scale (such as annual in the case of most water resources planning projects). The proposed approach addresses all these issues, and additionally attempts to correct for lag-1 dependence (and cross-dependence) attributes across multiple time scales. The approach is called multivariate recursive quantile nesting bias correction (MRQNBC). The fidelity of the approach is demonstrated by applying it to a vector of CSIRO Mk3 GCM atmospheric variables and comparing the results with the commonly used quantile-matching approach. Following this, the implications of the approach in hydrology- and water resources–related applications are demonstrated by feeding the bias-corrected data to a rainfall downscaling model and comparing the downscaled rainfall attributes for current and future climate. The proposed approach is shown to represent the variability and persistence related attributes better and can thus be expected to have important consequences for the simulation of occurrence and intensity of extreme events such as floods and droughts in downscaled simulations, of importance in various climate impact assessment applications.

scholarly journals Seasonal Synchronization of ENSO Events in a Linear Stochastic Model*

2010 ◽  
Vol 23 (21) ◽  
pp. 5629-5643 ◽  
Author(s):  
Karl Stein ◽  
Niklas Schneider ◽  
Axel Timmermann ◽  
Fei-Fei Jin
Keyword(s):  
Growth Rate ◽  
Seasonal Cycle ◽  
General Circulation ◽  
Heat Budget ◽  
Circulation Model ◽  
Angular Frequency ◽  
Model Parameters ◽  
Mean Flow ◽  
Enso Events ◽  
Seasonal Synchronization

Abstract A simple model of ENSO is developed to examine the effects of the seasonally varying background state of the equatorial Pacific on the seasonal synchronization of ENSO event peaks. The model is based on the stochastically forced recharge oscillator, extended to include periodic variations of the two main model parameters, which represent ENSO’s growth rate and angular frequency. Idealized experiments show that the seasonal cycle of the growth rate parameter sets the seasonal cycle of ENSO variance; the inclusion of the time dependence of the angular frequency parameter has a negligible effect. Event peaks occur toward the end of the season with the most unstable growth rate. Realistic values of the parameters are estimated from a linearized upper-ocean heat budget with output from a high-resolution general circulation model hindcast. Analysis of the hindcast output suggests that the damping by the mean flow field dominates the seasonal cycle of ENSO’s growth rate and, thereby, seasonal ENSO variance. The combination of advective, Ekman pumping, and thermocline feedbacks plays a secondary role and acts to enhance the seasonal cycle of the ENSO growth rate.

scholarly journals Sea Level Expression of Intrinsic and Forced Ocean Variabilities at Interannual Time Scales

2011 ◽  
Vol 24 (21) ◽  
pp. 5652-5670 ◽  
Author(s):  
Thierry Penduff ◽  
Mélanie Juza ◽  
Bernard Barnier ◽  
Jan Zika ◽  
William K. Dewar ◽  
...  
Keyword(s):  
Sea Level ◽  
Time Scales ◽  
General Circulation ◽  
Large Scale ◽  
Full Range ◽  
Circulation Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Intrinsic Variability ◽  
Boundary Current

Abstract This paper evaluates in a realistic context the local contributions of direct atmospheric forcing and intrinsic oceanic processes on interannual sea level anomalies (SLAs). A ¼° global ocean–sea ice general circulation model, driven over 47 yr by the full range of atmospheric time scales, is quantitatively assessed against altimetry and shown to reproduce most observed features of the interannual SLA variability from 1993 to 2004. Comparing this simulation with a second driven only by the climatological annual cycle reveals that the intrinsic part of the total interannual SLA variance exceeds 40% over half of the open-ocean area and exceeds 80% over one-fifth of it. This intrinsic contribution is particularly strong in eddy-active regions (more than 70%–80% in the Southern Ocean and western boundary current extensions) as predicted by idealized studies, as well as within the 20°–35° latitude bands. The atmosphere directly forces most of the interannual SLA variance at low latitudes and in most midlatitude eastern basins, in particular north of about 40°N in the Pacific. The interannual SLA variance is almost entirely due to intrinsic processes south of the Antarctic Circumpolar Current in the Indian Ocean sector, while half of this variance is forced by the atmosphere north of it. The same simulations were performed and analyzed at 2° resolution as well: switching to this laminar regime yields a comparable forced variability (large-scale distribution and magnitude) but almost suppresses the intrinsic variability. This likely explains why laminar ocean models largely underestimate the interannual SLA variance.

scholarly journals Global Meteorological Drought: A Synthesis of Current Understanding with a Focus on SST Drivers of Precipitation Deficits

2016 ◽  
Vol 29 (11) ◽  
pp. 3989-4019 ◽  
Author(s):  
Siegfried D. Schubert ◽  
Ronald E. Stewart ◽  
Hailan Wang ◽  
Mathew Barlow ◽  
Ernesto H. Berbery ◽  
...  
Keyword(s):  
Time Scales ◽  
Land Surface ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Meteorological Drought ◽  
Current Understanding ◽  
Northern Eurasia ◽  
Eastern Canada

Abstract Drought affects virtually every region of the world, and potential shifts in its character in a changing climate are a major concern. This article presents a synthesis of current understanding of meteorological drought, with a focus on the large-scale controls on precipitation afforded by sea surface temperature (SST) anomalies, land surface feedbacks, and radiative forcings. The synthesis is primarily based on regionally focused articles submitted to the Global Drought Information System (GDIS) collection together with new results from a suite of atmospheric general circulation model experiments intended to integrate those studies into a coherent view of drought worldwide. On interannual time scales, the preeminence of ENSO as a driver of meteorological drought throughout much of the Americas, eastern Asia, Australia, and the Maritime Continent is now well established, whereas in other regions (e.g., Europe, Africa, and India), the response to ENSO is more ephemeral or nonexistent. Northern Eurasia, central Europe, and central and eastern Canada stand out as regions with few SST-forced impacts on precipitation on interannual time scales. Decadal changes in SST appear to be a major factor in the occurrence of long-term drought, as highlighted by apparent impacts on precipitation of the late 1990s “climate shifts” in the Pacific and Atlantic SST. Key remaining research challenges include (i) better quantification of unforced and forced atmospheric variability as well as land–atmosphere feedbacks, (ii) better understanding of the physical basis for the leading modes of climate variability and their predictability, and (iii) quantification of the relative contributions of internal decadal SST variability and forced climate change to long-term drought.

Low-Frequency Pycnocline Variability in the Northeast Pacific

2005 ◽  
Vol 35 (8) ◽  
pp. 1403-1420 ◽  
Author(s):  
Antonietta Capotondi ◽  
Michael A. Alexander ◽  
Clara Deser ◽  
Arthur J. Miller
Keyword(s):  
Simple Model ◽  
General Circulation ◽  
Large Scale ◽  
Broad Band ◽  
Low Frequency ◽  
Circulation Model ◽  
Gulf Of Alaska ◽  
Ekman Pumping ◽  
Mean Flow ◽  
Northeast Pacific

Abstract The output from an ocean general circulation model (OGCM) driven by observed surface forcing is used in conjunction with simpler dynamical models to examine the physical mechanisms responsible for interannual to interdecadal pycnocline variability in the northeast Pacific Ocean during 1958–97, a period that includes the 1976–77 climate shift. After 1977 the pycnocline deepened in a broad band along the coast and shoaled in the central part of the Gulf of Alaska. The changes in pycnocline depth diagnosed from the model are in agreement with the pycnocline depth changes observed at two ocean stations in different areas of the Gulf of Alaska. A simple Ekman pumping model with linear damping explains a large fraction of pycnocline variability in the OGCM. The fit of the simple model to the OGCM is maximized in the central part of the Gulf of Alaska, where the pycnocline variability produced by the simple model can account for ∼70%–90% of the pycnocline depth variance in the OGCM. Evidence of westward-propagating Rossby waves is found in the OGCM, but they are not the dominant signal. On the contrary, large-scale pycnocline depth anomalies have primarily a standing character, thus explaining the success of the local Ekman pumping model. The agreement between the Ekman pumping model and OGCM deteriorates in a large band along the coast, where propagating disturbances within the pycnocline, due to either mean flow advection or boundary waves, appear to play an important role in pycnocline variability. Coastal propagation of pycnocline depth anomalies is especially relevant in the western part of the Gulf of Alaska, where local Ekman pumping-induced changes are anticorrelated with the OGCM pycnocline depth variations. The pycnocline depth changes associated with the 1976–77 climate regime shift do not seem to be consistent with Sverdrup dynamics, raising questions about the nature of the adjustment of the Alaska Gyre to low-frequency wind stress variability.

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