scholarly journals Annular Modes in a Multiple Migrating Zonal Jet Regime

2007 ◽  
Vol 64 (11) ◽  
pp. 4053-4068 ◽  
Author(s):  
Cegeon J. Chan ◽  
R. Alan Plumb ◽  
Ivana Cerovecki
Keyword(s):  
Ocean Model ◽  
Heat Fluxes ◽  
Residual Flow ◽  
Model Output ◽  
Residual Circulation ◽  
Annular Modes ◽  
Zonal Jets ◽  
Momentum Fluxes ◽  
Leading Mode ◽  
Baroclinic Zone

Abstract The authors investigate the dynamics of zonal jets in a semihemisphere zonally reentrant ocean model. The forcings imposed in the model are an idealized atmospheric wind stress and relaxation to a latitudinal temperature profile held constant in time. While there are striking similarities to the observed atmospheric annular modes, where the leading mode of variability is associated with the primary zonal jet’s meridional undulation, secondary (weaker) jets emerge and systematically migrate equatorward. The model output suggests the following mechanism for the equatorward migration: while the eddy momentum fluxes sustain the jets, the eddy heat fluxes have a poleward bias causing an anomalous residual circulation with poleward (equatorward) flow on the poleward (equatorward) flanks. By conservation of mass, there must be a rising residual flow at the jet. From the thermodynamics equation, the greatest cooling occurs at the jet core, thus creating a tendency to reduce the baroclinicity on the poleward flank, while enhancing it on the equatorward flank. Consequently, the baroclinic zone shifts, perpetuating the jet migration.

scholarly journals Detection of Fronts as a Metric for Numerical Model Accuracy

2019 ◽  
Vol 36 (8) ◽  
pp. 1547-1561
Author(s):  
Elizabeth M. Douglass ◽  
Andrea C. Mask
Keyword(s):  
Numerical Model ◽  
Region Of Interest ◽  
Ocean Model ◽  
Minimum Size ◽  
Model Output ◽  
Oceanic Fronts ◽  
Hybrid Coordinate Ocean Model ◽  
Quantitative Metrics ◽  
Coastal Ocean Model ◽  
Along Track

AbstractAs numerical modeling advances, quantitative metrics are necessary to determine whether the model output accurately represents the observed ocean. Here, a metric is developed based on whether a model places oceanic fronts in the proper location. Fronts are observed and assessed directly from along-track satellite altimetry. Numerical model output is then interpolated to the locations of the along-track data, and fronts are detected in the model output. Scores are determined from the percentage of observed fronts correctly simulated in the model and from the percentage of modeled fronts confirmed by observations. These scores depend on certain parameters such as the minimum size of a front, which will be shown to be geographically dependent. An analysis of two models, the Hybrid Coordinate Ocean Model (HYCOM) and the Navy Coastal Ocean Model (NCOM), is presented as an example of how this metric might be applied and interpreted. In this example, scores are found to be relatively stable in time, but strongly dependent on the mesoscale variability in the region of interest. In all cases, the metric indicates that there are more observed fronts not found in the models than there are modeled fronts missing from observations. In addition to the score itself, the analysis demonstrates that modeled fronts have smaller amplitude and are less steep than observed fronts.

scholarly journals Ocean Model Diagnosis of Interannual Coevolving SST Variability in the South Indian and South Atlantic Oceans

2005 ◽  
Vol 18 (15) ◽  
pp. 2864-2882 ◽  
Author(s):  
J. C. Hermes ◽  
C. J. C. Reason
Keyword(s):  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Large Scale ◽  
Ocean Model ◽  
South Atlantic ◽  
Heat Fluxes ◽  
Global Ocean ◽  
The South ◽  
Sst Variability ◽  
South Indian

Abstract A global ocean model (ORCA2) forced with 50 yr of NCEP–NCAR reanalysis winds and heat fluxes has been used to investigate the evolution and forcing of interannual dipolelike sea surface temperature (SST) variability in the South Indian and South Atlantic Oceans. Although such patterns may also exist at times in only one of these basins and not the other, only events where there are coherent signals in both basins during the austral summer have been chosen for study in this paper. A positive (negative) event occurs when there is a significant warm (cool) SST anomaly evident in the southwest of both the South Indian and South Atlantic Oceans and a cool (warm) anomaly in the eastern subtropics. The large-scale forcing of these events appears to consist of a coherent modulation of the wavenumber-3 or -4 pattern in the Southern Hemisphere atmospheric circulation such that the semipermanent subtropical anticyclone in each basin is shifted from its summer mean position and its strength is modulated. A relationship to the Antarctic Oscillation is also apparent, and seems to strengthen after the mid-1970s. The modulated subtropical anticyclones lead to changes in the tropical easterlies and midlatitude westerlies in the South Atlantic and South Indian Oceans that result in anomalies in latent heat fluxes, upwelling, and Ekman heat transports, all of which contribute to the SST variability. In addition, there are significant modulations to the strong Rossby wave signals in the South Indian Ocean. The results of this study confirm the ability of the ORCA2 model to represent these dipole patterns and indicate connections between large-scale modulations of the Southern Hemisphere midlatitude atmospheric circulation and coevolving SST variability in the South Atlantic and South Indian Oceans.

scholarly journals Rossby Wave Breaking and Transient Eddy Forcing during Euro-Atlantic Circulation Regimes

2017 ◽  
Vol 74 (6) ◽  
pp. 1735-1755 ◽  
Author(s):  
Erik T. Swenson ◽  
David M. Straus
Keyword(s):  
Rossby Wave ◽  
Wave Breaking ◽  
Ocean Model ◽  
Heat Fluxes ◽  
Boreal Winter ◽  
Transient Eddy ◽  
Flux Divergence ◽  
Western Atlantic ◽  
Rossby Wave Breaking

Abstract The occurrence of boreal winter Rossby wave breaking (RWB) along with the quantitative role of synoptic transient eddy momentum and heat fluxes directly associated with RWB are examined during the development of Euro-Atlantic circulation regimes using ERA-Interim. Results are compared to those from seasonal reforecasts made using the Integrated Forecast System model of ECWMF coupled to the NEMO ocean model. The development of both Scandinavian blocking and the Atlantic ridge is directly coincident with anticyclonic wave breaking (AWB); however, the associated transient eddy fluxes do not contribute to (and, in fact, oppose) ridge growth, as indicated by the local Eliassen–Palm (EP) flux divergence. Evidently, other factors drive development, and it appears that wave breaking assists more with ridge decay. The growth of the North Atlantic Oscillation (NAO) in its positive phase is independent of RWB in the western Atlantic but strongly linked to AWB farther downstream. During AWB, the equatorward flux of cold air at upper levels contributes to a westerly tendency just as much as the poleward flux of momentum. The growth of the negative phase of the NAO is almost entirely related to cyclonic wave breaking (CWB), during which equatorward momentum flux dominates at jet level, yet low-level heat fluxes dominate below. The reforecasts yield realistic frequencies of CWB and AWB during different regimes, as well as realistic estimates of their roles during development. However, a slightly weaker role of RWB is simulated, generally consistent with a weaker anomalous circulation.

Using an ocean model to validate ECMWF heat fluxes

1993 ◽  
Vol 119 (513) ◽  
pp. 1003-1021 ◽  
Author(s):  
David J. Carrington ◽  
David L. T. Anderson
Keyword(s):  
Ocean Model ◽  
Heat Fluxes

scholarly journals Model-Inspired Predictors for Model Output Statistics (MOS)*

2007 ◽  
Vol 135 (10) ◽  
pp. 3496-3505 ◽  
Author(s):  
Piet Termonia ◽  
Alex Deckmyn
Keyword(s):  
Linear Regression ◽  
Thermal Radiation ◽  
Degrees Of Freedom ◽  
Selection Procedure ◽  
Heat Fluxes ◽  
Scale Model ◽  
Model Output ◽  
Model Equations ◽  
Model Output Statistics ◽  
Temporal Accumulation

Abstract This article addresses the problem of the choice of the predictors for the multiple linear regression in model output statistics. Rather than devising a selection procedure directly aimed at the minimization of the final scores, it is examined whether taking the model equations as a guidance may render the process more rational. To this end a notion of constant fractional errors is introduced. Experimental evidence is provided that they are approximately present in the model and that their impact is sufficiently linear to be corrected by a linear regression. Of particular interest are the forcing terms in the coupling of the physics parameterization to the dynamics of the model. Because such parameterizations are estimates of subgrid processes, they are expected to represent degrees of freedom that are independent of the resolved-scale model variables. To illustrate the value of this approach, it is shown that the temporal accumulation of sensible and latent heat fluxes and net solar and thermal radiation utilized as predictors add a statistically significant improvement to the 2-m temperature scores.

scholarly journals Moisture and the Persistence of Annular Modes

2021 ◽  
Author(s):  
Nicholas J. Lutsko ◽  
Momme C. Hell
Keyword(s):  
High Frequency ◽  
Channel Model ◽  
Low Frequency ◽  
Direct Effects ◽  
Atmospheric Models ◽  
Annular Mode ◽  
Annular Modes ◽  
Leading Term ◽  
Leading Mode ◽  
Geometric Effects

AbstractAnnular modes are the leading mode of variability in extratropical atmospheres, and a key source of predictability at mid-latitudes. Previous studies of annular modes have primarily used dry atmospheric models, so that moisture’s role in annular mode dynamics is still unclear. In this study, a moist two-layer quasi-geostrophic channel model is used to study the effects of moisture on annular mode persistence. Using a channel model allows moisture’s direct effects to be studied, rather than changes in persistence due to geometric effects associated with shifts in jet latitude on the sphere. Simulations are performed in which the strength of latent heat release is varied, to investigate how annular mode persistence responds as precipitation becomes a leading term in the thermodynamic budget. At short lags (<20 model days ≈ 4 Earth days), moisture increases annular mode persistence, reflecting weaker eddy activity that is less effective at disrupting zonal-mean wind anomalies. Comparisons to dry simulations with weaker mean flows demonstrate that moisture is particularly effective at damping high frequency eddies, further enhancing short lag persistence. At long lags (>20 model days), moisture weakly increases persistence, though it decreases the amplitudes of low frequency annular mode anomalies. In the most realistic simulation, the greater short-lag persistence increases the e-folding time of the zonal index by 21 model days (≈4 Earth days). Moisture also causes a transition to propagating variability, though this does not seem to affect the leading mode’s persistence.

scholarly journals On the Dynamical Mechanism of the Southern Annular Mode, Including Seasonality: Inter-Annual Variability: and Trends

2021 ◽  
Author(s):  
◽  
Joseph Kidston
Keyword(s):  
Storm Track ◽  
South Pacific ◽  
La Niña ◽  
Pacific Region ◽  
Mean Flow ◽  
The South ◽  
Annular Modes ◽  
La Nina ◽  
Poleward Shift ◽  
Leading Mode

<p>This thesis considers the dynamics of the leading mode of extratropical atmospheric variability, the so-called annular modes, with a focus on the Southern Hemisphere (SH). Various aspects of the annular modes are addressed, from the underlying mechanism, to variability at progressively longer time-scales; ranging from the seasonality; to inter-annual variability; to the observed and predicted trends. The underlying mechanism of the annular modes is approached in the context of the recent theory that eddy-driven jets may be self-maintaining. We show that the leading mode of variability is associated with changes in the eddy source latitude, and that the latitude of the eddy source region is organised by the mean flow. This is consistent with the idea that the annular modes should be thought of as the meridional wandering of a self-maintaining jet, and that a positive baroclinic feedback prolongs these vacillations. Further, the degree to which the eddy-driven flow is self-maintaining determines the time-scale of the leading mode in a simplified general circulation model (GCM). Preliminary results indicate that the same dynamics are important in the real atmosphere. Secondly the seasonality of the southern annular mode (SAM) is investigated. As with previous studies, during summer the SAM is found to be largely zonally symmetric, whereas during winter it exhibits increased zonal wave number 2-3 variability. This is consistent with seasonal variations in the mean-state, and it is argued that the seasonal cycle of near-surface temperature over the Australian continent plays an important role, making the eddy driven jet, and hence the SAM, more zonally symmetric during summer than winter. During winter, the SAM exhibits little variability over the South Pacific and southeast of Australia. Dynamical reasons for this behaviour are discussed. This seasonality is discussed in the context of New Zealand climate, where it is shown that the variability in rainfall and temperature data are impacted by the large-scale seasonality of the SAM. Thirdly the zonally symmetric response of the SH to the El Nino Southern Oscillation (ENSO) is examined. Such a response is only observed in the mid-latitudes during austral summer and autumn, the same period when the climatological mean flow and storm-track is most zonally symmetric. During all seasons the ENSO stationary wave, or Pacific South American mode affects the baroclinicity at 850 hPa in the South Pacific region, so that during La Nina (El Nino) events the baroclinicity is increased (reduced). During summer La Nina events the anomalous transient eddy activity is increased over the entire meridional extent of the storm-track in the South Pacific region, whereas down-stream, over the Atlantic and Indian Oceans, the storm track moves poleward. It is suggested that during La Nina events, more vigorous eddy activity in the South Pacific leads to a poleward shift of the storm-track immediately down-stream, in the East Pacific. During summer and autumn the location of the storm-track in the Pacific region may be communicated around the hemisphere because there is a single climatological storm track, and so eddies can propagate from the Pacific region to the Atlantic region. There is some evidence of these dynamics in that the anomalous eddy activity associated with La Nina events begins in the South Pacific region and subsequently propagates zonally. Finally the cause of the poleward shift of the mid-latitude eddy-driven jet streams under global warming is considered. GCMs indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st Century. Here it is shown that the shift is associated with an increase in the eddy length-scale. The cause of the increase in eddy length-scale is discussed. Larger eddies are shown to propagate preferentially poleward, and it is argued that this may induce a corresponding shift in the mean flow that they maintain. The mechanism is investigated using a simplified GCM.</p>

scholarly journals Multiscale Applications of Two Online-Coupled Meteorology-Chemistry Models During Recent Field Campaigns in Australia, Part II: Comparison of WRF/Chem and WRF/Chem-ROMS and Impacts of Air-Sea Interactions and Boundary Conditions

Atmosphere ◽  
2019 ◽  
Vol 10 (4) ◽  
pp. 210 ◽  
Author(s):  
Zhang ◽  
Wang ◽  
Jena ◽  
Paton-Walsh ◽  
Guérette ◽  
...  
Keyword(s):  
Boundary Conditions ◽  
Temporal Variations ◽  
Ocean Model ◽  
Heat Fluxes ◽  
Atmospheric Pollutants ◽  
Spatial Distributions ◽  
Surface Heat Fluxes ◽  
Regional Ocean Model System ◽  
Field Campaigns ◽  
The Impact

Air-sea interactions play an important role in atmospheric circulation and boundary layer conditions through changing convection processes and surface heat fluxes, particularly in coastal areas. These changes can affect the concentrations, distributions, and lifetimes of atmospheric pollutants. In this Part II paper, the performance of the Weather Research and Forecasting model with chemistry (WRF/Chem) and the coupled WRF/Chem with the Regional Ocean Model System (ROMS) (WRF/Chem-ROMS) are intercompared for their applications over quadruple-nested domains in Australia during the three following field campaigns: The Sydney Particle Study Stages 1 and 2 (SPS1 and SPS2) and the Measurements of Urban, Marine, and Biogenic Air (MUMBA). The results are used to evaluate the impact of air-sea interaction representation in WRF/Chem-ROMS on model predictions. At 3, 9, and 27 km resolutions, compared to WRF/Chem, the explicit air-sea interactions in WRF/Chem-ROMS lead to substantial improvements in simulated sea-surface temperature (SST), latent heat fluxes (LHF), and sensible heat fluxes (SHF) over the ocean, in terms of statistics and spatial distributions, during all three field campaigns. The use of finer grid resolutions (3 or 9 km) effectively reduces the biases in these variables during SPS1 and SPS2 by WRF/Chem-ROMS, whereas it further increases these biases for WRF/Chem during all field campaigns. The large differences in SST, LHF, and SHF between the two models lead to different radiative, cloud, meteorological, and chemical predictions. WRF/Chem-ROMS generally performs better in terms of statistics and temporal variations for temperature and relative humidity at 2 m, wind speed and direction at 10 m, and precipitation. The percentage differences in simulated surface concentrations between the two models are mostly in the range of ±10% for CO, OH, and O3, ±25% for HCHO, ±30% for NO2, ±35% for H2O2, ±50% for SO2, ±60% for isoprene and terpenes, ±15% for PM2.5, and ±12% for PM10. WRF/Chem-ROMS at 3 km resolution slightly improves the statistical performance of many surface and column concentrations. WRF/Chem simulations with satellite-constrained boundary conditions (BCONs) improve the spatial distributions and magnitudes of column CO for all field campaigns and slightly improve those of the column NO2 for SPS1 and SPS2, column HCHO for SPS1 and MUMBA, and column O3 for SPS2 at 3 km over the Greater Sydney area. The satellite-constrained chemical BCONs reduce the model biases of surface CO, NO, and O3 predictions at 3 km for all field campaigns, surface PM2.5 predictions at 3 km for SPS1 and MUMBA, and surface PM10 predictions at all grid resolutions for all field campaigns. A more important role of chemical BCONs in the Southern Hemisphere, compared to that in the Northern Hemisphere reported in this work, indicates a crucial need in developing more realistic chemical BCONs for O3 in the relatively clean SH.

scholarly journals High-resolution nested model simulations of the climatological circulation in the southeastern Mediterranean Sea

2003 ◽  
Vol 21 (1) ◽  
pp. 267-280 ◽  
Author(s):  
S. Brenner
Keyword(s):  
High Resolution ◽  
Continental Shelf ◽  
Mediterranean Sea ◽  
Seasonal Cycle ◽  
Internal Variability ◽  
Ocean Model ◽  
Heat Fluxes ◽  
Intermediate Model ◽  
Grid Model ◽  
The Mediterranean

Abstract. As part of the Mediterranean Forecasting System Pilot Project (MFSPP) we have implemented a high-resolution (2 km horizontal grid, 30 sigma levels) version of the Princeton Ocean Model for the southeastern corner of the Mediterranean Sea. The domain extends 200 km offshore and includes the continental shelf and slope, and part of the open sea. The model is nested in an intermediate resolution (5.5 km grid) model that covers the entire Levantine, Ionian, and Aegean Sea. The nesting is one way so that velocity, temperature, and salinity along the boundaries are interpolated from the relevant intermediate model variables. An integral constraint is applied so that the net mass flux across the open boundaries is identical to the net flux in the intermediate model. The model is integrated for three perpetual years with surface forcing specified from monthly mean climatological wind stress and heat fluxes. The model is stable and spins up within the first year to produce a repeating seasonal cycle throughout the three-year integration period. While there is some internal variability evident in the results, it is clear that, due to the relatively small domain, the results are strongly influenced by the imposed lateral boundary conditions. The results closely follow the simulation of the intermediate model. The main improvement is in the simulation over the narrow shelf region, which is not adequately resolved by the coarser grid model. Comparisons with direct current measurements over the shelf and slope show reasonable agreement despite the limitations of the climatological forcing. The model correctly simulates the direction and the typical speeds of the flow over the shelf and slope, but has difficulty properly re-producing the seasonal cycle in the speed.Key words. Oceanography: general (continental shelf processes; numerical modelling; ocean prediction)

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