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John D. Horel
and
Carlos R. Mechoso

Abstract

The persistence of the planetary-scale simulation over the North Pacific Ocean is investigated during 18 Northern Hemisphere winters from 1965/66 to 1982/83. Quasi-stationary flow patterns dominate 20 periods during the 6 El Niño winters. In contrast, 29 such periods are observed during the remaining 12 winters. Nearly all of the quasi-stationary episodes during El Niño winters exhibit negative 500 mb geopotential height anomalies in the Gulf of Alaska-Aleutian Island region. During the other 12 winters, episodes characterized by positive height anomalies in that region occur as frequently as those exhibiting negative height anomalies.

The observed persistence of the planetary circulation is contrasted to that simulated by the UCLA general circulation model. Ten winters of model output are analyzed: during five winters, sea surface temperatures (SSTs) are prescribed to evolve through their climatological seasonal cycle while during the other five winters, SST anomalies corresponding to idealized or observed El Niño conditions are added to the climatological field. The model atmosphere has less intraseasonal variability, and quasi-stationary events are less frequent than observed. However, the model is successful in simulating the observed preponderance of quasi-stationary regimes which exhibit below-normal 500 mb geopotential height anomalies in the Gulf of Alaska during winters with positive SST anomalies in the equatorial Pacific. The evolution of the model's quasi-stationary events suggests that they result directly from dynamical processes in midlatitudes, but their characteristics are apparently affected by SST conditions.

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Douglas M. Sinton
and
Carlos R. Mechoso

Abstract

A two-layer, shallow-water frontal model on an f-plane is used to study the nonlinear evolution of frontal waves. The fluid is confined to a periodic channel with parallel vertical walls. It is found that, at an advanced stage in the evolution of frontal waves, small-scale disturbances develop along the cold front while the warm front evolves in a smooth fashion. It is shown that the motion field associated with the primary low advects kinetic energy and low potential vorticity into the cold-frontal region. That kinetic energy is transferred by barotropic processes to the secondary disturbances at locations along the cold front where advection of low potential vorticity results in an enhancement of the horizontal shears. On the other hand, kinetic energy is removed from the warm-frontal region, which remains undisturbed.

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Carlos R. Mechoso
and
Douglas M. Sinton

Abstract

The energy analysis of the two-layer frontal model of Kotchin (1932) and Orlanski (1968) is reformulated. The new formulation is based on separating the contributions to the eddy kinetic energy of the unstable waves by the changes in 1) the difference in relative momentum between the layers (multiplied by the shear), and in 2) the available potential energy. Such a separation results in a clear characterization of the instabilities, particularly near the Rayleigh, Helmholtz and baroclinic instability limits. The mean meridional circulation induced by the unstable waves is analyzed.

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Koji Yamazaki
and
Carlos R. Mechoso

Abstract

The evolution of the flow in the Southern Hemisphere during the period 31 August-10 November 1979 is examined. The final stratospheric warming of 1979 and the associated reversal of the flow above 10 mb occurred during this period. It is found that this warming processs was newly monotonic but modulated by a series of events with enhanced eddy activity.

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Carlos R. Mechoso
and
Dennis L. Hartmann

Abstract

Southern Hemisphere analyses from the surface to 2 mb and from 20 to 80°S for the period May-September 1979 have been used to study the structure of traveling planetary waves. Space-time cross- spectral analysis of the height field has been employed to define the amplitude and phase for both the eastward and westward moving components of particular combinations of zonal wavenumber and frequency band. Latitude-height contour plots of power, phase and coherence squared show that westward moving waves have structure characteristic of barotropic external modes and are coherent across a broad range of latitudes and from the surface to 2 mb, the highest level analyzed. Eastward-moving waves, on the other hand, have more rapid phase variations, especially in the troposphere, and appear more baroclinic. The tropospheric structure of the moving components of wavenumbers 1–4 is as one would expect for baroclinically unstable modes of the Charney type. Wavenumbers 1 and 2 both have double amplitude maxima in the troposphere, separated by ∼20° of latitude. These amplitude maxima are coherent with each other and are about 180° out of phase. The variances of the eastward components of wavenumbers 1 and 2 increase rapidly with altitude in the stratosphere, but the variance in the upper stratosphere is not coherent with that in the troposphere. To explain these observations it is suggested that two linearly independent eastward moving modes are present simultaneously in the Southern Hemisphere, and that these modes are manifestations of the baroclinic instability of the zonal mean flow. One of the modes dominates the variance in the troposphere (Charney mode) and the other dominates the variance in the stratosphere (Green mode).

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Arturo I. Quintanar
and
Carlos R. Mechoso

Abstract

This Part I presents selected major features of the quasi-stationary (monthly mean) wave field in the troposphere and stratosphere of the Southern Hemisphere. It is confirmed that the quasi-stationary wave with zonal wavenumber 1 (QS-wave 1) is by far the dominant component of the geopotential height field at tropospheric and stratospheric levels. The amplitude of this wave is largest at about 60°S all year round and reaches a maximum during September and October in the upper troposphere and stratosphere.

Analysis of the Elliasen-Palm flux vector suggests that at high latitudes the quasi-stationary wave field is primarily forced from lower latitudes, most prominently from the Indian Ocean region during June and October. Orographic and thermal forcing from Antarctic regions seem to also be important sources of wave activity in polar and high latitudes, particularly over southern South America and the Atlantic Ocean.

The contribution to the quasi-stationary flow by the transient component of the flow is also analyzed. This analysis suggests that at high latitudes, the low-frequency transients act to strengthen QS-wave 1, while high-frequency transients weaken it. The values found for these contributions suggest that the low-frequency component is dominant.

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Arturo I. Quintanar
and
Carlos R. Mechoso

Abstract

In this Part II the authors investigate the role that Antarctic elevations, the rest of the world orography, thermal forcing from lower latitudes, and the transient eddy component of the flow play on the generation of the quasi-stationary wave field in the Southern Hemisphere. An approach based on the UCLA GCM is followed. Results from a control simulation with full orography and from experiments without the Antarctic elevations and without the rest of the world orography, suggest that the quasi-stationary wave with zonal wavenumber 1 (QS-wave 1) around Antarctica is primarily generated by mechanisms other than the Antarctic elevations.

Comparison of a three-dimensional Eliassen-Palm flux vector in the control simulation, and those where the Antarctic elevation and the rest of the world orography are removed, suggests that wave activity propagates both from the subtropics and from polar latitudes. Although in qualitative agreement with results of Part I, the horizontal and vertical structure of these remote forcings is different in the simulations where a more barotropic wave train is generated from lower and polar latitudes. Antarctica is indeed a source of wave activity but unlike observations it is confined to polar regions at tropospheric levels. Additional evidence of thermal forcing was found in an experiment without orographic elevations and zonal asymmetries south of 45°S. It is found that QS-wave 2 is most affected by the zonal asymmetries in sea ice and SST.

The effects of the transient component of the flow were also analyzed. The heat transport by the transient eddies in the absence of Antarctic elevations is greater than in the control simulation consistent with a warming of the polar region. Analysis of the contribution by the low-pass and high-pass transients to QS-wave 1 in the control simulation reveals a very different behavior than in Part I. In the control simulation, the low-pass transients and QS-wave 1 are mostly in opposition of phase. High-frequency transients are uncorrelated with QS-wave 1 in all cases. In the experiments without Antarctic elevations or the rest of the world orography, low-pass transients are in phase with QS-wave 1 over high and polar latitudes. In summary, the results of this study suggest that the generation of QS-wave 1 at high latitudes is predominantly from lower latitudes.

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Carlos R. Mechoso
and
Gonzalo Perez Iribarren

Abstract

The relationship between the Southern Oscillation (SO) and streamflow in two major rivers of southeastern South America (Negro and Uruguay rivers) is explored for the period 1909–1989. It is found that streamflow in both rivers has a clear tendency to be below average in the period from June through December in high SO index years (cold events in the equatorial Pacific Ocean) and a slight tendency to be above average in the period from November through the next February in ENSO years. These findings are in broad agreement with previously proposed associations between extremes in the Southern Oscillation and rainfall variability in southeastern South America.

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Jin-Yi Yu
and
Carlos R. Mechoso

Abstract

This study examines interannual variability produced by a recent version of the University of California, Los Angeles, coupled atmosphere–ocean general circulation model (CGCM). The CGCM is shown to produce ENSO-like climate variability with reasonable frequency and amplitude. A multichannel singular spectrum analysis identifies the simulated ENSO cycle and permits examination of the associated evolution of atmospheric and oceanic states. During the cycle, the evolution of upper-ocean heat content in the tropical Pacific is characterized by a zonal oscillation between the western and eastern equatorial Pacific and a meridional oscillation between the equator and 10°N. The zonal oscillation is related to the amplification of the cycle, and the meridional oscillation is related to the transition between phases of the cycle. It is found that the north–south ocean heat content difference always reaches a threshold near the onset of a warm/cold event.

The three-dimensional evolution of ocean temperature anomalies in the tropical Pacific during the simulated ENSO cycle is characterized by four major features: 1) a build up in the subsurface of the western equatorial sector during the pre-onset stage, 2) a fast spread from the western subsurface to the eastern surface along the equator during the onset stage, 3) a zonal extension and amplification at the surface during the growth stage, and 4) a northward and downward spread during the decay stage.

Ocean temperature budget analyses show that the buildup of subsurface temperature anomalies is dominated by the vertical advection process in the western sector and the meridional advection process in the central sector. The former process is associated with vertical displacements of the thermocline, which is an important element of the delayed oscillator theory. The latter process is associated with a Sverdrup imbalance between trade wind and thermocline anomalies and is emphasized as the primary charge–discharge process by the recharge oscillator theory. It is argued that both processes play key roles in producing subsurface ocean memory for the phase transitions of the ENSO cycle.

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Jin-Yi Yu
and
Carlos R. Mechoso

Abstract

This paper contrasts the sea surface temperature (SST) and surface heat flux errors in the Tropical Pacific simulated by the University of California, Los Angeles, coupled atmosphere–ocean general circulation model (CGCM) and by its atmospheric component (AGCM) using prescribed SSTs. The usefulness of such a comparison is discussed in view of the sensitivities of the coupled system.

Off the equator, the CGCM simulates more realistic surface heat fluxes than the AGCM, except in the eastern Pacific south of the equator where the coupled model produces a spurious intertropical convergence zone. The AGCM errors are dominated by excessive latent heat flux, except in the stratus regions along the coasts of California and Peru where errors are dominated by excessive shortwave flux. The CGCM tends to balance the AGCM errors by either correctly decreasing the evaporation at the expense of cold SST biases or erroneously increasing the evaporation at the expense of warm SST biases.

At the equator, errors in simulated SSTs are amplified by the feedbacks of the coupled system. Over the western equatorial Pacific, the CGCM produces a cold SST bias that is a manifestation of a spuriously elongated cold tongue. The AGCM produces realistic values of surface heat flux. Over the cold tongue in the eastern equatorial Pacific, the CGCM simulates realistic annual variations in SST. In the simulation, however, the relationship between variations in SST and surface latent heat flux corresponds to a negative feedback, while in the observation it corresponds to a positive feedback. Such an erroneous feature of the CGCM is linked to deficiencies in the simulation of the cross-equatorial component of the surface wind. The reasons for the success in the simulation of SST in the equatorial cold tongue despite the erroneous surface heat flux are examined.

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