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Andrew W. Robertson
and
Carlos R. Mechoso

Abstract

The time series of annual streamflow of four rivers in southeastern and south-central South America (the Negro, Paraguay, Paraná, and Uruguay Rivers) for the period 1911–93 are analyzed. Application of the multitaper method shows that the following features are significant at the 95% level: 1) a nonlinear trend, 2) a near-decadal component, and 3) interannual peaks with ENSO timescales. The trend and near-decadal components are most marked in the two more central rivers, the Paraguay and Paraná, with ENSO timescale variability most pronounced in the Negro and Uruguay rivers in the southeast. Composites of SST are made for each of the statistically significant oscillatory components of river flow, by reconstructing each component using singular spectrum analysis. These composites confirm the influence of ENSO on the streamflow variability of the Negro and Uruguay Rivers, with El Niño associated with enhanced streamflow. On the decadal timescale, high river runoff is associated with anomalously cool SSTs over the tropical North Atlantic. A very similar near-decadal oscillation in SST over this region is identified separately from a rotated empirical orthogonal function analysis of gridded annual mean SSTs. The near-decadal component of the Paraguay and Paraná Rivers is strongest in the austral summer.

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Carlos R. Mechoso
and
Steven W. Lyons

Abstract

The impact of sea surface temperature (SST) anomalies observed during the Northern Hemisphere spring of 1984, which include the growing phase of an intense Atlantic warm event on the atmospheric circulation over the tropical Atlantic and Pacific is investigated using the nine-layer, low resolution version of the UCLA general circulation model. This impact is contrasted with that for the same period during 1983, when SST anomalies include the decaying phase of the strongest Pacific El Niño on record. Results obtained in control and anomaly simulations, consisting, respectively, of extended integrations with and without the observed SST anomalies, are analyzed.

It is found that simulated anomalies in the atmospheric circulation corresponding to 1984 include low-level westerlies over the equatorial Atlantic and easterlies over the equatorial Pacific. There are centers of anomalous low-level convergence and divergence off the northeast coast of Brazil and equatorial Brazil, respectively, which are associated with positive and negative precipitation anomalies. Differences between results corresponding to 1984 and 1983 show the impact of El Niño over the Pacific. Further, positive precipitation anomalies over the equatorial Atlantic shift from generally north of the equator in 1983 to south of the equator in 1994 (dry and wet years for northeast Brazil, respectively).

These simulated anomalies and interannual differences in the atmospheric circulation are in good general agreement with those observed. This agreement strongly suggests that the atmospheric anomalies observed during the northern springs of 1984 and 1983 over the tropical Atlantic and Pacific were primarily due to the SST anomalies.

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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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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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Jin-Yi Yu
and
Carlos R. Mechoso

Abstract

The hypothesis that Peruvian stratocumulus play an important role on both the annual mean and annual variations of sea surface temperature (SST) in the eastern equatorial Pacific is examined. The problem is addressed by performing sensitivity experiments using the University of California, Los Angeles, coupled atmosphere–ocean GCM with different idealized temporal variations of stratocumulus in a region along the coast of Peru.

The results obtained are consistent with the notion that Peruvian stratocumulus are a key component of the interhemispherically asymmetric features that characterize the annual mean climate of the eastern equatorial Pacific, including the cold SSTs off Peru and the absence of a southern ITCZ. The principal new finding of this study is that the annual variations (i.e., deviations from the annual mean) of Peruvian stratocumulus are linked to the differences between the amplitude, duration, and westward propagation of the warm and cold phases of the equatorial cold tongue. In the model’s context, only if the prescribed annual variations of Peruvian stratocumulus have the same phase as the observed variations are those differences successfully captured.

The impact of Peruvian stratocumulus on equatorial SST involves “dynamical” and “thermal” effects. The former develop through an enhancement of the northerly component of the surface wind from the Peruvian coast to the equator. The thermal effects develop through the special relationships between SST and surface evaporation over the equatorial cold tongue, which contributes to extend the cold phase until the end of the year. A successful portrayal of this behavior requires a realistic simulation of the annual variations of surface wind over the equatorial cold tongue.

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JoséL. Genta
,
Gonzalo Perez-Iribarren
, and
Carlos R. Mechoso

Abstract

This paper examines the records of streamflow during the period 1901–95 corresponding to four major rivers in southeastern South America: Uruguay, Negro, Paraná, and Paraguay. The emphasis is on the detection of long-term trends in the records. The authors demonstrate that the 30-yr running averaged streamflows increased after the mid-1960s at a rate that is approximately linear but not the same in all rivers. There seems to be a tendency toward leveling off in the most recent values. The increased streamflow is consistent with a significant decrease in the amplitude of the seasonal cycle in all rivers, except in the Negro River. An analysis of the sea surface temperature in the eastern equatorial Pacific Ocean suggests that an important component of such an increase in streamflows is consistent with a large-scale and low-frequency variability of the climate system.

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