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NASA NTRS2006

Comments on the Terminology of "Convectively-Coupled Kelvin Waves"

The terminology "convectively-coupled Kelvin waves" has been used frequently in the literature to refer to the 15 m/s eastward-moving planetary and large-scale waves in the tropics. This note points out that this terminology is not appropriate, since these waves contain Rossby waves and mixed Rossby-gravity waves also. The significance of pointing out this misnomer is that a better understanding of these waves may contribute to the search for their cause.

Chao, Winston C.↗

NASA NTRS1987

A new class of stratospheric vacillations in a highly truncated model due to wave interference

A new class of vacillations is obtained in the Holton and Mass model with a different bottom boundary condition. The model is a highly truncated spectral model describing wave-zonal flow interactions in a forced-dissipative system. The mean zonal wind and the wave change their vertical structures periodically with a period of the wave progression (5-10 days for the parameters used in this study). The vacillations are interpreted as an interference between a stationary wave and a topographically modified Rossby wave. The modified Rossby wave is an eigenmode of baroclinic flow in the presence of bottom topography within the framework of the highly truncated system. Time variations of the mean zonal wind are essential for the modification of the Rossby wave.

Yoden, Shigeo↗

NASA NTRS1982

The effect of islands on low frequency equatorial motions

A complete analytic solution is presented for the influence of equatorial islands on steady low-frequency waves. If the island is small (the meridional extent is much less than the equatorial radius of deformation, R), the waves pass it almost undisturbed, with the mass flux incident on the upstream side flowing around it nearly equally to the north and to the south and continuing on downstream in the lee of the island. For large islands (comparable in extent with R or larger), the principal response is organized as it would be if the island barrier were meridionally infinite. An incident Kelvin wave is largely reflected as long Rossby waves; symmetric long Rossby waves are reflected as equatorial Kelvin waves, while antisymmetric ones stop at the island barrier. In all cases, a boundary current composed of short Rossby waves forms at the eastern side of the island and accomplishes the required meridional redistribution of the zonal mass flux.

Cane, M. A.↗

NASA NTRS1993

Rossby-gravity waves in tropical total ozone data

Evidence for Rossby-gravity waves in tropical data fields produced by the European Center for Medium Range Weather Forecasts (ECMWF) was recently reported. Similar features are observable in fields of total column ozone from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument. The observed features are episodic, have zonal (east-west) wavelengths of 6,000-10,000 km, and oscillate with periods of 5-10 days. In accord with simple linear theory, the modes exhibit westward phase progression and eastward group velocity. The significance of finding Rossby-gravity waves in total ozone fields is that (1) the report of similar features in ECMWF tropical fields is corroborated with an independent data set and (2) the TOMS data set is demonstrated to possess surprising versatility and sensitivity to relatively smaller scale tropical phenomena.

Stanford, J. L.↗

NASA NTRS1983

A baroclinic quasigeostrophic open ocean model

A baroclinic quasigeostrophic open ocean model is presented, calibrated by a series of test problems, and demonstrated to be feasible and efficient for application to realistic mid-oceanic mesoscale eddy flow regimes. Two methods of treating the depth dependence of the flow, a finite difference method and a collocation method, are tested and intercompared. Sample Rossby wave calculations with and without advection are performed with constant stratification and two levels of nonlinearity, one weaker than and one typical of real ocean flows. Using exact analytical solutions for comparison, the accuracy and efficiency of the model is tabulated as a function of the computational parameters and stability limits set; typically, errors were controlled between 1 percent and 10 percent RMS after two wave periods. Further Rossby wave tests with realistic stratification and wave parameters chosen to mimic real ocean conditions were performed to determine computational parameters for use with real and simulated data. Finally, a prototype calculation with quasiturbulent simulated data was performed successfully, which demonstrates the practicality of the model for scientific use.

Miller, R. N.↗

NASA NTRS1993

Rossby-gravity waves in tropical total ozone

Randel (1992) has recently reported evidence for Rossby-gravity waves in tropical data fields produced by the European Centre for Medium Range Weather Forecasts (ECMWF). The purpose of this paper is to show that similar features are observable in fields of total column ozone from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument. The observed features are episodic, have zonal (east-west) wavelengths of approximately 6,000-10,000 km and oscillate with periods of 5-10 days. The modes exhibit westward phase progression and eastward group velocity. A simple linear model is used to estimate the magnitude of total ozone perturbations induced by Rossby-gravity waves. The model is able to account for the magnitude of observed TOMS features as well as their asymmetry about the equator. The significance of finding Rossby-gravity waves in total ozone fields is that: (1) the report of similar features in ECMWF tropical fields is corroborated with an independent data set; and (2) the TOMS data set is demonstrated to possess surprising versatility and sensitivity to relatively smaller scale tropical phenomena.

Stanford, J. L.↗

NASA NTRS1994

Geosat-derived sea level and surface current anomalies in the equatorial Pacific during the 1986-1989 El Nino and La Nina

Equatorial wave dynamics are essential in most oceanic models for reproducing aspects of the El Nino-Southern Oscillation (ENSO) phenomenon. Observational evidence of first baroclinic equatorial Kelvin and first symmetric meridional Rossby waves is found in Geosat-derived sea level anomalies (SLA) and surface zonal current anomalies (ZCA) in the equatorial Pacific ocean during the 1986-1987 El Nino and the ensuing 1988-1989 La Nina. This was made possible after extensive quality control and specific processing of the recently improved Geosat geophysical data records pertaining to the 17-day Exact Repeat Mission. In particular, the processing was made so that the Geosat-derived ZCA best fit near-surface zonal currents from three equatorial moorings at 165 deg E, 140 deg W, and 110 deg W. The Geosat-derived SLA and ZCA are decomposed into first baroclinic equatorial Kelvin and gravest Rossby modes. The emphasis is then put on the chronology of the ZCA at the equator where the currents are the most energetic and where Kelvin and first symmetric Rossby waves explain most of the variance in ZCA, in similar proportion. The 1986-1987 El Nino is mostly characterized by a strong downwelling Kelvin wave in December 1986, a series of downwelling Kelvin waves in March-October 1987, and a strong upwelling Rossby wave in March-September 1987. These waves are consistent with wind forcing, and all give rise to notable eastward ZCA for almost an entire year. During El Nino-La Nina transition period there is the occurrence of two downwelling Rossby waves originating from the eastern Pacific at times of favorable wind forcing. These Rossby waves switch the basin-wide ZCA from eastward to westward, terminating the warm event. Then, the 1988-1989 La Nina shows the generation of a series of upwelling Kelvin waves which are clearly identified in March/April and September/October 1988. These upwelling Kelvin waves are apparently related to the enhencement of the trade wind in the western half of the basin. These waves maintain the basin-wide westward ZCA for more than a year, although the ZCA is slightly reduced ba front of upwelling Rossby waves in phase with the normal seasonal cycle. The Kelvin wave forcing and sign of kelvin wave contribution versus Rossby wave contribution to ZCA in the western Pacific are discussed, and we conclude that neither pleads in favor of the delayed action oscillator mechanism.

Delcroix, Thierry↗

NASA NTRS1991

The structure and annual variation of antisymmetric fluctuations of tropical convection and their association with Rossby-gravity waves

The present study explores the signature of 4-5-day-period Rossby-gravity waves in the tropical convection field across the Indian and Pacific oceans. The convergence/divergence field of these waves in the lower troposphere is anticipated to produce an antisymmetric fluctuation in tropical convection. Antisymmetric fluctuations of tropical convection are shown to exhibit a pronounced spectral peak at a 4-5-day period only during boreal fall and only within about 30 deg longitude of the date line. The peak amplitude occurs around 7.5 deg latitude. These fluctuations propagate westward at 15-20 m/s with zonal wavelength of about 7000-9000 km. The fluctuations of convections are coherent and out of phase with the equatorial meridional wind, which also exhibits a pronounced spectral peak at a 4-5-day period in the lower troposphere near the date line. The horizontal distributions of the 4-5-day power and coherence of the winds and convection are consistent with that produced by a convectively coupled Rossby-gravity wave that is confined near the date line.

Hendon, Harry H.↗

NASA NTRS2022

Space Weather Modeling and Prediction for Intermediate Time-scales

Much of solar activity within a sunspot cycle occurs as bursts, or 'seasons' of strong activity over several months, separated by periods of much less activity. The most important space weather effects occur during these bursts. Previous modeling and forecasting efforts have focused on time-scales of hours-to-days and decades-to-centuries. The recent discovery of Rossby waves in the Sun, together with recently developed global models of solar MHD Rossby waves and their interactions with differential rotation and spot-producing magnetic fields, reveal the opportunity to simulate and predict the occurrence, strength and location of enhanced activity bursts a few weeks up to several months in advance. We now have a golden opportunity to fill in this gap in time-scales of forecasting space weather. This requires a) continuous observations of solar Rossby waves by various techniques; b) development of coupled nonlinear MHD models that simulate both global Rossby waves and the much smaller spatial scale emergence of new active regions; c) application of advanced data assimilation techniques to couple surface observations to update the model-system to integrate forward in time for creating forecasts months ahead. Then it will be possible to build operational prediction models to meet the needs of customers and stakeholders, including support of future NASA missions, regarding what kind and level of space weather to expect a few weeks to several months ahead.

SMD↗

NASA NTRS2002

On the Pathways of the Return Flow of the Meridional Overturning Circulation in the Tropical Atlantic

A numerical model of the tropical Atlantic ocean is used to investigate the upper layer pathways of the Meridional Overturning Circulation (MOC) in the tropical Atlantic. The main focus of this thesis is on those parts of the tropical circulation that are thought to be important for the MOC return flow, but whose dynamics have not been understood yet. It is shown how the particular structure of the tropical gyre and the MOO act to inhibit the flow of North Atlantic water into the equatorial thermocline. As a result, the upper layers of the tropical Atlantic are mainly fed by water from the South Atlantic. The processes that carry the South Atlantic water across the tropical Atlantic into the North Atlantic as part of the MOO are described here, and three processes that were hitherto not understood are explained as follows: The North Brazil Current rings are created as the result of the reflection of Rossby waves at the South American coast. These Rossby waves are generated by the barotropically unstable North Equatorial Countercurrent. The deep structure of the rings can be explained by merger of the wave's anticyclones with the deeper intermediate eddies that are generated as the intermediate western boundary current crosses the equator. The bands of strong zonal velocity in intermediate depths along the equator have hitherto been explained as intermediate currents. Here, an alternative interpretation of the observations is offered: The Eulerian mean flow along the equator is negligible and the observations are the signature of strong seasonal Rossby waves. The previous interpretation of the observations can then be explained as aliasing of the tropical wave field. The Tsuchyia Jets are driven by the Eliassen-Palm flux of the tropical instability waves. The equatorial current system with its strong shears is unstable and generates tropical instability waves.

Jochum, Markus↗

NASA NTRS1999

Dynamics of Quasi-biennial Oscillations in Tropical Ocean-Atmosphere Coupled System

In this study, quasi-biennial oscillation (QBO) in atmosphere-ocean coupled system is investigated using intermediate coupled model. Observation studies show that the easterly zonal winds anomalies prevail over the equatorial western Pacific during the warm phase of El Nino/Southern Oscillation (ENSO). At the time scale of QBO, SST variations and east Asia Summer monsoon rainfall are closely linked to the eastward propagating zonal winds anomalies originated from Indian ocean. To investigate the effect of zonal wind anomalies over western Pacific on the evolution of ENSO, simple anomalous winds are added to the western part of model domain as external forcing. Wind forcing is parameterized as a function of SST anomalies in the eastern Pacific with time lag. Time lag is adopted to mimic the relation between east Asian monsoon and ENSO. The results shows that the winds anomalies make coupled system oscillate through generating forced Kelvin waves even without the western boundary reflection of Rossby waves. Kelvin waves generated by external forcing are also crucial for the model to oscillate as well as Rossby wave reflections at the western boundary. When the monsoon forced Kelvin wave is strong during the northern winter, the coupled system damped out very quickly. In certain range of external winds amplitude and time lag, the model El Nino shows QBO features. It is suggested that the external wind forcing which is related to summer monsoon flow over western Pacific intensify the negative feedback process of off-equatorial Rossby waves and modify the ENSO periodicity.

Kim, K.-M.↗

NASA NTRS1990

Observations of planetary mixed Rossby-gravity waves in the upper stratosphere

Observational evidence is presented for planetary scale (zonal wave number 1-2) mixed Rossby-gravity (MRG) waves in the equatorial upper stratosphere (35-50 km). These waves are detected in LIMS measurements as coherently propagating temperature maxima of amplitude 0.1-0.3 K, which are antisymmetric (out of phase) about the equator, centered near 10-15 deg north and south latitude. These features have vertical wavelengths of order 10-15 km, periods near 2-3 days, and zonal phase velocities close to 200 m/s. Both eastward and westward propagating waves are found, and the observed vertical wavelengths and meridional structures are in good agreement with the MRG dispersion relation. Theoretical estimates of the zonal accelerations attributable to these waves suggest they do not contribute substantially to the zonal momentum balance in the middle atmosphere.

Randel, William J.↗

NASA NTRS1994

A preliminary evaluation of ocean topography from the TOPEX/POSEIDON mission

We have analyzed 50 ten-day cycles of TOPEX/POSEIDON (T/P) altimeter data to evaluate the ocean dynamic topography and its temporal variations. We have employed data from both the U.S. and French altimeters along with the NASA precision orbits in this analysis. Errors in the diurnal and semidiurnal components of the Cartwright-Ray tide model have been significantly reduced using a correction developed from a combination of JGM-2 and OSU91A was employed, as well as a geoid model based solely on OSU91A. The long wavelengths of the comparisons to historical data, although geoid error still corrupts the dynamic topography for wavelengths shorter than 2500 km. The root mean square (RMS) variability is similar to previous results from Geosat, with bakground 'noise' approaching 3 cm RMS. The computed annual and semiannual variations are also similar to previous Geosat results, although the hemispheric distribution of the annual heating cycle is much better presented in the T/P results. They also compare reasonably well with the Levitus hydrographic compilation in the northern hemisphere, although the T/P variations generally have larger amplitudes. Ten-day average maps of variations in sea level compare well with simulations measurements at ocean tide gauges, with RMS differences of less than 4 cm and correlations greater than 0.6 for most of the island gauges. Time-longitude plots of these sea level variations at different latitudes in the Pacific clearly show the presence of equatorial Kelvin waves and Rossby waves, with the wave speeds agreeing well with theoretical and observed values. Measurement of variations in global sea level over cycles 2-51 have an RMS variability of 6.3 mm and a rate of change of -3.5 +/- 8 mm/yr, the uncertainty primarily due to insufficient averaging of the interannual and periodic sea level variations. These results show that the accuracy of the T/P measurements of sea level has dramatically improved over previous missions, with estimated time variable errors of 4 cm or less. Although geographically correlated orbit errors have also been reduced to the few centimeter level, further improvement in determinations of the mean dynamic topography will be difficult to obtain until a more accurate model of the marine geoid is available.

Nerem, R. S.↗

NASA NTRS1993

The relationship between mixed Rossby-gravity waves and convection in a general circulation model

We investigate the relationship between mixed Rossby-gravity waves (MRGWs) and convection in a general circulation model. The experiments described are performed in a general circulation model with the lower boundary set to that of an ocean surface everywhere. Several experiments are run varying the convective parameterization scheme (using either a modified Kuo scheme or a moist convective adjustment scheme) and varying the tropical sea surface temperatures (specified to be zonally symmetric in all cases), thereby changing the location of the modeled intertropical convergence zones (ITCZs). The appearance of a robust MRGW occurs when the sea surface temperature is such that two ITCZs straddle the equator. The particular sea surface temperature distribution used and the parameterization scheme for convection also affect the structure and strength of the modeled MRGW. The vertical structure of MRGWs is analyzed in the experiment in which this wave mode is the most energetic. We show that MRGWs of several different zonal length scales exist in the troposphere in association with convection; however, it is only the longer length scales which can be discerned in the upper troposphere and lower stratosphere.

Hess, Peter G.↗

NASA NTRS1987

On the origin of the tropical intraseasonal oscillation

This study provides an explanation for the origin of the tropical intraseasonal (40-50 day) oscillation (TIO) based on a simple generalization of Gill's linear analytic model for tropical large-scale heat-induced circulation. The solution contains a convective region that excites an eastward-moving Kelvin wave and a westward-moving Rossby wave. The entire system moves eastward as a response to the circulation it excites at a speed at which the latent heat energy in the tropics is best extracted. The TIO speed is a weighted mean of the speed of the Kelvin wave and that of the Rossby wave.

Chao, Winston C.↗

NASA NTRS1991

The effects of transience on the propagation of stratospheric planetary waves

The propagation of planetary-scale Rossby waves in the stratosphere is investigated in a linear, time-dependent, primitive equation model. Two distinct maxima in the convergence of the Eliassen-Palm flux (EP flux) are found for steady and for transient waves in a variety of realistic northern hemisphere, winter season, zonal flows; one below the stratospheric polar jet and the other north of the zero wind line. These maxima appear at higher altitudes for wave 1 than for wave 2, especially in low latitudes. Different mechanisms cause the formation of these two maxima. The confinement of wave activity in high latitudes is mainly due to the dissipation of waves in that region under non-WKBJ conditions. The maximum in lower latitudes is related to the absorption of Rossby waves near their critical lines. For wave 2, the intensity of the high latitude maximum is sensitive to the transience of the wave forcing at the tropopause, while for wave 1 this sensitivity is reduced. For both waves the low latitude maximum shifts northward with increasing transience.

Chen, Ping↗

NASA NTRS1999

Investigation of Tropical Transport with UARS Data

Measurements of trace constituents obtained by instruments aboard the Upper Atmosphere Research Satellite (UARS) have been used to study transport processes associated with the quasi-biennial oscillation, laterally propagating Rossby waves, and upward propagating Kelvin waves in the tropical and subtropical upper troposphere and stratosphere. Mean vertical motions, vertical diffusivities and in-mixing rates were inferred from observations of the 'tape recorder' signal in near-equatorial stratospheric water vapor. The effect of the quasi-biennial oscillation (QBO) on tracer distributions in the upper half of the stratosphere was seen in a spectacular 'staircase' pattern, predominantly in the winter hemisphere, revealing the latitudinally asymmetric nature of QBO transport due to induced mean meridional circulations and modulation of lateral mixing associated with planetary Rossby waves. The propagation of Rossby waves across the equator in the westerly phase of the QBO was seen in tracer fields and corroborating United Kingdom Meteorological Office (UKMO) analyses; a modeling study of the effect of these waves on typical QBO wind profiles was performed. Water vapor in the upper troposphere and lower stratosphere was found to exhibit signatures of the tropical intraseasonal oscillation (TIO) and faster Kelvin waves in the two regions, respectively.

Dunkerton, Timothy J.↗

NASA NTRS2018

Simulation and Prediction of Warm Season Drought in North America

This presentation presents our recent work on model simulation and prediction of warm season drought in North America. The emphasis will be on the contribution from the leading modes of subseasonal atmospheric circulation variability, which are often present in the form of stationary Rossby waves. Here we take advantage of the results from observations, reanalyses, and simulations and reforecasts performed using the NASA Goddard Earth Observing System (GEOS-5) atmospheric and coupled General Circulation Model (GCM). Our results show that stationary Rossby waves play a key role in Northern Hemisphere (NH) atmospheric circulation and surface meteorology variability on subseasonal timescales. In particular, such waves have been crucial to the development of recent short-term warm season heat waves and droughts over North America (e.g. the 1988, 1998, and 2012 summer droughts) and northern Eurasia (e.g., the 2003 summer heat wave over Europe and the 2010 summer drought and heat wave over Russia). Through an investigation of the physical processes by which these waves lead to the development of warm season drought in North America, it is further found that these waves can serve as a potential source of drought predictability. In order to properly represent their effect and exploit this source of predictability, a model needs to correctly simulate the Northern Hemisphere (NH) mean jet streams and be able to predict the sources of these waves. Given the NASA GEOS-5 AGCM deficiency in simulating the NH jet streams and tropical convection during boreal summer, an approach has been developed to artificially remove much of model mean biases, which leads to considerable improvement in model simulation and prediction of stationary Rossby waves and drought development in North America. Our study points to the need to identify key model biases that limit model simulation and prediction of regional climate extremes, and diagnose the origin of these biases so as to inform modeling group for model improvement.

Rossby Waves↗