scholarly journals UPPER LAYER CIRCULATION IN THE BANDA SEA IN RESPONSE TO THE ONSET OF MONSOON WINDS

2018 ◽  
Vol 28 ◽  
pp. 81-95 ◽  
Author(s):  
W. Hutahean ◽  
R. E. Wilson
Keyword(s):  
Gravity Model ◽  
Rossby Waves ◽  
Kelvin Waves ◽  
Low Frequencies ◽  
Free Wave ◽  
Local Forcing ◽  
Upper Layer Circulation ◽  
Monsoon Winds ◽  
Basin Response ◽  
Onset Of Monsoon

Aspects of the upper layer circulation in the interior of the Banda Sea, Indonesia, associated with local forcing by monsoon winds are examined numerically through the use of a reduced gravity model. The basin is located between approximately 4°S and 8°S and is partially enclosed by chains of islands. The primary emphasis is an evaluation of the free wave response which contributes to the steady or slowly varying circulation. Basin response appears to be characterized by interacting Kelvin waves and Rossby at low frequencies, and by evanescent Poincare waves of higher frequencies. Passages between islands along the perimeter of the basin appear to be nearly, impermeable to Rossby waves, which contribute to a pattern of westward propagating quasi geostrophic eddies. This pattern would persist during periods of wind transition.

scholarly journals The Nature of the Stochastic Wind Forcing of ENSO

2018 ◽  
Vol 31 (19) ◽  
pp. 8081-8099 ◽  
Author(s):  
Antonietta Capotondi ◽  
Prashant D. Sardeshmukh ◽  
Lucrezia Ricciardulli
Keyword(s):  
El Niño ◽  
High Frequency ◽  
El Nino ◽  
Southern Oscillation ◽  
Low Frequency ◽  
Kelvin Waves ◽  
Wind Forcing ◽  
Alternative View ◽  
Low Frequencies ◽  
Wind Events

El Niño–Southern Oscillation (ENSO) is commonly viewed as a low-frequency tropical mode of coupled atmosphere–ocean variability energized by stochastic wind forcing. Despite many studies, however, the nature of this broadband stochastic forcing and the relative roles of its high- and low-frequency components in ENSO development remain unclear. In one view, the high-frequency forcing associated with the subseasonal Madden–Julian oscillation (MJO) and westerly wind events (WWEs) excites oceanic Kelvin waves leading to ENSO. An alternative view emphasizes the role of the low-frequency stochastic wind components in directly forcing the low-frequency ENSO modes. These apparently distinct roles of the wind forcing are clarified here using a recently released high-resolution wind dataset for 1990–2015. A spectral analysis shows that although the high-frequency winds do excite high-frequency Kelvin waves, they are much weaker than their interannual counterparts and are a minor contributor to ENSO development. The analysis also suggests that WWEs should be viewed more as short-correlation events with a flat spectrum at low frequencies that can efficiently excite ENSO modes than as strictly high-frequency events that would be highly inefficient in this regard. Interestingly, the low-frequency power of the rapid wind forcing is found to be higher during El Niño than La Niña events, suggesting a role also for state-dependent (i.e., multiplicative) noise forcing in ENSO dynamics.

scholarly journals Wind-Driven Oceanic Rossby Waves in the Tropical South Indian Ocean with and without an Active ENSO

2005 ◽  
Vol 35 (5) ◽  
pp. 729-746 ◽  
Author(s):  
Astrid Baquero-Bernal ◽  
Mojib Latif
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Time Scale ◽  
Rossby Waves ◽  
Heat Content ◽  
Kelvin Waves ◽  
South Indian Ocean ◽  
South Indian ◽  
Coastal Kelvin Waves ◽  
The Tropical Pacific

Abstract The interannual heat content variability in the tropical south Indian Ocean (SIO) and its relationship with El Niño–Southern Oscillation (ENSO) is studied. The baroclinic ocean response to stochastic wind stress predicted by a simple analytical model is compared with two integrations of the ECHO-G coupled general circulation model. In one integration, ocean–atmosphere interactions are suppressed in the tropical Pacific Ocean, so that this integration does not simulate ENSO. In the other integration, interactions are allowed everywhere and ENSO is simulated. The results show that basinwide variability in the SIO heat content can be produced by two mechanisms: 1) oscillatory forcing by ENSO-related wind stress and 2) temporally stochastic and spatially coherent wind stress forcing. Previous studies have shown that transmission of energy from the tropical Pacific to the southern Indian Ocean occurs through coastal Kelvin waves along the western coast of Australia. The results in this paper confirm the occurrence of such transmission. In the ECHO-G simulations, this transmission occurs both at the annual time scale and at interannual time scales. Generation of offshore Rossby waves by these coastal Kelvin waves at interannual time scales—and, in particular, at the ENSO time scale—was found.

Intraseasonal Variability of the Equatorial Indian Ocean Observed from Sea Surface Height, Wind, and Temperature Data

2007 ◽  
Vol 37 (2) ◽  
pp. 188-202 ◽  
Author(s):  
Lee-Lueng Fu
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Equatorial Indian Ocean ◽  
Sea Surface Height ◽  
Kelvin Waves ◽  
Wind Forcing ◽  
Sea Surface ◽  
Vertical Mode ◽  
Basin Mode

Abstract The forcing of the equatorial Indian Ocean by the highly periodic monsoon wind cycle creates many interesting intraseasonal variabilities. The frequency spectrum of the wind stress observations from the European Remote Sensing Satellite scatterometers reveals peaks at the seasonal cycle and its higher harmonics at 180, 120, 90, and 75 days. The observations of sea surface height (SSH) from the Jason and Ocean Topography Experiment (TOPEX)/Poseidon radar altimeters are analyzed to study the ocean’s response. The focus of the study is on the intraseasonal periods shorter than the annual period. The semiannual SSH variability is characterized by a basin mode involving Rossby waves and Kelvin waves traveling back and forth in the equatorial Indian Ocean between 10°S and 10°N. However, the interference of these waves with each other masks the appearance of individual Kelvin and Rossby waves, leading to a nodal point (amphidrome) of phase propagation on the equator at the center of the basin. The characteristics of the mode correspond to a resonance of the basin according to theoretical models. For the semiannual period and the size of the basin, the resonance involves the second baroclinic vertical mode of the ocean. The theory also calls for similar modes at 90 and 60 days. These modes are found only in the eastern part of the basin, where the wind forcing at these periods is primarily located. The western parts of the theoretical modal patterns are not observed, probably because of the lack of wind forcing. There is also similar SSH variability at 120 and 75 days. The 120-day variability, with spatial patterns resembling the semiannual mode, is close to a resonance involving the first baroclinic vertical mode. The 75-day variability, although not a resonant basin mode in theory, exhibits properties similar to the 60- and 90-day variabilities with energy confined to the eastern basin, where the SSH variability seems in resonance with the local wind forcing. The time it takes an oceanic signal to travel eastward as Kelvin waves from the forcing location along the equator and back as Rossby waves off the equator roughly corresponds to the period of the wind forcing. The SSH variability at 60–90 days is coherent with sea surface temperature (SST) with a near-zero phase difference, showing the effects of the time-varying thermocline depth on SST, which may affect the wind in an ocean–atmosphere coupled process governing the intraseasonal variability.

scholarly journals Trajectory Analysis of the Mechanism for Westward Propagation of Rossby Waves

2015 ◽  
Vol 72 (5) ◽  
pp. 2178-2182 ◽  
Author(s):  
Edwin K. Schneider
Keyword(s):  
Reference Frame ◽  
Trajectory Analysis ◽  
Rossby Waves ◽  
Phase Speed ◽  
Point Of View ◽  
Three Dimensions ◽  
The West ◽  
Free Wave ◽  
Wave Dynamics ◽  
Wave Solutions

Abstract An analysis of Rossby wave dynamics in two and three dimensions is carried from the point of view of a reference frame propagating with the zonal phase speed of the wave. Since trajectories and streamlines coincide in this reference frame, the mechanism for (westward) propagation of free waves has a different interpretation than in a reference frame fixed to the ground. In the wave reference frame, propagating free-wave solutions are possible only when parcels approach from the west. When parcels approach from the east, potential vorticity cannot be conserved along trajectories.

Horizontal wave propagation in the equatorial waveguide

1994 ◽  
Vol 271 ◽  
pp. 267-284 ◽  
Author(s):  
P. Ripa
Keyword(s):  
Rossby Waves ◽  
Normal Modes ◽  
Wave Radiation ◽  
Linear Wave ◽  
Low Frequencies ◽  
First Case ◽  
Horizontal Wave ◽  
Velocity Addition ◽  
Spurious Solution ◽  
Full Solution

The dispersive (i.e. non-Kelvin) linear wave field on the equatorial β-plane, in a single vertical mode, is fully described by a single potential φ. Long Rossby waves, which are weakly dispersive, are represented in this field. This description is free from the problem of the ‘spurious solution’ encountered when working with an evolution equation for the meridional velocity; addition of this unwanted solution represents a gauge transformation that leaves the physical fields unaltered.The general solution of the ray equations is found, including trajectories, and the amplitudes and phase fields. This solution is asymptotically valid for either high or low frequencies. The ray paths are identical in both limits, but the phase field is not, reflecting the isotropy of Poincaré waves, in one case, and the zonal anisotropy of Rossby waves, in the other.Two examples are studied by ray theory: meridional normal modes and wave radiation from a point source in the equator. In the first case, the exact dispersion relation is obtained. In the second one, northern and southern caustics bend towards the equator, meeting there at focal points. The full solution is the superposition of many leaves and has a structure that would be hard to find in a normal modes expansion.

scholarly journals The characteristics of the equatorial waves caused a record torrential rain event over western Sumatra

2021 ◽  
Vol 893 (1) ◽  
pp. 012015
Author(s):  
P Wu ◽  
Y Fukutomi ◽  
K Kikuchi
Keyword(s):  
Rossby Waves ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Precipitation Event ◽  
Western Coast ◽  
Rain Event ◽  
Equatorial Waves ◽  
Torrential Rain ◽  
Sumatra Island ◽  
Equatorial Rossby Waves

Abstract This study examined the cause of a record torrential rain event over the western coast of Sumatra Island in March 2016. The influence of atmospheric equatorial waves (EWs) and the characteristics of the EWs were investigated. Analysis of the Japanese 55-year Reanalysis data (JRA-55) and precipitation data from the Global Precipitation Measurement (GPM) satellite showed that the event was caused by the combined effects of Kelvin waves, equatorial Rossby waves, and westward inertio-gravity (WIG) waves. An examination of the characteristics of the EWs revealed that the Kelvin waves had longitudinal scales of ~6,000 km, with a period of ~6 days and phase speed of ~12 m s-1, which was typical of the convectively coupled Kelvin waves in this region. The WIG waves had a scale of ~2,500 km, with a period of 2.5 days and a relatively fast phase speed of 12~13 m s-1. Heavy precipitation occurred when an eastward Kelvin wave from the Indian Ocean encountered a westward inertio-gravity (WIG) over Sumatra Island. It was concluded that along with the Kelvin and equatorial Rossby waves, the WIG waves might have played a major role in the formation of the extreme precipitation event.

scholarly journals Combined Role of High- and Low-Frequency Processes of Equatorial Zonal Transport in Terminating an ENSO Event

2018 ◽  
Vol 31 (14) ◽  
pp. 5461-5483 ◽  
Author(s):  
Han-Ching Chen ◽  
Chung-Hsiung Sui ◽  
Yu-Heng Tseng ◽  
Bohua Huang
Keyword(s):  
El Niño ◽  
High Frequency ◽  
Rossby Waves ◽  
El Nino ◽  
Low Frequency ◽  
Kelvin Waves ◽  
La Niña ◽  
La Nina ◽  
Eastern Boundary ◽  
Peak Phase

This study investigates the sudden reversal of anomalous zonal equatorial transport above thermocline at the peak phase of ENSO. The oceanic processes associated with zonal transport are separated into low-frequency ENSO cycle and high-frequency oceanic wave processes. Both processes can generate a reversal of equatorial zonal current at the ENSO peak phase, which is a trigger for the rapid termination of ENSO events. For the low-frequency process, zonal transport exhibits slower and basinwide evolution. During the developing phase of El Niño (La Niña), eastward (westward) transport prevails in the central-eastern Pacific, which enhances ENSO. At the peak of ENSO, a basinwide reversal of the zonal transport resulting from the recharge–discharge process occurs and weakens the existing SST anomalies. High-frequency zonal transport presents clear eastward propagation related to Kelvin wave propagation at the equator, reflection at the eastern boundary, and the westward propagating Rossby waves. The major westerly wind bursts (easterly wind surges) occur in late boreal summer and fall with coincident downwelling (upwelling) Kelvin waves for El Niño (La Niña) events. After the peak of El Niño (La Niña), Kelvin waves reach the eastern boundary in boreal winter and reflect as off-equatorial Rossby waves; then, the zonal transport switches from eastward (westward) to westward (eastward). The high-frequency zonal transport can be represented by equatorial wave dynamics captured by the first three EOFs based on the high-pass-filtered equatorial thermocline. The transport anomaly during the decaying phase is dominated by the low-frequency process in El Niño. However, the transport anomaly is caused by both low- and high-frequency processes during La Niña.

Roles of Equatorial Waves and Western Boundary Reflection in the Seasonal Circulation of the Equatorial Indian Ocean

2006 ◽  
Vol 36 (5) ◽  
pp. 930-944 ◽  
Author(s):  
Dongliang Yuan ◽  
Weiqing Han
Keyword(s):  
Indian Ocean ◽  
Sea Level ◽  
Rossby Waves ◽  
Equatorial Indian Ocean ◽  
Kelvin Waves ◽  
Wind Forcing ◽  
Western Boundary ◽  
Central Basin ◽  
Sea Levels ◽  
Equatorial Rossby Waves

Abstract An ocean general circulation model (OGCM) is used to study the roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial Indian Ocean. The western boundary reflection is defined as the total Kelvin waves leaving the western boundary, which include the reflection of the equatorial Rossby waves as well as the effects of alongshore winds, off-equatorial Rossby waves, and nonlinear processes near the western boundary. The evaluation of the reflection is based on a wave decomposition of the OGCM results and experiments with linear models. It is found that the alongshore winds along the east coast of Africa and the Rossby waves in the off-equatorial areas contribute significantly to the annual harmonics of the equatorial Kelvin waves at the western boundary. The semiannual harmonics of the Kelvin waves, on the other hand, originate primarily from a linear reflection of the equatorial Rossby waves. The dynamics of a dominant annual oscillation of sea level coexisting with the dominant semiannual oscillations of surface zonal currents in the central equatorial Indian Ocean are investigated. These sea level and zonal current patterns are found to be closely related to the linear reflections of the semiannual harmonics at the meridional boundaries. Because of the reflections, the second baroclinic mode resonates with the semiannual wind forcing; that is, the semiannual zonal currents carried by the reflected waves enhance the wind-forced currents at the central basin. Because of the different behavior of the zonal current and sea level during the reflections, the semiannual sea levels of the directly forced and reflected waves cancel each other significantly at the central basin. In the meantime, the annual harmonic of the sea level remains large, producing a dominant annual oscillation of sea level in the central equatorial Indian Ocean. The linear reflection causes the semiannual harmonics of the incoming and reflected sea levels to enhance each other at the meridional boundaries. In addition, the weak annual harmonics of sea level in the western basin, resulting from a combined effect of the western boundary reflection and the equatorial zonal wind forcing, facilitate the dominance by the semiannual harmonics near the western boundary despite the strong local wind forcing at the annual period. The Rossby waves are found to have a much larger contribution to the observed equatorial semiannual oscillations of surface zonal currents than the Kelvin waves. The westward progressive reversal of seasonal surface zonal currents along the equator in the observations is primarily due to the Rossby wave propagation.

scholarly journals Influence of the Reflected Rossby Waves on the Western Arabian Sea Upwelling Region

2014 ◽  
Vol 44 (5) ◽  
pp. 1424-1438 ◽  
Author(s):  
Tomoki Tozuka ◽  
Motoki Nagura ◽  
Toshio Yamagata
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
General Circulation ◽  
Arabian Sea ◽  
Rossby Waves ◽  
Circulation Model ◽  
Equatorial Indian Ocean ◽  
Kelvin Waves ◽  
Eastern Boundary ◽  
Arabian Sea Upwelling

Abstract The sea surface temperature (SST) in the western Arabian Sea upwelling region is known to influence the amount of precipitation associated with the Indian summer monsoon. Thus, understanding what determines the SST in this region is an important issue. Using outputs from an ocean general circulation model with and without strong damping in the eastern equatorial Indian Ocean, this study examines how the reflection of semiannual Kelvin waves at the eastern boundary of the Indian Ocean may influence the western Arabian Sea upwelling region. The downwelling Kelvin waves generated in boreal spring are reflected at the eastern boundary and reach the western equatorial Indian Ocean as reflected Rossby waves about 6 months later. The resulting westward current along the equator in the western equatorial Indian Ocean transports warmer water to the western Arabian Sea upwelling region. Thus, the SST in this region becomes colder especially in boreal fall without the reflected Rossby waves. These results are further supported by the analysis of the mixed layer temperature balance. Surprisingly, vertical processes do not contribute to the SST difference, even though the thermocline becomes shallower without the downwelling Rossby waves. This is because the mixed layer is shoaling rapidly from September to November, and there is basically no entrainment of water from below. In contrast, the reflected Rossby waves do not have large impacts on the SST in other seasons mainly because the zonal SST gradient is not as strong and/or the amplitude of Rossby waves is weaker.

Why Are There Rossby Wave Maxima in the Pacific at 10°S and 13°N?

2003 ◽  
Vol 33 (8) ◽  
pp. 1549-1563 ◽  
Author(s):  
Antonietta Capotondi ◽  
Michael A. Alexander ◽  
Clara Deser
Keyword(s):  
Rossby Wave ◽  
General Circulation ◽  
Rossby Waves ◽  
Circulation Model ◽  
Wave Model ◽  
Dominant Factor ◽  
Small Scale ◽  
Thermocline Depth ◽  
Ekman Pumping ◽  
Low Frequencies

Abstract Observations indicate the existence of two bands of maximum thermocline depth variability centered at ∼10°S and 13°N in the tropical Pacific Ocean. The analysis of a numerical integration performed with the National Center for Atmospheric Research ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat, and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 10°S and 13°N are associated with first-mode baroclinic Rossby waves forced by anomalous Ekman pumping. In this study the factors that may be responsible for the Rossby wave maxima at 10°S and 13°N, including the amplitude and spatial coherency of the forcing at those latitudes, are systematically investigated. A simple Rossby wave model is used to interpret the OGCM variability and to help to discriminate between the different factors that may produce the tropical maxima. These results indicate that the dominant factor in producing the maximum variability at 10°S and 13°N is the zonal coherency of the Ekman pumping, a characteristic of the forcing that becomes increasingly more pronounced at low frequencies, maximizing at timescales in the decadal range. Local maxima in the amplitude of the forcing, while not explaining the origin of the centers of variability at 10°S and 13°N, appear to affect the sharpness of the variability maxima at low frequencies. Although the Rossby wave model gives an excellent fit to the OGCM, some discrepancies exist: the amplitude of the thermocline variance is generally underestimated by the simple model, and the variability along 13°N is westward intensified in the wave model but reaches a maximum in the central part of the basin in the OGCM. Short Rossby waves excited by small-scale Ekman pumping features, or the presence of higher-order Rossby wave modes may be responsible for the differences in the zonal variance distribution along 13°N.

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