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.

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.

Interannual Variability in Sea Surface Height at Southern Midlatitudes of the Indian Ocean

2021 ◽  
Vol 51 (5) ◽  
pp. 1595-1609
Author(s):  
Motoki Nagura ◽  
Michael J. McPhaden
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Sea Surface Height ◽  
Wind Forcing ◽  
Sea Surface ◽  
The South ◽  
South Indian Ocean ◽  
Eastern Boundary ◽  
South Indian ◽  
The Indian Ocean

AbstractThis study examines interannual variability in sea surface height (SSH) at southern midlatitudes of the Indian Ocean (10°–35°S). Our focus is on the relative role of local wind forcing and remote forcing from the equatorial Pacific Ocean. We use satellite altimetry measurements, an atmospheric reanalysis, and a one-dimensional wave model tuned to simulate observed SSH anomalies. The model solution is decomposed into the part driven by local winds and that driven by SSH variability radiated from the western coast of Australia. Results show that variability radiated from the Australian coast is larger in amplitude than variability driven by local winds in the central and eastern parts of the south Indian Ocean at midlatitudes (between 19° and 33°S), whereas the influence from eastern boundary forcing is confined to the eastern basin at lower latitudes (10° and 17°S). The relative importance of eastern boundary forcing at midlatitudes is due to the weakness of wind stress curl anomalies in the interior of the south Indian Ocean. Our analysis further suggests that SSH variability along the west coast of Australia originates from remote wind forcing in the tropical Pacific, as is pointed out by previous studies. The zonal gradient of SSH between the western and eastern parts of the south Indian Ocean is also mostly controlled by variability radiated from the Australian coast, indicating that interannual variability in meridional geostrophic transport is driven principally by Pacific winds.

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.

scholarly journals Intraseasonal Variability of the Equatorial Undercurrent in the Indian Ocean

2019 ◽  
Vol 49 (1) ◽  
pp. 85-101 ◽  
Author(s):  
Gengxin Chen ◽  
Weiqing Han ◽  
Yuanlong Li ◽  
Jinglong Yao ◽  
Dongxiao Wang
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Wind Forcing ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Equatorial Undercurrent ◽  
Eastern Basin ◽  
The Indian Ocean ◽  
Set Up

AbstractBy analyzing in situ observations and conducting a series of ocean general circulation model experiments, this study investigates the physical processes controlling intraseasonal variability (ISV) of the Equatorial Undercurrent (EUC) of the Indian Ocean. ISV of the EUC leads to time-varying water exchanges between the western and eastern equatorial Indian Ocean. For the 2001–14 period, standard deviations of the EUC transport variability are 1.92 and 1.77 Sv (1 Sv ≡ 106 m3 s−1) in the eastern and western basins, respectively. The ISV of the EUC is predominantly caused by the wind forcing effect of atmospheric intraseasonal oscillations (ISOs) but through dramatically different ocean dynamical processes in the eastern and western basins. The stronger ISV in the eastern basin is dominated by the reflected Rossby waves associated with intraseasonal equatorial zonal wind forcing. It takes 20–30 days to set up an intraseasonal EUC anomaly through the Kelvin and Rossby waves associated with the first and second baroclinic modes. In the western basin, the peak intraseasonal EUC anomaly is generated by the zonal pressure gradient force, which is set up by radiating equatorial Kelvin and Rossby waves induced by the equatorial wind stress. Directly forced and reflected Rossby waves from the eastern basin propagate westward, contributing to intraseasonal zonal current near the surface but having weak impact on the peak ISV of the EUC.

scholarly journals The Intraseasonal Variability of the Indian Summer Monsoon Using TMI Sea Surface Temperatures and ECMWF Reanalysis

2008 ◽  
Vol 21 (11) ◽  
pp. 2519-2539 ◽  
Author(s):  
Nicholas P. Klingaman ◽  
Hilary Weller ◽  
Julia M. Slingo ◽  
Peter M. Inness
Keyword(s):  
Life Cycle ◽  
Indian Ocean ◽  
General Circulation ◽  
Function Analysis ◽  
Intraseasonal Variability ◽  
Equatorial Indian Ocean ◽  
Sea Surface ◽  
Sea Surface Temperatures ◽  
Surface Temperatures ◽  
Sst Anomalies

Abstract The northward-propagating intraseasonal (30–40 day) oscillation (NPISO) between active and break monsoon phases exerts a critical control on summer-season rainfall totals over India. Advances in diagnosing these events and comprehending the physical mechanisms behind them may hold the potential for improving their predictability. While previous studies have attempted to extract active and break events from reanalysis data to elucidate a composite life cycle, those studies have relied on first isolating the intraseasonal variability in the record (e.g., through bandpass filtering, removing harmonics, or empirical orthogonal function analysis). Additionally, the underlying physical processes that previous studies have proposed have varied, both among themselves and with studies using general circulation models. A simple index is defined for diagnosing NPISO events in observations and reanalysis, based on lag correlations between outgoing longwave radiation (OLR) over India and over the equatorial Indian Ocean. This index is the first to use unfiltered OLR observations and so does not specifically isolate intraseasonal periods. A composite NPISO life cycle based on this index is similar to previous composites in OLR and surface winds, demonstrating that the dominance of the intraseasonal variability in the monsoon climate system eliminates the need for more complex methods (e.g., time filtering or EOF analysis) to identify the NPISO. This study is also among the first to examine the NPISO using a long-period record of high-resolution sea surface temperatures (SSTs) from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager. Application of this index to those SSTs demonstrates that SST anomalies exist in near quadrature with convection, as suggested by recent coupled model studies. Analysis of the phase relationships between atmospheric fields and SSTs indicates that the atmosphere likely forced the SST anomalies. The results of this lag-correlation analysis suggest that the oscillation serves as its own most reliable—and perhaps only—predictor, and that signals preceding an NPISO event appear first over the Indian subcontinent, not the equatorial Indian Ocean where the events originate.

scholarly journals Trapped Planetary (Rossby) Waves Observed in the Indian Ocean by Satellite Borne Altimeters

2017 ◽  
Author(s):  
Yair De-Leon ◽  
Nathan Paldor
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Low Frequency ◽  
Propagation Speed ◽  
Wave Theory ◽  
Planetary Waves ◽  
Sea Surface ◽  
Trapped Wave ◽  
The Indian Ocean ◽  
Frequency Variations

Abstract. Using 20 years of accurately calibrated, high resolution, observations of Sea Surface Height Anomalies (SSHA) by satellite ‎borne altimeters we show that in the Indian Ocean south of the Australian coast the low frequency variations of SSHA are ‎dominated by westward propagating, trapped, i.e. non-harmonic, planetary waves. Our results demonstrate that the ‎meridional-dependent amplitudes of the SSHA are large only within a few degrees of latitude next to the South-Australian ‎coast while farther in the ocean they are uniformly small. This meridional variation of the SSHA signal is typical of the ‎amplitude structure in the trapped wave theory. The westward propagation speed of the SSHA signals is analyzed by ‎employing three different methods of estimation. Each one of these methods yields speed estimates that can vary widely ‎between adjacent latitudes but the combination of at least two of the three methods yields much smoother variation. The ‎estimates obtained in this manner show that the observed phase speeds at different latitudes exceed the phase speeds of ‎harmonic Rossby (Planetary) waves by 140 % to 200 %. In contrast, the theory of trapped Rossby (Planetary) waves in a ‎domain bounded by a wall on its equatorward side yields phase speeds that approximate more closely the observed phase ‎speeds.‎

Variations of the Indian Ocean Walker circulation since the Last Glacial Maximum revealed by reconstructed and simulated zonal wind intensity

2021 ◽  
Author(s):  
Xinquan Zhou ◽  
Stéphanie Duchamp-Alphonse ◽  
Masa Kageyama ◽  
Franck Bassinot ◽  
Xiaoxu Shi ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Surface Temperature ◽  
Equatorial Indian Ocean ◽  
Walker Circulation ◽  
Sea Surface ◽  
The Indian Ocean ◽  
Sst Anomaly ◽  
Mean Circulation ◽  
The Last Glacial Maximum ◽  
The Last Glacial

<p>Today, precipitation and wind patterns over the equatorial Indian Ocean and surrounding lands are paced by monsoon and Walker circulations that are controlled by the seasonal land-sea temperature contrast and the inter-annual convection over the Indo-Pacific Warm Pool, respectively. The annual mean surface westerly winds are particularly tied to the Walker circulation, showing interannual variability coupled with the gradient of Sea Surface Temperature (SST) anomaly between the tropical western and southeastern Indian Ocean, namely, the Indian Ocean Dipole (IOD). While the Indian monsoon pattern has been widely studied in the past, few works deal with the evolution of Walker circulation despite its crucial impacts on modern and future tropical climate systems. Here, we reconstruct the long-term westerly (summer) and easterly (winter) wind dynamics of the equatorial Indian Ocean (10°S−10°N), since the Last Glacial Maximum (LGM) based on i) primary productivity (PP) records derived from coccolith analyses of sedimentary cores MD77-191 and BAR94-24, retrieved off the southern tip of India and off the northwestern tip of Sumatra, respectively and ii) the calculation of a sea surface temperature (SST) anomaly gradient off (south) western Sumatra based on published SST data. We compare these reconstructions with atmospheric circulation simulations obtained with the general coupled model AWI-ESM-1-1-LR (Alfred Wegener Institute Earth System Model).</p><p>Our results show that the Indian Ocean Walker circulation was weaker during the LGM and the early/middle Holocene than present. Model simulations suggest that this is due to anomalous easterlies over the eastern Indian Ocean. The LGM mean circulation state may have been comparable to the year 1997 with a positive IOD, when anomalously strong equatorial easterlies prevailed in winter. The early/mid Holocene mean circulation state may have been equivalent to the year 2006 with a positive IOD, when anomalously strong southeasterlies prevailed over Java-Sumatra in summer. The deglaciation can be seen as a transient period between these two positive IOD-like mean states.</p>

scholarly journals Dynamics of the seasonal variations in the Indian Ocean from TOPEX/POSEIDON sea surface height and an ocean model

1998 ◽  
Vol 25 (11) ◽  
pp. 1915-1918 ◽  
Author(s):  
Jiayan Yang ◽  
Lisan Yu ◽  
Chester J. Koblinsky ◽  
David Adamec
Keyword(s):  
Indian Ocean ◽  
Seasonal Variations ◽  
Sea Surface Height ◽  
Ocean Model ◽  
Sea Surface ◽  
The Indian Ocean
Sign in / Sign up

Export Citation Format

Share Document