Basin Resonances in the Equatorial Indian Ocean

2011 ◽  
Vol 41 (6) ◽  
pp. 1252-1270 ◽  
Author(s):  
Weiqing Han ◽  
Julian P. McCreary ◽  
Yukio Masumoto ◽  
Jérôme Vialard ◽  
Benét Duncan
Keyword(s):  
Indian Ocean ◽  
Sea Level ◽  
Rossby Wave ◽  
Time Scale ◽  
General Circulation ◽  
Equatorial Indian Ocean ◽  
Western Boundary ◽  
Kelvin Wave ◽  
Resonance Amplitude ◽  
The Impact

Abstract Previous studies have investigated how second-baroclinic-mode (n = 2) Kelvin and Rossby waves in the equatorial Indian Ocean (IO) interact to form basin resonances at the semiannual (180 day) and 90-day periods. This paper examines unresolved issues about these resonances, including the reason the 90-day resonance is concentrated in the eastern ocean, the time scale for their establishment, and the impact of complex basin geometry. A hierarchy of ocean models is used: an idealized one-dimensional (1D) model, a linear continuously stratified ocean model (LCSM), and an ocean general circulation model (OGCM) forced by Quick Scatterometer (QuikSCAT) wind during 2000–08. Results indicate that the eastern-basin concentration of the 90-day resonance happens because the westward-propagating Rossby wave is slower, and thus is damped more than the eastward-propagating Kelvin wave. Results also indicate that superposition with other baroclinic modes further enhances the eastern maximum and weakens sea level variability near the western boundary. Without resonance, although there is still significant power at 90 and 180 days, solutions have no spectral peaks at these periods. The key time scale for the establishment of all resonances is the time it takes a Kelvin wave to cross the basin and a first-meridional-mode (ℓ = 1) Rossby wave to return; thus, even though the amplitude of the 90-day winds vary significantly, the 90-day resonance can be frequently excited in the real IO, as evidenced by satellite-observed and OGCM-simulated sea level. The presence of the Indian subcontinent enhances the influence of equatorial variability in the north IO, especially along the west coast of India. The Maldives Islands weaken the 180-day resonance amplitude but have little effect on the 90-day resonance, because they fall in its “node” region. Additionally, resonance at the 120-day period for the n = 1 mode is noted.

scholarly journals Long-Wave Dynamics of Sea Level Variations during Indian Ocean Dipole Events

2009 ◽  
Vol 39 (5) ◽  
pp. 1115-1132 ◽  
Author(s):  
Dongliang Yuan ◽  
Hailong Liu
Keyword(s):  
Indian Ocean ◽  
Sea Level ◽  
Indian Ocean Dipole ◽  
Rossby Waves ◽  
Western Boundary ◽  
Kelvin Wave ◽  
Wave Pulse ◽  
Wave Dynamics ◽  
Long Wave ◽  
Equatorial Rossby Waves

Abstract Long-wave dynamics of the interannual variations of the equatorial Indian Ocean circulation are studied using an ocean general circulation model forced by the assimilated surface winds and heat flux of the European Centre for Medium-Range Weather Forecasts. The simulation has reproduced the sea level anomalies of the Ocean Topography Experiment (TOPEX)/Poseidon altimeter observations well. The equatorial Kelvin and Rossby waves decomposed from the model simulation show that western boundary reflections provide important negative feedbacks to the evolution of the upwelling currents off the Java coast during Indian Ocean dipole (IOD) events. Two downwelling Kelvin wave pulses are generated at the western boundary during IOD events: the first is reflected from the equatorial Rossby waves and the second from the off-equatorial Rossby waves in the southern Indian Ocean. The upwelling in the eastern basin during the 1997–98 IOD event is weakened by the first Kelvin wave pulse and terminated by the second. In comparison, the upwelling during the 1994 IOD event is terminated by the first Kelvin wave pulse because the southeasterly winds off the Java coast are weak at the end of 1994. The atmospheric intraseasonal forcing, which plays an important role in inducing Java upwelling during the early stage of an IOD event, is found to play a minor role in terminating the upwelling off the Java coast because the intraseasonal winds are either weak or absent during the IOD mature phase. The equatorial wave analyses suggest that the upwelling off the Java coast during IOD events is terminated primarily by western boundary reflections.

Impact of Rossby and Kelvin Wave Components on MJO Eastward Propagation

2018 ◽  
Vol 31 (17) ◽  
pp. 6913-6931 ◽  
Author(s):  
Lu Wang ◽  
Tim Li ◽  
Tomoe Nasuno
Keyword(s):  
Rossby Wave ◽  
General Circulation ◽  
Task Force ◽  
Phase Speed ◽  
General Circulation Models ◽  
Wave Component ◽  
Kelvin Wave ◽  
Free Atmosphere ◽  
The Impact ◽  
East West

There are contrasting views concerning the impact of Rossby wave component of MJO flow on its eastward propagation. One view (called “drag effect”) argues that because Rossby waves propagate westward, a stronger Rossby wave component slows down the eastward propagation. The other view (called “acceleration effect”) argues that a stronger Rossby wave enhances east–west asymmetry of moist static energy (MSE) tendency and thus favors the eastward propagation. This study aims to resolve this issue through diagnosis of both idealized aquaplanet simulations and 26 models from the MJO Task Force/GEWEX Atmospheric System Studies (MJOTF/GASS). In the aquaplanet experiments, three sets of zonally uniform, equatorially symmetric SST distributions are specified. The MJO phase speed is faster in the presence of a narrower SST meridional profile, in which both the Rossby and the Kelvin wave components are stronger and the east–west asymmetry of MSE tendency is larger. A further analysis of the 26 general circulation models reveals that the MJO propagation skill and phase speed are positively correlated to both the Rossby wave and the Kelvin wave strength in the lower free atmosphere (above 800 hPa). Models that have a stronger Rossby and Kelvin wave component tend to simulate realistic and faster eastward propagation. Therefore, both the aquaplanet and the multimodel simulations support the Rossby wave acceleration effect hypothesis.

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.

Origins and Dynamics of the 90-Day and 30–60-Day Variations in the Equatorial Indian Ocean

2005 ◽  
Vol 35 (5) ◽  
pp. 708-728 ◽  
Author(s):  
Weiqing Han
Keyword(s):  
Indian Ocean ◽  
Sea Level ◽  
Equatorial Indian Ocean ◽  
Ocean Model ◽  
Forced Response ◽  
Western Boundary ◽  
Reflected Waves ◽  
Equatorial Wave ◽  
Baroclinic Modes ◽  
And Sea Level

Abstract Sea level observations in the equatorial Indian Ocean show a dominant spectral peak at 90 days and secondary peaks at 30–60 days over an intraseasonal period (20–90 days). A detailed investigation of the origins and dynamics of these variations is carried out using an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Two parallel experiments are performed in the tropical Indian Ocean basin for the period 1988–2001: one is forced by NCEP 3-day mean forcing fields together with the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) pentad precipitation, and the other is forced by monthly mean fields. To help to understand the role played by the wind-driven equatorial wave dynamics, a linear continuously stratified ocean model is also used. Both the observed and modeled 90-day sea level anomaly fields and HYCOM surface current clearly show equatorial Kelvin and first-meridional-mode Rossby wave structures that are forced by the 90-day winds. The wind amplitude at the 90-day period, however, is weaker than that for the 30–60-day period, suggesting that the equatorial Indian Ocean selectively responds to the 90-day winds. This selective response arises mainly from the resonant excitation of the second-baroclinic-mode (n = 2) waves by the 90-day winds. In this case, Rossby waves reflected from the eastern ocean boundary enhance the directly forced response in the ocean interior, strengthening the 90-day peak. In addition, the directly forced response increases monotonically with the increase of forcing period, contributing to the larger variances of currents and sea level at 90 days. Two factors account for this monotonic increase in directly forced response. First, at lower frequency, both Rossby and Kelvin waves associated with the low-order baroclinic modes have longer wavelengths, which are more efficiently excited by the larger-scale winds. Second, responses of the high-order modes directly follow the local winds, and their amplitudes are proportional to both forcing period and wind strength. Although most energy is surface trapped, there is a significant amount that propagates through the pycnocline into the deep ocean. The dominance of the 90-day peak occurs not only at the surface but also in the deeper layers down to 600 m. In the deeper ocean, both the directly forced response and reflected waves associated with the first two baroclinic modes contribute to the 90-day variation. Spectra of the observed sea surface temperature (SST) also show a 90-day peak, likely a result of the selective response of the equatorial Indian Ocean at the 90-day period. Near the surface, the spectral peaks of currents and sea level at the 30–60-day period are directly forced by winds that peak at 30–60 days. In the deeper layers, both directly forced and reflected waves associated with the first two baroclinic modes contribute. Oceanic instabilities can have significant contributions only near the western boundary and near 5°N south of Sri Lanka.

scholarly journals Seasonality and Dynamics of Atmospheric Teleconnection from the Tropical Indian Ocean and the Western Pacific to West Antarctica

Atmosphere ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 849
Author(s):  
Hyun-Ju Lee ◽  
Emilia-Kyung Jin
Keyword(s):  
Indian Ocean ◽  
Rossby Wave ◽  
Tropical Indian Ocean ◽  
Western Pacific ◽  
Tropical Region ◽  
Formation Mechanisms ◽  
West Antarctica ◽  
Background Flow ◽  
The Impact ◽  
The Western Pacific

The global impact of the tropical Indian Ocean and the Western Pacific (IOWP) is expected to increase in the future because this area has been continuously warming due to global warming; however, the impact of the IOWP forcing on West Antarctica has not been clearly revealed. Recently, ice loss in West Antarctica has been accelerated due to the basal melting of ice shelves. This study examines the characteristics and formation mechanisms of the teleconnection between the IOWP and West Antarctica for each season using the Rossby wave theory. To explicitly understand the role of the background flow in the teleconnection process, we conduct linear baroclinic model (LBM) simulations in which the background flow is initialized differently depending on the season. During JJA/SON, the barotropic Rossby wave generated by the IOWP forcing propagates into the Southern Hemisphere through the climatological northerly wind and arrives in West Antarctica; meanwhile, during DJF/MAM, the wave can hardly penetrate the tropical region. This indicates that during the Austral winter and spring, the IOWP forcing and IOWP-region variabilities such as the Indian Ocean Dipole (IOD) and Indian Ocean Basin (IOB) modes should paid more attention to in order to investigate the ice change in West Antarctica.

scholarly journals A 17-My-old whale constrains onset of uplift and climate change in east Africa

2015 ◽  
Vol 112 (13) ◽  
pp. 3910-3915 ◽  
Author(s):  
Henry Wichura ◽  
Louis L. Jacobs ◽  
Andrew Lin ◽  
Michael J. Polcyn ◽  
Fredrick K. Manthi ◽  
...  
Keyword(s):  
Indian Ocean ◽  
East Africa ◽  
Sea Level ◽  
Drainage System ◽  
Beaked Whale ◽  
East African ◽  
Surface Uplift ◽  
The Indian Ocean ◽  
Mantle Processes ◽  
The Impact

Timing and magnitude of surface uplift are key to understanding the impact of crustal deformation and topographic growth on atmospheric circulation, environmental conditions, and surface processes. Uplift of the East African Plateau is linked to mantle processes, but paleoaltimetry data are too scarce to constrain plateau evolution and subsequent vertical motions associated with rifting. Here, we assess the paleotopographic implications of a beaked whale fossil (Ziphiidae) from the Turkana region of Kenya found 740 km inland from the present-day coastline of the Indian Ocean at an elevation of 620 m. The specimen is ∼17 My old and represents the oldest derived beaked whale known, consistent with molecular estimates of the emergence of modern strap-toothed whales (Mesoplodon). The whale traveled from the Indian Ocean inland along an eastward-directed drainage system controlled by the Cretaceous Anza Graben and was stranded slightly above sea level. Surface uplift from near sea level coincides with paleoclimatic change from a humid environment to highly variable and much drier conditions, which altered biotic communities and drove evolution in east Africa, including that of primates.

Indian Ocean Dipole Response to Global Warming: Analysis of Ocean–Atmospheric Feedbacks in a Coupled Model*

2010 ◽  
Vol 23 (5) ◽  
pp. 1240-1253 ◽  
Author(s):  
Xiao-Tong Zheng ◽  
Shang-Ping Xie ◽  
Gabriel A. Vecchi ◽  
Qinyu Liu ◽  
Jan Hafner
Keyword(s):  
Global Warming ◽  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
General Circulation ◽  
Equatorial Indian Ocean ◽  
Thermocline Depth ◽  
Easterly Wind ◽  
Thermocline Feedback ◽  
Ocean Atmosphere ◽  
Atmospheric Feedbacks

Abstract Low-frequency modulation and change under global warming of the Indian Ocean dipole (IOD) mode are investigated with a pair of multicentury integrations of a coupled ocean–atmosphere general circulation model: one under constant climate forcing and one forced by increasing greenhouse gas concentrations. In the unforced simulation, there is significant decadal and multidecadal modulation of the IOD variance. The mean thermocline depth in the eastern equatorial Indian Ocean (EEIO) is important for the slow modulation, skewness, and ENSO correlation of the IOD. With a shoaling (deepening) of the EEIO thermocline, the thermocline feedback strengthens, and this leads to an increase in IOD variance, a reduction of the negative skewness of the IOD, and a weakening of the IOD–ENSO correlation. In response to increasing greenhouse gases, a weakening of the Walker circulation leads to easterly wind anomalies in the equatorial Indian Ocean; the oceanic response to weakened circulation is a thermocline shoaling in the EEIO. Under greenhouse forcing, the thermocline feedback intensifies, but surprisingly IOD variance does not. The zonal wind anomalies associated with IOD are found to weaken, likely due to increased static stability of the troposphere from global warming. Linear model experiments confirm this stability effect to reduce circulation response to a sea surface temperature dipole. The opposing changes in thermocline and atmospheric feedbacks result in little change in IOD variance, but the shoaling thermocline weakens IOD skewness. Little change under global warming in IOD variance in the model suggests that the apparent intensification of IOD activity during recent decades is likely part of natural, chaotic modulation of the ocean–atmosphere system or the response to nongreenhouse gas radiative changes.

Precursor Signals and Processes Associated with MJO Initiation over the Tropical Indian Ocean*

2013 ◽  
Vol 26 (1) ◽  
pp. 291-307 ◽  
Author(s):  
Chongbo Zhao ◽  
Tim Li ◽  
Tianjun Zhou
Keyword(s):  
Indian Ocean ◽  
Rossby Wave ◽  
Lower Troposphere ◽  
Equatorial Indian Ocean ◽  
Specific Humidity ◽  
Boreal Winter ◽  
Mean Flow ◽  
Wave Response ◽  
The Mean ◽  
Convection Initiation

Abstract The precursor signals of convection initiation associated with the Madden–Julian oscillation (MJO) in boreal winter were investigated through the diagnosis of the 40-yr ECMWF Re-Analysis (ERA-40) data for the period 1982–2001. The western equatorial Indian Ocean (WIO) is a key region of the MJO initiation. A marked increase of specific humidity and temperature in the lower troposphere appears 5–10 days prior to the convection initiation. The increased moisture and temperature cause a convectively more unstable stratification, leading to the onset of convection. A diagnosis of lower-tropospheric moisture (heat) budgets shows that the moisture (temperature) increase is caused primarily by the horizontal advection of the mean specific humidity (temperature) by the MJO flow. The anomalous flow is primarily determined by the downstream Rossby wave response to a preceding suppressed-phase MJO over the eastern Indian Ocean, whereas the upstream Kelvin wave response to the previous eastward-propagating convective-phase MJO is not critical. An idealized numerical experiment further supports this claim. The Southern Hemisphere (SH) midlatitude Rossby wave train and associated wave activity flux prior to the MJO initiation were diagnosed. It is found that SH midlatitude Rossby waves may contribute to MJO initiation over the western Indian Ocean through wave energy accumulation. Idealized numerical experiments confirm that SH midlatitude perturbations play an important role in affecting the MJO variance in the tropics. A barotropic energy conversion diagnosis indicates that there is continuous energy transfer from the mean flow to intraseasonal disturbances over the initiation region, which may help trigger MJO development.

scholarly journals Water Mass Export from Drake Passage to the Atlantic, Indian, and Pacific Oceans: A Lagrangian Model Analysis

2005 ◽  
Vol 35 (7) ◽  
pp. 1206-1222 ◽  
Author(s):  
Yann Friocourt ◽  
Sybren Drijfhout ◽  
Bruno Blanke ◽  
Sabrina Speich
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Drake Passage ◽  
General Circulation Models ◽  
Lagrangian Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Intermediate Water ◽  
Boundary Current ◽  
The Indian Ocean

Abstract The northward export of intermediate water from Drake Passage is investigated in two global ocean general circulation models (GCMs) by means of quantitative particle tracing diagnostics. This study shows that a total of about 23 Sv (Sv ≡ 106 m3 s−1) is exported from Drake Passage to the equator. The Atlantic and Pacific Oceans are the main catchment basins with 7 and 15 Sv, respectively. Only 1–2 Sv of the water exported to the Atlantic equator follow the direct cold route from Drake Passage without entering the Indian Ocean. The remainder loops first into the Indian Ocean subtropical gyre and flows eventually into the Atlantic Ocean by Agulhas leakage. The authors assess the robustness of a theory that relates the export from Drake Passage to the equator to the wind stress over the Southern Ocean. Our GCM results are in reasonable agreement with the theory that predicts the total export. However, the theory cannot be applied to individual basins because of interocean exchanges through the “supergyre” mechanism and other nonlinear processes such as the Agulhas rings. The export of water from Drake Passage starts mainly as an Ekman flow just northward of the latitude band of the Antarctic Circumpolar Current south of South America. Waters quickly subduct and are transferred to the ocean interior as they travel equatorward. They flow along the eastern boundaries in the Sverdrup interior and cross the southern basins northwestward to reach the equator within the western boundary current systems.

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