Deep Signatures of Southern Tropical Indian Ocean Annual Rossby Waves*

2011 ◽  
Vol 41 (10) ◽  
pp. 1958-1964 ◽  
Author(s):  
Gregory C. Johnson
Keyword(s):  
Indian Ocean ◽  
Annual Cycle ◽  
Surface Wind ◽  
Phase Speed ◽  
Tropical Indian Ocean ◽  
Energy Propagation ◽  
Deep Basin ◽  
Zonal Velocity ◽  
Vertical Displacements ◽  
Phase Propagation

Abstract The southern tropical Indian Ocean contains a striking forced annual Rossby wave studied previously using satellite altimeter sea surface height data, surface wind fields, expendable bathythermograph ocean temperature data, and models. Here, the deep reach of this wave and its velocity are analyzed using density–depth profiles and 1000-dbar horizontal drift data from Argo. Significant annual cycles in isopycnal vertical displacements and zonal velocity persist to the deepest pressures to which Argo data can be mapped reliably in the region, 1600–1900 dbar. Phase propagation of the annual cycle of the directly measured zonal velocities at 1000 dbar suggests a zonal wavelength of about 6000 km—about the length of the deep basin in which the wave is found—and a westward phase speed of ~0.2 m s−1. Apparent upward phase propagation in isopycnal vertical displacements suggests energy propagation downward into the abyss. This pattern is clearer when accounting for both the potential and kinetic energy of the wave. The largest zonal current associated with this wave has a middepth maximum that decays rapidly up through the pycnocline and less rapidly with increasing depth, suggesting a first-vertical-mode structure. The anomalous zonal volume transport of this annually reversing current is ~27 × 106 m3 s−1 across 80°E in mid-November. The peak zonal velocity of 0.06 m s−1 implies a maximum zonal excursion of about 600 km associated with the wave over an annual cycle.

scholarly journals Zonal Propagation of Near-Surface Zonal Currents in Relation to Surface Wind Forcing in the Equatorial Indian Ocean

2016 ◽  
Vol 46 (12) ◽  
pp. 3623-3638 ◽  
Author(s):  
Motoki Nagura ◽  
Michael J. McPhaden
Keyword(s):  
Indian Ocean ◽  
Surface Wind ◽  
Phase Speed ◽  
Equatorial Indian Ocean ◽  
Wind Forcing ◽  
Western Basin ◽  
Ocean Reanalysis ◽  
Near Surface ◽  
Zonal Velocity ◽  
Zonal Propagation

AbstractZonal propagation of zonal velocity along the equator in the Indian Ocean and its relationship with wind forcing are investigated with a focus on seasonal time scales using in situ observations from four acoustic Doppler current profilers (ADCPs) and an ocean reanalysis dataset. The results show that the zonal phase speed of zonal currents varies depending on season and depth in a very complicated way in relation to surface wind forcing. Surface layer zonal velocity propagates to the west in northern spring but to the east in fall in response to zonally propagating surface zonal winds, while in the pycnocline zonal phase speed is related to wind-forced ocean wave dynamics. In the western half of the analysis domain (78°–83°E), zonal phase speed in the pycnocline is eastward all year, which is attributed to the radiation of Kelvin waves forced in the western basin. In the eastern half of the domain (80°–90°E), zonal phase speed is westward at 50- to 100-m depths in northern fall, but eastward above and below, most likely due to Rossby waves generated at the eastern boundary.

scholarly journals Impact of the Madden–Julian Oscillation on Summer Rainfall in Southeast China

2009 ◽  
Vol 22 (2) ◽  
pp. 201-216 ◽  
Author(s):  
Lina Zhang ◽  
Bizheng Wang ◽  
Qingcun Zeng
Keyword(s):  
Indian Ocean ◽  
Tropical Indian Ocean ◽  
Summer Rainfall ◽  
Western Pacific ◽  
Madden Julian Oscillation ◽  
Energy Propagation ◽  
Southeast China ◽  
The Indian Ocean ◽  
The Impact ◽  
The Western Pacific

Abstract The impact of the Madden–Julian oscillation (MJO) on summer rainfall in Southeast China is investigated using the Real-time Multivariate MJO (RMM) index and the observational rainfall data. A marked transition of rainfall patterns from being enhanced to being suppressed is found in Southeast China (east of 105°E and south of 35°N) on intraseasonal time scales as the MJO convective center moves from the Indian Ocean to the western Pacific Ocean. The maximum positive and negative anomalies of regional mean rainfall are in excess of 10% relative to the climatological regional mean. Such different rainfall regimes are associated with the corresponding changes in physical fields such as the western Pacific subtropical high (WPSH), moisture, and vertical motions. When the MJO is mainly over the Indian Ocean, the WPSH shifts farther westward, and the moisture and upward motions in Southeast China are increased. In contrast, when the MJO enters the western Pacific, the WPSH retreats eastward, and the moisture and upward motions in Southeast China are decreased. It is suggested that the MJO may influence summer rainfall in Southeast China through remote and local dynamical mechanisms, which correspond to the rainfall enhancement and suppression, respectively. The remote role is the energy propagation of the Rossby wave forced by the MJO-related heating over the Indian Ocean through the low-level westerly waveguide from the tropical Indian Ocean to Southeast China. The local role is the northward shift of the upward branch of the anomalous meridional circulation when the MJO is over the western Pacific, which causes eastward retreat of the WPSH and suppressed moisture transport toward Southeast China.

scholarly journals Features of the Equatorial Intermediate Current Associated with Basin Resonance in the Indian Ocean

2018 ◽  
Vol 48 (6) ◽  
pp. 1333-1347 ◽  
Author(s):  
Ke Huang ◽  
Weiqing Han ◽  
Dongxiao Wang ◽  
Weiqiang Wang ◽  
Qiang Xie ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Equatorial Indian Ocean ◽  
Reanalysis Data ◽  
Ocean Model ◽  
Baroclinic Mode ◽  
Energy Propagation ◽  
Equatorial Wave ◽  
The Indian Ocean ◽  
Phase Propagation ◽  
Equatorial Intermediate Current

AbstractThis paper investigates the features of the Equatorial Intermediate Current (EIC) in the Indian Ocean and its relationship with basin resonance at the semiannual time scale by using in situ observations, reanalysis output, and a continuously stratified linear ocean model (LOM). The observational results show that the EIC is characterized by prominent semiannual variations with velocity reversals and westward phase propagation and that it is strongly influenced by the pronounced second baroclinic mode structure but with identifiable vertical phase propagation. Similar behavior is found in the reanalysis data and LOM results. The simulation of wind-driven equatorial wave dynamics in the LOM reveals that the observed variability of the EIC can be largely explained by the equatorial basin resonance at the semiannual period, when the second baroclinic Rossby wave reflected from the eastern boundary intensifies the directly forced equatorial Kelvin and Rossby waves in the basin interior. The sum of the first 10 modes can reproduce the main features of the EIC. Among these modes, the resonant second baroclinic mode makes the largest contribution, which dominates the vertical structure, semiannual cycle, and westward phase propagation of the EIC. The other 9 modes, however, are also important, and the superposition of the first 10 modes produces downward energy propagation in the equatorial Indian Ocean.

scholarly journals Contrasting Eastern-Pacific and Central-Pacific Types of ENSO

2009 ◽  
Vol 22 (3) ◽  
pp. 615-632 ◽  
Author(s):  
Hsun-Ying Kao ◽  
Jin-Yi Yu
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
Surface Wind ◽  
Tropical Indian Ocean ◽  
Eastern Pacific ◽  
Structure Evolution ◽  
Interdecadal Change ◽  
Central Pacific ◽  
Enso Events ◽  
Dominant Period

Abstract Surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of El Niño–Southern Oscillation (ENSO) in the tropical Pacific: an eastern-Pacific (EP) type and a central-Pacific (CP) type. An analysis method combining empirical orthogonal function (EOF) analysis and linear regression is used to separate these two types. Correlation and composite analyses based on the principal components of the EOF were performed to examine the structure, evolution, and teleconnection of these two ENSO types. The EP type of ENSO is found to have its SST anomaly center located in the eastern equatorial Pacific attached to the coast of South America. This type of ENSO is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical Indian Ocean. In contrast, the CP type of ENSO has most of its surface wind, SST, and subsurface anomalies confined in the central Pacific and tends to onset, develop, and decay in situ. This type of ENSO appears less related to the thermocline variations and may be influenced more by atmospheric forcing. It has a stronger teleconnection with the southern Indian Ocean. Phase-reversal signatures can be identified in the anomaly evolutions of the EP-ENSO but not for the CP-ENSO. This implies that the CP-ENSO may occur more as events or epochs than as a cycle. The EP-ENSO has experienced a stronger interdecadal change with the dominant period of its SST anomalies shifted from 2 to 4 yr near 1976/77, while the dominant period for the CP-ENSO stayed near the 2-yr band. The different onset times of these two types of ENSO imply that the difference between the EP and CP types of ENSO could be caused by the timing of the mechanisms that trigger the ENSO events.

scholarly journals Contributions of Indian Ocean and Monsoon Biases to the Excessive Biennial ENSO in CCSM3

2009 ◽  
Vol 22 (7) ◽  
pp. 1850-1858 ◽  
Author(s):  
Jin-Yi Yu ◽  
Fengpeng Sun ◽  
Hsun-Ying Kao
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
Surface Wind ◽  
Tropical Indian Ocean ◽  
Global Ocean ◽  
Climate System Model ◽  
Wind Pattern ◽  
Enso Variability ◽  
Monsoon Variability ◽  
Ocean Atmosphere

Abstract The Community Climate System Model, version 3 (CCSM3), is known to produce many aspects of El Niño–Southern Oscillation (ENSO) realistically, but the simulated ENSO exhibits an overly strong biennial periodicity. Hypotheses on the cause of this excessive biennial tendency have thus far focused primarily on the model’s biases within the tropical Pacific. This study conducts CCSM3 experiments to show that the model’s biases in simulating the Indian Ocean mean sea surface temperatures (SSTs) and the Indian and Australian monsoon variability also contribute to the biennial ENSO tendency. Two CCSM3 simulations are contrasted: a control run that includes global ocean–atmosphere coupling and an experiment in which the air–sea coupling in the tropical Indian Ocean is turned off by replacing simulated SSTs with an observed monthly climatology. The decoupling experiment removes CCSM3’s warm bias in the tropical Indian Ocean and reduces the biennial variability in Indian and Australian monsoons by about 40% and 60%, respectively. The excessive biennial ENSO is found to reduce dramatically by about 75% in the decoupled experiment. It is shown that the biennial monsoon variability in CCSM3 excites an anomalous surface wind pattern in the western Pacific that projects well into the wind pattern associated with the onset phase of the simulated biennial ENSO. Therefore, the biennial monsoon variability is very effective in exciting biennial ENSO variability in CCSM3. The warm SST bias in the tropical Indian Ocean also increases ENSO variability by inducing stronger mean surface easterlies along the equatorial Pacific, which strengthen the Pacific ocean–atmosphere coupling and enhance the ENSO intensity.

scholarly journals Extreme IOD induced tropical Indian Ocean warming in 2020

2021 ◽  
Vol 8 (1) ◽  
Author(s):  
Ying Zhang ◽  
Yan Du
Keyword(s):  
Indian Ocean ◽  
Surface Wind ◽  
Tropical Indian Ocean ◽  
First Record ◽  
Early Summer ◽  
Wind Anomaly ◽  
Surface Warming ◽  
Positive Indian Ocean Dipole ◽  
Indian Ocean Warming ◽  
The 1960S

AbstractThe tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer. This study reveals the important role of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode, which allows us to rethink the dynamic connections between the Indo-Pacific climate modes.

scholarly journals Diagnosing Heat and Vorticity Budgets of Annual Coupled Rossby Waves

2005 ◽  
Vol 35 (7) ◽  
pp. 1173-1189 ◽  
Author(s):  
Warren B. White ◽  
Jeffrey L. Annis
Keyword(s):  
Indian Ocean ◽  
Latent Heat ◽  
Rossby Waves ◽  
Surface Wind ◽  
Phase Speed ◽  
Western Indian Ocean ◽  
Heat Fluxes ◽  
Eastern Indian Ocean ◽  
Wind Stress Curl ◽  
Upper Level

Abstract Annual coupled Rossby waves are generated at the west coast of Australia and propagate westward across the eastern Indian Ocean from 10° to 30°S in covarying sea level height (SLH), sea surface temperature (SST), and meridional surface wind (MSW) residuals, generally traveling slower than uncoupled Rossby waves while increasing amplitude. The waves decouple in the western Indian Ocean as SST and SLH residuals become decorrelated, with wave amplitudes decreasing and westward phase speeds increasing. Here, the ocean and atmosphere thermal and vorticity budgets of the coupled Rossby waves in the eastern Indian Ocean along 20°S are diagnosed. In the upper ocean, these diagnostics find the residual SST tendency driven by the residual meridional geostrophic advection of mean temperature with warm SST residuals dissipated by upward latent heat flux to the atmosphere. In the troposphere, these upward latent heat fluxes drive mid-to-upper-level residual diabatic heating via excess condensation, balanced there by upward residual vertical thermal advection. The resulting upward residual vertical velocity drives residual upper-level divergence and lower-level convergence, the latter balanced in the troposphere vorticity budget by the residual meridional advection of planetary vorticity. This yields poleward MSW residuals collocated with warm SST residuals, as observed. The SLH tendency is modified by a positive feedback from wind stress curl residuals, the latter acting to increase the amplitude and decrease the westward phase speed of the wave. These diagnostics allow a more exact analytical model for coupled Rossby waves to be constructed, yielding wave characteristics as observed.

scholarly journals Annual Cycle in Southern Tropical Indian Ocean Bottom Pressure

2014 ◽  
Vol 44 (6) ◽  
pp. 1605-1613 ◽  
Author(s):  
Christopher G. Piecuch ◽  
Rui M. Ponte
Keyword(s):  
Indian Ocean ◽  
Rossby Wave ◽  
Annual Cycle ◽  
Tropical Indian Ocean ◽  
Thermocline Depth ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Good Correspondence ◽  
Ocean Bottom Pressure ◽  
Baroclinic Rossby Wave

Abstract The seasonal monsoon drives a dynamic response in the southern tropical Indian Ocean, previously observed in baroclinic Rossby wave signatures in annual sea level and thermocline depth anomalies. In this paper, monthly mass grids based on Release-05 Gravity Recovery and Climate Experiment (GRACE) data are used to study the annual cycle in southern tropical Indian Ocean bottom pressure. To interpret the satellite data, a linear model of the ocean’s response to wind forcing—based on the theory of vertical normal modes and comprising baroclinic and barotropic components—is considered. The model is evaluated using stratification from an ocean atlas and winds from an atmospheric reanalysis. Good correspondence between model and data is found over the southern tropical Indian Ocean: the model explains 81% of the annual variance in the data on average between 10° and 25°S. Model solutions suggest that, while the annual baroclinic Rossby wave has a seafloor signature, the annual cycle in the deep sea generally involves important barotropic dynamics, in contrast to the response in the upper ocean, which is largely baroclinic.

scholarly journals On the semi-diagnostic computation of climatological circulation in the western tropical Indian Ocean

MAUSAM ◽  
2021 ◽  
Vol 51 (4) ◽  
pp. 329-348
Author(s):  
C. SHAJI ◽  
A. D. RAO ◽  
S. K. DUBE ◽  
N. BAHULAYAN
Keyword(s):  
Indian Ocean ◽  
Surface Wind ◽  
Diagnostic Technique ◽  
Circulation Model ◽  
Tropical Indian Ocean ◽  
Salt Transport ◽  
3 Dimensional ◽  
Sea Surface Topography ◽  
Salinity Data ◽  
Climatological Circulation

The seasonal mean climatological circulation in the Indian Ocean north of 20°S and west of 80°E during the summer and winter has been investigated using a 3-dimensional, fully non-linear, semi-diagnostic circulation model. The model equations include the basic ocean hydrothermodynamic  equations of momentum, hydrostatics, continuity, sea surface topography and temperature and salt transport equations. Model is driven with the seasonal mean data on wind stress at the ocean surface and thermohaline forcing at different levels. The circulation in the upper levels of the ocean at 20, 50, 150, 300, 500 and 1000 m depths during the two contrasting seasons has been obtained using the model, and the role of steady, local forcing of wind and internal density field on the dynamical balance of circulation in the western tropical Indian Ocean is explained. The climatological temperature and salinity data used to drive the model is found to be hydrodynamically adjusted with surface wind, flow field and bottom relief during the adaptation stages. Semi-diagnostic technique is found to be very effective for the smoothening of climatic temperature and salinity data and also to obtain the 3-dimensional steady state circulation, which would serve as initial condition in simulation models of circulation.

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