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.

Reinitiation of the Boreal Summer Intraseasonal Oscillation in the Tropical Indian Ocean*

2005 ◽  
Vol 18 (18) ◽  
pp. 3777-3795 ◽  
Author(s):  
Xian-An Jiang ◽  
Tim Li
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Intraseasonal Oscillation ◽  
Circulation Model ◽  
Equatorial Indian Ocean ◽  
Specific Humidity ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Temperature Advection

Abstract The characteristic features of the boreal summer intraseasonal oscillation (BSISO) during its reinitiation period are studied using NCEP–NCAR reanalysis. Based on these observations and with the aid of an anomalous atmospheric general circulation model (AGCM), a possible mechanism responsible for the BSISO reinitiation is elucidated. The western equatorial Indian Ocean along the eastern African coast tends to be a key region for the phase transition of the BSISO from an enhanced to suppressed convective phase, or vise versa. The major precursory feature associated with reinitiation of suppressed convection is found in the divergence and reduced specific humidity in the boundary layer. Numerical experiments indicate that the low-level divergence is caused by the cold horizontal temperature advection and associated adiabatic warming (descending motion) in situ. The summer mean state is found to be important for the cold horizontal temperature advection through the modulation of a Gill-type response to an intraseasonal oscillation (ISO) heating in the eastern equatorial Indian Ocean. The results in this study suggest a self-sustained paradigm in the Indian Ocean for the BSISO; that is, the BSISO could be a basinwide phenomenon instead of a global circumstance system as hypothesized for the boreal winter ISO (i.e., the Madden–Julian oscillation).

scholarly journals MJO Initiation Processes over the Tropical Indian Ocean during DYNAMO/CINDY2011*

2015 ◽  
Vol 28 (6) ◽  
pp. 2121-2135 ◽  
Author(s):  
Tim Li ◽  
Chongbo Zhao ◽  
Pang-chi Hsu ◽  
Tomoe Nasuno
Keyword(s):  
Indian Ocean ◽  
Large Scale ◽  
Intraseasonal Variability ◽  
Lower Troposphere ◽  
Vertical Motion ◽  
Equatorial Indian Ocean ◽  
Tropical Indian Ocean ◽  
Final Analysis ◽  
Specific Humidity ◽  
Zonal Scale

Abstract A multination joint field campaign called the Dynamics of MJO/Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011 (DYNAMO/CINDY2011) took place in the equatorial Indian Ocean (IO) in late 2011. During the campaign period, two strong MJO events occurred from the middle of October to the middle of December (referred to as MJO I and MJO II, respectively). Both the events were initiated over the western equatorial Indian Ocean (WIO) around 50°–60°E. Using multiple observational data products (ERA-Interim, the ECMWF final analysis, and NASA MERRA), the authors unveil specific processes that triggered the MJO convection in the WIO. It is found that, 10 days prior to MJO I initiation, a marked large-scale ascending motion anomaly appeared in the lower troposphere over the WIO. The cause of this intraseasonal vertical motion anomaly was attributed to anomalous warm advection by a cyclonic gyre anomaly over the northern IO. The MJO II initiation was preceded by a low-level specific humidity anomaly. This lower-tropospheric moistening was attributed to the advection of mean moisture by anomalous easterlies over the equatorial IO. The contrast of anomalous precursor winds at the equator (westerly versus easterly) implies different triggering mechanisms for the MJO I and II events. It was found that upper-tropospheric circumnavigating signals did not contribute the initiation of both the MJO events. The EOF-based real-time multivariate MJO (RMM) indices should not be used to determine MJO initiation time and location because they are primarily used to capture large zonal scale and eastward-propagating signals, not localized features.

scholarly journals On the Wave Spectrum Generated by Tropical Heating

2011 ◽  
Vol 68 (9) ◽  
pp. 2042-2060 ◽  
Author(s):  
David A. Ortland ◽  
M. Joan Alexander ◽  
Alison W. Grimsdell
Keyword(s):  
Linear Models ◽  
Nonlinear Models ◽  
Wave Spectrum ◽  
Convective Heating ◽  
Temperature Structure ◽  
Mean Flow ◽  
Quasi Biennial Oscillation ◽  
Wave Response ◽  
Zonal Wavenumber ◽  
The Mean

Abstract Convective heating profiles are computed from one month of rainfall rate and cloud-top height measurements using global Tropical Rainfall Measuring Mission and infrared cloud-top products. Estimates of the tropical wave response to this heating and the mean flow forcing by the waves are calculated using linear and nonlinear models. With a spectral resolution up to zonal wavenumber 80 and frequency up to 4 cpd, the model produces 50%–70% of the zonal wind acceleration required to drive a quasi-biennial oscillation (QBO). The sensitivity of the wave spectrum to the assumed shape of the heating profile, to the mean wind and temperature structure of the tropical troposphere, and to the type of model used is also examined. The redness of the heating spectrum implies that the heating strongly projects onto Hough modes with small equivalent depth. Nonlinear models produce wave flux significantly smaller than linear models due to what appear to be dynamical processes that limit the wave amplitude. Both nonlinearity and mean winds in the lower stratosphere are effective in reducing the Rossby wave response to heating relative to the response in a linear model for a mean state at rest.

scholarly journals Response of the South Asian Summer Monsoon to Global Warming: Mean and Synoptic Systems*

2009 ◽  
Vol 22 (4) ◽  
pp. 1014-1036 ◽  
Author(s):  
Markus Stowasser ◽  
H. Annamalai ◽  
Jan Hafner
Keyword(s):  
Indian Ocean ◽  
Arabian Sea ◽  
Climate Model ◽  
Asian Summer Monsoon ◽  
Equatorial Indian Ocean ◽  
South Asian Summer Monsoon ◽  
Eastern Equatorial Indian Ocean ◽  
Asian Summer ◽  
The Mean ◽  
Gfdl Model

Abstract Recent diagnostics with the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1), coupled model’s twentieth-century simulations reveal that this particular model demonstrates skill in capturing the mean and variability associated with the South Asian summer monsoon precipitation. Motivated by this, the authors examine the future projections of the mean monsoon and synoptic systems in this model’s simulations in which quadrupling of CO2 concentrations are imposed. In a warmer climate, despite a weakened cross-equatorial flow, the time-mean precipitation over peninsular parts of India increases by about 10%–15%. This paradox is interpreted as follows: the increased precipitation over the equatorial western Pacific forces an anomalous descending circulation over the eastern equatorial Indian Ocean, the two regions being connected by an overturning mass circulation. The spatially well-organized anomalous precipitation over the eastern equatorial Indian Ocean forces twin anticyclones as a Rossby wave response in the lower troposphere. The southern component of the anticyclone opposes and weakens the climatological cross-equatorial monsoon flow. The patch of easterly anomalies centered in the southern Arabian Sea is expected to deepen the thermocline north of the equator. Both these factors limit the coastal upwelling along Somalia, resulting in local sea surface warming and eventually leading to a local maximum in evaporation over the southern Arabian Sea. It is shown that changes in SST are predominantly responsible for the increase in evaporation over the southern Arabian Sea. The diagnostics suggest that in addition to the increased CO2-induced rise in temperature, evaporation, and atmospheric moisture, local circulation changes in the monsoon region further increase SST, evaporation, and atmospheric moisture, leading to increased rainfall over peninsular parts of India. This result implies that accurate observation of SST and surface fluxes over the Indian Ocean is of urgent need to understand and monitor the response of the monsoon in a warming climate. To understand the regional features of the rainfall changes, the International Pacific Research Center (IPRC) Regional Climate Model (RegCM), with three different resolution settings (0.5° × 0.5°, 0.75° × 0.75°, and 1.0° × 1.0°), was integrated for 20 yr, with lateral and lower boundary conditions taken from the GFDL model. The RegCM solutions confirm the major results obtained from the GFDL model but also capture the orographic nature of monsoon precipitation and regional circulation changes more realistically. The hypothesis that in a warmer climate, an increase in troposphere moisture content favors more intense monsoon depressions is tested. The GFDL model does not reveal any changes, but solutions from the RegCM suggest a statistically significant increase in the number of storms that have wind speeds of 15–20 m s−1 or greater, depending on the resolution employed. Based on these regional model solutions a possible implication is that in a CO2-richer climate an increase in the number of flood days over central India can be expected. The model results obtained here, though plausible, need to be taken with caution since even in this “best” model systematic errors still exist in simulating some aspects of the tropical and monsoon climates.

Overtransmission of Rossby Waves at a Lower-Layer Critical Latitude in the Two-Layer Model

2020 ◽  
Vol 77 (3) ◽  
pp. 859-870 ◽  
Author(s):  
Matthew T. Gliatto ◽  
Isaac M. Held
Keyword(s):  
Rossby Waves ◽  
Lower Layer ◽  
Scattering Problem ◽  
Lower Troposphere ◽  
Vertical Shear ◽  
Mean Flow ◽  
External Mode ◽  
The Mean ◽  
Critical Latitude ◽  
Pv Gradient

Abstract Rossby waves, propagating from the midlatitudes toward the tropics, are typically absorbed by critical latitudes (CLs) in the upper troposphere. However, these waves typically encounter CLs in the lower troposphere first. We study a two-layer linear scattering problem to examine the effects of lower CLs on these waves. We begin with a review of the simpler barotropic case to orient the reader. We then progress to the baroclinic case using a two-layer quasigeostrophic model in which there is vertical shear in the mean flow on which the waves propagate, and in which the incident wave is assumed to be an external-mode Rossby wave. We use linearized equations and add small damping to remove the critical-latitude singularities. We consider cases in which either there is only one CL, in the lower layer, or there are CLs in both layers, with the lower-layer CL encountered first. If there is only a CL in the lower layer, the wave’s response depends on the sign of the mean potential vorticity gradient at this lower-layer CL: if the PV gradient is positive, then the CL partially absorbs the wave, as in the barotropic case, while for a negative PV gradient, the CL is a wave emitter, and can potentially produce overreflection and/or overtransmission. Our numerical results indicate that overtransmission is by far the dominant response in these cases. When an upper-layer absorbing CL is encountered, following the lower-layer encounter, one can still see the signature of overtransmission at the lower-layer CL.

scholarly journals Westerly Wind Bursts and Their Relationship with Intraseasonal Variations and ENSO. Part I: Statistics

2007 ◽  
Vol 135 (10) ◽  
pp. 3325-3345 ◽  
Author(s):  
Ayako Seiki ◽  
Yukari N. Takayabu
Keyword(s):  
Indian Ocean ◽  
Rossby Wave ◽  
El Nino ◽  
Westerly Wind ◽  
Intraseasonal Variations ◽  
Wave Response ◽  
Rossby Wave Response ◽  
The Indian Ocean ◽  
Wind Bursts ◽  
Westerly Wind Bursts

Abstract Statistical features of the relationship among westerly wind bursts (WWBs), the El Niño–Southern Oscillation (ENSO), and intraseasonal variations (ISVs) were examined using 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis data (ERA-40) for the period of January 1979–August 2002. WWBs were detected over the Indian Ocean and the Pacific Ocean, but not over the Atlantic Ocean. WWB frequencies for each region were lag correlated with a sea surface temperature anomaly over the Niño-3 region. WWBs tended to occur in sequence, from the western to eastern Pacific, leading the El Niño peak by 9 months to 1 month, respectively, and after around 11 months, over the Indian Ocean. These results suggest that WWB occurrences are not random, but interactive with ENSO. Composite analysis revealed that most WWBs were associated with slowdowns of eastward-propagating convective regions like the Madden–Julian oscillation (MJO), with the intensified Rossby wave response. However, seasonal and interannual variations in MJO amplitude were not correlated with WWB frequency, while a strong MJO event tended to bear WWBs. It is suggested that the strong MJO amplitude promotes favorable conditions, but it is not the only factor influencing WWB frequency. An environment common to WWB generation in all regions was the existence of background westerlies around the WWB center near the equator. It is inferred that ENSO prepares a favorable environment for the structural transformation of an MJO, that is, the intensified Rossby wave response, that results in WWB generations. The role of the background wind fields on WWB generations will be discussed in a companion paper from the perspective of energetics.

scholarly journals Strong Intraseasonal Variability of Meridional Currents near 5°N in the Eastern Indian Ocean: Characteristics and Causes

2017 ◽  
Vol 47 (5) ◽  
pp. 979-998 ◽  
Author(s):  
Gengxin Chen ◽  
Weiqing Han ◽  
Yuanlong Li ◽  
Michael J. McPhaden ◽  
Ju Chen ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Upper Ocean ◽  
Mean Flow ◽  
Eastern Boundary ◽  
Typical Period ◽  
The Indian Ocean ◽  
The Mean

AbstractThis paper reports on strong, intraseasonal, upper-ocean meridional currents observed in the Indian Ocean between the Bay of Bengal (BOB) and the equator and elucidates the underlying physical processes responsible for them. In situ measurements from a subsurface mooring at 5°N, 90.5°E reveal strong intraseasonal variability of the meridional current with an amplitude of ~0.4 m s−1 and a typical period of 30–50 days in the upper 150 m, which by far exceeds the magnitudes of the mean flow and seasonal cycle. Such prominent intraseasonal variability is, however, not seen in zonal current at the same location. Further analysis suggests that the observed intraseasonal flows are closely associated with westward-propagating eddylike sea surface height anomalies (SSHAs) along 5°N. The eddylike SSHAs are largely manifestations of symmetric Rossby waves, which result primarily from intraseasonal wind stress forcing in the equatorial waveguide and reflection of the equatorial Kelvin waves at the eastern boundary. Since the wave signals are generally symmetric about the equator, similar variability is also seen at 5°S but with weaker intensity because of the inclined coastline at the eastern boundary. The Rossby waves propagate westward, causing pronounced intraseasonal SSHA and meridional current in the upper ocean across the entire southern BOB between 84° and 94°E. They greatly weaken in the western Indian Basin, but zonal currents near the equator remain relatively strong.

scholarly journals Propagating and Nonpropagating MJO Events over Maritime Continent*

2015 ◽  
Vol 28 (21) ◽  
pp. 8430-8449 ◽  
Author(s):  
Jing Feng ◽  
Tim Li ◽  
Weijun Zhu
Keyword(s):  
Rossby Wave ◽  
Longwave Radiation ◽  
Meridional Wind ◽  
Specific Humidity ◽  
Maritime Continent ◽  
Budget Analysis ◽  
Positive Tendency ◽  
Moisture Advection ◽  
Longitudinal Zone ◽  
The Mean

Abstract The observed outgoing longwave radiation (OLR) and ERA-Interim data during 1979–2008 (from November to April) were analyzed to reveal fundamental differences between eastward-propagating (EP) and nonpropagating (NP) MJO events across the Maritime Continent (MC). It was found that when the maximum MJO convection arrives near 120°E, a positive moisture tendency lies in a longitudinal zone (10°S–10°N, 130°–170°E) for the EP cases, whereas a negative tendency appears in the same region for the NP cases. In the latter cases, there are clearly detectable westward-propagating Rossby wave–type dry signals over the equatorial central-western Pacific. The dry Rossby-wave signal may hinder the development of new convection to the east of the MJO convective center, preventing the eastward propagation of the MJO. A moisture budget analysis shows that the positive tendency of specific humidity in the EP composite is mainly attributed to the advection of the mean moisture by an intraseasonal ascending motion anomaly, whereas the negative tendency in the NP composite arises from the advection of anomalous dry air by the mean easterly and the advection of the mean moisture by the anomalous easterly. The EP cases were further separated into two groups: a group with, and a group without, a clear suppressed convective phase of OLR to the east of the MJO convection. In the former (latter), the column-integrated moisture anomaly is negative (positive) to the east of the convection. Nevertheless, MJO crosses the MC in both of the groups, in which anomalous moisture tendency is always positive to the east of the MJO convection. Such positive tendencies are caused by different processes. In the former, anomalous horizontal advections associated with eddy moisture transport and mean moisture advection by intraseasonal meridional wind play an important role. In the latter, it is mainly attributed to mean moisture advection by anomalous vertical velocity.

scholarly journals The Shift of the Northern Node of the NAO and Cyclonic Rossby Wave Breaking

2012 ◽  
Vol 25 (22) ◽  
pp. 7973-7982 ◽  
Author(s):  
Yi-Hui Wang ◽  
Gudrun Magnusdottir
Keyword(s):  
Rossby Wave ◽  
Wave Breaking ◽  
Geographical Location ◽  
Mean Flow ◽  
Decadal Time Scale ◽  
Rossby Wave Breaking ◽  
The North ◽  
The Mean ◽  
The Difference ◽  
The North Atlantic Oscillation

Abstract Several studies have found an eastward shift in the northern node of the North Atlantic Oscillation (NAO) during the winters of 1978–97 compared to 1958–77. This study focuses on the connection between this shift of the northern node of the NAO and Rossby wave breaking (RWB) for the period 1958–97. It is found that the region of frequent cyclonic RWB underwent a northeastward shift at high latitudes in the latter 20-yr period. On a year-to-year basis, the cyclonic RWB region moves along a southwest–northeast (SW–NE)-directed axis. Both latitude and longitude of the winter maximum frequency of cyclonic RWB occurrence are positively correlated with the NAO index. To investigate the role of location of cyclonic RWB in influencing the NAO pattern, the geographical location of frequent cyclonic RWB is divided into two subdomains located along the SW–NE axis, to the south (SW domain) and east (NE domain) of Greenland. Two composites are assembled as one cyclonic RWB occurrence is detected in one of the two subdomains in 6-hourly instantaneous data. The forcing of the mean flow due to cyclonic RWB within individual subdomains is found to be locally restricted to where the breaking occurs, which is usually near the jet exit region and far removed from the jet core. The difference in the jet between the NE and SW composites resembles the difference in the mean jet between the 1978–97 and 1958–77 periods, which suggests that the change in cyclonic RWB occurrence in the two subdomains is associated with the wobbling of the jet on the decadal time scale.

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.

Sign in / Sign up

Export Citation Format

Share Document