| dc.description.abstract | The Intraseasonal Oscillation (ISO) plays an important role to modulate deep convective activity
in the tropical region. In this thesis, I aim to understand the role of land and warm oceans
in ISO, using a general circulation model. For this, I conduct a series of experiments in the
Community Atmosphere Model (CAM) with various idealized and realistic surface boundary
conditions to study tropical ISO. To investigate the influence of tropical sea surface temperature
(SST) on ISO and convectively coupled equatorial waves in the global atmosphere, I conduct
experiments with idealized, zonally symmetric SST profiles having different widths of warm
ocean centered at the equator. I use the model in its basic “Aquaplanet” configuration, with
the sun at the equator, i.e. perpetual spring equinox forcing; with idealized zonally symmetric
SST, the aquaplanet model produces a double Intertropical Convergence Zone (ITCZ) on
either side of the equator, and an eastward propagating Madden Julian oscillation (MJO)like
mode with variance at intraseasonal (30 to 96 day) periods and zonal wavenumber one. In the
experiment with the narrowest meridional width of warm SST, the variance of moist convective
activity lies predominantly in equatorially trapped Kelvin wave band. As the width of the
warm equatorial SST is increased, the eastward propagating speed of the MJO-like signal decreases;
for the broadest SST profile (warm SST covering 20 degrees of latitude), the speed of
the model MJO is about 5.5 m s−1, close to the observed speed. This is because the latitudinal
extent of warm SST is comparable to the equatorial Rossby radius, and the model produces off equatorial
Rossby waves of sufficient strength to interact with the Kelvin wave and slow down
the MJO-like mode. The model also generates westward propagating waves with intraseasonal periods and zonal wavenumber 1–3; the structure of these signals, which extend well into the
mid-latitudes, projects onto equatorially trapped Rossby waves with meridional mode numbers
1, 3 and 5, associated with convection that is symmetric about the equator. In addition, the
model generates 30–80 day westward moving signals with zonal wavenumber 4–7, particularly
in the experiment with a narrow region of warm SST. Although these waves are seen in
the wavenumber-frequency spectra in the equatorial region, they have the largest amplitude in
the middle and high latitudes. Thus, our study shows that wider, meridionally symmetric SST
profiles support a strong MJO-like eastward propagation, and even in an aquaplanet setting,
westward propagating Rossby waves comprise a large portion of tropical intraseasonal variability.
In the observations (ERA-Interim daily reanalysis), the MJO signal lies in the range of
zonal wavenumbers 1 to 5. The variance of MJO at higher wavenumbers (2–5) is absent in
the aquaplanet model. For this, I design model experiments in order to study how model MJO
responds to the introduction of continents in the presence of zonally symmetric SST, and a realistic
SST distribution with the Indo-Pacific warm pool and cool SST in the eastern Pacific.
As before, the model is in the aquaplanet-like configuration, to eliminate the effects of seasonality.
Model results are compared with 21 years (1995–2015) ERA-Interim reanalysis data
and analyzed in terms of the moist static energy (MSE) budget to study the growth and propagation
of MJO. When I introduce continents with realistic orography and interactive surface
temperature, soil moisture, and albedo, the variance of model MJO is reduced due to weaker
boundary layer moisture convergence. However,MJO variance extends to higher wavenumbers.
With prescribed climatological January SST boundary condition in the presence of continents,
the variance of model MJO is enhanced by a factor of 2–3, and it is distributed across zonal
wavenumbers 1 to 5, in closer agreement with observations. Thus, I find that the presence of
land by itself is not enough to produce realistic MJO in CAM, but realistic SST distribution is
also necessary to simulate MJO with improved spacetime characteristics. Both in simulations
and ERA-Interim data, column-integrated longwave radiation plays a key role in the growth of
MSE anomaly associated with MJO; in general, meridional and vertical advection of MSE both
acts to promote eastward movement of MJO. In the model experiments, meridional advection of low-level MSE anomaly is most significant in the vicinity of the ITCZ. This indicates that
the physical processes which determine the location of (single or double) ITCZ are linked to
MJO dynamics.
The westward propagating “quasi-biweekly” oscillation (QBWO) with 10–25 day period is
an important intraseasonal mode of the Asian summer monsoon, yet very few model studies focus
on this mode. I study QBWO in the northern and southern tropics in the model and compare
it with ERA-Interim reanalysis data. The pure aquaplanet model produces a double Intertropical
Convergence Zone (ITCZ), winds that are predominantly zonal, and weak quasi-biweekly variance.
When continents are introduced in the model with zonally symmetric SST, the northern
ITCZ, as well as quasi-biweekly variance between 10◦N to 24◦N are strengthened in the Pacific
Ocean, bringing model results closer to observations. In the model with continents, the QBWO
signal dwells inside the mean envelope of high atmospheric moisture, or total precipitable water
(TPW), in agreement with observations. However, in the presence of zonally symmetric SST,
the model fails to simulate sufficiently high precipitable water in the region extending from the
north Indian Ocean to East Asia, resulting in very weak QBWO variance. When the model includes
continents and realistic (January) SST boundary conditions, the spatial structure of both
TPW and QBWO variance becomes more realistic. I study the mechanisms of propagation and
maintenance of the quasi-biweekly mode using vorticity budget and moist static energy (MSE)
budget analysis. Advection due to the effect is responsible for the northwestward propagation
of QBWO vorticity, while the propagation of column MSE anomaly is mainly due to horizontal
advection. Surface turbulent heat fluxes and vertical MSE advection are the dominant contributors
to the growth and maintenance of column MSE anomaly in observations and model
respectively. Surface heat flux makes a significant contribution to the growth of quasi-biweekly
MSE anomaly in the presence of land, in association with the enhanced meridional wind, and
vortical structures that resemble moist Rossby waves with a wavelength of about 4000 kilometers. | en_US |
