The effects of a non-zero background state on atmospheric dynamics is explored through simple models and observations. Firstly, we examine the effects of moisture gradients on the stability and propagation of Rossby waves in a mid-latitude -plane. We begin by a consistent derivation of the forced quasi-geostrophic equations on a -plane to understand the constraints placed by geostrophy on the time scale of condensation. We see that the presence of meridional gradients of moisture results in a slowdown of the waves. On the introduction of zonal gradients of moisture, the waves become unstable, and for certain parameters which are representative of the real atmosphere, they propagate eastward and mature on an intra-seasonal timescale. The mechanism of the in hence of moisture on waves is understood by thinking of condensation as providing an \equivalent" potential vorticity (PV) gradient which opposes the dynamical PV gradient. Secondly, we look at the effects of a mean background ow on the Matsuno-Gill response in the spherical shallow water system. The mean ow is prescribed to resemble the climatological upper tropospheric zonal wind structure in the atmosphere. As the strength of the ow increases, the equatorially trapped Matsuno-Gill response rst transforms into a poleward propagating Rossby wavetrain. As the strength of the mean ow reaches values similar to that observed in the atmosphere, the stationary wave response becomes a zonally oriented quadrupole structure. This structure bears a striking resemblance to the observed upper level structure of the Madden-Julian oscillation (MJO). The time evolution of this quadrupole structure is quick enough to be relevant on MJO timescales, and the structure is quite robust across a range of values for the drag coefficient. Finally, we look at the role played by low frequency variability in the Pacific in the recent expansion of the Hadley cell. We find that the dominant effect of the low frequency variability is a stationary dispersive Rossby wavetrain extending from the tropical Paci. We further find that most of the observed expansion of the Hadley cell can be accounted for by this low frequency variability. We nd that large scale changes such as the changes in the equator-pole temperature gradient or midlatitude static stability need not be invoked to understand the observed expansion.

In this thesis we examine intra-seasonal oscillations (ISO) in the aqua-planet setup of the Community Atmospheric Model (CAM) version 5.1, mainly based on July and January climatological sea surface temperature (SST). We investigate mainly two questions -what should be the SST distribution for the existence of (a) northward moving ISO in summer, and (b) eastward moving MJO-like modes in winter. In the first part of the thesis we discuss the northward propagation. A series of experiments were performed with zonally symmetric and asymmetric SST distributions. The basic lower boundary condition is specified from zonally averaged observed July and January SST. The zonally symmetric July SST experiment produced an inter tropical convergence zone (ITCZ) on both sides of the equator. Poleward movement is not clear, and it is confined to the region between the double ITCZ. In July, the Bay of Bengal (BOB) and West Pacific SST is high compared to the rest of the northern tropics. When we impose a zonally asymmetric SST structure with warm SST spanning about 80 of longitude, the model shows a monsoon-like circulation, and some northward propagating convective events. Analysis of these events shows that two adjacent cells with cyclonic and anticyclonic vorticity are created over the warm SST anomaly and to the west. The propagation occurs due to the convective region drawn north in the convergence zone between these vortices. Zonally propagating Madden-Julian oscillations (MJO) are discussed in the second part of the thesis. All the experiments in this part are based on the zonally symmetric SST. The zonally symmetric January SST configuration gives an MJO-like mode, with zonal wave number 1 and a period of 40-90 days. The SST structure has a nearly meridionally symmetric structure, with local SST maxima on either side of the equator, and a small dip in the equatorial region. If we replace this dip with an SST maximum, the time-scale of MJO becomes significantly smaller (20-40 days). The implication is that an SST maximum in the equatorial region reduces the strength of MJO, and a flat SST profile in the equatorial region is required for more energetic of MJO. This result was tested and found to be valid in a series of further experiments.