The Role of Air Masses and the Intertropical Convergence Zone on Seasonal Climatic Variations

The influence of an air masses is regarded as one of the most fundamental elements of weather and climate regimes, determining an area’s associated temperature and precipitation development.   Defined, an air mass is an exceedingly large mass of air that shares a relatively homogeneous temperature and moisture content at any given altitude, or horizontal direction, that is a part of the mass. Though small variations do exist within these masses, as to be expected, as they usually exceed over 1,500 kilometers. As the mass moves away from its point of origin, the characteristics of that location are transferred to the locations it travels over.  Movements of these masses are driven from pressure differentials from latent heat flux on the surface, for Bangalore this movement coincides with that of the intertropical convergence zone (ITCZ).  Masses are classified under two categories, their relation to the equator, which consists of arctic, polar, tropical, or equatorial; and their topographical surface of their origin, either maritime over large bodies of water, or continental over landmasses. For instance in the case of Bangalore a continental polar (cP) and continental arctic (cA) mass is predominant in the fall and winter, which produces dry conditions, while a maritime equatorial (mE) is dominant in the spring and summer, bringing moister conditions. Though seemingly vastly different, these two masses do hold similarities, like most masses they are formed on relatively flat, uniform surfaces with stagnant surface circulation. The complexity of these topics can be quite extensive, but can be somewhat clarified by examining the following graphics.

The graphic above shows the approximate region of origin for the major air masses of the world. Of these, two have a significant impact on the precipitation characteristics of Bangalore, the continental polar and continental arctic masses originating in Siberia, and the maritime equatorial mass over the Indian Ocean.  These masses traverse in and out of the Indian subcontinent, largely because of three main reasons. First the surface temperature differential shaped by the seasons, followed by the difference in specific heat of water molecules and molecules incorporating the makeup of land surfaces, and finally the influence of the intertropical convergence zone.  

The intertropical convergence zone (ITCZ) is a band that represents the convergence of the NE and SE trade winds, within a close proximity to the equator.  Also referred to as the equatorial low, this band brings high levels of precipitation from the release of latent heat from the persistent warm rising air.  The ITCZ loosely follows the sub-solar point over India, which is representative of the seasonal shift between the wet and dry. As seen above the ITCZ is deep within the the Indian Ocean in January, drawing in the cool dry continental air from the northern Asian continent.  The opposite is true in July, as the land is warmed more rapidly than the water from the ocean, it draws in the humid unstable air, resulting in heavy precipitation. 

The above graphics help demonstrate the typical direction and extent of the ITCZ and how and where it affects Bangalore.

The above map shows the amount of precipitation across the world in the month of January.  As can be seen the India receives minimal amounts during this period. as the dry continental Siberian mass influence the convective and thermal forces above the land.  The temperature however remains relatively mild due to Bangalore's positioning within the tropics.

A vastly different scenario develops in Bangalore soon after the spring equinox when heavy precipitation occurs from the end of April to the beginning of October. As can be seen above the movement of the maritime equatorial mass from the Indian Ocean moves north, inudating the land with some of the highest precipitaion totals on the entire planet.

Another factor that accounts for the amount of precipitation and the movements and development of air masses within the atmosphere in Bangalore, is the Indian Ocean Oscillations. Similar to the movements of El Nino and La Nina oscillations, except to a much lesser extent, depending on the phase rainy or drought conditions can fall upon Bangalore or the Indian sub continent as a whole.

Current Atmospheric Conditions

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