C2 Monsoons, Cyclones and ENSO

January monsoon

1. Difference in surface atmospheric pressure

   1. Asia is experiencing winter and intense cooling, it develops an area of high pressure

   2. Australia is experiencing summer and heated air rises, developing an area of low pressure

   3. Due to pressure gradient force, air moves from Central Asia to Australia

2. Deflection of winds

   1. Due to differing specific heat capacities of land and sea, Pacific Ocean forms a region of lower pressure

   2. Winds moving towards Pacific Ocean are deflected to the right in NH due to Coriolis force, forming northeast monsoon winds away from Central Asia

   3. When winds cross the equator, it moves into the SH and deflects to the left, forming northwest monsoon winds towards Australia

3. Weather patterns

   1. Central Asia experiences a dry and cool winter monsoon season as dry air blows off the continent 

   2. Singapore and Malaysia experience heavy rainfall as northeast monsoon winds blow over the South China Sea and pick up moisture

   3. Northwest Australia experiences heavy rainfall as north west monsoon winds pick up moisture from Indian Ocean

July monsoon

1. Difference in surface atmospheric pressure

   1. Australia is experiencing winter and intense cooling, it develops an area of high pressure

   2. Central Asia is experiencing summer and heated air rises, developing an area of low pressure

   3. Due to pressure gradient force, air moves from Australia to Central Asia

2. Deflection of winds

   1. Due to differing specific heat capacities of land and sea, Central Asia forms a region of lower pressure compared to Pacific Ocean

   2. Winds moving towards Central Asia are deflected to the left in SH due to Coriolis force, forming southeast monsoon winds away from Australia

   3. When winds cross the equator, it moves into the NH and deflects to the right, forming southwest monsoon winds towards Central Asia

3. Weather patterns

   1. Australia experiences a dry and cool winter monsoon season as dry air blows off the continent 

   2. Most of Asia experiences heavy rainfall as southwest monsoon winds pick up moisture from Indian Ocean

   3. ITCZ dominates over the Asian continent in July, creates an even lower pressure zone

Tropical cyclone conditions

Surface Conditions for Development of Cyclones

1. Huge body of warm water

   1. Creates an area of low pressure

   2. Ensures there is sufficient energy for storm to maintain its intensity

   3. Sea temperatures beyond 26ºC and waters deeper than 60m

2. Sufficient Coriolis force

   1. Results in spinning of winds around the area of low pressure (the eye)

Atmospheric Conditions for Development of Cyclones

3. Low (or absence of) vertical wind shear

   1. Little change in velocity of wind across altitudes

   2. Allows for upward spiralling winds to form thick vertical wall of clouds

   3. High vertical wind shear will disrupt flow of heat and moisture

4. Divergence of air aloft

   1. Rising air currents will be pumped out and low pressure is maintained at the centre

Impact of cyclones

Torrential rainfall

Walker’s circulation

1. Surface ocean currents

   1. Easterly trade winds blow from the high pressure area off the western coast of South America to the low pressure area off East Asia. Generates surface ocean currents travelling in the same direction as the easterly trade winds

2. Upwelling

   1. Surface waters off Peruvian coast is pushed westward, hence deeper colder water rises up from below to fill the gap

3. Walker Circulation (atmospheric)

   1. Rising air in western Pacific and sinking air in eastern Pacific connect to form a continuous loop

4. Weather patterns

   1. Warmer water in western Pacific results in higher air temperatures and more convective precipitation (eg Indonesia)

   2. Colder water in eastern Pacific leads to lower air temperatures and dry weather conditions in South America

El Niño Phase

1. Reversal of trade winds

   1. Every 3-7 years periodically, easterly trade winds weaken and even reverse and flow eastwards

   2. Warm water piled up in the west sloshes back towards the east

2. Weather pattern in eastern Pacific

   1. Sea surface temperatures in the east increases by 2-3ºC

   2. Experiences warmer and wetter weather conditions

3. Weather pattern in western Pacific

   1. Experiences drier than usual conditions

   2. Upwelling is repressed

La Niña phase

1. Intensification of Walker’s Circulation

   1. If waters cool to below normal temperatures, La Nina occurs

   2. Walker Circulation intensifies with greater convection over western Pacific due to stronger trade winds, causing surface of eastern Pacific to cool

2. Weather patterns

   1. Droughts in South America

   2. Heavy rainfall and floods in eastern Australia