Eastern Depression

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Easterly waves or eastern depressions are roughs of low pressure embedded in north-east trade winds & they move from east to west.They develop in broad convergence zone formed by an aggregation of several cloud clusters within ITCZ. Since,the resultant vigorous convection takes place too close to equator where Coriolis force is too weak to generate circular atmospheric motions & major synoptic-scale disturbances comparable with mid-latitude depressions.the tropical convection becomes associated with pronounced wave like undulations & thwart trade winds.


These troughs alternate with high pressure ridges (divergence zones) with wavelengths between 2000Km & 4000 Km.these waves have a life span of upto 2 weeks & travel westwards  (in trade wind flow) 6(degrees) - 7 (degrees) long/day at a speed of 5 - 7 m/sec. ,in deep trade wind flow of the southern limb of the Azores & North Pacific sub-tropical highs.The shallow weak troughs extend from equator to the sub-tropics.their origin is unclear but they develop over the tropical oceans like - Carribean,where trade wind inversion is weak or absent during summer 7 autumn especially.the absence of this stabilizing upper-air inversion within ITCZ allows the surface heating & lapse rate  steepening to develop into unstable,freely convected systems with considerable amounts of latent heat released due to condensation.Ahead of the tough axis,trade wind inversion is low & strong,prohibiting free convection & producing stable,clear weather.
this zone is also one of the strong divergence as the air moves equatorwards & curves anticyclonically with vertical contraction of the air column & descending (at Dry Adiabatic Lapse rate ),drying & cloud free air.Behind the trough axis,inversion is much higher or absent,allowing deep,free convection with cumulonimbus clouds in the unstable,humid air & heavy thundery showers.this is a zone of convergence as air moves polewards & moves cyclonically with vertical expansion of air column & ascending cooling air ( at Saturated adiabatic Lapse rate).these depressions tend to act as parent 'vortices' for the generation of tropical cyclone 'seedlings' although only 10% of these disturbances become 'mature' hurricanes.  

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