the doldrums is the area near the equator where winds die out.
area near the equator where winds die out?
the doldrums
as part of the hadley cell, surface air flows toward the equator while the flow aloft is towards the poles. a low-pressure area of calm, light variable winds near the equator is known as the doldrums, near-equatorial trough, intertropical front, or the intertropical convergence zone. when located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough. around 30° in both hemispheres, air begins to descend toward the surface in subtropical high-pressure belts known as subtropical ridges. the subsident (sinking) air is relatively dry because as it descends, the temperature increases, but the absolute humidity remains constant, which lowers the relative humidity of the air mass. this warm, dry air is known as a superior air mass and normally resides above a maritime tropical (warm and moist) air mass. an increase of temperature with height is known as a temperature inversion. when it occurs within a trade wind regime, it is known as a trade wind inversion.
trade winds
seasonal variability is the dominant time scale of tav, which is due to the seasonal march of the sun. this seasonal variability is related to the movement of intertropical convergence zone (itcz),which is the convergent zone of trade winds from south and north near the equator. it has strong vertical convection resulting to a redundant participation band and weak winds. the mean location of the itcz over the atlantic ocean is 5~10 degrees north of the geographical equator. all this asymmetric of itcz is the ultimate cause of the annual cycle in equatorial sea surface temperature (sst) in atlantic by maintaining southerly cross-equatorial winds that intensify in boreal summer/fall and relax in boreal spring. from march to april, during which the temperature of the equator reaches maximum, winds are weakest and the sun shines directly over the equator. so, sst is uniformly warm near the equator, which makes the itcz really sensitive to even small disturbance of sst and explains the relaxation of cross-equator winds in spring. from july to september, during which the temperature of the equator reaches its minimum, itcz reaches its northernmost location, which explains the intensification of cross-equator winds. it can be seen that the process of cooling takes 3 months while that of warming takes 7 months, which are asymmetric. this seasonal asymmetry is due to influence of seasonal continental monsoon. because of the narrow width of atlantic, continental monsoon has a much more important influence on its variation compared to the wide pacific ocean, whose leading factor is air-sea interaction.
tropical atlantic variability