Changes in the stratospheric composition during the major stratospheric warming of 2013

English Abstract:

Volume mixing ratios (VMR) of different chemical components obtained from Microwave Limb Sounder onboard Aura satellite show distinct variations at high and low latitudes in the upper stratosphere (30-50 km) during the occurrence of a major stratospheric sudden warming (SSW) in January 2013. Zonally averaged ozone VMR as well as chlorine monoxide (ClO) VMR are found to increase over higher latitudes (60-90°N) whereas hydrogen chloride (HCl) VMR shows decrement as the warming proceeds. At higher latitudes, as O3/O partitioning controls the ClO/Cl ratio, the increase of ozone during SSW shifts the ClO/Cl partitioning in favour of ClO, thereby decreasing Cl which indirectly results in decrease of HCl. In the equatorial stratosphere, the ozone VMR is found to increase because of weakening of Brewer-Dobson circulation (BDC) due to reduction in planetary wave activity with the onset of warming. However, methane is expected to decrease due to weakening of upwelling branch of BDC resulting in decrease of HCl VMR. Cl i.e. active chlorine converts to ClO by reacting with ozone molecule and this active Cl comes from the reaction of HCl and water vapour (H2O). As both HCl and water vapour are found to decrease during the SSW, the production of active Cl is also getting reduced. As a result, the amount of ClO also reduces during the SSW thereby partly decreasing the depletion of ozone. The decrement of ClO persists for quite a long time even after the end of the stratwarm event.