Winter time Changes in the Strength of Tropical Circulation and its Impact on Tropospheric Ozone over Pacific under Global Warming Scenario

Abstract:

Ozone is of great importance to climate due to its radiative effects and is the primary source of hydroxyl radical, which acts as an oxidizing agent of the atmosphere. It can be a regional pollutant and harmful to human health and vegetation. Climate change has diverse impacts on O3 and its budgets through changes in circulation and meteorological variables. On the other hand, an enhanced stratosphere-troposphere exchange (STE) of ozone, which increases by 80% due to doubling CO2, contributes to ozone increases in the extratropical free troposphere which subsequently propagate to the surface. Moreover, stratospheric circulation is projected to intensify over the coming century, which could lead to an increase in the flux of ozone from the stratosphere to the troposphere.

Downward transport stratospheric ozone is an important source of tropospheric ozone, particularly in the upper troposphere, where the radiative effect due to changes in ozone is maxima. Upper tropospheric (UT, 200 hpa) equatorial westerly ducts over the Pacific and Atlantic oceans are common locations for incursions of extratropical air deep into the tropics. During boreal winter to spring when the equatorial westerly is strong, Rossby wave breaking occurs within these ducts and intrusions of dry and ozone rich stratospheric air can be seen on maps of potential vorticity (PV) of UT. These intrusions can affect UT ozone, especially in the northern tropical Pacific Ocean and its interannual variability is suggested to be linked to ENSO. We mainly focus on the increasing multidecadal trend and westward shift in Rossby wave breaking events over Pacific Ocean, drawn from multiple reanalysis datasets, particularly in boreal winter months under global warming scenerio. Therefore, we also address the changes in the strength of UT equatorial westerly wind and the subtropical jets, associated with the strength of Pacific circulations in longer time scale. Secondly, we observe depletion in ozone concentration in midlatitude stratosphere and equatorward propagation to the lower troposphere, due to wave intrusions. Therefore multidecadal strengthening in wave intrusions will enhance the flux of ozone from the stratosphere to the troposphere in the subtropical northern hemispheric central Pacific, which should have significant climate impact globally.