Abstract
Upper tropospheric equatorial westerly ducts over the Pacific Ocean are the preferred location for Rossby wave breaking events during boreal winter and spring. These subtropical wave breaking events lead to the intrusion of high PV (potential vorticity) air along the extra-tropical tropopause and transport ozone rich dry stratospheric air into the tropics. The intrusion frequency has strong interannual variability due to ENSO (El-Niño/Southern Oscillation), with more events under La-Niña and less under El-Niño conditions. This may result from stronger equatorial westerly ducts and subtropical jets during La-Niña and weaker during El-Niño. It was previously suggested that the interannual variability of the tropospheric ozone distribution over the central-eastern Pacific Ocean is mainly driven by convective activity related to ENSO and that the barotropic nature of the subtropical intrusions restricts the tracers within the UT. However, our analysis shows that tropospheric ozone concentration and subtropical intrusions account ~65% of the co- variability (below 5 km) in the outer tropical (10–25°N) central Pacific Ocean, particularly during La-Niña conditions. Additionally, we find a two-fold increase and westward shift in the intrusion frequency over the Pacific Ocean, due to the climate regime shift in SST pattern during 1997/98.
Highlights
The extra-tropical-tropical interactions and the equatorward transport of stratospheric dry air have significant impact on the tropical dynamics, in the central and eastern Pacific[1]
Previous studies reported that the impact of El-Niño/Southern Oscillation (ENSO) on the Pacific upper tropospheric (UT) field[30,31] is maximum during boreal winter and spring months (November-March), much larger than in any other season of the year
We examined the climatology of PV intrusions and its impact on the tropospheric ozone distribution over the outer tropical Pacific Ocean during the two phases of ENSO
Summary
Variability of PV intrusion events in two phases of ENSO. ENSO has significant impact on lateral transport of tracers into the tropics by means of intrusion and the interannual variability of Pacific PVI events correlates strongly with the Niño 3.4 index. Zonal mean stratospheric overturning circulation organizes the transport of ozone rich air poleward and downward to the high and midlatitudes leading there to higher ozone concentration[16] In addition to this well described mechanism, an increase in ozone flux has been reported over the northern hemispheric outer tropical (10–25°N) central Pacific Ocean. During LN, the subtropical wave guide (7–17 km) is stronger and exhibits an equatorward and downward trajectory from the midlatitudes (UT–LS) to the outer tropics (UT–LT) This strengthening of the subtropical waveguide during LN is primarily due to the weakening of the subtropical jet (10–15 km & 30–40°N) and strengthening of the equatorial westerly duct (10–15 km), resulting from the weakening of the central-eastern Pacific Hadley circulation and Pacific Walker circulation, respectively[8]. We may argue that the interannual oceanic and atmospheric variability over the Pacific Ocean itself is being altered by the recent CRS around 1997/1998 and that a LN like condition dominates the trends
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