Intrusions into the tropical upper troposphere: Three-dimensional structure and accompanying ozone and OLR distributions
Résumé
The evolution and structure of stratospheric intrusions into the upper troposphere (UT) over the northern tropical Pacific is examined in terms of both potential vorticity (PV) and ozone (O3). Analysis of 20 years of NCEP–NCAR reanalysis PV shows that the intrusion events have remarkably similar evolution and structure at 350 K, with all events producing narrow tongues of high PV that have an almost north–south orientation and last around 3 days. Nearly all events extend up into the lower stratosphere, but only for a small percentage is there deep downward penetration. The intrusions explain a large amount of the observed variability in upper tropospheric O3 above Hilo, Hawaii, with large values occurring when a tongue of high PV passes over Hilo and low values when Hilo is just upstream of a high-PV tongue. There is also an increase in total column ozone within the PV tongues, but for most intrusions the increase is relatively small. The relationship between deep convection, as diagnosed by satellite observations of outgoing longwave radiation (OLR), and intrusions is also examined. It is shown that transient convection and intrusions in the central and eastern northern Pacific nearly always occur together, with the convection at the leading edge of the PV tongue. This confirms the results of previous studies that have shown a close link between Rossby wave activity and transient convection, and supports the hypothesis that the ascent and reduced static stability due to anomalous PV in the UT initiates and supports the convection.