Transport of potential vorticity and Eliassen - Palm fluxes for two contrasting years : 1995 – 1996 ( La Niña ) and 1997 – 1998 ( El Niño )

Potential vorticity transport (PV)-Eliassen and Palm (EP) cross sections are studied for two contrasting years: 1995–1996 (La Niña) and 1997–1998 (El Niño). The results show that the largest differences in PV transport-EP fluxes between El Niño and La Niña events occur in winter in both hemispheres, but the changes are higher in the Northern Hemisphere. PV transport-EP fluxes are stronger in both July 1997 and January 1998 than in July 1995 and January 1996, respectively, indicating stronger baroclinic activity in the El Niño year compared to the La Niña year. The changes in PV transport seem to be due mainly to the changes in eddy heat fluxes. Due to the increase in the wind shear the Eady growth rate is stronger in the 1997–1998 El Niño compared to 1995– 1996 La Niña. Consequently, the zonal winds are stronger in the El Niño event, although the location of the jet streams is almost the same in both the contrasting years. However, in the Southern Hemisphere, there are two regions of maxima Eady growth rate in July 1997 and a double jet is observed while a single jet is seen in July 1995 associated with only one maximum of Eady growth rate. In the case of summer, there is little difference in PV transport-EP fluxes between the two contrasting years in the hemispheres, although they are slightly higher in the El Niño event than in the La Niña event.