Can changes in ENSO activity help to explain increasing stratospheric water vapor?

Abstract

[1] Observational studies indicate that stratospheric water vapor has been increasing by 1% per year on average over the past few decades. The only well established mechanism for this increase is the increasing concentration of atmospheric methane and its oxidation in the stratosphere. However, this effect accounts for no more than half of the observed trend in water vapor. Here we use a middle atmosphere general circulation model (GCM) and the observed record of past El Nino/Southern Oscillation (ENSO) activity to test whether observed increases in water vapor can be explained by changes in ENSO activity. In our model, El Nino conditions typically result in about a 10% increase in the concentration of water vapor entering the stratosphere. The observed record also shows an increase of almost one extra year of El Nino conditions per decade over the past half century. The model and observational results taken together therefore imply a positive trend in water vapor of around 0.1% per year and ENSO effects appear to explain no more than about one tenth of the long term trend. We also find variability in the water vapor entry values related to the Quasi-Biennial Oscillation (QBO) in our model. As ENSO and the QBO are variable in nature, this helps to explain some of the intermittency in the observed trend of increasing water vapor, particularly during periods such as the first part of the 1990s when ENSO was relatively active and water vapor increased rapidly despite a reduction in the rate of methane increase.

Extrapolating future Arctic ozone losses

“…Figure 5 shows that halving the PSC trends would lead to a much faster recovery of the ozone layer (magenta dash-dotted line). In the last years H 2 O has decreased (Rosenlof et al, 2003, Randel et al, 2004, but this may be related to changes in the SOI (Scaife et al, 2003). It should be noted that there is no consensus about the attribution of the observed downward trend of the Arctic winter temperatures to the trends in ozone, WMGHG, and water vapour (WMO, 2003).…”

“…Scaife et al (2003) showed that part of the trend is due to the upward trend in the Southern Oscillation Index (SOI). According to Boer et al (2004) the change in the SOI may be due to increased well-mixed greenhouse gases (WMGHG) and may thus continue in the future.…”

The relationship between tropospheric wave forcing and tropical lower stratospheric water vapor

“…An explanation for the extreme departure from the linear relationship for both satellite data is not known. The increase in water vapor at the TTL have been related to El Niño events (Gettelman et al, 2002;Hatsushika and Yamazaki, 2000;Scaife et al, 2003). In early 1997 the Southern Oscillation Index (SOI) shows very low values indicating the beginning of a strong El Niño event, however, in JFM of 1998 the TLS water vapor VMRs appears to be normal, although the Southern Oscillation Index (SOI) remained very low.…”

Modeling Water Vapor in the Upper Troposphere and Lower Stratosphere

“…This is closely connected to the Walker Circulation (Walker 1924), which describes large east west shifts of mass between the Indian and west Pacific oceans and the east Pacific Ocean. El Niño events are largely correlated with enhanced water vapor values in the mid troposphere (Jackson et al 1996;Scaife et al 2003;Fernandez et al 2004 and references therein). This signal propagates into the stratosphere with a time delay associated with tropical mean upwelling.…”

“…This signal propagates into the stratosphere with a time delay associated with tropical mean upwelling. Scaife et al (2003) showed in a general circulation model that there was an increase in water vapor entering the stratosphere during El Niño events. There is no clear evidence of a similar decrease in water vapor transport across the tropical tropopause during La Niña events.…”