Abstract
Tropopause temperatures (TPTs) control the amount of stratospheric water vapour, which influences chemistry, radiation and circulation in the stratosphere, and is also an important driver of surface climate. Decadal variability and long-term trends in tropical TPTs as well as stratospheric water vapour are largely unknown. Here, we present for the first time evidence, from reanalysis and state-of-the-art climate model simulations, of a link between decadal variability in tropical TPTs and the Pacific Decadal Oscillation (PDO). The negative phase of the PDO is associated with anomalously cold sea surface temperatures (SSTs) in the tropical east and central Pacific, which enhance the zonal SST gradient across the equatorial Pacific. The latter drives a stronger Walker Circulation and a weaker Hadley Circulation, which leads to less convection and subsequently a warmer tropopause over the central equatorial Pacific. Over the North Pacific, positive sea level pressure anomalies occur, which damp vertical wave propagation into the stratosphere. This in turn slows the Brewer-Dobson circulation, and hence warms the tropical tropopause, enabling more water vapour to enter the stratosphere. The reverse chain of events holds for the positive phase of the PDO. Such ocean-troposphere-stratosphere interactions may provide an important feedback on the Earth’s global surface temperature.
Highlights
The tropical tropopause temperature (TPT), as shown by the Modern Era Retrospective-Analysis for Research and Applications (MERRA) reanalysis data, exhibits clear decadal variability between 1979 and 2014. This decadal variability in tropical TPTs may be related to the Pacific Decadal Oscillation (PDO), the leading mode of monthly sea surface temperatures (SSTs) anomalies over the North Pacific
This has been shown by the MERRA TPTs and observed SSTs (HadISST) for the real climate behaviour, and by means of a number of long-term climate model simulations with the CESM-WACCM model
We suggest that tropical TPTs, which control the amount of stratospheric water vapour, could be in part forced by decadal to multidecadal variability in the equatorial Pacific and the North Pacific sea surface temperatures (SSTs)
Summary
Interannual and decadal variability in TPTs and SSTs. Figure 1a shows deseasonalized tropical TPT anomalies averaged over the region 20°S–20°N from both GPS-RO (2001–2014, blue line) and MERRA data (1979–2014, red line). The reduced vertical motion in the equatorial upper troposphere and lower stratosphere, which is a result from a combination of the weaker Hadley circulation in the upper tropical troposphere and the weaker Brewer-Dobson circulation in the tropical lower stratosphere, leads to a warming around the tropical tropopause at around 18 km, which is strongest near 15°N and 15°S (Fig. 7c) This mechanism is very similar to one previously reported with regard to the ENSO influences on the stratosphere[25,39,41], but is applied here to the PDO and to the decadal timescale. This can explain about 15–30% of the water vapour variance on decadal timescales
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