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Signals of El Ni o Modoki in the tropical tropopause layer and stratosphere

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Abstract:

The effects of El Ni o Modoki events on the tropical tropopause layer (TTL) and on the stratosphere were investigated using European Center for Medium Range Weather Forecasting (ECMWF) reanalysis data, oceanic El Ni o indices, and general climate model outputs. El Ni o Modoki events tend to depress convective activities in the western and eastern Pacific but enhance convective activities in the central and northern Pacific. Consequently, during El Ni o Modoki events, negative water vapor anomalies occur in the western and eastern Pacific upper troposphere, whereas there are positive anomalies in the central and northern Pacific upper troposphere. The spatial patterns of the outgoing longwave radiation (OLR) and upper tropospheric water vapor anomalies exhibit a tripolar form. The empirical orthogonal function (EOF) analysis of the OLR and upper tropospheric water vapor anomalies reveals that canonical El Ni o events are associated with the leading mode of the EOF, while El Ni o Modoki events correspond to the second mode. The composite analysis based on ERA-interim data indicate that El Ni o Modoki events have a reverse effect on middle-high latitudes stratosphere, as compared with the effect of typical El Ni o events, i.e., the northern polar vortex is stronger and colder but the southern polar vortex is weaker and warmer during El Ni o Modoki events. According to the simulation' results, we found that the reverse effect on the middle-high latitudes stratosphere is resulted from a complicated interaction between quasi-biennial oscillation (QBO) signal of east phase and El Ni o Modoki signal. This interaction is not a simply linear overlay of QBO signal and El Ni o Modoki signal in the stratosphere, it is El Ni o Modoki that leads to different tropospheric zonal wind anomalies with QBO forcing from that caused by typical El Ni o, thus, the planetary wave propagation from troposphere to the stratosphere during El Ni o Modoki events is different from that during canonical El Ni o events. However, when QBO is in its west phase, El Ni o Modoki events have the same effect on middle-high latitudes stratosphere as the typical El Ni o events. Our simulations also suggest that canonical El Ni o and El Ni o Modoki activities actually have the same influence on the middle-high latitudes stratosphere when in the absence of QBO forcing.

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