Abstract
Solar signals in the atmosphere and the ocean, especially in tropopause temperatures and lower stratospheric water vapour are investigated using recent observational and reanalyses data sets for the period from 1958 through 2013. Previous observational and modeling studies demonstrated solar influences in the lower stratosphere resembling a positive Northern Annular Mode due to the top-down mechanism involving enhanced solar UV radiation in the stratosphere during solar maxima and dynamical amplification mechanisms in the atmosphere. We found that these stratospheric changes might propagate down to the troposphere and become zonally asymmetric with characteristic pressure and wind pattern over the North Atlantic and North Pacific. Such changes in tropospheric circulation are related to anomalous positive SST anomalies in the central Pacific which resemble an El Niño Modoki event. We show for the first time with ocean reanalysis data that these SST anomalies are amplified by a positive feedback through oceanic subsurface currents and heat transport in the equatorial Pacific. Anomalous warm SSTs in the equatorial central Pacific change the zonal SST gradient and lead to anomalous westerly winds and currents in the western Pacific and easterly winds and currents in the eastern Pacific. This indicates a convergence and less upwelling and therefore enhances the positive SST anomalies in the equatorial central Pacific. Such a positive feedback results in a peak of El Niño Modoki events about 2 years after the solar maximum. These solar-induced signals in the ocean in turn modify the circulation and convection in the troposphere, resulting in lagged solar signals of anomalous high tropopause heights and negative anomalies in tropopause temperatures as well as in lower stratospheric water vapour over the equatorial Pacific which are in agreement with a time evolving solar-induced El Niño Modoki-like SST pattern. We demonstrate a solar modulation of intrinsic decadal climate variability over the Pacific which is amplified by positive feedbacks between the ocean and the atmosphere.
Highlights
The Sun is the energy source of the climate system
At 50 hPa (Fig. 1a), the anomalous westerlies are evident in mid-latitudes and subpolar while easterly anomalies can be seen in lower latitudes, which resembles the Northern Annular Mode (NAM) (Thompson and Wallace 2000; Baldwin and Dunkerton 2005; Kodera et al 2016)
Solar signals in Sea surface temperature (SST), the ocean and tropopause temperatures are investigated using a combination of both observations and oceanic and atmospheric reanalyses data from 1958 through 2013
Summary
The Sun is the energy source of the climate system. The total solar irradiance (TSI) varies with a clear 11 year solar cycle but a small amplitude of about 0.1% (1.3 W m−2 ) (Kopp and Lean 2011). There is still a debate whether this secondary warming is related to solar and to aliasing effects with volcanoes and the El Niño Southern Oscillation (ENSO) (Marsh and Garcia 2007; Gray et al 2010; Chiodo et al 2014). Another amplification mechanism is the so-called “bottom-up” mechanism which involves an amplification of the small direct signal at the Earth’s and ocean surface, in particular in the equatorial Pacific through atmosphere–ocean coupling (White et al 1997; Meehl et al 2009)
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