26 December 2014

Interactions Between Externally-forced Climate Signals from Sunspot Peaks and the Internally-generated Pacific Decadal and North Atlantic Oscillations


The key to knowing how future climate may evolve is to better understand historical climate fluctuations that involve a mixture of naturally-occurring variability (e.g. the Pacific Decadal Oscillation, PDO; or North Atlantic Oscillation, NAO)  and externally-forced responses (e.g. from humans such as increasing greenhouse gases, or natural such as from solar variability).  Part of the mystery in how the climate system responds to the solar forcing 11 year sunspot cycle is that the signals appear to be intermittent.  Here we provide some insight as to why that may be by showing that when the PDO is in phase with the 11 year sunspot cycle there are positive sea level pressure (SLP) anomalies  in the Gulf of Alaska, nearly no anomalous zonal SLP gradient across the equatorial Pacific, and a mix of small positive and negative SST anomalies there.  When the two indices are out of phase, positive SLP anomalies extend farther south in the Gulf of Alaska and west into eastern Russia, with a strengthened anomalous zonal equatorial Pacific SLP gradient and larger magnitude and more extensive negative SST anomalies along the equatorial Pacific.  In the North Atlantic, when the NAO is in phase with the sunspot peaks, there is an intensified positive NAO SLP pattern.  When the NAO is out of phase with the peaks, there is the opposite pattern (negative NAO).  Thus we make the argument that the phasing of the internally-generated PDO and NAO can interact with the externally forced response from the solar cycle to produce different climate signals depending on the phase of the solar cycle.

H van Loon