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Solar influences show up in sea level rise, El Nino events and oceanic climatic cycles

Dr David Whitehouse, Science editor

The Sun’s energy affects our climate but its influence is often ignored as changes in its intensity are very small. Its effect might be subtle but over decadal periods it adds up to being significant as a series of recent papers show.

Scientists from the University of California, Irvine, the National Taiwan Normal University, and the Institute of Atmospheric Physics, of the Chinese Academy of Sciences, Beijing, find that the 11-year solar cycle has a significant correlation with sea surface temperature variations in the North-eastern Pacific. They believe that the Sun’s influence is first seen and then amplified in the lower stratosphere, but it then alters the circulation in the troposphere which then affects the temperature of the ocean.

They note that the changes have a structure similar to that of the Pacific meridional mode – an interaction between trade winds and ocean evaporation which is an important trigger of the central Pacific (CP) type of the El Nino-Southern Oscillation (ENSO).

It seems that the 11-year solar cycle modulates the CP ENSO and, in particular, is associated with more CP El Nino events during the active phase of the cycle and more La Nina events when the solar cycle undergoes a downturn.

The solar influence is also apparent in other aspects of airflow in the tropics. A team from Oxford University, Aarhus University, the Max Planck Institute for Meteorology, Hamburg, Germany; Imperial College London, and the Gantham Institute, of Imperial College London, recently provided observational evidence that the solar cycle affects atmospheric circulation over the Pacific on decadal timescales finding that there is a reduction of east–west sea-level pressure gradients over the Indo-Pacific Ocean during solar maxima and the following few years.

This reduction is associated with westerly wind anomalies at the surface and throughout the equatorial troposphere in the western/central Pacific as well as an eastward shift of precipitation that brings more rainfall to the central Pacific. It’s an effect that shows up in some climate models that use simulations considering only solar irradiance variations.

A Missed Connection

Another recent study shows a correlation between the end of solar cycles and a switch from El Nino to La Nina conditions suggesting that solar variability can drive seasonal weather variability on Earth. If the connection outlined in the journal Earth and Space Science holds up, it could significantly improve the predictability of the largest El Nino and La Nina events. According to Scott McIntosh, a scientist at the National Center for Atmospheric Research (NCAR) and co-author of the paper. “The scientific community has been unclear on the role that solar variability plays in influencing weather and climate events here on Earth. This study shows there’s reason to believe it absolutely does and why the connection may have been missed in the past.”

The paper does not examine what physical connection between the Sun and Earth could be responsible for the correlation, but that there are several possibilities such as the influence of the Sun’s magnetic field on the incidence of cosmic rays that bombard Earth.

A team from the Australian National University, the Australian Bureau of Meteorology and Australian Centre of Excellence for Climate Extremes find a solar influence on the Southern Annular Mode (SAM) – an important pattern of climate variability in the extratropical Southern Hemisphere, with major regional climate impacts.

Whilst the available evidence shows changes in the SAM since the 1960s can be accounted for by climate models earlier trends in palaeo-climate SAM reconstructions cannot be reconciled with more recent simulations.

The researchers find that the mean SAM state can be significantly altered by solar irradiance changes. They suggest that the effects of solar forcing on high-latitude climate may not be adequately incorporated in most last millennium simulations.

Researchers from the Jeju National University in Korea and NASA’s Jet Propulsion Laboratory have looked at the various factors that influence the rate of increase of global mean sea level and detect the influence of the global temperature hiatus.

One suggestion for the hiatus was that the oceans absorbed more heat reducing the heating of the surface. Recently however observations have shown that ocean uptake actually slowed down during the hiatus.

They found a “distinct decade-long fluctuation with a peak period of ~12 years,” that was in phase with the hiatus. They also saw a “strong relationship” between global mean sea level and the PDO – a pattern of ocean-atmosphere climate variability centred over the mid-latitude Pacific – with the PDO undergoing a change around 2011 coincident with what they consider to be the end of the hiatus. Others consider that the hiatus went on somewhat longer.

They conclude, “there is an oceanic response to the solar cycle on decadal timescales.”