THE SOLAR CYCLE AND GLOBAL MEAN SEA LEVEL

1. INTRODUCTION AND BRIEF SUMMARY

In the present era of global warming, the steady rise in sea level is widely recognized to be a process of critical societal importance as it carries the potential to affect the lives of millions of people. But in order to unequivocally identify man-made contributions to this trend, we must first be able to define the natural rhythm of sea-level variability over both short and long terms. Our paper “Origin of the Solar-Cycle Imprint on Global Sea Level Change” (Scientific Reports - Nature, give details and date etc.) focusses on a hitherto poorly defined 11-year periodic variation in the rate of change of global mean sea level that stems from fundamental adjustments taking place in the atmosphere-ocean system during the course of the solar cycle. We have been able to identify the source of these sea-level fluctuations following a careful assessment of a variety of long-term environmental datasets. Much to our surprise, we found their origin to lie within systematic variations in the water-mass balance (basically the loss or gain of water) between the oceanic and terrestrial realms (Fig. 1). These water-mass redistributions are in turn driven by a subtle evolution that takes place within a sequence of atmospheric processes during the sun’s 11-year cycle, the consequences of which eventually impact on the El Niño Southern Oscillation (ENSO) and thus on global rainfall patterns and so on the global water-mass balance itself. The observed solar-cyclic oscillations in the rate of change of mean sea level that form the focus of our study are thus a manifestation of the subtle relations that exist between ongoing variations in the sun’s activity and the subsequent responses these engender within the earth’s atmosphere and oceans.

2. KEY POINTS

• Earlier work has shown that global mean sea level change rate fluctuates in synchrony with the 11-year cycle of solar activity. However, prior to our research, the causal origin of this link had not been clarified.
• We carefully re-examined long-term historical datasets using a variety of earth observation records and found that the solar-cyclic influence on global mean sea level change rates reflects water-mass redistributions that take place between the land surfaces and the oceans. In short, during periods of high solar activity, global mean sea level change rates become higher as enhanced net water discharge takes place from the land surfaces into the oceans.
• These regular changes in the global water cycle reflect a fundamental aspect of the interaction between the sun and the upper atmosphere, although the solar influence we identify here occurs selectively, since it depends on the phase of the Quasi-Biennial Oscillation (QBO), a series of quasiperiodic wind reversals taking place in the tropical stratosphere. Modulation of the behavior of the QBO wind phenomenon during the course of the solar cycle influences intra-seasonal variability at lower altitudes within the tropical troposphere, which in turn modifies the activity of the El Niño Southern Oscillation (ENSO).
• The discovery of this solar-cyclic impact on the ENSO represents the major breakthrough of this research program since it enables us to interpret the systematic changes taking place in global precipitation patterns and therefore understand the observed evolution of the land-sea water-mass balance during the course of the solar cycle. It is the latter that dictates the observed change rates in global mean sea level.

3. OVERVIEW

The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) principal researcher Shuhei Masuda, Kyoto University Professor Emeritus John Philip Matthews (Environmental Satellite Applications, UK) and Professor Yosuke Yamashiki (Kyoto University Graduate School of Advanced Integrated Studies in Human Survivability) have published new research results showing how the sun’s 11-year cycle of activity can induce a similar periodicity in the rates of change of global sea-level. These “solar-cyclic” variations in sea level change rate are best defined during periods of high solar activity (when sunspot numbers on the solar surface are high), as then global mean sea level tends to increase more rapidly. This correlation cannot be quantitatively explained on the basis of the thermal expansion of seawater, as one might naively anticipate at solar maximum (due to enhanced solar radiation fluxes entering the oceanic surface layers).

However, an early indication of the factors driving this solar-cyclic variability in sea level change rate became apparent when the authors examined long-term earth-observation data and found that the Palmer Drought Severity Index (PDSI), an index used in hydrological monitoring to assess the relative dryness of terrestrial conditions, is synchronized with the 11-year solar activity. This suggested that the observed solar-cyclic influence on sea level change rate is caused by the movement of water between the oceans and land i.e. changes to the water-mass balance established between the terrestrial and oceanic realms. In particular, during periods of high solar activity the net discharge of water from land surfaces into the oceans increases substantially relative to that during quiet-sun periods. We then confirmed the qualitative validity of this result in an analysis of nearly 30 years of precise satellite observations which involved both altimetric sea-height observations and satellite gravity-field measurements (from which estimates of terrestrial water storage were derived).

The next phase of our research involved an examination of the underlying causative factors driving these water transfers in relation to changes in solar activity. This was carried out using historical climate records and computer-based reconstructions spanning an analysis period of approximately 160 years. An essential step toward solving the problem of how the sun’s variability can effect sea-level change rates involved the sorting of the various atmospheric and environmental data according to the Quasi-Biennial-Oscillation (QBO) phase. Our analysis approach then revealed that a sequence of atmospheric processes is involved in the creation of the solar-cyclic height signature on the sea surface. This sequence commences within the stratospheric wind-reversals of the QBO, a phenomenon that exhibits a clear solar-cyclic dependence. Evolution of the QBO during the course of the sun’s cycle has a well-defined impact on the development of the Madden Julian Oscillation (MJO), while changes to the MJO can in turn influence the evolution of the El Niño Southern Oscillation (ENSO). As the latter has a controlling influence on global rainfall patterns, the modifications to the land-sea water-mass balance that we identified earlier in the study (as responsible for the solar-cyclic variations in sea level change rate) must inevitably take place. The discovery that the strength and rate of evolution of the ENSO exhibits a QBO-phase-related dependence on solar activity represents a significant research advance. This is because the ENSO phenomenon is well known to have a substantial influence not only on global rainfall patterns (considered here in terms of its impact on the land-sea water-mass balance), but also on a wide range of environmental processes.

Our analysis of the mechanisms underlying the solar-cyclic modulation of sea level change rates has not only advanced our understanding of the factors determining historical and future variability in global mean sea level but has also opened up new possibilities in environmental and climate research. In essence, our work tells the story of how a linked sequence of climate phenomena comprised of the QBO, MJO and ENSO, can eventually result in the formation of a footprint of solar activity on the global mean sea level.

Fig. 1　Schematic diagram of the systematic variations in the water-mass balance during the course of the solar cycle.

The results will be published in the Scientific Reports on 16 May, 2025 (10:00 UTC).