Association Between Solar-Irradiance Variations and Hydroclimatology of Selected Regions of the USA

Charles A. Perry, U.S. Geological Survey, Lawrence, Kansas, USA
Proceedings of the 6th International Meeting on Statistical Climatology, 19-23 June, 1995, Galway, Ireland

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Abstract

Hydrologic time series can be indicators of climate trends and cyclic variations. Although the mechanisms of climatic trends and variations are not completely understood, extraterrestrial solar-irradiance variations may be associated with variations in regional hydroclimatology. A proposed mechanism responsible for the association involves three basic processes: varying amounts of solar energy are (1) absorbed by tropical oceans creating ocean temperature anomalies that (2) move with the ocean currents to locations where they can (3) alter regional atmospheric moisture and pressure patterns which affect regional precipitation and temperature and, consequently, the hydroclimatology of a region. Hydrologic time series in selected regions in the western two-thirds of the United States of America (USA) are shown to correlate with solar-irradiance variations lagged 4-5 years.

Introduction

Water availability is becoming a critical issue as supplies are pressured by an increasing world population; information on hydroclimatic trends and variations is important for wise management of water resources. Hydrologic information, such as streamflow peak discharges and annual flow volumes, lake levels, glacier volumes, and groundwater levels, provide convenient measures of regional climate. Whereas meteorologic data such as precipitation, temperature, radiation, and wind are usually point measurements, hydrologic information is the result of temporal and spatial integration of environmental factors that affect the accumulation of water.

Time series of hydrological data can provide much information on climate trends and variations, and possible forcing factors. For example, the floods in the upper Mississippi River Basin (upstream from the confluence of the Missouri and Mississippi Rivers) were of historic proportions in 1993. Peak discharges and annual flow volumes measured at many stream-gaging stations throughout the basin exceeded previously observed extremes. A peak flow of 30,590 cubic meters per second (m/s) occurred on the Mississippi River at St. Louis, Missouri, on August 1, 1993. Peak streamflow records show that the 1993 peak discharge was the largest since 1861 and has been exceeded only by an estimated discharge of 36,820 m/s for the historic flood of 1844. Discharge for the flood of 1903, 28,860 m/s, was only slightly less than that of 1993. Examination of volume data at the same location since continuous records began in 1934 reveal high volumes of runoff during 1993, 1983, 1973, and 1951. A pattern of major runoff years occurring approximately every 10, 20, 42, and 90 years emerges from this flow data and other flood data within the upper Mississippi River Basin.

Solar activity displays a cyclic pattern of increasing to a peak every 10-12 years. Other multiples of this cyclic pattern are seen in the 20- to 24-year Hale cycle, the 40- to 45-year Double-Hale cycle, and the 80- to 90-year Gleissberg cycle. The floods of 1993, 1983, 1973, 1951, and 1903 tend to follow this pattern. However, projections of solar-activity cycles for estimating future hydroclimatic events have been suspect because there has been no identified physical mechanism to explain the connection between activity on the Sun and regional hydroclimatic patterns on Earth.

Measurements of the Sun's total energy output (total solar irradiance) by Earth satellites show small changes in total energy that generally follow the solar-activity cycle and are providing new information on the controversial solar/climate relation. These variations have been shown to correlate significantly with regional precipitation in various locations in the USA (Perry, 1994). Association between solar-irradiance variations and the hydroclimatology of selected regions of the United States will be presented in this paper.

Proposed Solar/Hydroclimate Mechanism

The mechanism proposed for the coupling of solar-irradiance variations with regional hydroclimate consists of three main components. These are: (1) absorption of solar energy by the tropical oceans in a deep surface layer, (2) transport of that energy by major ocean currents of the Pacific Gyre, and (3) transfer of that energy by evaporation into atmospheric moisture and atmospheric pressure systems that can be advantageous for precipitation formation (Perry, 1994). The components are depicted in Figure 1.

A. Absorption of Energy

Although sea surface temperatures (SST) show significant coupling with atmospheric components (Wallace et al. 1990), it is not the ocean's surface that stores the majority of the solar energy. The visible part of the solar spectrum contains about one-half of the total energy available from the Sun at the Earth's surface (Loiv, 1980), and those wavelengths penetrate well below the ocean's surface. Lewis et al. (1990) showed that solar radiation in visible frequencies, usually assumed to be absorbed at the sea surface, penetrates to a significant depth below the upper mixed layer of the ocean that interacts directly with the atmosphere. In clear water, the blue wavelengths, where the greatest amount of energy is available, penetrate the deepest, to nearly 100 meters (m). Energy injected into the ocean at this depth can be stored for a substantial period of time.

The transparency of the tropical oceans is dependent upon the amount of biogenic material, phytoplankton pigments, and degradation products that are present. In the Pacific Ocean, transparency increases from east to west, with the greatest penetration of solar energy occurring in the western tropical Pacific. This stored energy eventually returns to the ocean's surface to interact with the atmosphere as the general circulation of the Pacific Ocean transports water to the north and east toward North America (Figure 2).

Other factors that affect the amount of solar energy absorbed by the oceans include latitude, season, sea-surface roughness, atmospheric particulates, and cloud conditions. All of these factors contribute in various degrees to the development of anomalous ocean temperatures. However, extraterrestrial solar-irradiance variations may be a significant factor.

B. Transportation of Ocean Temperature Anomalies

The North Pacific Ocean currents take approximately 4 years to move temperature anomalies from the western tropical Pacific to near North America (Favorite and McLain, 1973). In addition, the Equatorial Countercurrent flows eastward between the westward flowing currents of the North Pacific and South Pacific Gyres. Upon this countercurrent rides the elusive oceanic warming of El Nino and the oceanic cooling of La Nina. Another important minor circulation is the counterclockwise flow of the Gulf of Alaska Gyre. The North Pacific Gyre and its minor circulations are the conveyors of absorbed solar energy from the central and western tropical Pacific to locations north and east. If incoming solar energy varies on time scales of months to years, then different parts of the gyre will receive different amounts of solar energy as the water moves through the tropics. During a period of diminished irradiance, a part or pool of the tropical ocean would receive less energy and be anomalously cool, whereas increased irradiance would result in an anomalously warm pool. These pools of slightly warmer or cooler water would move around the Pacific Gyre like riders on a carousel.

C. Transfer of Energy to the Atmosphere

In the tropical Pacific Ocean, the net flux of energy is downward into the ocean, whereas at the higher latitudes this flux is upward. Energy is transferred from the ocean to the atmosphere primarily by evaporation. Evaporation from the surface of the ocean is a mechanism that could amplify the effect of solar-irradiance variations. The vapor pressure of water increases by approximately 7% for each 1 ÷C of increase in water temperature between 5 and 15 ÷C. Therefore, a 1 ÷C positive anomaly in SST could result in about a 7% increase in the amount of energy available to the atmosphere. Variations in the vapor pressure at the sea surface can significantly alter atmospheric moisture fields, from which further amplification of solar-irradiance variations can occur through dynamic atmospheric processes. Energy patterns in the upper atmosphere are responsible for the persistent upper air flow patterns that interact on a global scale. The strength, shape, and location of the jet stream and other motions of the atmosphere, which are critical factors in precipitation formation, are largely dependent upon the temperature patterns of the oceans.

Solar-Irradiance Variations

Solar irradiance has been measured in space by sensors on several spacecraft including the Earth Radiation Budget Experiment (ERBE) on the Nimbus-7 satellite, the Active Cavity Radiometer (ACRIM I) that flew on the Solar Maximum Mission (SSM), and an ACRIM II that is presently aboard the Upper Atmospheric Research Satellite (UARS). In Figure 3, the Nimbus-7 data show a sinusoidal variation of more than 3 watts per square meter (W/mî) over the 14-year period shown, with annual variations of more than 1 W/mî. Presently (1995), these measurements account for more than 16 years of data. However, 16 years is a short time for comparison with climatic data. Fortunately, two solar-irradiance investigators have developed empirical models for estimating total solar irradiance before 1978 (Foukal and Lean, 1990).

One solar-irradiance model is based on changes in excess radiation from bright magnetic faculae (areas of higher irradiance on the Sun's disk) and on changes in reduced radiation from dark sunspots. Using this model, estimates of bright magnetic faculae were made back to 1954 using daily flux 10.7-centimeter (cm) wavelength (Lean and Foukal, 1988). Estimates of irradiance were later extended back to 1874 by Foukal and Lean (1990) in a second model using monthly means of the sunspot number in place of the 10.7-cm flux. Because the 10.7-cm flux was not measured before 1947, the 42-year period (1947-88) of monthly values of irradiance is assumed to have a greater reliability than the values for the period from 1874-1946. Irradiance values generated by the 1990 model are depicted in Figure 4.

This paper is based on the hypothesis that increases or decreases in solar irradiance have an effect on regional hydroclimate. Increased irradiance creates warmer pools of Pacific Ocean water that are transported to a location where increased evaporation leads to increased precipitation and ultimately increased terrestrial water. The opposite is hypothesized for decreasing irradiance. Because of the process of ocean temperature-anomaly formation, a time series of differences of average annual solar irradiance were determined (Figure 5), based on the model-generated values (Foukal and Lean, 1990).

To obtain a series of annual solar-irradiance values, a series of 13-month averages of monthly irradiance values centered on July were determined. An annual difference was computed by subtracting one year's average from the previous year's average.

Irradiance Variations Compared with Hydroclimatologic Time Series

Solar-irradiance variations and hydroclimatic time series in selected regions within the USA were correlated to test the proposed hypothesis that a solar/hydroclimate association exists. Examples of time series for precipitation, glacier mass, groundwater levels, and flow volumes of a major river basin were compared to a time series of solar-irradiance variations.

D. River Volume

The average annual flow of a river is the total volume of water produced by runoff and groundwater discharge and is an integration of many climatic as well as hydrologic factors over the river basin. The Mississippi River Basin measured at St. Louis, Missouri, encompasses slightly more than 2 million kmî, draining more than one-fourth of the area of the 48 contiguous United States. Figure 9 shows the annual average flow of the Mississippi River at St. Louis and annual solar-irradiance differences. The best correlation is obtained with a 5-year lag time. The correlation coefficient for these data is R=0.52, significant at the 1% level. An approximate 5-year lag is apparent for other basins and subbasins between the Rocky Mountains and the Appalachian Mountains, as opposed to the 4-year lag time observed in the Pacific Northwest. A possible explanation for the greater lag time may be in the more eastward location of the ocean temperature anomalies in the Pacific Ocean that affect atmospheric dynamics over the midsection of the United States than over the Pacific Northwest. An additional year may be needed to move the ocean temperature anomalies farther to this location.

An intriguing observation of individual graphical correlations of annual solar-irradiance variations with annual precipitation and streamflow in the United States is an apparent change in effective lag times for a specific location. Prior to the mid-1970's, lag times were slightly less than lag times after the mid-1970's. This point in time coincides with an apparent shift in atmospheric pressure patterns and with an increase in global surface air temperatures recorded at land-based stations (Graham, 1995).

Conclusions

Comparison of hydroclimatologic times series from selected regions in the USA with variations in annual solar irradiance, provides apparent support for a solar/hydroclimate association. The mechanism for this association is thought to begin with variations in the amount of solar-irradiance energy absorbed by the tropical Pacific Ocean, which result in ocean temperature anomalies that are transported by major oceanic currents to locations where the process of evaporation transfers latent-energy variations to the atmosphere. Variations in atmospheric moisture and pressure patterns ultimately affect the amount of regional precipitation. Variations in precipitation, along with the effects of temperature, humidity, wind, and insolation, determine the long-term regional water budget and, therefore, the hydroclimate of a region.

There is a tendency for hydroclimatologic time series to lag solar-irradiance variations by 4 years in the Pacific Northwest and 5 years in the midsection of the United States. These lag times are similar to the traveltimes of water from the tropical Pacific Ocean to the Gulf of Alaska. Although variations in solar irradiance may be only one component of the complex process of precipitation formation, it is a component that could prove valuable in long-term hydrologic risk assessment due to the 4-5 year lag time between solar-irradiance input and the resultant response by a hydrologic basin.

References

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Foukal, P., and Lean, J., 1990, An empirical model of total solar irradiance variation between 1874 and 1988: Science, Volume 247, pp. 556-559.
Graham, N.E., 1995, Simulation of recent global temperature trends: Science, Volume 267, pp. 666-671.
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Association Between Solar-Irradiance Variations and Hydroclimatology of Selected Regions of the USA

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