April showers bring May flowers. Well, not always. The climate of the Chesapeake Bay area, as elsewhere, varies from year to year as anyone living in the watershed the last few years can attest.

That variability, though, is more than just a conversation ice-breaker. Fluctuations in seasonal rainfall affect river flows into the Bay and can hinder nutrient reduction efforts aimed at restoring the Chesapeake ecosystem because nutrient flows into the Bay are closely tied to river discharge.

While once viewed primarily as random acts of nature, recent research suggests that - to some extent - historical river flows into the Bay may be linked to complex climate patterns that may originate half a world away. As efforts improve to predict future climate change months in advance using computer models, the potential exists for Bay watershed managers, farmers and others to take weather into account when making their plans.

To restore the Chesapeake, the Bay Program has a goal of reducing the amount of the nutrients phosphorus and nitrogen that enter the Bay during an "average" flow year by 40 percent. Recent years, though, have been anything but average. Last year, rivers delivered an average daily flow of 87.5 billion gallons into the Bay - 1.7 times higher than normal. Three of the last four years have ranked among the six highest flow years since 1951.

Heavier precipitation can lead to greater amounts of nutrients being washed off farms, lawns, roads and other surfaces, ultimately ending up in the Bay. Those "nonpoint" sources of pollution, which vary with rainfall, and the more predictable amounts of nutrients discharged from sewage treatment plants, help spur large algae blooms which contribute to reduced oxygen levels in the Bay.

Low levels of oxygen can have a severe impact on the health of bottom-dwelling species and the Bay ecosystem as a whole. In addition, sediment inputs, which diminish environmental quality in the Chesapeake by burying bottom-dwellers and clouding the water, are influenced by tributary discharge in many parts of the Bay.

Should managers be concerned that flows of recent years may have started a new trend that could complicate cleanup efforts? Or will discharge return to the lower levels of the 1950s and '60s? To evaluate the impact of past, present and future climate changes on the Bay, one must know the answers to many questions:


  • What natural factors control climate change and influence the Bay ecosystem over different time scales?



  • Are year-to-year differences in spring and summer discharge related to broader, perhaps hemispheric or global climate patterns?



  • How might changes in climate and river discharge over the last few decades be related to changing climate?



  • What might future climate changes hold in store?


Factors Controlling Climate Change

Climate refers to long-term temperature and precipitation patterns of an area. But it also includes upper atmospheric conditions, soil moisture and vegetation, cloudiness, and the variability of these parameters, including year-to-year fluctuations in storms and rainfall. We can put climate change into a temporal framework to distinguish among the different factors that cause it to change.

Long-term Climate Change: Chesapeake Bay owes its very existence to the 125-meter global sea level rise that began about 15,000 years ago when the last glacial period ended and melting continental ice sheets flooded the ancient Susquehanna River Valley. Such changes are part of periodic climatic cycles, occurring over tens of thousands of years, which have brought alternating ice ages and intervening periods of global warmth to the Earth for the last few million years.

According to the Astronomical Theory of Climate, "paleoclimate" records demonstrate that long-term climatic cycles are due to changes in the seasonal and geographical distribution of the sun's radiation caused by gravitational effects of the planets on the eccentricity of Earth's orbit, its axial tilt and its wobble. Orbitally driven changes in solar radiation are amplified by natural changes in Earth's atmospheric greenhouse gas concentrations that result from interactions between the atmosphere, oceans and the productivity of the world's biota.

Centennial and Millennial Climate Change: Climate variability over shorter time periods, such as hundreds to thousands of years, is due to other factors. The most commonly cited causes are ocean circulation and hydrography; changes in glaciers, solar luminosity, short-term changes in atmospheric greenhouse gas concentrations and random variability in the climate system. One widely supported theory holds that millennial-scale climate changes are linked to the global circulation of the oceans, which acts like a great conveyor belt carrying heat from the ocean surface to great depths and, eventually, from the northern to southern hemisphere and around the world.

Decadal Climate Change: Variability in climate over the past few decades has also been observed, but its causes are not understood. Some weather changes over the past 50 years have been linked to middle and high latitude ocean-atmosphere changes, solar luminosity and other factors. Scientists have developed several climate indexes to measure decadal climate change in various regions, among them the North Atlantic Oscillation (NAO) and the Pacific North American (PNA) indexes.

Additional evidence for decadal climate change is provided by "dendroclimatology" - the study of climate change using tree rings; trapped air bubbles in ice cores; and tropical coral skeletons that, like tree rings, have distinct annual growth bands. Some decadal climate records provide evidence for 11-year climatic cycles of precipitation that seem to correspond with sunspot cycles. However, causes of decadal climate change in general, and the direct link between solar activity and climate in particular, are not understood.

Interannual Climate Changes: Year-to-year climate changes, known as interannual changes, are due to several factors. For example, occasional volcanic activity like the 1991 eruption of Mt. Pinatubo in the Philippines can cause two to three years of global cooling as tiny, sun-blocking particles are spewed high into Earth's atmosphere. But the major cause of Earth's interannual trends is the El Nino-Southern Oscillation (collectively called ENSO) phenomenon, a climate anomaly usually occurring every two to six years. An ENSO "event" originates in the tropical ocean-atmosphere system, usually in the fall. Then it "matures" during winter months, lasting a total of about 12-18 months.

The El Nino part of ENSO is an ocean phenomenon, originally named by a South American fishermen for an unusual wintertime warming of the eastern Pacific Ocean when the normal upwelling of cool, nutrient rich waters is suppressed by warm surface water. When ocean temperatures are relatively cool, the opposite of El Nino, referred to by scientists as La Nina, occurs.

The Southern Oscillation is the atmospheric part of ENSO. During El Nino ocean conditions, the Asian summer monsoon weakens and the usually heavy rainfall in the western Pacific diminishes while rainfall intensifies in the eastern Pacific; the opposite occurs during the cool La Nina phase. ENSO events can therefore be viewed as two extremes of seesawing tropical ocean-atmosphere conditions.

ENSO events can have major impacts on weather throughout the globe - known as "teleconnections." The 1982-83 El Nino, described as the greatest ocean-atmospheric disturbance in recorded history, caused an estimated $8 billion in worldwide damage stemming from floods and drought.

Weather patterns in the United States during the winter and spring after an El Nino or La Nina event can also be severely altered. For example, North American weather anomalies show that El Nino teleconnections bring increased precipitation to southern California and to the southeast during winter and early spring. La Nina events bring increased rainfall to coastal Washington and Oregon and winter/early spring rainfall to the Ohio and lower Mississippi River valleys. ENSO events can severely affect river discharge in western North America, oceanography off California, and tropical coral reefs in the Pacific. Although the impacts are still not fully understood, especially in temperate regions, abundant evidence exists that regional climate and hydrology can be impacted by ENSO.

The Bay and Climate Change

Where does the climate of the Chesapeake Bay watershed fit into this? Because weather influences river discharge, one way to address this question is to examine long-term monthly river discharge records to see if they change during years when strong ENSO events developed in the Pacific Ocean or NAO events in the Atlantic Ocean.

Discharge data from U.S. Geological Survey stream flow gauging stations for five rivers in the Bay watershed (Susquehanna at Harrisburg, Pa., Little Patuxent, Potomac near Washington, Rappahannock near Fredericksburg, Va., and James at Cartersville, Va.) provide a record of flow back to the early part of the century. The trends presented in Figure 1 show that there is extreme variability from year to year and over decades. It is also easy to identify the impact on discharge of extreme events like Hurricane Agnes in June 1972. The figure also shows a slight difference in flow among tributaries because of local factors.

Total mean annual stream flow to the Bay is available since 1950 and is shown in Figure 2. A statistical technique that averages every three points successively was used to smooth out the scatter in both trends. It is clear that discharge varies a great deal, in one year it can be almost three times as large as in the next.

High flow years like 1984 contrast strongly with low flow years like 1985; the year 1986 saw moderate discharge. These three years, 1984-1986, were used by the Chesapeake Bay Program to calibrate the Bay's Water Quality Model that predicts the response of the Bay to different levels of nutrient loading.

It is also clear that the 1950s and 1960s were decades of much reduced flow, a time approaching drought in some regions, compared with the 1970s, and the 1990s have had extremely high flows. A major shift toward increased discharge occurred in the early 1970s.

What caused these interannual and decadal trends in flow? What factors account for the differences between high flow and low flow years? While scientists cannot yet be certain, there are several possible large-scale climate patterns that could influence bay discharge. For example, El Nino events in the Pacific Ocean often correspond with high wintertime rainfall in large parts of the southeastern United States. It is possible that parts of the Bay watershed are influenced to moderate degrees by this rainfall anomaly, but it should be remembered that not all ENSO events have the same impact.

Furthermore, ENSO events can be complicated by other climate patterns like the NAO. The change to increased flow around 1970 is coincident with a shift toward positive values in the NAO climate index. Oscillations in large-scale climate patterns across the North Atlantic Ocean might be related to precipitation and temperature changes that bring about changes in discharge. Alternatively, a changing climate in the Pacific Ocean can also impact rainfall across North America and the mid-Atlantic region, and may also be a factor.

When comparing climate and discharge trends, it should be emphasized that climatic "noise" may obscure some of the cycles and trends and that rigorous statistical tests are required to determine the significance of seasonal and decadal discharge trends and climate patterns related to ENSO, NAO and PNA. Extreme events like Hurricane Agnes can offset the discharge curve and obscure more general trends. Moreover, important processes like evapotranspiration (the flow of water up through trees into the atmosphere) occur between the time of precipitation and discharge in to the Bay, influencing the magnitude of river runoff. There is nonetheless evidence for both year-to-year and decade-to-decade variability in tributary discharge, perhaps related to regional or global climate anomalies.

Planning for the Weather

Where does this leave us with regard to future climate and hydrological events? Climate modelers can sometimes successfully estimate weather patterns for some tropical regions related to El Nino and La Nina events several months to as much as a year in advance using "coupled" ocean-atmosphere climate models.

Future research might give scientists a better understanding of ENSO impacts on midlatitude regions such as the Eastern United States. With improved models and continued observations, predictive capabilities may improve to give at least reasonable probabilities for positive or negative seasonal rainfall anomalies expected during the ENSO events. Also, we may expect future advances in establishing trends and modeling of decadal climate change related to increasing greenhouse gas concentrations.

The Bay community has an opportunity to integrate our increasing understanding of global and regional climate change and its impact on the Mid-Atlantic region with advancing knowledge about other environmental stresses, into management strategies for the Chesapeake Bay ecosystem. Can we continue to "talk about the weather but do nothing about it?"

It is incumbent upon scientists to establish the relationship between the Bay system and climate change over short and long time scales by establishing natural levels of climate variability and their relationship to Bay water quality. Precipitation and discharge trends must be examined for possible relationships with trends in Bay habitats and living resources.

Although scientists will not make policy decisions, they can present information to resource managers, politicians, and the public at large about the potential impact of short-term and long-term climate change.

Thomas M. Cronin is a research scientist with the U.S. Geological Survey in Reston, Va., who specializes in the study of paleoclimatology.

Climate Indexes

Scientists have devised indexes of atmospheric conditions to track weather trends.



  • Pacific/North American Index: The Pacific-North American Index has been used to examine rainfall patterns across the United States as it relates to weather in the Pacific Ocean.The PNA reflects the combination of atmospheric pressure conditions at three places: Gulf of Alaska, western Canada and southeastern United States. High PNA values mean south-to-north atmospheric motion in the southeastern United States; low values mean west-to-east flow. A shift in the PNA Index occurred in the 1970s.



  • North Atlantic Oscillation: The North Atlantic Oscillation Index records atmospheric pressure changes in Iceland and the Azores Islands, which fluctuate every two to eight years and affect precipitation and temperatures over Europe and perhaps eastern North America. High NOA values correspond with cold winters in Northern Europe. A shift from negative to positive values in the NAO Index occurred about 1970 and the extreme positive phase of the last decade corresponds to warm winters in Europe and cold weather in the northwest Atlantic Ocean.



  • Southern Oscillation: The Southern Oscillation is the atmospheric portion of the El-Nino-Southern Oscillation - or ENSO - phenomenon. It measures the difference in atmospheric pressure between Tahiti and Darwin, Australia in the tropical Pacific Ocean. Other commonly used measures of ENSO involve sea-surface temperatures in the tropical Pacific Ocean.

Long-term Forecasts

An improved ability to predict El Nino, La Nina and other regional climate variables could bring a range of benefits. First, understanding such climate variables is essential to determining whether a long-term pattern of global warming is under way.

But in parts of the world, climate models are being used to predict weather patterns months to a year in advance. With adequate refinements, it is possible that some of these tools may eventually be available to managers here as well.

In Peru, farmers are using El Nino forecasts for potential flood or drought events to help determine planting schedules, and what crops will be grown. Other countries are beginning similar efforts.

The National Oceanic and Atmospheric Administration, which is heavily engaged in El Nino and climate prediction, anticipates that more accurate predictions for the United States and other nations may help not only agriculture, but also improve management for water supplies, fisheries - climate can make big differences in fish catches - and of other resources.