Hydrological Cycle

Understanding the processes that control changes in the hydrological cycle is one of the most pressing issues in climate-change research because changes in precipitation and evaporation impact the salinity/density distribution in the surface ocean, ecosystems on land, floods, aridification, water resources, and climate-vegetation feedbacks.

Although discussed in the context of extreme events (see section 2.1.4), a comprehensive examination of the climatic controls on the hydrological cycle requires the study of long-term trends and variability over a range of climatic states. In fact, even small global temperature changes impact the energy balance in tropical regions, resulting in changes in evaporation and storm and hurricane activity. In the mid-latitudes, changes in the atmospheric thermal gradients can impact atmospheric circulation, the location of storm tracks, and the intensity of storms. For example, recent global trends in surface humidity (Willett et al., 2007), precipitation rates including an intensified hydrological cycle with more rainfall in tropical and temperate regions and lower rainfall in the subtropics (Zhang et al., 2007), and changes in large-scale atmospheric circulation, specifically the widening of Hadley circulation (Johanson and Fu, 2009), have been attributed to anthropogenic influences. In addition, trends in extreme events (storms and hurricanes) have also been observed (Trenberth and Fasullo, 2008) and modeled for future global warming scenarios (Emanuel et al., 2008).

The current generation of climate models needs to be improved to more accurately predict changes in the hydrological cycle. For example, both the prediction of changes in horizontal temperature gradients (Gastineau et al., 2009; Karnauskas et al., 2009) and the treatment of tropical atmospheric convection (O‘Gorman and Schneider, 2009) have a large impact on the ability of models to accurately predict changes in the

hydrological cycle and regional precipitation patterns, and need improvement. As such, records of past climate and oceanic changes provide the necessary data to validate climate-change models and to provide the needed context of natural variability, particularly for changes that occur on decadal or longer timescales. For example, in a study of megadrought in North America, Cook et al. (2010) make a strong argument for the use of paleoclimate data to understand the global context of North American megadroughts in order to better predict future changes. This is true at all timescales. In fact, paleoceanographic and paleoclimatic studies using older drilled ocean sediments, from times when Earth‘s climate was different than today, provide the only opportunity we have to study the processes that control precipitation across a large dynamic range of conditions.

There was not an INVEST working group that focused explicitly on the hydrological cycle; however, because of the strong impact of precipitation on the environment and its habitability (e.g., water resources, floods, etc.), and thus the urgent need to develop a deeper understanding of the factors that control regional precipitation, many of the climate-related INVEST working groups emphasized the need to study the

“Testing models with

hydrological cycle. Specifically, discussion in many working groups emphasized the need to understand controls on the Intertropical Convergence Zone (ITCZ), to study climate oscillation modes and their behavior during different mean climate states, to focus on wind-driven upper ocean circulation and its coupling to atmospheric forcing, to understand the relationship between precipitation patterns, density stratification, and biogeochemical processes, to examine past changes in monsoons, among other topics.

Pressing questions that can only be answered by studying geological archives of past climate change are focused on three large-scale climatic features that influence continental patterns of precipitation.

2.3.1 The Intertropical Convergence Zone

What controls the position of the ITCZ? The ITCZ, the tropical zone of deep atmospheric convection and rainfall, is highly variable in character and seasonal behavior in each ocean basin. While the seasonal migration of the ITCZ is dictated to a large extent by seasonal change in the temperature gradient between low and high latitudes, its location, variability, and intensity is also strongly controlled by other processes whose importance varies depending on ocean dynamics and land-sea distribution. In the Indian Ocean, land-sea interactions have a dominant control on monsoons, which strongly influence the seasonal position of the ITCZ. In the Atlantic Ocean, the position of the ITCZ is influenced by both oceanic processes and land-sea temperature contrasts, whereas in the Pacific Ocean, the position of the ITCZ is dominantly controlled by global and regional tropical sea-surface temperature spatial patterns. Contrasting the behavior of the ITCZ in different ocean basins during periods with different global boundary conditions can shed light on the importance of global versus regional processes on the tropical hydrological cycle. In addition, the detailed examination of how the position of the ITCZ in one region varies through time as solar heating, greenhouse gas forcing, and global climate changed in the past can provide time series of hydrological changes needed in order to apply statistical tools to deconvolve multiple mechanistic sources of variability.

Studies of the response of the ITCZ to climate conditions during periods of warmth relative to today are particularly critical for making predictions of changes in precipitation patterns when the hydrological cycle is enhanced during globally warm periods. The strategies for studies of the ITCZ involve drilling north-south transects of sites to trace north-south shifts in the ITCZ; however, to gain insight into the mechanisms of changes in the ITCZ, drilling must also focus on key locations (which may be remote) where changes in temperature could have a profound influence on the position and intensity of the ITCZ. To devise such a drilling strategy would involve a detailed planning group with deep knowledge of tropical climate models and theory who could help define locations and predict the amplitude of change that would be diagnostic of each mechanism of change. The strategic planning would also involve geologists and geophysicists with expertise in tropical sediment distributions, possible drilling targets, and the newest methods and techniques for determining past changes in tropical precipitation and temperature patterns of the sea surface and on land.

2.3.2 Monsoons

What controls the strength the monsoons, and therefore seasonal rainfall in low-latitude continental regions? Monsoon strength is strongly dependent on subtle differences in ocean and continental temperatures and/or the land-sea temperature contrasts. As there is a plethora of climatic and oceanic processes that can cause changes in land and sea temperatures, an examination of the major controlling factors of monsoon strength requires a comprehensive approach that would start with reconstructions of rainfall strength and sea-surface temperatures, but would also need to include strategies for using changes in spatial patterns of multiple oceanic variables to identify underlying ocean dynamical causes. In addition, there are clearly orographic influences on the monsoon, such that the evolution of continental mountains must play a role in the large-scale development and establishment of the modern-day monsoon system.

As with studies of tropical rainfall changes associated with variations in the ITCZ, investigating the causes of changes in the monsoon requires an examination of past time periods with climate forcings different than today. To make major advances, sediment records from continental margins where sediments of continental origin contain needed information regarding changes in precipitation, run-off, and continental climate need to be collected. Strategies for studying the evolution of Asian, Indian, and African monsoons should be designed by multi-disciplinary detailed planning groups with knowledge of monsoon climate dynamics, continental mountain tectonics, river basin and geomorphological evolution, continental margin, slope, and river delta sedimentological processes, and technological and analytical advances in reconstructing continental climate, sea-surface salinity and precipitation-evaporation balances.

2.3.3 Mid-latitude storm tracks

What controls the strength and position of mid-latitude storm tracks? Climate models predict that with warmer climate due to greenhouse gas forcing, extratropical rainfall including extreme events should increase and storm tracks should move poleward as the Hadley Cell expands. In addition, this enhancement in the hydrological cycle includes intensification and poleward expansion of subtropical arid desert zones.

Changes in large-scale extratropical precipitation–evaporation patterns are linked to warmer sea-surface temperatures and concomitant increases in water-vapor content of the atmosphere, but they are also strongly controlled by changes in the low–high latitude temperature gradient. As such, the regional processes that determine the sensitivity of temperature changes to radiative forcing play a big role in controlling the intensity and position of mid-latitude storm tracks.

Past changes in the spatial extent of climate zones can be detected using geologic proxies; thus, the geological perspective that is provided by ocean drilling is one of the most powerful ways to test model predictions of the mechanisms that control Hadley Cell dimensions and associated precipitation patterns. Regional geological studies can be further applied to studying the interdependence between regional climate and large-scale global precipitation patterns. As with geological studies of the tropical hydrological cycle (ITCZ and monsoon), studies of extra-tropical climate change need to be approached with a drilling strategy that integrates climate system model predictions with knowledge of the best localities on continental margins to find continuous sections with

well-preserved fossils of continental and oceanic origin. The biggest challenge will be to find locations and analytical strategies to extract information about seasonal or extreme precipitation events; however, even information on how climate zone boundaries evolve over longer timescales will provide valuable insight into the behavior of, and fidelity of models of, the hydrological cycle.