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Comprehensive and up-to-date information on Earth’s most dominant year-to-year climate variation The El Niño Southern Oscillation (ENSO) in the Pacific Ocean has major worldwide social and economic consequences through its global scale effects on atmospheric and oceanic circulation, marine and terrestrial ecosystems, and other natural systems. Ongoing climate change is projected to significantly alter ENSO's dynamics and impacts. El Niño Southern Oscillation in a Changing Climate presents the latest theories, models, and observations, and explores the challenges of forecasting ENSO as the climate continues to change. Volume highlights include: Historical background on ENSO and its societal consequences Review of key El Niño (ENSO warm phase) and La Niña (ENSO cold phase) characteristics Mathematical description of the underlying physical processes that generate ENSO variations Conceptual framework for understanding ENSO changes on decadal and longer time scales, including the response to greenhouse gas forcing ENSO impacts on extreme ocean, weather, and climate events, including tropical cyclones, and how ENSO affects fisheries and the global carbon cycle Advances in modeling, paleo-reconstructions, and operational climate forecasting Future projections of ENSO and its impacts Factors influencing ENSO events, such as inter-basin climate interactions and volcanic eruptions The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals. Find out more about this book from this Q&A with the editors.
Despite continual improvements in both model physics and resolution, accurate simulation of the tropical Pacific mean state has been an elusive goal for many coupled global climate models (GCMs) for decades. Because temperature and pressure gradients are weak in the deep tropics, small errors in simulating these fields can lead to large circulation biases. Any initial bias may be further amplified through feedbacks involving ocean circulations, cloud radiative forcings, and surface turbulent heat or momentum fluxes. The interrelated nature of these processes in the deep tropics has long complicated our ability to understand and correct the initial source of a given model bias. In this dissertation, we examine two persistent tropical biases within the National Center for Atmospheric Research's Community Earth System Model (CESM): the Pacific cold tongue bias and the double-Intertropical Convergence Zone (ITCZ) bias. The Pacific cold tongue bias refers to the tendency for coupled GCMs to simulate sea surface temperatures (SSTs) which are too cold along the equator in the tropical Pacific. The double-ITCZ bias refers the tendency for coupled GCMs to simulate too much precipitation in a zonal band south of the equator. In Chapter 2, a series of six-month coupled hindcasts show the strength of the rapidly developing Pacific cold tongue bias in CESM version 1 (CESM1) to be sensitive to the convective parameterization employed. In the standard configuration of the model, too strong equatorial surface easterlies drive cooling of up to 1~K in the first two months of coupled integration. In a simulation wherein the deep convective parameterization is disabled, the cold tongue bias intensifies due to an increase in the zonal pressure gradient and associated easterlies. Superparameterized hindcasts show improvements in the cold tongue bias and reduced surface easterlies despite an increase in the zonal pressure gradient. The superparameterized model neglects convective momentum fluxes as the embedded two dimensional cloud resolving models are unable to accurately simulate turbulent momentum flux tendencies. Thus, rather than increasing near-surface wind speeds, the increased zonal pressure gradient drives anomalously strong easterly jet at 1-1.5~km elevation as surface drag effects are incorrectly concentrated in the lowest model levels. A series of sensitivity tests confirm the role of shallow convective momentum transport in determining the low-level zonal wind shear. The simulations presented in this chapter suggest shallow convective momentum fluxes may be an underappreciated mechanism for controlling both the equatorial cold tongue strength and the relationship between the large scale surface pressure gradient and surface easterlies. Despite differences in central Pacific SST of nearly 2~K across these hindcasts, the double-ITCZ bias persists in all model configurations. While the double-ITCZ bias is robust across all simulations presented in Chapter 2, the simulations presented in Chapter 3 show the east Pacific manifestation of the double-ITCZ bias to be greatly improved in the newest version of CESM: CESM version~2 (CESM2). In Chapter 3, we examine the state of the double-ITCZ bias across ten versions of CESM created as part of the development process for CESM2. In CESM1, a warm SSTs bias in the southeast Pacific forces zonal and meridional surface pressure gradients that are favorable for increased convergence and convection in this region. This SST bias is reduced in CESM2 due to an increase in overlying low cloud fraction and a corresponding strengthening of the shortwave cloud forcing (SWCF). Between two model versions with similar configurations but differing ITCZ bias strengths, this cloud change is driven by the removal of the dependence of liquid autoconversion and accretion rates on cloud water variance and by the removal of a secondary condensation scheme. These changes reduce the drizzle production rate in the low liquid clouds of the southeast Pacific which in turn delays their breakup and dissipation. As a result, cloud fraction and SWCF increase to more realistic values in the stratocumulus to trade cumulus transition region. The improvements in SWCF and the double-ITCZ bias persist through subsequent modifications to the liquid microphysics parameterizations. Despite the local improvement in the east Pacific rainfall climatology, neither the Pacific cold tongue bias nor global measures of the double-ITCZ bias show a consistent improvement across the model development process from CESM1 to CESM2.
This book discusses the sources of uncertainty in future model projections of the tropical Pacific SST warming pattern under global warming. It mainly focuses on cloud radiation feedback and ocean dynamical effect, which reveal to be the two greatest sources of uncertainty in the tropical Pacific SST warming pattern. Moreover, the book presents a correction for model projections of the tropical Pacific SST warming pattern based on the concept of “observational constraints”; the corrected projection exhibits a more El Niño-like warming pattern.
Numerical model and assimilation experiments were conducted in the tropical Pacific Ocean to obtain a better understanding of the processes that control the cold tongue surface mixed layer temperature balance during August 1999 to July 2004. The numerical model was first applied to test two hypotheses (asymmetric background currents and asymmetric wind forcing) for the observed asymmetry of annual equatorial Rossby waves. The model with asymmetric background currents perturbed with symmetric annually-varying winds consistently produced asymmetric Rossby waves, and simulations with symmetric background currents perturbed by asymmetric annually-varying winds failed to produce the observed Rossby wave structure unless the perturbation winds were strong enough for nonlinear interactions to become important. The observed latitudinal asymmetry of the westward phase speed was found to be critically dependent on the inclusion of realistic coastline boundaries. To measure the cold tongue sensitivity to errors in wind forcing, the next study compared the seasonal cycle response of the model driven by different wind stress products. The FSU wind stress produced the least realistic cold tongue, and both the ECMWF and QuikSCAT wind stress driven model runs exhibited cold tongue annual cycles, tropical instability waves, and annual equatorial Rossby waves that compared well with observations. The highest realism, however, was obtained with QuikSCAT wind forcing. In the final modeling study, mean dynamic height biases resulting from climatological drift away from the Levitus initialization were discovered in the waveguide. The assimilation experiments combined the model driven by 5-day QuikSCAT winds with 5-day Tropical Atmosphere Ocean dynamic height anomalies via a reduced state space Kalman filter. Assimilation improved the interannual and intraseasonal variability of sea surface height, reduced the cold tongue bias in the waveguide, increased the core strength of the Equatorial Undercurrent, and produced more realistic albeit weak tropical instability waves. An autoregressive model added to the innovation sequence further optimized the assimilation scheme, but did not correct the pre-existing cold tongue thermal biases. Despite the decrease in positive (warming) high-frequency horizontal advection associated with TIWs, the assimilation run with the autoregressive model did not alter the mean balance significantly as there was a compensatory decrease in magnitude of the cooling by the low-frequency horizontal advection. Based on comparisons with observations, the annual cycle of the model tendency was too weak in the eastern Pacific giving rise to sea surface temperatures that were too cold in the spring and summer months and during the 2002-2003 El Nino event. Errors in the simulated net surface heat flux, vertical entrainment, and diffusion were identified as sources for the unrealistically low annual amplitudes of sea surface temperature and tendency in the model cold tongue.
El Nino and the Southern Oscillation is by far the most striking phenomenon caused by the interplay of ocean and atmosphere. It can be explained neither in strictly oceanographic nor strictly meteorological terms. This volume provides a brief history of the subject, summarizes the oceanographic and meteorological observations and theories, and discusses the recent advances in computer modeling studies of the phenomenon. - Includes a comprehensive and up-to-date research survey - Discusses in detail sophisticated computer models - Provides a clear exposition of the major problems which prevent more accurate predictions of El Nino
As the nation's economic activities, security concerns, and stewardship of natural resources become increasingly complex and globally interrelated, they become ever more sensitive to adverse impacts from weather, climate, and other natural phenomena. For several decades, forecasts with lead times of a few days for weather and other environmental phenomena have yielded valuable information to improve decision-making across all sectors of society. Developing the capability to forecast environmental conditions and disruptive events several weeks and months in advance could dramatically increase the value and benefit of environmental predictions, saving lives, protecting property, increasing economic vitality, protecting the environment, and informing policy choices. Over the past decade, the ability to forecast weather and climate conditions on subseasonal to seasonal (S2S) timescales, i.e., two to fifty-two weeks in advance, has improved substantially. Although significant progress has been made, much work remains to make S2S predictions skillful enough, as well as optimally tailored and communicated, to enable widespread use. Next Generation Earth System Predictions presents a ten-year U.S. research agenda that increases the nation's S2S research and modeling capability, advances S2S forecasting, and aids in decision making at medium and extended lead times.
Methods are assembled for calculating the components of ocean heating/cooling from above, viz. short-wave radiation, albedo, long-wave radiation, and evaporative and sensible heat exchange with the atmosphere.
Society today may be more vulnerable to global-scale, long-term, climate change than ever before. Even without any human influence, past records show that climate can be expected to continue to undergo considerable change over decades to centuries. Measures for adaption and mitigation will call for policy decisions based on a sound scientific foundation. Better understanding and prediction of climate variations can be achieved most efficiently through a nationally recognized "dec-cen" science plan. This book articulates the scientific issues that must be addressed to advance us efficiently toward that understanding and outlines the data collection and modeling needed.
Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 147. It is more than 30 years since the publication of Jacob Bjerknes' groundbreaking ideas made clear the importance of ocean-atmosphere interaction in the tropics. It is now more than 20 years since the arrival of a massive El Niño in the fall of 1982 set off a cascade of observational and theoretical studies. During the following decades, the climate research community has made exceptional progress in refining our capacity to observe earth's climate and theorize about it, including new satellite-based and in situ monitoring systems and coupled ocean-atmosphere predictive numerical models. Of equal importance. is the expanding scope ofresearch, which now reaches far beyond the Pacific El Niño and includes climate phenomena in other ocean basins. In order to cover the now global context of ocean-atmosphere interaction we have organized this monograph around five principal themes, each introduced by one or more broad overview papers. Theme I covers interaction and climate variability in the Pacific sector, with extensive discussion of El Niño-Southern Oscillation, and with the possible causes and consequences of variability on both shorter and longer timescales. Theme II is devoted to interaction in the Atlantic sector. This basin exhibits complex behavior, reflecting its geographic location between two major zones of convection as well as neighboring the tropical Pacific. Theme III reviews the recent, exciting progress in our understanding of climate variability in the Indian sector. Theme IV addresses the interaction between the tropics and the extratropics, which are linked through the presence of shallow meridional overturning cells in the ocean. Finally, Theme V discusses overarching issues of cross-basin interaction.