CHAPTER 8 BATTISTI ET AL. 8.1 Chapter 8 100 Years of Progress in Understanding the Dynamics of Coupled Atmosphere–Ocean Variability DAVID S. BATTISTI Department of Atmospheric Sciences, University of Washington, Seattle, Washington DANIEL J. VIMONT Atmospheric and Oceanic Science, University of Wisconsin–Madison, Madison, Wisconsin BENJAMIN P. KIRTMAN Department of Atmospheric Science, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida ABSTRACT In situ observation networks and reanalyses products of the state of the atmosphere and upper ocean show well-defined, large-scale patterns of coupled climate variability on time scales ranging from seasons to several decades. We summarize these phenomena and their physics, which have been revealed by analysis of ob- servations, by experimentation with uncoupled and coupled atmosphere and ocean models with a hierarchy of complexity, and by theoretical developments. We start with a discussion of the seasonal cycle in the equatorial tropical Pacific and Atlantic Oceans, which are clearly affected by coupling between the atmosphere and the ocean. We then discuss the tropical phenomena that only exist because of the coupling between the atmo- sphere and the ocean: the Pacific and Atlantic meridional modes, the El Niño–Southern Oscillation (ENSO) in the Pacific, and a phenomenon analogous to ENSO in the Atlantic. For ENSO, we further discuss the sources of irregularity and asymmetry between warm and cold phases of ENSO, and the response of ENSO to forcing. Fundamental to variability on all time scales in the midlatitudes of the Northern Hemisphere are preferred patterns of uncoupled atmospheric variability that exist independent of any changes in the state of the ocean, land, or distribution of sea ice. These patterns include the North Atlantic Oscillation (NAO), the North Pacific Oscillation (NPO), and the Pacific–North American (PNA) pattern; they are most active in wintertime, with a temporal spectrum that is nearly white. Stochastic variability in the NPO, PNA, and NAO force the ocean on days to interannual times scales by way of turbulent heat exchange and Ekman transport, and on decadal and longer time scales by way of wind stress forcing. The PNA is partially responsible for the Pacific decadal oscillation; the NAO is responsible for an analogous phenomenon in the North Atlantic subpolar gyre. In models, stochastic forcing by the NAO also gives rise to variability in the strength of the Atlantic meridional overturning circulation (AMOC) that is partially responsible for multidecadal anomalies in the North Atlantic climate known as the Atlantic multidecadal oscillation (AMO); observations do not yet exist to adequately determine the physics of the AMO. We review the progress that has been made in the past 50 years in understanding each of these phenomena and the implications for short-term (seasonal-to-in- terannual) climate forecasts. We end with a brief discussion of advances of things that are on the horizon, under the rug, and over the rainbow. Corresponding author: David S. Battisti, [email protected] DOI: 10.1175/AMSMONOGRAPHS-D-18-0025.1 Ó 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 8.2 METEOROLOGICAL MONOGRAPHS VOLUME 59 1. Introduction significantly enhanced the observing system in the tropical Pacific [for an overview, see McPhaden et al. There was little discussion of coupled atmosphere– (1998)] and ushered in intermediate complexity models ocean variability in the first two-thirds of the twentieth of the coupled atmosphere–ocean system that led to an century. In an early study, Sir Gilbert Walker analyzed understanding of the essential aspects of the canonical station data around the world and coined the term ENSO cycle, including the spatial structure and ampli- ‘‘Southern Oscillation’’ for a large-scale coherent oscil- tude of the warm and cold (El Niño and La Niña) events, lation in sea level pressure and in precipitation in the the period between warm events, and the seasonality in Maritime Continent (Walker 1924).1 Decades later, the variance of ENSO. In turn, analyses of these models Berlage (1966) linked the Southern Oscillation to epi- led to the view that ENSO is an intrinsic mode of the sodic, localized warming off Peru and Ecuador: this dynamically coupled atmosphere–ocean system in the warming was the El Niño phenomenon documented tropical Pacific, in which tropical ocean dynamics are nearly 75 years earlier (Carranza 1892; Carrillo 1893; essential for the evolution of SST through a rich mix- Pezet 1895, 1896). Bjerknes (1969) showed that changes ture of ocean dynamics and surface fluxes. Following in the trade winds along the equator in the Pacific were on the TOGA program and the World Ocean Climate associated with the Southern Oscillation through the Experiment (WOCE), the World Climate Research Walker circulation. Bjerknes presented evidence that Programme outlined a science plan to advance under- changes in the strength of the trade winds were in- standing and predictability of climate variability arising timately related to the large-scale east–west sea surface from coupled ocean–atmosphere interactions (U.S. temperature (SST)2 gradient across the Pacific; today CLIVAR Scientific Steering Committee 2013). The Cli- this tight relationship is recognized as the Bjerknes mate Variability and Predictability Program (CLIVAR) feedback and is one of two central ingredients to the program (both International CLIVAR in 1995 and U.S. large-scale El Niño–Southern Oscillation (ENSO) phe- CLIVAR in 1997) continued to advance understanding nomenon (the other is the ocean adiabatic adjustment to of coupled ocean–atmosphere variations with a more the changes in wind stress; see section 4). While earlier global focus, including coupled ocean–atmosphere phe- examples of this necessary two-way coupling may exist, nomena in the tropical Atlantic and Indian Oceans and in the description by Bjerknes (1969) marks a beginning to the mid- and high latitudes. the now widespread recognition of coupled ocean– Today, a quarter century after TOGA, further obser- atmosphere phenomena, as defined herein. vations and modeling studies have confirmed this view of In the 1960s and 1970s, observational studies of the ENSO (but important questions remain; see section 8). In tropical Pacific atmosphere and ocean gave rise to a addition, other modes of climate variability have been ‘‘canonical’’ view of ENSO as a pan-Pacific phenome- identified in observations and simulated by climate non, with warm El Niño events lasting 12–18 months or models that likely exist only because of a coupling be- so, usually followed by cold La Niña events of lesser tween the atmosphere and ocean. For example, the sea- amplitude and lasting a few years (Rasmusson and sonal cycle along the equator in the tropical Atlantic and Carpenter 1982). El Niño events occurred every 4–7 Pacific is now understood as being born from a coupling years and tended to peak at the end of the calendar year. of atmosphere and ocean in response to the seasonal Also in the 1960s and 1970s, theoretical advances led to cycle in insolation (section 2). The meridional modes an understanding of the response of the tropical oceans describe patterns of intraseasonal to interannual vari- to wind stress forcing, and to the response of the atmo- ability in the tropical Pacific and Atlantic basins that are sphere to changes in SST (see section 4b). intrinsically forced by (stochastic) atmosphere variability, The 1982/83 El Niño event (the warm phase of ENSO) but with important feedbacks between the atmosphere was remarkable for its amplitude and duration. It in- and ocean in the subtropical Northern Hemisphere spired meteorologists and oceanographers to come to- (section 3). And there is growing evidence that coupling gether and plan the 10-yr program Tropical Oceans on between the atmosphere and oceans in the Northern the Global Atmosphere (TOGA) to study the impact of Hemisphere extratropics contributes to the climate vari- the oceans on the atmosphere over 1985–94. TOGA ability on decadal to multidecadal time scales. This review paper contains a brief summary of the observational and modeling evidence for climate vari- 1 The interested reader can find more information on the pre- ability that is intrinsic to the coupling of the atmosphere 1960 observational studies that lead to an understanding of the Southern Oscillation in Wallace et al. (1998), Clarke (2008), and and oceans in the tropics. The review is organized by other readily available resources. phenomenon (e.g., the meridional modes, ENSO), and 2 A glossary of acronyms is presented in the appendix. begins with the seasonal cycle in section 2 (because it is, CHAPTER 8 BATTISTI ET AL. 8.3 in itself, a coupled atmosphere–ocean phenomenon and the climate of the global tropics and the Western it is the background state that gives rise to ENSO) and Hemisphere through teleconnections. For brevity, we the meridional modes in section 3.Insection 4a we do not focus on these teleconnections. We discuss the present the observations of ENSO, followed by a brief interaction between the coupled phenomenon only introduction to the essential uncoupled atmosphere and when the connections are essential (e.g., the impact of ocean dynamics that are relevant to ENSO in section 4b. the Pacific meridional mode on ENSO). Phenomena We present the theory and further observations in sup- that are intrinsic to the atmosphere but modified by port of ENSO as a true coupled atmosphere–ocean their interaction with the ocean, or vice versa, are not mode in sections 4c and 4d, respectively, and discuss covered in this review: review papers on the impact of the reasons for asymmetry between the warm and cold the ocean on the atmospheric convection and on the phases of ENSO in section 4e. A discussion of the Madden–Julian phenomenon—both phenomena in- sources of irregularity of ENSO is presented in section trinsic to the atmospheric—can be found in Hirons 4f, and a discussion of the response of ENSO to external et al.