David S. Battisti

ENSO-like Interdecadal Variability: 1900–93

A number of recent studies have reported an ENSO-like EOF mode in the global sea surface temperature (SST) field, whose time variability is marked by an abrupt change toward a warmer tropical eastern Pacific and a colder extratropical central North Pacific in 1976‐77. The present study compares this pattern with the structure of the interannual variability associated with the ENSO cycle and documents its time history back to 1900. The analysis is primarily based on the leading EOFs of the SST anomaly and ‘‘anomaly deviation’’ fields in various domains and the associated expansion coefficient (or principal component) time series, which are used to construct global regression maps of SST, sea level pressure (SLP), and a number of related variables. The use of ‘‘anomaly deviations’’ (i.e., departures of local SST anomalies from the concurrent global-mean SST anomaly) reduces the influence of global-mean SST trends upon the structure of the EOFs and their expansion coefficient time series. An important auxiliary time series used in this study is a ‘‘Southern Oscillation index’’ based on marine surface observations. By means of several different analysis techniques, the time variability of the leading EOF of the global SST field is separated into two components: one identified with the ‘‘ENSO cycle-related’’ variability on the interannual timescale, and the other a linearly independent ‘‘residual’’ comprising all the interdecadal variability in the record. The two components exhibit rather similar spatial signatures in the global SST, SLP, and wind stress fields. The SST signature in the residual variability is less equatorially confined in the eastern Pacific and it is relatively more prominent over the extratropical North Pacific. The corresponding SLP signature is also stronger over the extratropical North Pacific, and its counterpart in the cold season 500-mb height field more closely resembles the PNA pattern. The amplitude time series of the ENSO-like pattern in the residual variability reflects the above-mentioned shift in 1976‐77, as well as a number of other prominent features, including a shift of opposite polarity during the 1940s.

Historical Warnings of Future Food Insecurity with Unprecedented Seasonal Heat

Higher growing season temperatures can have dramatic impacts on agricultural productivity, farm incomes, and food security. We used observational data and output from 23 global climate models to show a high probability (>90%) that growing season temperatures in the tropics and subtropics by the end of the 21st century will exceed the most extreme seasonal temperatures recorded from 1900 to 2006. In temperate regions, the hottest seasons on record will represent the future norm in many locations. We used historical examples to illustrate the magnitude of damage to food systems caused by extreme seasonal heat and show that these short-run events could become long-term trends without sufficient investments in adaptation.

Interannual variability in a tropical atmosphere−ocean model: influence of the basic state, ocean geometry and nonlinearity

Abstract The behavior of a tropical coupled atmosphere/ocean model is analyzed for a range of different background states and ocean geometries. The model is essentially that of Cane and Zebiak for the tropical Pacific, except only temporally constant background states are considered here. For realistic background states and ocean geometry, the model solutions feature oscillations of period of 3–5 yr. By comparing the full model solution with a linearized version of the model, it is shown that the basic mechanism of the oscillation is contained within linear theory. A simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere–ocean system. The analog model highlights the properties that produce coupled atmosphere–ocean instability in the eastern ocean basin, and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed, negative feedback into this instability growth. The growth rate of the local instabil...

The Basic Effects of Atmosphere-Ocean Thermal Coupling on Midlatitude Variability*

Abstract Starting from the assumption that the atmosphere is the primary source of variability internal to the midlatitude atmosphere–ocean system on intraseasonal to interannual timescales, the authors construct a simple stochastically forced, one-dimensional, linear, coupled energy balance model. The coupled system is then dissected into partially coupled and uncoupled systems in order to quantify the effects of coupling. The simplicity of the model allows for analytic evaluation of many quantities of interest, including power spectra, total variance, lag covariance between atmosphere and ocean, and surface flux spectra. The model predicts that coupling between the atmosphere and ocean in the midlatitudes will enhance the variance in both media and will decrease the energy flux between the atmosphere and the ocean. The model also demonstrates that specification of historical midlatitude sea surface temperature anomalies as a boundary condition for an atmospheric model will not generally lead to a correc...

Dynamics and Thermodynamics of a Warming Event in a Coupled Tropical Atmosphere–Ocean Model

Abstract A simple coupled ocean–atmosphere model, similar to that of Zebiak and Cane, is used to examine the dynamic and thermodynamic processes associated with El Nino/Southern Oscillation (ENSO). The model is run for 300 years. The interannual variability which results is regular, with a period of either 3 or 4 years, quantized by the annual cycle. The amplitude (∼1.5 m s−1 wind and 2°C SST anomalies), period and structure of the interannual variability compare well with observations. The model warm event is initiated in the spring prior to the event peak, and is well described as an instability of the coupled system. During instability growth, the sea surface temperature (SST) anomaly is primarily generated by vertical upwelling processes. The SST anomaly can be approximately described by the expression ∂T/∂t = KTh − α*T, where T is the SST anomaly, t time, h the upper layer thickness (pycnocline) perturbation and α* an effective damping time which includes heat loss to the atmosphere. KT parameterizes...

Radically Rethinking Agriculture for the 21st Century

Population growth, arable land and fresh water limits, and climate change have profound implications for the ability of agriculture to meet this century’s demands for food, feed, fiber, and fuel while reducing the environmental impact of their production. Success depends on the acceptance and use of contemporary molecular techniques, as well as the increasing development of farming systems that use saline water and integrate nutrient flows.

Interannual (ENSO) and Interdecadal (ENSO-like) Variability in the Southern Hemisphere Tropospheric Circulation*

Abstract Recent work has identified variability in the Pacific Ocean SST with a structure qualitatively similar to ENSO, but at lower frequencies than ENSO. Zhang et al. have documented the atmospheric circulation anomalies in the Tropics and Northern Hemisphere that are associated with decadal ENSO-like variability and compared these anomalies to those associated with the (interannual) ENSO cycle. Here the authors extend the study of Zhang et al. to the Southern Hemisphere using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data for 1958–96. Consistent with previous studies, the Southern Hemisphere circulation anomalies associated with ENSO display a teleconnection pattern from the central tropical Pacific into the far southeastern Pacific Ocean. Comparatively larger circulation anomalies are found in the Southern Hemisphere associated with the decadal ENSO-like variability, though aloft the structure of the anomalies emphasizes the polar vortex wit...

The Seasonal Footprinting Mechanism in the Pacific: Implications for ENSO*

Abstract Midlatitude atmospheric variability is identified as a particularly effective component of the stochastic forcing of ENSO. This forcing is realized via a seasonal footprinting mechanism (SFM), in which the tropical atmosphere is forced during the spring and summer by SST anomalies generated by midlatitude atmospheric variability during the previous winter. The strong relationship between the SFM and ENSO may serve to enhance ENSO predictability and supports the view that ENSO is linearly stable in nature.

Footprinting: A seasonal connection between the tropics and mid‐latitudes

A connection between the mid-latitude and tropical Pacific is identified in a coupled general circulation model (CGCM). The connection involves a seasonal coupling between winter mid-latitude atmospheric circulation anomalies, and summer equatorial wind stress anomalies. The seasonal coupling results from a “footprinting” mechanism, in which the summer tropical atmosphere responds to subtropical sea surface temperature (SST) anomalies that are generated by the mid-latitude atmospheric variability during the previous winter. Details of the connection, and of the footprinting mechanism are presented. Implications for interannual ENSO and decadal ENSO-like variability are discussed.

An interpretation of the results from atmospheric general circulation models forced by the time history of the observed sea surface temperature distribution

Recent studies using atmospheric general circulation models forced by the observed time history of global sea surface temperature anomalies have been used to hind-cast the temporal history of the North Atlantic Oscillation. They find that the mean of a large ensemble of integrations using slightly different initial atmospheric conditions reproduces the observed variability surprisingly well, especially on time scales longer than a few years. However, they also find that amplitude of the atmospheric variability is considerably reduced and the air-sea heat fluxes are of the reverse sign to those observed. Here, a linear model of midlatitude atmosphere/ocean interaction forced only by high-frequency atmospheric stochastic variability is shown to reproduce all of these findings. This model suggests that despite the hind-cast skill, the useful predictability associated with midlatitude SST anomalies may be limited to one or two seasons.