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Richard Seager

Abstract

A simple model of the low-level wind field in the entire tropics is presented. The dynamics are the same as those within the familiar Gill model, i.e., linear, steady state, contained within a single vertical mode and damped by Rayleigh friction. Convective atmospheric heating can occur if a lifted air parcel is buoyant relative to its surroundings, and the heating is computed with reference to the cloud model of Yanai et al. Radiative cooling is represented by a Newtonian cooling to an equilibrium lapse rate. The model is forced by surface temperature and humidity. A qualitatively correct representation of the climatological flow is achieved. The main differences between model and observations relate to the model's inability to reproduce the intensity and limited spatial scale of the convergence zones. Model simulations of anomalous circulations are subject to the same limitations. Problems related to the lack of an explicit boundary layer in the model, the poor representation of radiation, and the cumulus parameterization are discussed, together with suggestions for future work.

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Richard Seager

Abstract

An attempt is made to model the sea surface temperature (SST) of the tropical Pacific Ocean between January 1970 and August 1987. The SST is computed using the model and heat flux parameterization discribed in the climatological study of Seager et al. The model is forced with the observed winds as given by the Florida State University analysis.

The results indicate that while short period variability in the model and observations is largely uncorrelated, variability with periods greater than about one year is well represented in the model. Each El Niño that occurred in this time period is captured by the model. The main discrepancies in the evolution of the 1982/83 and 1986/87 model El Niño events are the inability of the model equatorial SST anomaly to cool in early 1983 and the disappearance of the 1986/87 anomaly during 1987.

The results suggest a number of conclusions. In the east Pacific equatorial Kelvin waves, excited by variations in the trade wind strength in the central and west Pacific, increase the SST via depression of the thermocline. In this region the surface heat flux acts as a negative feedback on the SST anomaly. However, in the central Pacific the surface heat flux anomalies are reinforcing heating through suppression of latent heat loss as a result or weakened trades. Zonal advection of warm water from the west, associated with Rossby waves excited by trade wind relaxation, contributes to warming in both the central and west Pacific. Anomalous cooling by entrainment is a negative feedback on the SST anomaly in the central Pacific.

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Richard Seager

Abstract

The causes and global context of the North American drought between 1998 and 2004 are examined using atmospheric reanalyses and ensembles of atmosphere model simulations variously forced by global SSTs or tropical Pacific SSTs alone. The drought divides into two distinct time intervals. Between 1998 and 2002 it coincided with a persistent La Niña–like state in the tropical Pacific, a cool tropical troposphere, poleward-shifted jet streams, and, in the zonal mean, eddy-driven descent in midlatitudes. During the winters reduced precipitation over North America in the climate models was sustained by anomalous subsidence and reductions of moisture convergence by the stationary flow and transient eddies. During the summers reductions of evaporation and mean flow moisture convergence drove the precipitation reduction, while transient eddies acted diffusively to oppose this. During these years the North American drought fitted into a global pattern of circulation and hydroclimate anomalies with noticeable zonal and hemispheric symmetry.

During the later period of the drought, from 2002 to 2004, weak El Niño conditions prevailed and, while the global climate adjusted accordingly, western North America remained, uniquely among midlatitude regions, in drought. The ensemble mean of the climate model simulations did not simulate the continuation of the drought in these years, suggesting that the termination of the drought was largely unpredictable in terms of global ocean conditions.

The global context of the most recent, turn of the century, drought is compared to the five prior persistent North American droughts in the instrumental record from the mid-nineteenth century on. A classic La Niña pattern of ocean temperature in the Pacific is common to all. A cold Indian Ocean, also typical of La Niña, is common to all five prior droughts, but not the most recent one. Except in southern South America the global pattern of precipitation anomalies of the turn of the century drought is similar to that during the five prior droughts. These comparisons suggest that the earlier period of this most recent drought is the latest in a series of multiyear droughts forced by persistent changes in tropical Pacific Ocean temperatures. Warm tropical North Atlantic Ocean temperatures may play a secondary role.

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Richard Seager and Naomi Naik

Abstract

Both naturally occurring La Niña events and model-projected anthropogenic-driven global warming are associated with widespread drying in the subtropics to midlatitudes. Models suggest anthropogenic drying should already be underway but climate variability on interannual to multidecadal time scales can easily obscure any emerging trend, making it hard to assess the validity of the simulated forced change. Here, the authors address this problem by using model simulations and the twentieth-century reanalysis to distinguish between natural variability of, and radiatively forced change in, hydroclimate on the basis of the mechanisms of variations in the three-dimensional moisture budget that drive variations in precipitation minus evaporation (PE). Natural variability of PE is dominated by the El Niño–Southern Oscillation (ENSO) cycle and is “dynamics dominated” in that the associated global PE anomalies are primarily driven by changes in circulation. This is quite well reproduced in the multimodel mean of 15 models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/Coupled Model Intercomparison Project 3 (CMIP3). In contrast, radiatively forced PE change is “thermodynamics mediated” in that the rise in specific humidity leads to intensified patterns of moisture transport and PE. But, as for ENSO, the poleward shift of the storm tracks and mean meridional circulation cells also contribute to changes in PE. However, La Niña and radiatively forced changes in the zonal mean flow are distinct in the tropics. These distinctions are applied to the post-1979 record of PE in the twentieth-century reanalysis. ENSO-related variations strongly influence the observed PE trend since 1979, but removal of this influence leaves an emerging pattern of PE change consistent with the predictions of the IPCC AR4/CMIP3 models over this period together with, to some extent, consistent contributions from dynamical and thermodynamical mechanisms and consistent changes in the zonal mean circulation. The forced trends are currently weak compared to those caused by internal variability.

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Richard Seager and Naomi Henderson

Abstract

The diagnostic evaluation of moisture budgets in archived atmosphere model data is examined. Sources of error in diagnostic computation can arise from the use of numerical methods different from those used in the atmosphere model, the time and vertical resolution of the archived data, and data availability. These sources of error are assessed using the climatological moisture balance in the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) that archives vertically integrated moisture fluxes and convergence. The largest single source of error arises from the diagnostic evaluation of divergence. The chosen second-order accurate centered finite difference scheme applied to the actual vertically integrated moisture fluxes leads to significant differences from the ERA-Interim reported moisture convergence. Using daily data, instead of 6-hourly data, leads to an underestimation of the patterns of moisture divergence and convergence by midlatitude transient eddies. A larger and more widespread error occurs when the vertical resolution of the model data is reduced to the 8 levels that is quite common for daily data archived for the Coupled Model Intercomparison Project (CMIP). Dividing moisture divergence into components due to the divergent flow and advection requires bringing the divergence operator inside the vertical integral, which introduces a surface term for which a means of accurate evaluation is developed. The analysis of errors is extended to the case of the spring 1993 Mississippi valley floods, the causes of which are discussed. For future archiving of data (e.g., by CMIP), it is recommended that monthly means of time-step-resolution flow–humidity covariances be archived at high vertical resolution.

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Richard Seager and Ragu Murtugudde

Abstract

The role of tropical Pacific ocean dynamics in regulating the ocean response to thermodynamic forcing is investigated using an ocean general circulation model (GCM) coupled to a model of the atmospheric mixed layer. It is found that the basin mean sea surface temperature (SST) change is less in the presence of varying ocean heat transport than would be the case if the forcing was everywhere balanced by an equivalent change in the surface heat flux. This occurs because the thermal forcing in the eastern equatorial Pacific is partially compensated by an increase in heat flux divergence associated with the equatorial upwelling. This constitutes a validation of a previously identified “ocean dynamical thermostat.”

A simple two-box model of subtropical–equatorial interaction shows that the SST regulation mechanism crucially depends on spatial variation in the sensitivity of the surface fluxes to SST perturbations. In the GCM, this sensitivity increases with latitude, largely a result of the wind speed dependence of the latent heat flux, so that a uniform forcing can be balanced by a smaller SST change in the subtropics than in equatorial latitudes. The tropical ocean circulation moves heat to where the ocean more readily loses it to the atmosphere. Water that subducts in subtropical latitudes and returns to the equatorial thermocline therefore has a smaller temperature perturbation than the surface equatorial waters. The thermocline temperature adjusts on timescales of decades to the imposed forcing, but the adjustment is insufficient to cancel the thermostat mechanism.

The results imply that an increase in the downward heat flux at the ocean surface, as happens with increasing concentrations of greenhouse gases, should be accompanied by a stronger equatorial SST gradient. This contradicts the results of coupled atmosphere–ocean GCMs. Various explanations are offered. None are conclusive, but the possibility that the discrepancy lies in the low resolution of the ocean GCMs typically used in the study of climate change is discussed.

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Richard Seager and Martin Hoerling

Abstract

The atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada. Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. Oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of internal atmospheric variability. Up to half of the soil moisture deficits during severe droughts in the southeast United States in 2000, Texas in 2011, and the central Great Plains in 2012 were related to SST forcing, although SST forcing was an insignificant factor for northern Great Plains drought in 1988. During the early twenty-first century, natural decadal swings in tropical Pacific and North Atlantic SSTs have contributed to a dry regime for the United States. Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts.

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Richard Seager and Naomi Henderson

Abstract

The causes of the high pressure ridge at the North American west coast during winter 2013/14, the driest winter of the recent California drought, are examined. The ridge was part of an atmosphere–ocean state that included anomalies, defined relative to a 1979–2014 mean, of circulation across the Northern Hemisphere, warm sea surface temperatures (SSTs) in the tropical western and northeastern Pacific and the south Indian Ocean, and cool SSTs in the central tropical Pacific. The SST anomalies differ sufficiently between datasets that, when used to force atmosphere models, the simulated circulation anomalies vary notably in realism. Recognizing uncertainty in the SST field, the authors use idealized tropical SST anomaly experiments to identify an optimal combination of SST anomalies that forces a circulation response that best matches observations. The optimal SST pattern resembles that observed but the associated circulation pattern is much weaker than observed, suggesting an important but limited role for ocean forcing. Analysis of the equilibrium and transient upper-troposphere vorticity balance indicates that the SST-forced component of the ridge arose as a summed effect of Rossby waves forced by SST anomalies across the tropical Indo-Pacific oceans and drives upper-troposphere convergence and subsidence at the west coast. The ridge, in observations and model, is associated with northward and southward diversion of storms. The results suggest that tropical Indo-Pacific ocean SSTs helped force the west coast ridge and drought of winter 2013/14.

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Amy Clement and Richard Seager

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An attempt is made to determine the role of the ocean in establishing the mean tropical climate and its sensitivity to radiative perturbations. A simple two-box energy balance model is developed that includes ocean heat transports as an interactive component of the tropical climate system. It is found that changes in the zonal mean ocean heat transport can have a considerable affect on the mean tropical sea surface temperature (SST) through their effect on the properties of subtropical marine stratus clouds or on the water vapor greenhouse effect of the tropical atmosphere. The way that the tropical climate adjusts to changes in the ocean heat transport is primarily through the atmospheric heat transport, without changing the net top of the atmosphere radiative balance. Thus, the total amount of low-latitude poleward heat transport is invariant with respect to changes in ocean circulation in this model. These results are compared with analogous experiments with general circulation models.

Doubled CO2 experiments are performed with different values of ocean heat transport. It is found that the sensitivity of the mean tropical SST to doubled CO2 depends on the strength of the ocean heat transport due to feedbacks between the ocean and subtropical marine stratus clouds and the water vapor greenhouse effect. In this model, the results are the same whether the ocean heat transports are determined interactively or are fixed.

Some recent studies have suggested that an increased meridional overturning in the ocean due to changes in the zonally asymmetric circulation can reduce the sensitivity of the tropical climate to increased CO2. It is found that, in equilibrium, this is not that case, but rather an increase in ocean heat transport, which involves increased equatorial upwelling, actually warms the tropical climate.

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Richard Seager and Stephen E. Zebiak

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The tropical climate simulated with a new global atmosphere model is presented. The model is purposely designed for climate studies and is still under development. It is designed to bridge the gap between very efficient but simple models of the tropical atmosphere and sophisticated but inefficient general circulation models (GCMs). In this paper the authors examine the sensitivity of the model's climate to specific formulations of convection, boundary-layer physics, and radiation.

The model uses the Betts–Miller convection scheme and a parameterization of the planetary boundary layer (PBL) that combines similarity theory for computation of surface fluxes with a simple scheme for diagnosing PBL depth. Radiative cooling is specified and land surface processes are bypassed by relaxing modeled low-level values to observed quantities. Orography is ignored. The model contains six vertical layers and has a horizontal resolution of about 3° × 5.625°.

The authors compare the climate simulated with two different versions of the Betts–Miller convection scheme. More realistic simulations of rainfall are obtained with the later version, which includes the effects of convective downdrafts. These, by cooling and drying the PBL, act to restrict the areas of convection while strengthening the intertropical convergence zone. The sensitivity to choice of PBL physics is less, and quite similar results were obtained when the PBL scheme was replaced with constant exchange coefficients and PBL depth. In contrast, the amount of precipitation varied strongly with the prescribed radiative cooling. The important role that shallow convection and cloud-radiation interactions play in the spatial organization of deep convection is demonstrated, by default, in an experiment using clear-sky radiative transfer.

The modeled climate, as judged qualitatively by its simulation of quantities of importance to air–sea interaction and climate, such as the low-level wind field and precipitation, is in many ways comparable to that achieved by much more complex GCMs. Indeed the rainfall simulation appears better than obtained by many models that use other convection parameterizations. This adds to the accumulating evidence that the Betts–Miller scheme is a quite reliable scheme, at least for simulation of convection in the current climate. A major model flaw is a very poor Asian summer monsoon, which is attributed to lack of orography in the model. It is demonstrated, by inclusion of a specified monsoonal forcing, that this also has an effect, though modest, on the simulation of the trade winds over the Pacific.

The results suggest there is hope for development of models of intermediate complexity that achieve a degree of realism exceeding the simple models that have often been used in El Niño studies while retaining much of their efficiency.

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