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Thomas L. Delworth

Abstract

The primary mode of sea surface temperature variability in the North Atlantic on interannual timescales during winter is examined in a coupled ocean–atmosphere model. The model, developed at die Geophysical Fluid Dynamics Laboratory, is global in domain with realistic geography and a seasonal cycle of insulation. Analyses performed on a 1000-year integration of this model show that this mode is characterized by zonal bands of SST anomalies in the North Atlantic and bears a distinct resemblance to observational results. The largest anomalies in the model are to the southeast of Newfoundland.

The model SST variations appear to be related to a north–south dipole in the atmospheric 500-mb geopotential height field, which resembles the North Atlantic oscillation and the Western Atlantic pattern. Analyses are presented that show that this mode of SST variability is primarily driven by perturbations to the surface heat fluxes, which are largely governed by atmospheric variability. Changes in model ocean circulation also contribute to this mode of variability but appear to be of secondary importance.

Additional integrations are analyzed to examine the above conclusion. The same atmospheric model used in the above integration was coupled to a 50-m slab ocean and integrated for 500 years. The primary mode of SST variability in this model, in which there were no effects of ocean dynamics, resembles the primary mode from the coupled model, strengthening the conclusion that the surface fluxes are the primary mechanism generating this oceanic variability. One notable difference between the two models is related to the presence of deep vertical mixing at high latitudes in the model with a fully dynamic ocean. An additional 500-year integration of the atmospheric model with a prescribed seasonal cycle of SSTs lends further support to this conclusion, as do additional diagnostic calculations in which a 50-m slab ocean was forced by the time series of surface fluxes from both the prescribed SST and fully coupled model.

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Liping Zhang and Thomas L. Delworth

Abstract

North Pacific decadal oceanic and atmospheric variability is examined in a suite of coupled climate models developed at the Geophysical Fluid Dynamics Laboratory (GFDL). The models have ocean horizontal resolutions ranging from 1° to 0.1° and atmospheric horizontal resolutions ranging from 200 to 50 km. In all simulations the dominant pattern of decadal-scale sea surface temperature (SST) variability over the North Pacific is similar to the observed Pacific decadal oscillation (PDO). Simulated SST anomalies in the Kuroshio–Oyashio Extension (KOE) region exhibit a significant spectral peak at approximately 20 yr.

Sensitivity experiments are used to show that (i) the simulated PDO mechanism involves extratropical air–sea interaction and oceanic Rossby wave propagation; (ii) the oscillation can exist independent of interactions with the tropics, but such interactions can enhance the PDO; and (iii) ocean–atmosphere feedback in the extratropics is critical for establishing the approximately 20-yr time scale of the PDO. The spatial pattern of the PDO can be generated from atmospheric variability that occurs independently of ocean–atmosphere feedback, but the existence of a spectral peak depends on active air–sea coupling. The specific interdecadal time scale is strongly influenced by the propagation speed of oceanic Rossby waves in the subtropical and subpolar gyres, as they provide a delayed feedback to the atmosphere. The simulated PDO has a realistic association with precipitation variations over North America, with a warm phase of the PDO generally associated with positive precipitation anomalies over regions of the western United States. The seasonal dependence of this relationship is also reproduced by the model.

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Rong Zhang and Thomas L. Delworth

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In this study, a mechanism is demonstrated whereby a large reduction in the Atlantic thermohaline circulation (THC) can induce global-scale changes in the Tropics that are consistent with paleoevidence of the global synchronization of millennial-scale abrupt climate change. Using GFDL’s newly developed global coupled ocean–atmosphere model (CM2.0), the global response to a sustained addition of freshwater to the model’s North Atlantic is simulated. This freshwater forcing substantially weakens the Atlantic THC, resulting in a southward shift of the intertropical convergence zone over the Atlantic and Pacific, an El Niño–like pattern in the southeastern tropical Pacific, and weakened Indian and Asian summer monsoons through air–sea interactions.

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Thomas L. Delworth and Syukuro Manabe

Abstract

An atmospheric general circulation model with prescribed sea surface temperature and cloudiness was integrated for 50 years in order to study atmosphere-land surface interactions. The temporal variability of model soil moisture and precipitation have been studied in an effort to understand the interactions of these variables with other components of the climate system.

Temporal variability analysis has shown that the spectra of monthly mean precipitation over land are close to white at all latitudes, with total variance decreasing poleward. In contrasts, the spectra of soil moisture are red, and become more red with increasing latitude. As a measure of this redness, half of the total variance of a composite tropical soil moisture spectrum occurs at periods longer than nine months, while at high latitudes, half of the total variance of a composite soil moisture spectrum occurs at periods longer than 22 months. The spectra of soil moisture also exhibit marked longitudinal variations.

These spectral results may be viewed in the light of stochastic theory. The formulation of the GFDL soil moisture parameterization is mathematically similar to a stochastic process. According to this model, forcing of a system by an input white noise variable (precipitation) will yield an output variable (soil moisture) with a red spectrum, the redness of which is controlled by a damping term (potential evaporation). Thus, the increasingly red nature of the soil moisture spectra at higher latitudes is a result of declining potential evaporation values at higher latitudes. Physically, soil moisture excesses are dissipated more slowly at high latitudes where the energy available for evaporation is small.

Some of the longitudinal variations in soil moisture spectra result from longitudinal variations in potential evaporation, while others are explicable in terms of the value of the ratio of potential evaporation to precipitation. Regions where this value is less than one are characterized by frequent runoff and short time scales of soil moisture variability. By preventing excessive positive anomalies of soil moisture, the runoff process hastens the return of soil moisture values to their mean state, thereby shortening soil moisture time scales.

Through the use of a second GCM integration with prescribed soil moisture, it was shown that interactive sod moisture may substantially increase summer surface air temperature variability. Soil moisture interacts with the atmosphere primarily through the surface energy balance. The degree of soil saturation strongly influences the atmosphere of outgoing energy from the surface between the latent and sensible heat fluxes. Interactive soil moisture allows larger variations of these fluxes, thereby increasing the variance of surface air temperature. Because the flux of latent heat is directly proportional to potential evaporation under conditions of sufficient moisture, the influence of soil moisture on the atmosphere is greatest when the potential evaporation value is large. This occurs most frequently in the tropics and summer hemisphere extratropics.

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Thomas L. Delworth and Fanrong Zeng

Abstract

The impact of the North Atlantic Oscillation (NAO) on the Atlantic meridional overturning circulation (AMOC) and large-scale climate is assessed using simulations with three different climate models. Perturbation experiments are conducted in which a pattern of anomalous heat flux corresponding to the NAO is added to the model ocean. Differences between the perturbation experiments and a control illustrate how the model ocean and climate system respond to the NAO. A positive phase of the NAO strengthens the AMOC by extracting heat from the subpolar gyre, thereby increasing deep-water formation, horizontal density gradients, and the AMOC. The flux forcings have the spatial structure of the observed NAO, but the amplitude of the forcing varies in time with distinct periods varying from 2 to 100 yr. The response of the AMOC to NAO variations is small at short time scales but increases up to the dominant time scale of internal AMOC variability (20–30 yr for the models used). The amplitude of the AMOC response, as well as associated oceanic heat transport, is approximately constant as the time scale of the forcing is increased further. In contrast, the response of other properties, such as hemispheric temperature or Arctic sea ice, continues to increase as the time scale of the forcing becomes progressively longer. The larger response is associated with the time integral of the anomalous oceanic heat transport at longer time scales, combined with an increased impact of radiative feedback processes. It is shown that NAO fluctuations, similar in amplitude to those observed over the last century, can modulate hemispheric temperature by several tenths of a degree.

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Riccardo Farneti and Thomas L. Delworth

Abstract

It has been suggested that a strengthening of the Southern Hemisphere winds would induce a more vigorous overturning through an increased northward Ekman flux, bringing more light waters into the oceanic basins and enhancing the upwelling of North Atlantic Deep Water in the Southern Ocean, thereby increasing ocean ventilation. Simulations from a coarse- and a fine-resolution version of a coupled model, subject to idealized wind stress changes in the Southern Ocean, are presented. In the fine-resolution eddy-permitting model, changes in poleward eddy fluxes largely compensate for the enhanced equatorward Ekman transport in the Southern Ocean. As a consequence, northward transport of light waters, pycnocline depth, Northern Hemisphere overturning, and Southern Ocean upwelling anomalies are much reduced compared with simulations in the coarse-resolution model with parameterized eddies. These results suggest a relatively weak sensitivity of present-day global ocean overturning circulation to the projected strengthening of the Southern Hemisphere winds.

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Liping Zhang and Thomas L. Delworth

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The impact of climate change on the Pacific decadal oscillation (PDO) is studied using a fully coupled climate model. The model results show that the PDO has a similar spatial pattern in altered climates, but its amplitude and time scale of variability change in response to global warming or cooling. In response to global warming the PDO amplitude is significantly reduced, with a maximum decrease over the Kuroshio–Oyashio Extension (KOE) region. This reduction appears to be associated with a weakened meridional temperature gradient in the KOE region. In addition, reduced variability of North Pacific wind stress, partially due to reduced air–sea feedback, also helps to weaken the PDO amplitude by reducing the meridional displacements of the subtropical and subpolar gyre boundaries. In contrast, the PDO amplitude increases in response to global cooling.

In the control simulations the model PDO has an approximately bidecadal peak. In a warmer climate the PDO time scale becomes shorter, changing from ~20 to ~12 yr. In a colder climate the time scale of the PDO increases to ~34 yr. Physically, global warming (cooling) enhances (weakens) ocean stratification. The increased (decreased) ocean stratification acts to increase (reduce) the phase speed of internal Rossby waves, thereby altering the time scale of the simulated PDO.

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Honghai Zhang and Thomas L. Delworth

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Regional hydroclimate changes on decadal time scales contain substantial natural variability. This presents a challenge for the detection of anthropogenically forced hydroclimate changes on these spatiotemporal scales because the signal of anthropogenic changes is modest, compared to the noise of natural variability. However, previous studies have shown that this signal-to-noise ratio can be greatly improved in a large model ensemble where each member contains the same signal but different noise. Here, using multiple state-of-the-art large ensembles from two climate models, the authors quantitatively assess the detectability of anthropogenically caused decadal shifts in precipitation-minus-evaporation (PmE) mean state against natural variability, focusing on North America during 2000–50. Anthropogenic forcing is projected to cause detectable (signal larger than noise) shifts in PmE mean state relative to the 1950–99 climatology over 50%–70% of North America by 2050. The earliest detectable signals include, during November–April, a moistening over northeastern North America and a drying over southwestern North America and, during May–October, a drying over central North America. Different processes are responsible for these signals. Changes in submonthly transient eddy moisture fluxes account for the northeastern moistening and central drying, while monthly atmospheric circulation changes explain the southwestern drying. These model findings suggest that despite the dominant role of natural internal variability on decadal time scales, anthropogenic shifts in PmE mean state can be detected over most of North America before the middle of the current century.

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Kirsten L. Findell and Thomas L. Delworth

Abstract

Climate model simulations run as part of the Climate Variability and Predictability (CLIVAR) Drought Working Group initiative were analyzed to determine the impact of three patterns of sea surface temperature (SST) anomalies on drought and pluvial frequency and intensity around the world. The three SST forcing patterns include a global pattern similar to the background warming trend, a pattern in the Pacific, and a pattern in the Atlantic. Five different global atmospheric models were forced by fixed SSTs to test the impact of these SST anomalies on droughts and pluvials relative to a climatologically forced control run.

The five models generally yield similar results in the locations of drought and pluvial frequency changes throughout the annual cycle in response to each given SST pattern. In all of the simulations, areas with an increase in the mean drought (pluvial) conditions tend to also show an increase in the frequency of drought (pluvial) events. Additionally, areas with more frequent extreme events also tend to show higher intensity extremes. The cold Pacific anomaly increases drought occurrence in the United States and southern South America and increases pluvials in Central America and northern and central South America. The cold Atlantic anomaly increases drought occurrence in southern Central America, northern South America, and central Africa and increases pluvials in central South America. The warm Pacific and Atlantic anomalies generally lead to reversals of the drought and pluvial increases described with the corresponding cold anomalies. More modest impacts are seen in other parts of the world. The impact of the trend pattern is generally more modest than that of the two other anomaly patterns.

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Salil Mahajan, Rong Zhang, and Thomas L. Delworth

Abstract

The simulated impact of the Atlantic meridional overturning circulation (AMOC) on the low-frequency variability of the Arctic surface air temperature (SAT) and sea ice extent is studied with a 1000-year-long segment of a control simulation of the Geophysical Fluid Dynamics Laboratory Climate Model version 2.1. The simulated AMOC variations in the control simulation are found to be significantly anticorrelated with the Arctic sea ice extent anomalies and significantly correlated with the Arctic SAT anomalies on decadal time scales in the Atlantic sector of the Arctic. The maximum anticorrelation with the Arctic sea ice extent and the maximum correlation with the Arctic SAT occur when the AMOC index leads by one year. An intensification of the AMOC is associated with a sea ice decline in the Labrador, Greenland, and Barents Seas in the control simulation, with the largest change occurring in winter. The recent declining trend in the satellite-observed sea ice extent also shows a similar pattern in the Atlantic sector of the Arctic in the winter, suggesting the possibility of a role of the AMOC in the recent Arctic sea ice decline in addition to anthropogenic greenhouse-gas-induced warming. However, in the summer, the simulated sea ice response to the AMOC in the Pacific sector of the Arctic is much weaker than the observed declining trend, indicating a stronger role for other climate forcings or variability in the recently observed summer sea ice decline in the Chukchi, Beaufort, East Siberian, and Laptev Seas.

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