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Michael A. Alexander

Abstract

The influence of midlatitude air-sea interaction on the atmospheric anomalies associated with El Niño is investigated by coupling the Community Climate Model to a mixed-layer ocean model in the North Pacific. Prescribed El Niño conditions, warm sea surface temperatures (SST) in the tropical Pacific, cause a southward displacement and strengthening of the Aleutian Low. This results in enhanced (reduced) advection of cold Asian air over the west-central (northwest) Pacific and northward advetion of warm air over the eastern Pacific. Allowing air-sea feedback in the North Pacific slightly modified the El Niño–induced neu-surface wind, air temperature, and precipitation anomalies. The anomalous cyclonic circulation over the North Pacific is more concentric and shifted slightly to the east in the coupled simulations. Air-sea feedback also damped the air temperature anomalies over most of the North Pacific and reduced the precipitation rate above the cold SST anomaly that develops in the central Pacific.

The simulated North Pacific SST anomalies and the resulting Northern Hemisphere atmospheric anomalies are roughly one-third as large as those related to the prescribed El Niño conditions in a composite of five cases. The composite geopotential height anomalies associated with changes in the North Pacific SSTs have an equivalent baretropic structure and range from −65 m to 50 m at the 200-mb level. Including air-sea feedback in the North Pacific tended to damp the atmospheric anomalies caused by the prescribed El Niño conditions in the tropical Pacific. As a result, the zonally elongated geopotential height anomalies over the West Pacific are reduced and shifted to the cast. However, the atmospheric changes associated with the North Pacific SST anomalies vary widely among the five cases.

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Michael A. Alexander

Abstract

Atmosphere-ocean experiments are experiments are used to investigate the formation of sea surface temperature (SST) anomalies in the North Pacific Ocean during fall and winter of the El Niño year. Experiments in which the NCAR Community Climate Model (CCM) surface fields am used to force a mixed-layer ocean model in the North Pacific (no air-sea feedback) are compared to simulations in which the CCM and North Pacific Ocean model are coupled. Anomalies in the atmosphere and the North Pacific Ocean during El Niño are obtained from the difference between simulations with and without prescribed warm SST anomalies in the tropical Pacific. In both the forced and coupled experiments, the anomaly pattern resembles a composite of the actual SST anomaly field during El Niño: warm SSTs develop along the cost of North America and cold SSTs form in the central Pacific. In the coupled simulations, air-sea interaction results in a 25% to 50% reduction in the magnitude of the SST and mixed-layer depth anomalies resulting in more realistic SST fields. Coupling also decreases the SST anomaly variance; as a result, the anomaly centers remain statistically significant even though the magnitude of the anomalies is reduced.

Three additional sensitivity studies indicate that air-sea feedback and entrainment act to damp SST anomalies while Ekman pumping has a negligible effect on mixed-layer depth and SST anomalies in midlatitudes.

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Michael A. Alexander and Cecile Penland

Abstract

A stochastic model of atmospheric surface conditions, developed from 30 years of data at Ocean Weather Station P in the northeast Pacific, is used to drive a mixed layer model of the upper mean. The spectral characteristics of anomalies in the four atmospheric variables: air and dewpoint temperature, wind speed and solar radiation, and many ocean features, including the seasonal cycle are reasonably well reproduced in a 500-year model simulation. However, the ocean model slightly underestimates the range of the mean and standard deviation of both temperature and mixed layer depth over the course of the year. The spectrum of the monthly SST anomalies from the model simulation are in close agreement with observations, especially when atmospheric forcing associated with El Niño is included. The spectral characteristics of the midlatitude SST anomalies is consistent with stochastic climate theory proposed by Frankignoul and Hasselmann (1977) for periods up to ∼6 months.

Lead/lag correlations and composites indicate a clear connection between the observed SST anomalies in spring and the following fall, as anomalous warm or cold water created in the deep mixed layer during winter/spring remain below the shallow mixed layer in summer and is then reentrained into the surface layer in the following fall and winter. This re-emergence mechanism also occurs in the model but the temperature anomaly pattern is more diffuse and influences the surface layer over a longer period compared with observations.

A detailed analysis of the simulated mixed layer temperature tendency indicates that the anomalous net surface heat flux plays an important role in the growth of SST anomalies throughout the year and is the dominant term during winter. Entrainment of water into the mixed layer from below strongly influences SST anomalies in fall when the mixed layer is relatively shallow and thus has little thermal inertia. Mixed layer depth anomalies are highly correlated with the anomalous surface mechanical mixing in summer and surface buoyancy forcing in winter.

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Michael A. Alexander and Clara Deser

Abstract

In the early 1970s, Namias and Born speculated that ocean temperature anomalies created over the deep mixed layer in winter could be preserved in the summer thermocline and reappear at the surface in the following fall or winter. This hypothesis is examined using upper-ocean temperature observations and simulations with a mixed layer model. The data were collected at six ocean weather stations in the North Atlantic and North Pacific. Concurrent and lead-lag correlations are used to investigate temperature variations associated with the seasonal cycle in both the observations and the model simulations.

Concurrent correlations between the surface and subsurface temperature anomalies in both the data and the model indicate that the penetration of temperature anomalies into the ocean is closely tied to the seasonal cycle in mixed layer depth: high correlations extend to relatively deep (shallow) depths in winter (summer). Lead-lag correlations in both the data and the model, at some of the stations, indicate that temperature anomalies beneath the mixed layer in summer are associated with the temperature anomalies in the mixed layer in the previous winter/spring and following fall/winter but are unrelated or weakly opposed to the temperature anomalies in the mixed layer in summer. These results suggest that vertical mixing processes allow ocean temperature anomalies created over a deep mixed layer in winter to be preserved below the surface in summer and reappear at the surface in the following fall, confirming the Namias–Born hypothesis.

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Antonietta Capotondi and Michael A. Alexander

Abstract

Multicentury preindustrial control simulations from six of the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) models are used to examine the relationship between low-frequency precipitation variations in the Great Plains (GP) region of the United States and global sea surface temperatures (SSTs). This study builds on previous work performed with atmospheric models forced by observed SSTs during the twentieth century and extends it to a coupled model context and longer time series. The climate models used in this study reproduce the precipitation climatology over the United States reasonably well, with maximum precipitation occurring in early summer, as observed. The modeled precipitation time series exhibit negative “decadal” anomalies, identified using a 5-yr running mean, of amplitude comparable to that of the twentieth-century droughts. It is found that low-frequency anomalies over the GP are part of a large-scale pattern of precipitation variations, characterized by anomalies of the same sign as in the GP region over Europe and southern South America and anomalies of opposite sign over northern South America, India, and Australia. The large-scale pattern of the precipitation anomalies is associated with global-scale atmospheric circulation changes; during wet periods in the GP, geopotential heights are raised in the tropics and high latitudes and lowered in the midlatitudes in most models, with the midlatitude jets displaced toward the equator in both hemispheres. Statistically significant correlations are found between the decadal precipitation anomalies in the GP region and tropical Pacific SSTs in all the models. The influence of other oceans (Indian and tropical and North Atlantic), which previous studies have identified as potentially important, appears to be model dependent.

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Hye-Mi Kim and Michael A. Alexander

Abstract

The vertically integrated water vapor transport (IVT) over the Pacific–North American sector during three phases of ENSO in boreal winter (December–February) is investigated using IVT values calculated from the Climate Forecast System Reanalysis (CFSR) during 1979–2010. The shift of the location and sign of sea surface temperature (SST) anomalies in the tropical Pacific Ocean leads to different atmospheric responses and thereby changes the seasonal mean moisture transport into North America. During eastern Pacific El Niño (EPEN) events, large positive IVT anomalies extend northeastward from the subtropical Pacific into the northwestern United States following the anomalous cyclonic flow around a deeper Aleutian low, while a southward shift of the cyclonic circulation during central Pacific El Niño (CPEN) events induces the transport of moisture into the southwestern United States. In addition, moisture from the eastern tropical Pacific is transported from the deep tropical eastern Pacific into Mexico and the southwestern United States during CPEN. During La Niña (NINA), the seasonal mean IVT anomaly is opposite to that of two El Niño phases. Analyses of 6-hourly IVT anomalies indicate that there is strong moisture transport from the North Pacific into the northwestern and southwestern United States during EPEN and CPEN, respectively. The IVT is maximized on the southeastern side of a low located over the eastern North Pacific, where the low is weaker but located farther south and closer to shore during CPEN than during EPEN. Moisture enters the southwestern United States from the eastern tropical Pacific during NINA via anticyclonic circulation associated with a ridge over the southern United States.

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James D. Scott and Michael A. Alexander

Abstract

Net surface shortwave fluxes (Q sw) computed from National Aeronautics and Space Administration/Langley satellite data are compared with Q sw from reanalyses of the European Centre for Medium-Range Weather Forecasts (ERA) and the National Centers for Environmental Prediction (NCEP). The mean and variability of Q sw is examined for the period 1983–91, with a focus on the tropical and summer hemisphere oceans during June, July, August (JJA) and December, January, February (DJF). Both reanalyses exhibit a positive bias, indicating too much sunlight is absorbed at the surface, in regions where low-level stratiform clouds are most common, but a negative bias in regions where cumuliform clouds are the dominant cloud type. The ERA has a greater intermonthly variability during JJA than the satellite data over most of the Pacific, especially north of 40°N and in the central and eastern equatorial Pacific. The NCEP variability in JJA is also larger than the satellite estimates over the North Pacific and the eastern equatorial Pacific, but is smaller over most of the western tropical and subtropical Pacific. During DJF, the ERA has more realistic variability in shortwave fluxes over the tropical oceans than the NCEP reanalysis, which underestimates the variability in the tropical Pacific and the Indian Ocean by a factor of 2. Ocean models using atmospheric forcing from reanalyses will be impacted not only by regional and seasonal Q sw biases but also by differences in Q sw variability. It is estimated that the largest impacts on SST due to differences in variability are in the North Pacific, eastern tropical Pacific, and western Atlantic during JJA and in the Indian Ocean and the tropical Pacific and Atlantic during DJF.

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Michael A. Alexander and James D. Scott
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Michael A. Alexander and James D. Scott

Abstract

The influence of oceanic Ekman heat transport (Q ek) on air–sea variability associated with ENSO teleconnections is examined via a pair of atmospheric general circulation model (AGCM) experiments. In the mixed layer model (MLM) experiment, observed sea surface temperatures (SSTs) for the years 1950–99 are specified over the tropical Pacific, while a grid of mixed layer models is coupled to the AGCM elsewhere over the global oceans. The same experimental design was used in the Ekman transport/mixed layer model (EKM) experiment with the addition of Q ek in the mixed layer ocean temperature equation. The ENSO signal was evaluated using differences between composites of El Niño and La Niña events averaged over the 16 ensemble members in each experiment.

In both experiments the Aleutian low deepened and the resulting surface heat fluxes cooled the central North Pacific and warmed the northeast Pacific during boreal winter in El Niño relative to La Niña events. Including Qek amplified the ENSO-related SSTs by ∼⅓ in the central and northeast North Pacific, producing anomalies comparable to those in nature. Differences between the ENSO-induced atmospheric circulation anomalies in the EKM and MLM experiments were not significant over the North Pacific. The sea level pressure (SLP) and SST response to ENSO over the Atlantic strongly projects on the North Atlantic Oscillation (NAO) and the SST tripole pattern in observations and both model experiments. The La Niña anomalies, which are stronger than during El Niño, include high pressure and positive SSTs in the central North Atlantic. Including Ekman transport enhanced the Atlantic SST anomalies, which in contrast to the Pacific, appeared to strengthen the overlying atmospheric circulation.

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Christophe Cassou, Clara Deser, and Michael A. Alexander

Abstract

Extratropical SSTs can be influenced by the “reemergence mechanism,” whereby thermal anomalies in the deep winter mixed layer persist at depth through summer and are then reentrained into the mixed layer in the following winter. The impact of reemergence in the North Atlantic Ocean (NAO) upon the climate system is investigated using an atmospheric general circulation model coupled to a mixed layer ocean/thermodynamic sea ice model.

The dominant pattern of thermal anomalies below the mixed layer in summer in a 150-yr control integration is associated with the North Atlantic SST tripole forced by the NAO in the previous winter as indicated by singular value decomposition (SVD). To isolate the reemerging signal, two additional 60-member ensemble experiments were conducted in which temperature anomalies below 40 m obtained from the SVD analysis are added to or subtracted from the control integration. The reemerging signal, given by the mean difference between the two 60-member ensembles, causes the SST anomaly tripole to recur, beginning in fall, amplifying through January, and persisting through the following spring. The atmospheric response to these SST anomalies resembles the circulation that created them the previous winter but with reduced amplitude (10–20 m at 500 mb per °C), modestly enhancing the winter-to-winter persistence of the NAO. Changes in the transient eddies and their interactions with the mean flow contribute to the large-scale equivalent barotropic response throughout the troposphere. The latter can also be attributed to the change in occurrence of intrinsic weather regimes.

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