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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

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 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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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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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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Michael A. Alexander and James D. Scott

Abstract

Daily fields obtained from a 17-yr atmospheric GCM simulation are used to study the surface sensible and latent heat flux variability and its relationship to the sea level pressure (SLP) field. The fluxes are analyzed over the North Pacific and Atlantic Oceans during winter. The leading mode of interannual SLP variability consists of a single center associated with the Aleutian low in the Pacific, and a dipole pattern associated with the Icelandic low and Azores high in the Atlantic. The surface flux anomalies are organized by the low-level atmospheric circulation associated with these modes in agreement with previous observational studies.

The surface flux variability on all of the timescales examined, including intraseasonal, interannual, 3–10 day, and 10–30 day, is maximized along the north and west edges of both oceans and between Japan and the date line at ∼35°N in the Pacific. The intraseasonal variability is approximately 3–5 times larger than the interannual variability, with more than half of the total surface flux variability occuring on timescales of less than 1 month. Surface flux variability in the 3–10-day band is clearly associated with midlatitude synoptic storms. Composites indicate upward (downward) flux anomalies that exceed |30 W m−2| occur to the west (east) of storms, which move eastward across the oceans at 10°–15° per day. The SLP and surface flux anomalies are also strong and coherent in the 10–30-day band but are located farther north, are broader in scale, and propagate ∼3–4 times more slowly eastward than the synoptic disturbances.

The sensible and latent heat flux are proportional to the wind speed multiplied by the air–sea temperature and humidity difference, respectively. The anomalous wind speed has the greatest influence on surface flux anomalies in the subtropics and western Pacific, while the air temperature and moisture anomalies have the greatest impact in the northeast Pacific and north of 40°N in the Atlantic. The covariance between the wind speed and the air temperature or humidity anomalies, while generally small, is nonnegligible on synoptic timescales.

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Philip Sura, Matthew Newman, and Michael A. Alexander

Abstract

The classic Frankignoul–Hasselmann hypothesis for sea surface temperature (SST) variability of an oceanic mixed layer assumes that the surface heat flux can be simply parameterized as noise induced by atmospheric variability plus a linear temperature relaxation rate. It is suggested here, however, that rapid fluctuations in this rate, as might be expected, for example, from gustiness of the sea surface winds, are large enough that they cannot be ignored. Such fluctuations cannot be fully modeled by noise that is independent of the state of the SST anomaly itself. Rather, they require the inclusion of a state-dependent (i.e., multiplicative) noise term, which can be expected to affect both persistence and the relative occurrence of high-amplitude anomalies. As a test of this hypothesis, daily observations at several Ocean Weather Stations (OWSs) are examined. Significant skewness and kurtosis of the distributions of SST anomalies is found, which is shown to be consistent with a multiplicative noise model. The observed wintertime SST distribution at OWS P is reproduced using a single-column variable-depth mixed layer model; the resulting non-Gaussianity is found to be largely due to the state dependence of rapidly varying (effectively stochastic) sensible and latent heat flux anomalies. The authors’ model for the non-Gaussianity of anomalous SST variability (counterintuitively) implies that the multiplicative noise increases the persistence, predictability, and variance of midlatitude SST anomalies. The effect is strongest on annual and longer time scales and may, therefore, be important to the understanding and modeling of interannual and interdecadal SST and related climate variability.

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Antonietta Capotondi, Michael A. Alexander, and Clara Deser

Abstract

Observations indicate the existence of two bands of maximum thermocline depth variability centered at ∼10°S and 13°N in the tropical Pacific Ocean. The analysis of a numerical integration performed with the National Center for Atmospheric Research ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat, and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 10°S and 13°N are associated with first-mode baroclinic Rossby waves forced by anomalous Ekman pumping. In this study the factors that may be responsible for the Rossby wave maxima at 10°S and 13°N, including the amplitude and spatial coherency of the forcing at those latitudes, are systematically investigated. A simple Rossby wave model is used to interpret the OGCM variability and to help to discriminate between the different factors that may produce the tropical maxima. These results indicate that the dominant factor in producing the maximum variability at 10°S and 13°N is the zonal coherency of the Ekman pumping, a characteristic of the forcing that becomes increasingly more pronounced at low frequencies, maximizing at timescales in the decadal range. Local maxima in the amplitude of the forcing, while not explaining the origin of the centers of variability at 10°S and 13°N, appear to affect the sharpness of the variability maxima at low frequencies. Although the Rossby wave model gives an excellent fit to the OGCM, some discrepancies exist: the amplitude of the thermocline variance is generally underestimated by the simple model, and the variability along 13°N is westward intensified in the wave model but reaches a maximum in the central part of the basin in the OGCM. Short Rossby waves excited by small-scale Ekman pumping features, or the presence of higher-order Rossby wave modes may be responsible for the differences in the zonal variance distribution along 13°N.

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Michael A. Alexander and Klaus M. Weickmann

Abstract

Recent observational analyses have indicated that tropospheric quasi-biennial oscillations (QBs) may play a fundamental role in regulating the timing and strength of El Niño and the Southern Oscillation. The biennial variability is examined in the tropical troposphere of a 35-year general circulation model (GCM) simulation forced by observed sea surface temperatures (SSTs). The results of spectral analyses and temporal filtering applied to the SST boundary conditions and the simulated lower- and upper-tropospheric zonal winds, precipitation, and sea level pressure anomalies are compared with observations and used to investigate the relationship between variables.

The GCM obtains regions of coherent biennial variability over the tropical Indian and Pacific Oceans in close correspondence with observations. In addition, the evolution of the stronger QBs and the physical relationship between variables are fairly well simulated. Zonal wind anomalies, with a simple baroclinic structure, tend to propagate eastward from the Indonesian region to the central Pacific where they increase in strength. The amplitude of the zonal wind and SST anomalies in the central Pacific vary together, with the largest anomalies occurring during the mid-1960s, mid-1970s, and early 1980s. During the time of the warmest SSTs, low pressure is found in the east Pacific with high pressure over Indonesia, and precipitation is enhanced between the date line and 120°W. However, the model underestimates the low-frequency variability in general and has approximately one-half to two-thirds of the observed variability in the biennial range. In addition, the observed phasing of the biennial and annual cycles in the zonal winds over the eastern Indian Ocean is not reproduced by the model.

The authors have also compared the amount of biennial variability of the near-surface zonal winds in the 35-year run with observed SSTs to two 35-year periods in a 100-year control run with climatological SSTs that repeat the seasonal cycle. Only the simulation with observed SSTs has an organized region of enhanced biennial variability near the equator, suggesting a strong oceanic component to the forcing of the QB.

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Sang-Ik Shin and Michael A. Alexander

Abstract

Projected climate changes along the U.S. East and Gulf Coasts were examined using the eddy-resolving Regional Ocean Modeling System (ROMS). First, a control (CTRL) ROMS simulation was performed using boundary conditions derived from observations. Then climate change signals, obtained as mean seasonal cycle differences between the recent past (1976–2005) and future (2070–99) periods in a coupled global climate model under the RCP8.5 greenhouse gas trajectory, were added to the initial and boundary conditions of the CTRL in a second (RCP85) ROMS simulation. The differences between the RCP85 and CTRL simulations were used to investigate the regional effects of climate change. Relative to the coarse-resolution coupled climate model, the downscaled projection shows that SST changes become more pronounced near the U.S. East Coast, and the Gulf Stream is further reduced in speed and shifted southward. Moreover, the downscaled projection shows enhanced warming of ocean bottom temperatures along the U.S. East and Gulf Coasts, particularly in the Gulf of Maine and the Gulf of Saint Lawrence. The enhanced warming was related to an improved representation of the ocean circulation, including topographically trapped coastal ocean currents and slope water intrusion through the Northeast Channel into the Gulf of Maine. In response to increased radiative forcing, much warmer than present-day Labrador Subarctic Slope Waters entered the Gulf of Maine through the Northeast Channel, warming the deeper portions of the gulf by more than 4°C.

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