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M. J. Fennessy, L. Marx, and J. Shukla

Abstract

Three control and anomaly simulation pairs run with the Goddard Laboratory for Atmospheric Sciences (GLAS) climate model have been analyzed in order to investigate the atmospheric response to the 1982–83 tropical sea surface temperature anomalies. The observed 1982–83 SST anomalies obtained from the Climate Analysis Center were applied to two separate 75‐day control simulations, starting on 16 December 1982 and 16 December 1979, respectively, and a third 60‐day control simulation starting on 1 January 1975.

In each experiment the equatorial Pacific precipitation increased significantly in a wide band stretching from just east of the dateline to the South American coast, in agreement with observed outgoing longwave radiation (OLR) anomalies. West of this region the precipitation was reduced in the anomaly simulations. As in previous GCM experiments, the major contributor to the tropical precipitation changes was the low‐level moisture convergence The largest evaporation differences were wound 4 mm day−1 and occurred over the regions of highest SST in the anomaly simulations. The tropical sea‐level pressure field showed a marked Southern Oscillation pattern, with a magnitude of roughly 2 millibars and a node at the dateline. There was a strong (∼10 m s−1) increase in the equatorial eastern Pacific 850 mb westerlies as well as a large (approximately −20 m s−1) easterly wind anomaly at 200 nib. The latter anonmaly was flanked by strong (∼20 m s−1) westerly anomalies at roughly 30°S and 30°N.

In agreement with earlier simulations with composite SST anomalies, the tropical precipitation anomalies for 1982–83 were also closely related to the extent of very warm (≥29°C) sea surface waters.

Each experiment had anomalous anticyclonic circulations aloft straddling the equator in the eastern Pacific, although they were weaker and more eastward than those observed. The extratropical response varied between the three experiments, as well as between months of a given experiment. Over North America the ensemble average anomaly minus control 300 mb geopotential height difference field resembled the observed February or March anomaly field more than the typical PNA‐like pattern. Other extratropical response were difficult to interpret, although they were clearly equivalent barotropic in structure and showed a much stronger dependence on initial conditions than was noted for the tropics.

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Paul A. Dirmeyer, Michael J. Fennessy, and L. Marx

Abstract

Ensemble integrations of three general circulation models (Center for Ocean–Land–Atmosphere Studies, NCAR, and NCEP) have been performed over five different boreal summer seasons (June through September of 1986–88 and 1993–94) with prescribed observed sea surface temperature to assess the predictability of seasonal climate during the boreal summer. Beyond some inconsistent initialization of soil wetness among the models, there is no land surface contribution to predictability that can be assessed. The models show a rapid degradation of skill in global terrestrial surface temperature after the first month, and no skill in precipitation over land. Potential predictability is assessed by examining in tandem the models' skill as measured by their anomaly correlation coefficients, and the models' signal-to-noise ratio (essentially interannual versus intraensemble variance) as a measure of confidence in the results. Collocation of skill in anomaly simulation and a robust signal is a strong indicator of potential predictability. Predictability of interannual climate variations is found to be low outside the deep Tropics, and nil over land. With only SST as a driving boundary condition, the poor performance of these models during summer may indicate that one must turn to the land surface in order to harvest potential predictability.

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Vasubandhu Misra, L. Marx, M. Brunke, and X. Zeng

Abstract

A set of multidecadal coupled ocean–atmosphere model integrations are conducted with different time steps for coupling between the atmosphere and the ocean. It is shown that the mean state of the equatorial Pacific does not change in a statistically significant manner when the coupling interval between the atmospheric general circulation model (AGCM) and the ocean general circulation model (OGCM) is changed from 1 day to 2 or even 3 days. It is argued that because the coarse resolution of the AGCM precludes resolving realistic “weather” events, changing the coupling interval from 1 day to 2 or 3 days has very little impact on the mean coupled climate.

On the other hand, reducing the coupling interval to 3 h had a much stronger impact on the mean state of the equatorial Pacific and the concomitant general circulation. A novel experiment that incorporates a (pseudo) interaction of the atmosphere with SST at every time step of the AGCM was also conducted. In this unique coupled model experiment, the AGCM at every time step mutually interacts with the skin SST. This skin SST is anchored to the bulk SST, which is updated from the OGCM once a day. Both of these experiments reduced the cold tongue bias moderately over the equatorial Pacific Ocean with a corresponding reduction in the easterly wind stress bias relative to the control integration. It is stressed from the results of these model experiments that the impact of high-frequency air–sea coupling is significant on the cold tongue bias.

The interannual variation of the equatorial Pacific was less sensitive to the coupling time step between the AGCM and the OGCM. Increasing (reducing) the coupling interval of the air–sea interaction had the effect of weakening (marginally strengthening) the interannual variations of the equatorial Pacific Ocean.

It is argued that the low-frequency response of the upper ocean, including the cold tongue bias, is modulated by the atmospheric stochastic forcing on the coupled ocean–atmosphere system. This effect of the atmospheric stochastic forcing is affected by the frequency of the air–sea coupling and is found to be stronger than the rectification effect of the diurnal variations of the air–sea interaction on the low frequency. This may be a result of a limitation in the coupled model used in this study in which the OGCM has an inadequate vertical resolution in the mixed layer to sustain diurnal variations in the upper ocean.

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J. L. Kinter III, J. Shukla, L. Marx, and E. K. Schneider

Abstract

The medium range forecast model of the National Meteorological Center (NMC) has been integrated to produce winter and summer simulations. For the winter simulation, the model was initialized with the NMC analysis of 1200 UTC 15 December 1985 and integrated for 110 days to simulate the December–March period (referred to hereinafter as the DJFM simulation). For the summer simulation, the model was initialized with NMC analysis of 0000 UTC 1 May 1986 and was integrated for 110 days to simulate the May–August period (MJJA simulation). In each case, seasonally varying boundary conditions of sea surface temperature, soil moisture and sea ice were used. The computer code used was nearly identical to that used by NMC for the operational ten-day forecast during the period 16 April 1985 through 30 May 1986. Both simulations have been compared to the NMC analyses for the corresponding period.

It was found that the model climatology, defined as the average over the last 3 months of each run, is similar to that of the observed atmosphere as well as climatologies of other general circulation models. Notably, the model maintains a reasonable horizontal temperature gradient and circulation distribution, but the model is colder than observed in the troposphere nearly everywhere and cools in the lower stratosphere in the tropics and near the poles in both simulations. A detailed description of the stationary and transient features of the model circulation including both tropical and extratropical regions is also given. In addition, the model hydrological cycle, radiative balance and surface heat budget are discussed. The secondary circulations in the tropics including the zonally symmetric Hadley cell, as simulated by the model, are also compared with the observations. The upper branch of the Hadley cell appears to be only poorly simulated in DJFM.

Generally, the simulations demonstrated reasonable agreement with the observations in sea level pressure, the structure of the tropospheric zonal jets and the winter hemispheric stationary waves. The tropical rainfall is very different from climatology or surrogates for precipitation observations—such as outgoing long radiation—particularly in the excessive amount of rain produced by the model over subtropical deserts. The summer hemisphere in both simulations does not agree with observations as well as the winter hemisphere; this may be related to the tropical rainfall problem.

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A. Kumar, M. Chen, L. Zhang, W. Wang, Y. Xue, C. Wen, L. Marx, and B. Huang

Abstract

For long-range predictions (e.g., seasonal), it is a common practice for retrospective forecasts (also referred to as the hindcasts) to accompany real-time predictions. The necessity for the hindcasts stems from the fact that real-time predictions need to be calibrated in an attempt to remove the influence of model biases on the predicted anomalies. A fundamental assumption behind forecast calibration is the long-term stationarity of forecast bias that is derived based on hindcasts.

Hindcasts require specification of initial conditions for various components of the prediction system (e.g., ocean, atmosphere) that are generally taken from a long reanalysis. Trends and discontinuities in the reanalysis that are either real or spurious can arise due to several reasons, for example, the changing observing system. If changes in initial conditions were to persist during the forecast, there is a potential for forecast bias to depend over the period it is computed, making calibration even more of a challenging task. In this study such a case is discussed for the recently implemented seasonal prediction system at the National Centers for Environmental Prediction (NCEP), the Climate Forecast System version 2 (CFS.v2).

Based on the analysis of the CFS.v2 for 1981–2009, it is demonstrated that the characteristics of the forecast bias for sea surface temperature (SST) in the equatorial Pacific had a dramatic change around 1999. Furthermore, change in the SST forecast bias, and its relationship to changes in the ocean reanalysis from which the ocean initial conditions for hindcasts are taken is described. Implications for seasonal and other long-range predictions are discussed.

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Vasubandhu Misra, L. Marx, M. Fennessy, B. Kirtman, and J. L. Kinter III

Abstract

This study compares an ensemble of seasonal hindcasts with a multidecadal integration from the same global coupled climate model over the tropical Pacific Ocean. It is shown that the annual mean state of the SST and its variability are different over the tropical Pacific Ocean in the two operating modes of the model.

These differences are symptoms of an inherent difference in the physics of coupled air–sea interactions and upper ocean variability. It is argued that in the presence of large coupled model errors and in the absence of coupled data assimilation, the competing and at times additive influence of the initialization and model errors can change the behavior of the air–sea interaction physics and upper ocean dynamics.

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J. L. Kinter III, M. J. Fennessy, V. Krishnamurthy, and L. Marx

Abstract

Recent decadal regime shifts in the large-scale circulation of the tropical atmosphere are examined using analyses and independent observations of the circulation and precipitation. Comparisons between reanalysis products and independent observations suggest that the shifts that are apparent and significant in the reanalysis products may be artifacts of changes in the observing system and/or the data assimilation procedures.

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Otha H. Vaughan Jr., Richard Blakeslee, William L. Boeck, Bernard Vonnegut, Marx Brook, and Jorn McKune Jr.

Abstract

No abstract available.

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Bohua Huang, Chul-Su Shin, J. Shukla, Lawrence Marx, Magdalena A. Balmaseda, Subhadeep Halder, Paul Dirmeyer, and James L. Kinter III

Abstract

A set of ensemble seasonal reforecasts for 1958–2014 is conducted using the National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2. In comparison with other current reforecasts, this dataset extends the seasonal reforecasts to the 1960s–70s. Direct comparison of the predictability of the ENSO events occurring during the 1960s–70s with the more widely studied ENSO events since then demonstrates the seasonal forecast system’s capability in different phases of multidecadal variability and degrees of global climate change. A major concern for a long reforecast is whether the seasonal reforecasts before 1979 provide useful skill when observations, particularly of the ocean, were sparser. This study demonstrates that, although the reforecasts have lower skill in predicting SST anomalies in the North Pacific and North Atlantic before 1979, the prediction skill of the onset and development of ENSO events in 1958–78 is comparable to that for 1979–2014. In particular, the ENSO predictions initialized in April during 1958–78 show higher skill in the summer. However, the skill of the earlier predictions declines faster in the ENSO decaying phase, because the reforecasts initialized after boreal summer persistently predict lingering wind and SST anomalies over the eastern equatorial Pacific during such events. Reforecasts initialized in boreal fall overestimate the peak SST anomalies of strong El Niño events since the 1980s. Both phenomena imply that the model’s air–sea feedback is overly active in the eastern Pacific before ENSO event termination. Whether these differences are due to changes in the observing system or are associated with flow-dependent predictability remains an open question.

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M.J. Fennessy, J.L. Kinter III, B. Kirtman, L. Marx, S. Nigam, E. Schneider, J. Shukla, D. Straus, A. Vernekar, Y. Xue, and J. Zhou

Abstract

A series of sensitivity experiments are conducted in an attempt to understand and correct deficiencies in the simulation of the seasonal mean Indian monsoon with a global atmospheric general circulation model. The seasonal mean precipitation is less than half that observed. This poor simulation in seasonal integrations is independent of the choice of initial conditions and global sea surface temperature data used. Experiments are performed to test the sensitivity of the Indian monsoon simulation to changes in orography, vegetation, soil wetness, and cloudiness.

The authors find that the deficiency of the model precipitation simulation may be attributed to the use of an enhanced orography in the integrations. Replacement of this orography with a mean orography results in a much more realistic simulation of Indian monsoon circulation and rainfall. Experiments with a linear primitive equation model on the sphere suggest that this striking improvement is due to modulations of the orographically forced waves in the lower troposphere. This improvement in the monsoon simulation is due to the kinematic and dynamical effects of changing the topography, rather than the thermal effects, which were minimal.

The magnitude of the impact on the Indian monsoon of the other sensitivity experiments varied considerably, but was consistently less than the impact of using the mean orography. However, results from the soil moisture sensitivity experiments suggest a possibly important role for soil moisture in simulating tropical precipitation, including that associated with the Indian monsoon.

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