Nicholas E. Graham

Persistent Positive North Atlantic Oscillation Mode Dominated the Medieval Climate Anomaly

The Medieval Climate Anomaly (MCA) was the most recent pre-industrial era warm interval of European climate, yet its driving mechanisms remain uncertain. We present here a 947-year-long multidecadal North Atlantic Oscillation (NAO) reconstruction and find a persistent positive NAO during the MCA. Supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker NAO conditions into the Little Ice Age (LIA). Globally distributed proxy data suggest that this NAO shift is one aspect of a global MCA-LIA climate transition that probably was coupled to prevailing La Niña–like conditions amplified by an intensified Atlantic meridional overturning circulation during the MCA.

Decadal-scale climate variability in the tropical and North Pacific during the 1970s and 1980s: observations and model results

An abrupt change in the large-scale boreal winter circulation pattern over the North Pacific was observed during the mid-1970s. Most notably, this change was marked by a southward shift and intensification of the Aleutian Low and prevailing westerlies over the mid-latitude central and eastern Pacific. Associated changes in diverse North Pacific climatological, hydrological, and biological variables have been noted by numerous researchers. Intriguingly, the timing of these changes in the extra-tropical circulation was coincident with a shift in the background state of the coupled ocean-atmosphere system over the tropical Pacific. These changes include increases in SST over broad regions of the central and eastern tropical Pacific and an eastward displacement of the region of persistent convection in the western Pacific. This paper presents a variety of observed data and model results to describe the climate shift, and to understand some of the links within the coupled climate system that produced it. Five main findings are emphasized: (1) evidence of abrupt, simultaneous, and apparently related changes can be found in many fields and in many model results; the climate shift is not an artifact, (2) over the tropical Pacific the climate change represents a shift in the state of the coupled ocean-atmosphere system, some aspects of which resemble features associated with El Niño episodes. However, the shift in state is not well characterized as due to a change in the frequency of intensity of El Ni~no episodes; it is better described as a change in background mean state, (3) when forced with observed SSTs, both a very simple atmospheric model and a full general circulation model (GCM) qualitatively simulate aspects of the decadal-scale shift over the tropical Pacific, (4) when forced with observed surface wind stress, two ocean models of the tropical Pacific, in which surface heat fluxes are parameterized as Newtonian damping, reproduce some aspects of the near-equatorial decadal SST signal. However, the models do not reproduce the large changes in SST observed at higher latitudes of the tropical Pacific, suggesting that altered surface heat fluxes dominated in producing these changes, and (5) an important new finding of this study is the success of a GCM in reproducing important aspects of the observed mid-1970s shift in winter northern hemisphere circulation. Comparative analyses of the observed and GCM simulated circulation suggest the altered patterns of tropical Pacific SST and convection were important in forcing the changes in the mid-latitude circulation, a finding corroborated by recent GCM experiments.

Sea Surface Temperature, Surface Wind Divergence, and Convection over Tropical Oceans

Large-scale convection over the warm tropical oceans provides an important portion of the driving energy for the general circulation of the atmosphere. Analysis of regional associations between ocean temperature, surface wind divergence, and convection produced two important results. First, over broad regions of the Indian and Pacific oceans, sea surface temperatures (SSTs) in excess of 27.5�C are required for large-scale deep convection to occur. However, SSTs above that temperature are not a sufficient condition for convection and further increases in SST appear to have little effect on the intensity of convection. Second, when SSTs are above 27.5�C, surface wind divergence is closely associated with the presence or absence of deep convection. Although this result could have been expected, it was also found that areas of persistent divergent surface flow coincide with regions where convection appears to be consistently suppressed even when SSTs are above 27.5�C. Thus changes in atmospheric stability caused by remotely forced changes in subsidence aloft may play a major role in regulating convection over warm tropical oceans.

Areas beneath the relative operating characteristics (ROC) and relative operating levels (ROL) curves: Statistical significance and interpretation

The areas beneath the relative (or receiver) operating characteristics (ROC) and relative operating levels (ROL) curves can be used as summary measures of forecast quality, but statistical significance tests for these areas are conducted infrequently in the atmospheric sciences. A development of signal-detection theory, the ROC curve has been widely applied in the medical and psychology fields where significance tests and relationships to other common statistical methods have been established and described. This valuable literature appears to be largely unknown to the atmospheric sciences where applications of ROC and related techniques are becoming more common. This paper presents a survey of that literature with a focus on the interpretation of the ROC area in the field of forecast verification. We extend these foundations to demonstrate that similar principles can be applied to the interpretation and significance testing of the ROL area. It is shown that the ROC area is equivalent to the Mann–Whitney U-statistic testing the significance of forecast event probabilities for cases where events actually occurred with those where events did not occur. A similar derivation shows that the ROL area is equivalent to the Mann–Whitney U-statistic testing the magnitude of events with respect to whether or not an event has been forecast. Because the Mann–Whitney U-statistic follows a known probability distribution, under certain assumptions it can be used to define the statistical significance of ROC and ROL areas and for comparing the areas of competing forecasts. For large samples the significance of either measure can be accurately assessed using a normal-distribution approximation. Copyright

Importance of the Indian Ocean for simulating rainfall anomalies over eastern and southern Africa

The relative contributions of the Indian Ocean and Pacific Ocean sea surface temperatures (SSTs) to the rainfall variability over eastern central, and southern Africa during the austral spring-summer are examined. The variability of African rainfall is statistically related to both oceans, but the variability in the two oceans is also related. To separate the effects of the Indian and Pacific Oceans, a suite of numerical model simulations is presented: GOGA, the atmosphere is forced by observed SSTs globally; IOGA, the atmosphere is forced by observed SSTs only in the Indian Ocean basin; and POGA, the atmosphere is forced by observed SSTs only in the tropical Pacific basin. While the SST variability of the tropical Pacific exerts some influence over the African region, it is the atmospheric response to the Indian Ocean variability that is essential for simulating the correct rainfall response over eastern, central, and southern Africa. Analyses of the dynamical response(s) seen in the numerical experiments and in the observations indicate that the Pacific and Indian Oceans have a competing influence over the Indian Ocean/African region. This competition is related to the influence of the two oceans on the Walker circulation and the consequences of that variability on low-level fluxes of moisture over central and southern Africa. Finally, given the high correlation found between SST variability in the Indian and Pacific Oceans with the Pacific leading by ∼3 months, we speculate on an approach to long-lead dynamical climate prediction over central-east and southern Africa.

Decadal Variability of Precipitation over Western North America

Abstract Decadal (>7- yr period) variations of precipitation over western North America account for 20%–50% of the variance of annual precipitation. Spatially, the decadal variability is broken into several regional [O(1000 km)] components. These decadal variations are contributed by fluctuations in precipitation from seasons of the year that vary from region to region and that are not necessarily concentrated in the wettest season(s) alone. The precipitation variations are linked to various decadal atmospheric circulation and SST anomaly patterns where scales range from regional to global scales and that emphasize tropical or extratropical connections, depending upon which precipitation region is considered. Further, wet or dry decades are associated with changes in frequency of at least a few short-period circulation “modes” such as the Pacific–North American pattern. Precipitation fluctuations over the southwestern United States and the Saskatchewan region of western Canada are associated with extensiv...

Interdecadal variability of the Pacific Ocean: Model response to observed heat flux and wind stress anomalies

Variability of the Pacific Ocean is examined in numerical simulations with an ocean general circulation model forced by observed anomalies of surface heat flux, wind stress and turbulent kinetic energy (TKE) over the period 1970-88. The model captures the 1976-77 winter time climate shift in sea surface temperature, as well as its monthly, seasonal and longer term variability as evidenced in regional time series and empirical orthogonal function analyses. Examination of the surface mixed-layer heat budget reveals that the 1976-77 shift was caused by a unique concurrance of sustained heat flux input anomalies and very strong horizontal advection anomalies during a multi-month period preceding the shift in both the central Pacific region (where cooling occurred) and the California coastal region (where warming occurred). In the central Pacific, the warm conditions preceding and the cold conditions following the shift tend to be maintained by anomalous vertical mixing due to increases in the atmospheric momentum flux (TKE input) into the mixed layer (which deepens in the model after the shift) from the early 1970s to the late 1970s and 1980s. Since the ocean model does not contain feedback to the atmosphere and it succeeds in capturing the major features of the 1976-77 shift, it appears that the midlatitude part of the shift was driven by the atmosphere, although effects of midlatitude ocean-atmosphere feedback are still possible. The surface mixed-layer heat budget also reveals that, in the central Pacific, the effects of heat flux input and vertical mixing anomalies are comparable in amplitude while horizontal advection anomalies are roughly half that size. In the California coastal region, in contrast, where wind variability is much weaker than in the central Pacific, horizontal advection and vertical mixing effects on the mixed layer heat budget are only one-quarter the size of typical heat flux input anomalies.

Conditional Probabilities, Relative Operating Characteristics, and Relative Operating Levels

Abstract The relative operating characteristic (ROC) curve is a highly flexible method for representing the quality of dichotomous, categorical, continuous, and probabilistic forecasts. The method is based on ratios that measure the proportions of events and nonevents for which warnings were provided. These ratios provide estimates of the probabilities that an event will be forewarned and that an incorrect warning will be provided for a nonevent. Some guidelines for interpreting the ROC curve are provided. While the ROC curve is of direct interest to the user, the warning is provided in advance of the outcome and so there is additional value in knowing the probability of an event occurring contingent upon a warning being provided or not provided. An alternative method to the ROC curve is proposed that represents forecast quality when expressed in terms of probabilities of events occurring contingent upon the warnings provided. The ratios used provide estimates of the probability of an event occurring give...

Evidence for Intensification of North Pacific Winter Cyclones since 1948

Abstract Using NCEP–NCAR reanalysis and in situ data, evidence of important changes in the winter (December–March) cyclone climatology of the North Pacific Ocean over the past 50 years is found. The frequency and intensity of extreme cyclones has increased markedly, with associated upward trends in extreme surface winds between 25° and 40°N and major changes in cyclone–related circulation patterns in the Gulf of Alaska. Related increases in extreme wave heights are inferred from wave measurements and wave–model hindcast results. The more vigorous cyclone activity has apparently resulted from increasing upper–tropospheric winds and vertical wind shear over the central North Pacific. Such changes, which create an environment more favorable for cyclone formation and intensification, may be related to the observed modulation of El Nino–related teleconnections at decadal and longer timescales. It is intriguing that this trend has been relatively steady rather than the sudden or stepwise shifts documented for o...

Long-Lead Seasonal Forecasts—Where Do We Stand?

Abstract The National Weather Service intends to begin routinely issuing long-lead forecasts of 3-month mean U.S. temperature and precipitation by the beginning of 1995. The ability to produce useful forecasts for certain seasons and regions at projection times of upto 1 yr is attributed to advances in data observing and processing, computer capability, and physical understanding-particularly, for tropical ocean-atmosphere phenomena. Because much of the skill of the forecasts comes from anomalies of tropical SST related to ENSO, we highlight here long-lead forecasts of the tropical Pacific SST itself, which have higher skill than the U.S forecasts that are made largely on their basis. The performance of five ENSO prediction systems is examined: Two are dynamical [the Cane-Zebiak simple coupled model of Lamont-Doherty Earth Observatory and the nonsimpie coupled model of the National Centers for Environmental Prediction (NCEP)]; one is a hybrid coupled model (the Scripps Institution for Oceanography-Max Pla...