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Seasonality of sea surface temperature (SST) predictions in the tropical Indian Ocean (TIO) was investigated using hindcasts (1982-2009) made with the NCEP Climate Forecast System version 2 (CFSv2). CFSv2 produced useful predictions of the TIO SST with lead times up to several months. A substantial component of this skill was attributable to signals other than the Indian Ocean dipole (IOD). The prediction skill of the IOD index, defined as the difference between the SST anomaly (SSTA) averaged over 10°S-0°, 90°-110°E and 10°S-10°N, 50°-70°E, had strong seasonality, with high scores in the boreal autumn. In spite of skill in predicting its two poles with longer leads, CFSv2 did not have skill significantly better than persistence in predicting IOD. This was partly because the seasonal nature of IOD intrinsically limits its predictability. The seasonality of the predictable patterns of the TIO SST was further explored by applying the maximum signal-to-noise (MSN) empirical orthogonal function (EOF) method to the predicted SSTA in March and October, respectively. The most predictable pattern in spring was the TIO basin warming, which is closely associated with El Niño. The basin mode, including its associated atmospheric anomalies, can be predicted at least 9 months ahead, even though some biases were evident. On the other hand, the most predictable pattern in fall was characterized by the IOD mode. This mode and its associated atmospheric variations can be skillfully predicted only 1-2 seasons ahead. Statistically, the predictable IOD mode coexists with El Niño; however, the 1994 event in a non-ENSO year (at least not a canonical ENSO year) can also be predicted at least 3 months ahead by CFSv2. [ABSTRACT FROM AUTHOR]

Rainfall and circulation changes accompanying the Indian monsoon onset are examined, focusing on the African continent and neighboring areas. The Indian Meteorological Department official monsoon onset dates over Kerala (MOK; on average on 1 June) are used. Composites are formed at a pentad (5 days) time scale to compare pre- and postonset conditions. Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) data for 1979–2007 indicate that a substantial rainfall decrease over several parts of Africa is associated with MOK. Significant rainfall anomalies, after removal of the mean seasonal cycle, are found in eastern Africa and the nearby western Indian Ocean. Indian monsoon onset dates over the period 1958–2001 are correlated at 0.55 with the cessation dates of the March–May rainy season (the long rains) averaged over Kenya and northern Tanzania. The rainy season cessation leads the onset by 12 days, on average. An early cessation of the March–May rains tends to precede an early onset over India. Over East Africa, the rainfall decrease is associated with a strengthening of the southerly winds (Somali jet) shortly before MOK, enhancing wind divergence and wind shear. A weaker rainfall signal is noted over western Africa and the Gulf of Guinea. MOK coincides with a pause in the progression of the West African monsoon. The pause is associated with anomalous descent over the Sahel, culminating two to three pentads after MOK. Composite maps further indicate that MOK is followed over much of the African continent by a sea level pressure rise and, between India and North Africa, a westward propagation of easterly wind and positive geopotential height anomalies. Many of these signals are associated with Madden–Julian oscillations, but independent features, suggesting Rossby wave propagation over North Africa, are also found to connect MOK and West Africa. Overall, these results are indicative of a large-scale adjustment of the atmospheric dynamics across both eastern and western Africa in conjunction with the monsoon onset over India. [ABSTRACT FROM AUTHOR]

A continuous record of 65 987 daily-mean temperature observations since 1828 has been developed for Manhattan, Kansas, by screening and correcting original station records of the U.S. Army Surgeon General, the Smithsonian Institution, and the Signal Service. Hourly, minimum, and maximum temperature observations from seven discontinuous historical stations and four modern stations in Kansas, Missouri, and Oklahoma were used to compile this unbroken record of daily-mean temperature. The historical temperature data were linked with the modern temperature record for Manhattan after these data were adjusted for time of observation differences, station movements, and changes in the environment around the station. The new daily-mean temperature reconstruction for Manhattan now extends with confidence back to 1 July 1855 and with more uncertainty back to 1 July 1828. The uncertainty prior to 1855 is due to instrumentation changes in 1843 and changes in observation practices in 1855 that occurred at many stations. The error estimates reported in this paper do not reflect these potential inhomogeneities and should be considered lower limits. Nonetheless, this new daily record indicates significant warming in all seasons; in heating and cooling degree-days; in the warmest and coldest days of the year; in extremes above the 90th percentile and below the 10th percentile; in the frequency of winter cold waves and summer heat waves; and in the overall annual-mean temperature, which has warmed by 1.57° ± 0.23°C since 1855 (1.27° ± 0.23°C since 1829). The warm Dust Bowl event in the summer of the 1930s and cold winters of the 1870s and 1880s dominate the reconstruction and included some of the warmest and coldest daily extremes, respectively, of the last 154–180 yr. This new reconstruction is currently the longest unbroken daily corrected record in the Americas. These data indicate that the nineteenth century was fundamentally cooler than the twentieth and early twenty-first century. [ABSTRACT FROM AUTHOR]

A consistent analysis of natural variability and secular trend in Pacific SSTs in the twentieth century is presented. By focusing on spatial and temporal recurrence, but without imposition of periodicity constraints, this single analysis discriminates between biennial, ENSO, and decadal variabilities, leading to refined evolutionary descriptions, and between these natural variability modes and secular trend, all without advance filtering (and potential aliasing) of the SST record. SST anomalies of all four seasons are analyzed together using the extended-EOF technique. Canonical ENSO variability is encapsulated in two modes that depict the growth (east-to-west along the equator) and decay (near-simultaneous amplitude loss across the basin) phases. Another interannual mode, energetic in recent decades, is shown linked to the west-to-east SST development seen in post–climate shift ENSOs: the noncanonical ENSO mode. The mode is closely related to Chiang and Vimont’s meridional mode, and leads to some reduction in canonical ENSO’s oscillatory tendency. Pacific decadal variability is characterized by two modes: the Pan-Pacific mode has a horseshoe structure with the closed end skirting the North American coast, and a quiescent eastern equatorial Pacific. The mode exhibits surprising connections to the tropical/subtropical Atlantic, with correlations there resembling the Atlantic multidecadal oscillation. The second decadal mode—the North Pacific mode—captures the 1976/77 climate shift and is closer to Mantua’s Pacific decadal oscillation. This analysis shows, perhaps for the first time, the striking links of the North Pacific mode to the western tropical Pacific and Indian Ocean SSTs. The physicality of both modes is assessed from correlations with the Pacific biological time series. Finally, the secular trend is characterized: implicit accommodation of natural variability leads to a nonstationary SST trend, including midcentury cooling. The SST trend is remarkably similar to the global surface air temperature trend. Geographically, a sliver of cooling is found in the central equatorial Pacific in the midst of widespread but nonuniform warming in all basins. An extensive suite of sensitivity tests, including counts of the number of observational analogs of the modes in test analyses, supports the robustness of this analysis. [ABSTRACT FROM AUTHOR]

Cluster analysis was used to study seasonal forecast skills of the winter season NCEP seasonal forecast model (SFM) hindcasts over the Pacific–North America (PNA) sector. Two skill scores based on cluster mean and ensemble mean are compared. It was shown that the anomaly correlation coefficients (ACCs) of cluster mean are generally higher than those of the simple ensemble mean. The results indicated that the skill was affected by the existence of multiple atmospheric regimes. Multiple regimes tend to appear more often in near-normal tropical Pacific sea surface temperature (SST) episodes, while a single regime tends to appear during warm/cold episodes. The dissimilarity among the cluster members is small and the number of the dominant cluster members is also small when the tropical SST anomaly is large, suggesting that the external forcing reduces the frequency of occurrence of the multiple regimes. The ACC improvements from the ensemble mean ACCs to the cluster mean ACCs are statistically significant. Thus, the cluster mean can be used as a supplementary tool for seasonal forecasting. [ABSTRACT FROM AUTHOR]

It is suggested that the seasonal dependence of interannual variability displayed by observed sea surface temperature (SST) over the north tropical Atlantic primarily reflects the seasonality of the remote forcing associated with the North Atlantic and Southern Oscillations and is controlled by the mean damping time scale of SST anomaly. A stochastic model including a seasonally dependent forcing is used to test this hypothesis against observations. [ABSTRACT FROM AUTHOR]

The natural variability over the North Pacific, where the influence of tropical El Niño—Southern Oscillation (ENSO) events is substantial, is examined to determine whether there is a large change owing to a difference in the ENSO forcing anomaly. The hindcast ensemble runs of the Seasonal Forecast Model of the National Centers for Environmental Prediction are analyzed for this assessment. Four sets of 10-member ensemble hind-casts out to 7 months with T42 horizontal resolution and another four sets with T62 resolution are examined in detail. The results consistently indicate that the natural variability, on both seasonal and monthly time scales, is significantly smaller during El Niño boreal winters than during La Niña boreal winters. The implication is that the predictability on both seasonal and monthly time scales over the North Pacific is potentially higher during El Niño winters than during La Niña winters. [ABSTRACT FROM AUTHOR]