Ileana Bladé

The Effective Number of Spatial Degrees of Freedom of a Time-Varying Field

The authors systematically investigate two easily computed measures of the effective number of spatial degrees of freedom (ESDOF), or number of independently varying spatial patterns, of a time-varying field of data. The first measure is based on matching the mean and variance of the time series of the spatially integrated squared anomaly of the field to a chi-squared distribution. The second measure, which is equivalent to the first for a long time sample of normally distributed field values, is based on the partitioning of variance between the EOFs. Although these measures were proposed almost 30 years ago, this paper aims to provide a comprehensive discussion of them that may help promote their more widespread use. The authors summarize the theoretical basis of the two measures and considerations when estimating them with a limited time sample or from nonnormally distributed data. It is shown that standard statistical significance tests for the difference or correlation between two realizations of a field (e.g., a forecast and an observation) are approximately valid if the number of degrees of freedom is chosen using an appropriate combination of the two ESDOF measures. Also described is a method involving ESDOF for deciding whether two time-varying fields are significantly correlated to each other. A discussion of the parallels between ESDOF and the effective sample size of an autocorrelated time series is given, and the authors review how an appropriate measure of effective sample size can be computed for assessing the significance of correlations between two time series.

Observed and simulated impacts of the summer NAO in Europe: implications for projected drying in the Mediterranean region

Climate models predict substantial summer precipitation reductions in Europe and the Mediterranean region in the twenty-first century, but the extent to which these models correctly represent the mechanisms of summertime precipitation in this region is uncertain. Here an analysis is conducted to compare the observed and simulated impacts of the dominant large-scale driver of summer rainfall variability in Europe and the Mediterranean, the summer North Atlantic Oscillation (SNAO). The SNAO is defined as the leading mode of July–August sea level pressure variability in the North Atlantic sector. Although the SNAO is weaker and confined to northern latitudes compared to its winter counterpart, with a southern lobe located over the UK, it significantly affects precipitation in the Mediterranean, particularly Italy and the Balkans (correlations of up to 0.6). During high SNAO summers, when strong anticyclonic conditions and suppressed precipitation prevail over the UK, the Mediterranean region instead is anomalously wet. This enhanced precipitation is related to the presence of a strong upper-level trough over the Balkans—part of a hemispheric pattern of anomalies that develops in association with the SNAO—that leads to mid-level cooling and increased potential instability. Neither this downstream extension nor the surface influence of the SNAO is captured in the two CMIP3 models examined (HadCM3 and GFDL-CM2.1), with weak or non-existent correlations between the SNAO and Mediterranean precipitation. Because these models also predict a strong upward SNAO trend in the future, the error in their representation of the SNAO surface signature impacts the projected precipitation trends. In particular, the attendant increase in precipitation that, based on observations, should occur in the Mediterranean and offset some of the non-SNAO related drying does not occur. Furthermore, the fact that neither the observed SNAO nor summer precipitation in Europe/Mediterranean region exhibits any significant trend so far (for either the full century or the recent half of the record) does not increase our confidence in these model projections.

The Influence of Midlatitude Ocean–Atmosphere Coupling on the Low-Frequency Variability of a GCM. Part I: No Tropical SST Forcing*

Abstract This study examines the extent to which the thermodynamic interactions between the midlatitude atmosphere and the underlying oceanic mixed layer contribute to the low-frequency atmospheric variability. A general circulation model, run under perpetual northern winter conditions, is coupled to a motionless constant-depth mixed layer in midlatitudes, while elsewhere the sea surface temperature (SST) is kept fixed; interannual tropical SST forcing is not included. It is found that coupling does not modify the spatial organization of the variability. The influence of coupling is manifested as a slight reddening of the spectrum of 500-mb geopotential height and a significant enhancement of the lower-tropospheric thermal variance over the oceans at very low frequencies by virtue of the mixed-layer adjustment to surface air temperature variations that occurs on those timescales. This adjustment effectively reduces the thermal damping of the atmosphere associated with surface heat fluxes (or negative ocea...

INFLUENCE OF CHOICE OF TIME PERIOD ON GLOBAL SURFACE TEMPERATURE TREND ESTIMATES

Annual global surface temperature and global land surface temperature trends are calculated for all possible periods of the historical record between 1850 and 2009. Two-dimensional parameter diagrams show the critical influence of the choice of start and end years on the calculated trend and associated temperature changes and suggest time scales required to establish robust trends. The largest trends and associated temperature changes are all positive and have occurred over periods ending in recent years. Substantial positive changes also occurred from the early twentieth century until the mid-1940s. The continents exhibit greater long-term warming than the global average overall, but less warming in the early part of the century (segments ending in the 1940s). The recent period of short-term cooling beginning in the late 1990s is neither statistically significant nor unusual in the context of trend variability in the full historical record. Global-mean and land surface temperature changes for periods end...

Seasonality of African Precipitation from 1996 to 2009

AbstractA precipitation climatology of Africa is documented using 12 years of satellite-derived daily data from the Global Precipitation Climatology Project (GPCP). The focus is on examining spatial variations in the annual cycle and describing characteristics of the wet season(s) using a consistent, objective, and well-tested methodology. Onset is defined as occurring when daily precipitation consistently exceeds its local annual daily average and ends when precipitation systematically drops below that value. Wet season length, rate, and total are then determined. Much of Africa is characterized by a single summer wet season, with a well-defined onset and end, during which most precipitation falls. Exceptions to the single wet season regime occur mostly near the equator, where two wet periods are usually separated by a period of relatively modest precipitation. Another particularly interesting region is the semiarid to arid eastern Horn of Africa, where there are two short wet seasons separated by nearly...

Understanding recent eastern Horn of Africa rainfall variability and change

AbstractObservations and sea surface temperature (SST)-forced ECHAM5 simulations are examined to study the seasonal cycle of eastern Africa rainfall and its SST sensitivity during 1979–2012, focusing on interannual variability and trends. The eastern Horn is drier than the rest of equatorial Africa, with two distinct wet seasons, and whereas the October–December wet season has become wetter, the March–May season has become drier.The climatological rainfall in simulations driven by observed SSTs captures this bimodal regime. The simulated trends also qualitatively reproduce the opposite-sign changes in the two rainy seasons, suggesting that SST forcing has played an important role in the observed changes. The consistency between the sign of 1979–2012 trends and interannual SST–precipitation correlations is exploited to identify the most likely locations of SST forcing of precipitation trends in the model, and conceivably also in nature. Results indicate that the observed March–May drying since 1979 is due ...

Origin of Convectively Coupled Kelvin Waves over South America

Convectively coupled Kelvin waves over the South American continent are examined through the use of temporal and spatial filtering of reanalysis, satellite, and gridded rainfall data. They are most prominent from November to April, the season analyzed herein. The following two types of events are isolated: those that result from preexisting Kelvin waves over the eastern Pacific Ocean propagating into the continent, and those that apparently originate over Amazonia, forced by disturbances propagating equatorward from central and southern South America. The events with precursors in the Pacific are mainly upper-level disturbances, with almost no signal at the surface. Those events with precursors over South America, on the other hand, originate as upper-level synoptic wave trains that pass over the continent and resemble the ‘‘cold surges’’ documented by Garreaud and Wallace. As the wave train propagates over the Andes, it induces a southerly low-level wind that advects cold air to the north. Precipitation associated with a cold front reaches the equator a few days later and subsequently propagates eastward with the characteristics of a Kelvin wave. The structures of those waves originating over the Pacific are quite similar to those originating over South America as they propagate to eastern South America and into the Atlantic. South America Kelvin waves that originate over neither the Pacific nor the midlatitudes of South America can also be identified. In a composite sense, these form over the eastern slope of the Andes Mountains, close to the equator. There are also cases of cold surges that reach the equator yet do not form Kelvin waves. The interannual variability of the Pacific-originating events is related to sea surface temperatures in the central–eastern Pacific Ocean. When equatorial oceanic conditions are warm, there tends to be an increase in the number of disturbances that reach South America from the Pacific.

The Influence of Midlatitude Ocean–Atmosphere Coupling on the Low-Frequency Variability of a GCM. Part II: Interannual Variability Induced by Tropical SST Forcing*

Abstract This study extends the investigation of the impact of midlatitude ocean–atmosphere interactions on the atmospheric circulation to the interannual timescale by incorporating SST variability in the tropical Pacific representative of observed conditions. Two perpetual January GCM simulations are performed to examine the changes in the low-frequency atmospheric variability brought about by the inclusion of an interactive slab mixed layer in midlatitudes, in particular the changes in the extratropical response to ENSO-like tropical 90-day mean SST anomalies. It is found that midlatitude coupling alters the spatial organization of the low-frequency variability in qualitatively the same manner (but not to the same extent) as tropical SST variability—namely, by selectively enhancing (in terms of amplitude, persistence, and/or frequency of occurrence) certain of the preexisting (natural) dominant modes without significantly modifying them or generating new ones. While tropical SST forcing results in a not...

Predicting East African spring droughts using Pacific and Indian Ocean sea surface temperature indices

Introduction Conclusions References

Characteristics of North American Summertime Rainfall with Emphasis on the Monsoon

The core region of the North American summer monsoon is examined using spatially averaged daily rainfall observations obtained from gauges, with the objective of improving understanding of its climatology and variability. At most grid points, composite and interannual variations of the onset and end of the wet season are well defined, although, among individual stations that make up a grid average, variability is large. The trigger for monsoon onset in southern and eastern Mexico appears to be related to a change in vertical velocity, while for northwestern Mexico, Arizona, and New Mexico it is related to a reduction in stability, as indicated by a decrease in the lifted index. The wet-season rain rate is a combination of the wet-day rain rate, which decreases with distance from the coast, and the wet-day frequency, which is largest over the Sierra Madre Occidental. Thus the maximum total rate lies slightly to the west of the highest orography. As has been previously noted, onset is not always well correlated with total seasonal precipitation, so in these areas, variations of wet-day frequency and wet-day rain rate must be important. Correlations are small between the wet-day frequency and the wet-day rate, and the former is better correlated than the latter with the seasonal rain rate. Summer rainfall in central to southern Mexico exhibits moderate negative correlations with the leading pattern of sea surface temperature (SST) anomalies in the equatorial Pacific, which projects strongly onto El Nino. The influence of equatorial SSTs on southern Mexico rainfall seems to operate mainly through variability of the wet-day frequency, rather than through variations of the wet-day rain rate.