Rob Allan

A New Globally Complete Monthly Historical Gridded Mean Sea Level Pressure Dataset (HadSLP2): 1850–2004

Abstract An upgraded version of the Hadley Centre’s monthly historical mean sea level pressure (MSLP) dataset (HadSLP2) is presented. HadSLP2 covers the period from 1850 to date, and is based on numerous terrestrial and marine data compilations. Each terrestrial pressure series used in HadSLP2 underwent a series of quality control tests, and erroneous or suspect values were either corrected, where possible, or removed. Marine observations from the International Comprehensive Ocean Atmosphere Data Set were quality controlled (assessed against climatology and near neighbors) and then gridded. The final gridded form of HadSLP2 was created by blending together the processed terrestrial and gridded marine MSLP data. MSLP fields were made spatially complete using reduced-space optimal interpolation. Gridpoint error estimates were also produced. HadSLP2 was found to have generally stronger subtropical anticyclones and higher-latitude features across the Northern Hemisphere than an earlier product (HadSLP1). Duri...

Experimental Dendroclimatic Reconstruction of the Southern Oscillation

Abstract Exactly dated tree-ring chronologies from ENSO-sensitive regions in subtropical North America and Indonesia together register the strongest ENSO signal yet detected in tree-ring data worldwide and have been used to reconstruct the winter Southern Oscillation index (SOI) from 1706 to 1977. This reconstruction explains 53% of the variance in the instrumental winter SOI during the boreal cool season (December-February) and was verified in the time, space, and frequency domains by comparisons with independent instrumental SOI and sea surface temperature (SST) data. The large-scale SST anomaly patterns associated with ENSO in the equatorial and North Pacific during the 1879-1977 calibration period are reproduced in detail by this reconstruction. Cross-spectral analyses indicate that the reconstruction reproduces over 70% of the instrumental winter SOI variance at periods between 3.5 and 5.6 yr, and over 88% in the 4-yr frequency band. Oscillatory modes of variance identified with singular spectrum ana...

ENSO and climatic signals across the Indian Ocean Basin in the global context: part I, interannual composite patterns

This study focuses on the interplay between mean sea level pressure (MSLP), sea surface temperature (SST), and wind and cloudiness anomalies over the Indian Ocean in seasonal composite sequences prior to, during, and after strong, near-global El Nino and La Nina episodes. It then examines MSLP and SST anomalies in the 2–2.5-year quasi-biennial (QB) and 2.5–7-year low-frequency (LF) bands that carry the bulk of the raw ENSO signal. Finally, these fields were examined in conjunction with patterns of correlations between rainfall and joint spatiotemporal empirical orthogonal function (EOF) time series band pass filtered in the QB and LF bands. The seasonal composites indicate that the El Nino-1 (La Nina-1) pattern tends to display a more robust and coherent (weaker and less organized) structure during the evolution towards the mature stage of the event. The reverse tends to be apparent in the cessation period after the peak phase of an event, when El Nino events tend to collapse quite quickly. Climatic variables over the Indian Ocean Basin linked to El Nino and La Nina events show responses varying from simultaneous, to about one seasons lag. In general, SSTs tend to evolve in response to changes in cloud cover and wind strength over both the north and south Indian Ocean. There are also strong indications that the ascending (descending) branch of the Walker circulation is found over the African continent (central Indian Ocean) during La Nina phases, and that the opposite configuration occurs in El Nino events. These alternations are linked to distinct warm–cool (cool–warm) patterns in the north–south SST dipole over the western Indian Ocean region during the El Nino (La Nina) events. An examination of MSLP and SST anomaly patterns in the QB and LF bands shows that signals are more consistent during El Nino-1 and El Nino sequences than they are during La Nina-1 and La Nina sequences. The QB band has a tendency to display the opposite anomaly patterns to that seen on the LF band during the early stages of event onset, and later stage of event cessation, during both El Nino–Southern Oscillation (ENSO) phases. El Nino events tend to be reinforced by signals on both bands up to their mature phase, but are then seen to erode rapidly, as a result of the presence of distinct La Nina anomalies on the QB band after their peak phase. During La Nina events, the opposite is observed during their cessation phase. Both QB and LF bands often display SST dipole anomalies that are not clearly evident in the raw composites alone. An eastern Indian Ocean SST dipole shows a tendency to occur during the onset phase of particular El Nino or La Nina episodes, especially during the austral autumn–winter (boreal spring–summer) and, when linked to tropical-temperate cloud bands, can influence Australian rainfall patterns. Analyses of seasonal correlations between rainfall and joint MSLP and SST EOF time series on QB and LF bands and their dynamical relationship with MSLP and SST anomalies during El Nino and La Nina events, show that the interplay between atmospheric circulation and SST anomalies dictates the observed rainfall response. Instances where either, or both, QB and LF bands are the prime influence on observed rainfall regimes are evident. This ability to discriminate the finer structure of physical relationships, correlations and patterns provides a deeper insight into Indian Ocean responses to ENSO phases. Copyright

A Further Extension of the Tahiti-Darwin SOI, Early ENSO Events and Darwin Pressure

Abstract An extension of the Tahiti minus Darwin Southern Oscillation Index (SOI) from 1882 back to 1876 is reported following the recovery of early Darwin mean sea-level pressure data spanning the period 1865–81. As a result, we are able to compare, for the first time, the major 1877–78 and 1982–83 ENSO events on the basis of this commonly used index. Early Darwin and Jakarta data are also examined in terms of a measure of the Australian response to documented El Nino and/or ENSO events in 1866, 1868, 1871, 1873, 1874 and 1875. The SOI during the 1877–78 ENSO event has a similar temporal response to that in 1982–83, but the index is slightly weaker than in the recent event. Examination of documentary evidence confirms the severity of the drought conditions that affected the Australian continent during the 1877–78 ENSO, and shows that this response is in line with the wider Indo-Pacific impacts reported in the literature. Earlier El Nino phases in 1868 and 1873 are not resolved distinctly in either the Da...

Daily mean sea level pressure reconstructions for the European-North Atlantic region for the period 1850-2003

Abstract The development of a daily historical European–North Atlantic mean sea level pressure dataset (EMSLP) for 1850–2003 on a 5° latitude by longitude grid is described. This product was produced using 86 continental and island stations distributed over the region 25°–70°N, 70°W–50°E blended with marine data from the International Comprehensive Ocean–Atmosphere Data Set (ICOADS). The EMSLP fields for 1850–80 are based purely on the land station data and ship observations. From 1881, the blended land and marine fields are combined with already available daily Northern Hemisphere fields. Complete coverage is obtained by employing reduced space optimal interpolation. Squared correlations (r 2) indicate that EMSLP generally captures 80%–90% of daily variability represented in an existing historical mean sea level pressure product and over 90% in modern 40-yr European Centre for Medium-Range Weather Forecasts Re-Analyses (ERA-40) over most of the region. A lack of sufficient observations over Greenland and...

Multidecadal Variability in the Climate System over the Indian Ocean Region during the Austral Summer

Abstract Several independent historical studies of global atmospheric and oceanic parameters have identified low-frequency fluctuations in the global climate system. Much of this research has focused on Europe, the Atlantic Ocean, and North America. However, recent interest has begun to encompass decadal to multidecadal variability across the Indo-Pacific region. Such variability has been detected in sea surface temperature (SST), mean sea level pressure (MSLP), and surface wind fields over both the landmasses and the oceans. Around the Indian Ocean basin, the broad periods before and after the 1940s show important differences in features such as Indian southwest monsoonal rainfall and circulation patterns, relationships between austral summer rainfall in southern Africa and the El Ni˜o–Southern Oscillation phenomenon, and Australasian MSLP. Very little is known about this variability, particularly during the austral summer. In an effort to isolate such fluctuations and work toward understanding the physi...

Circulation Features Associated with the Winter Rainfall Decrease in Southwestern Australia

Abstract A study of atmospheric and oceanic circulation features in the wider Australian region is undertaken in an attempt to establish the cause(s) of the observed decrease in austral winter (JJA) rainfall over the southwestern portion of Western Australia. Basic regional analyses reveal that long-term mean sea level pressure (MSLP) at Perth, Western Australia, is negatively correlated with southwestern Australian rainfall in JJA over the period 1876–1989. This significant MSLP-rainfall relationship is also observed when using smoothed data series, which resolve a decadal-multidecadal signal embedded within a long-term fluctuation. The latter is punctuated by a downward (upward) rainfall (MSLP) trend over the last 50–60 years that is most pronounced since the mid-1960s. Such relationships are examined further using Southern Hemisphere gridded MSLP, sea surface temperature (SST), and cloudiness data in the Australian sector during the period 1911–1989. On decadal to multidecadal time frames (MSLP bandpas...

Pre-1866 Extensions of the Southern Oscillation Index Using Early Indonesian and Tahitian Meteorological Readings

Pressure data from Indonesia and Tahiti for years before 1866 are used to extend the Southern Oscillation index (SOI) back to 1841, with a gap between 1861 and 1865. Further extension is possible using an index of Jakarta rainday counts back to 1829. Rainday counts correlate (r = -0.60) with average Jakarta pressure for the June-November dry season over the 1876-1944 period. Although low, this correlation is still better than the correlation of tree rings with pressure or SOI. After 1950 the rainday count-pressure relationship alters, and by the 1990s 18% more raindays (an increase of seven per dry season) occur than the pressure would indicate. The dramatic increase in the size and population of Jakarta since 1950 is considered the most likely reason.

Trends and low frequency variability of extra-tropical cyclone activity in the ensemble of twentieth century reanalysis

An objective cyclone tracking algorithm is applied to twentieth century reanalysis (20CR) 6-hourly mean sea level pressure fields for the period 1871–2010 to infer historical trends and variability in extra-tropical cyclone activity. The tracking algorithm is applied both to the ensemble-mean analyses and to each of the 56 ensemble members individually. The ensemble-mean analyses are found to be unsuitable for accurately determining cyclone statistics. However, pooled cyclone statistics obtained by averaging statistics from individual members generally agree well with statistics from the NCEP-NCAR reanalyses for 1951–2010, although 20CR shows somewhat weaker cyclone activity over land and stronger activity over oceans. Both reanalyses show similar cyclone trend patterns in the northern hemisphere (NH) over 1951–2010. Homogenized pooled cyclone statistics are analyzed for trends and variability. Conclusions account for identified inhomogeneities, which occurred before 1949 in the NH and between 1951 and 1985 in the southern hemisphere (SH). Cyclone activity is estimated to have increased slightly over the period 1871–2010 in the NH. More substantial increases are seen in the SH. Notable regional and seasonal variations in trends are evident, as is profound decadal or longer scale variability. For example, the NH increases occur mainly in the mid-latitude Pacific and high-latitude Atlantic regions. For the North Atlantic-European region and southeast Australia, the 20CR cyclone trends are in agreement with trends in geostrophic wind extremes derived from in-situ surface pressure observations. European trends are also consistent with trends in the mean duration of wet spells derived from rain gauge data in Europe.

The International Atmospheric Circulation Reconstructions over the Earth (ACRE) Initiative

In 2006, climate applications scientists in Queensland, Australia, asked the lead author if a longer and more complete historical weather record could be created and fed directly into various crop, pasture, and production models. Existing dynamical reanalyses were steps toward such a product, but they spanned only the last six decades and had well-known shortcomings. To meet the needs of application scientists, new reanalyses would have to extend much further back in time while maintaining accuracy with limited observations. They would also need to be disseminated in a way that is easy to use directly and to downscale to small regions.