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B. Bhaskaran
,
J. M. Murphy
, and
R. G. Jones

Abstract

Simulations of the intraseasonal oscillation (ISO) in the Indian summer monsoon by a general circulation model (GCM) and a nested regional climate model (RCM) are described. The ISO is the leading mode of subseasonal variability in both models. It is shown to be associated with circulation and precipitation anomalies that propagate northward from the equatorial Indian Ocean to the foothills of the Himalayas on the 30–50-day timescale. The spatial structure, timescale, and propagation characteristics of the simulated ISO are found to be similar to those of the leading observed intraseasonal mode. In particular, both of the simulated versions and the observed version all involve periodic deepening and filling of the monsoon trough resulting from northward propagation of troughs and ridges from the equatorial region. Some differences do occur, however: the GCM version of the ISO is too zonally symmetric and the ISO is too strong in both models. During the positive phase of the ISO (i.e., when the ISO acts to enhance the monsoon trough), composite low-level circulation anomalies in the monsoon trough region are found to be somewhat weaker in the RCM than in the GCM because the RCM signal is obscured to a greater degree by noise associated with other modes of variability.

In the GCM, large precipitation anomalies are found to be associated with the positive and negative phases of the ISO in many areas, particularly at the latitudes of the monsoon trough. However, the use of a fine-resolution nested RCM leads to the identification of important spatial detail not present in the GCM distributions. This is particularly true in mountainous regions, most notably in the foothills of the Himalayas: here the RCM simulates a strong precipitation signal, which appears to represent an orographic component of the response to circulation anomalies associated with the ISO, whereas this precipitation signal is absent in the GCM.

The use of a nested RCM also allows the phase relationship between the oscillations in the two models to be studied. The relationship is found to be close in most years, suggesting that the regional ISO in the RCM is modulated by the driving GCM circulation via the lateral boundary forcing on the 30–50-day timescale. Several examples are also found, however, where the GCM and RCM diverge, showing that the northward-propagating mode can occur independently of any global forcing on the same timescale, in agreement with observational evidence.

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T. R. Marthews
,
F. E. L. Otto
,
D. Mitchell
,
S. J. Dadson
, and
R. G. Jones
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Y. P. Meillier
,
R. G. Frehlich
,
R. M. Jones
, and
B. B. Balsley

Abstract

Constant altitude measurements of temperature and velocity in the residual layer of the nocturnal boundary layer, collected by the Cooperative Institute for Research in Environmental Sciences (CIRES) Tethered Lifting System (TLS), exhibit fluctuations identified by previous work (Fritts et al.) as the signature of ducted gravity waves. The concurrent high-resolution TLS turbulence measurements (temperature structure constant C 2 T and turbulent kinetic energy dissipation rate ε) reveal the presence of patches of enhanced turbulence activity that are roughly synchronized with the troughs of the temperature and velocity fluctuations. To investigate the potentially dominant role ducted gravity waves might play on the modulation of atmospheric stability and therefore, on turbulence, time series of the wave-modulated gradient Richardson number (Ri) and of the vertical gradient of potential temperature ∂θ/∂z(t) are computed numerically and compared to the TLS small-scale turbulence measurements. The results of this study agree with the predictions of previous theoretical studies (i.e., wave-generated fluctuations of temperature and velocity modulate the gradient Richardson number), resulting in periodic enhancements of turbulence at Ri minima. The patches of turbulence observed in the TLS dataset are subsequently identified as convective instabilities generated locally within the unstable phase of the wave.

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G-C. Yuan
,
L. J. Pratt
, and
C. K. R. T. Jones

Abstract

Cross-stream mixing and Lagrangian transport caused by chaotic advection within a baroclinic (2½ layer) meandering jet are investigated. The quasi-steady meanders arise as a result of evolution from an initial small-amplitude instability. The investigation keys on the proposition, made in earlier work, that the cross-jet mixing and transport resulting from the meandering motions are maximized at a subsurface level. It is found that the results depend largely on the size of the shear between the two active layers (which are referred to as the upper and lower layer), as measured by a parameter α. For weak vertical shear (α greater than about 0.5) the primary instability is barotropic and there is no cross-jet transport in either of the active layers. Barriers to transport are identified as plateaus in the probability density function (PDF) of potential vorticity distributions. For stronger shear (α less than about 0.4), baroclinic instability comes into play, and the lower layer experiences barrier destruction followed by cross-jet exchange and mixing. The upper-layer barrier remains intact. The barrier destruction has a dynamical effect as evidenced by the decay of total variance of potential vorticity in the lower layer. Of interest is that the value of α estimated for the Gulf Stream lies in the range 0.4–0.5.

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G. P. Können
,
P. D. Jones
,
M. H. Kaltofen
, and
R. J. Allan

Abstract

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.

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G. van der Schrier
,
K. R. Briffa
,
P. D. Jones
, and
T. J. Osborn

Abstract

Maps of monthly self-calibrating Palmer Drought Severity Index (SC-PDSI) have been calculated for the period of 1901–2002 for Europe (35°–70°N, 10°W–60°E) with a spatial resolution of 0.5° × 0.5°. The recently introduced SC-PDSI is a convenient means of describing the spatial and temporal variability of moisture availability and is based on the more common Palmer Drought Severity Index. The SC-PDSI improves upon the PDSI by maintaining consistent behavior of the index over diverse climatological regions. This makes spatial comparisons of SC-PDSI values on continental scales more meaningful.

Over the region as a whole, the mid-1940s to early 1950s stand out as a persistent and exceptionally dry period, whereas the mid-1910s and late 1970s to early 1980s were very wet. The driest and wettest summers on record, in terms of the amplitude of the index averaged over Europe, were 1947 and 1915, respectively, while the years 1921 and 1981 saw over 11% and over 7% of Europe suffering from extreme dry or wet conditions, respectively.

Trends in summer moisture availability over Europe for the 1901–2002 period fail to be statistically significant, both in terms of spatial means of the drought index and in the area affected by drought. Moreover, evidence for widespread and unusual drying in European regions over the last few decades is not supported by the current work.

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P. M. Kelly
,
P. D. Jones
,
C. B. Sear
,
B. S. G. Cherry
, and
R. K. Tavakol

Abstract

We describe annual and seasonal changes in air temperatures over high latitudes of the Northern Hemisphere during the period 1881–1980. Trends (that is, fluctuations on time scales greater than 20 years) in the average temperature of the Arctic are compared with those of the Northern Hemisphere. Seasonal and regional departures from the long-term trends in the average temperature of the Arctic are identified. Spatial patterns of variation in the Arctic temperature field are determined by principal component analysis and the major characteristics of the time series of the dominant patterns are summarized.

Trends in Arctic temperatures have been broadly similar to those for the Northern Hemisphere during the study period. The Arctic variations were, however, greater in magnitude and more rapid. The spatial pattern of change associated with the trend in Arctic temperatures is clearly identified by principal component analysis. It shows that the trends have, in general, been Arctic-wide, but that certain regions are particularly sensitive to long-term variations, most notably northwest Greenland and around the Kara Sea. There is some evidence that the pattern of Arctic cooling that occurred after 1940 was more complex than the warming that affected the whole Arctic during the 1920's and 1930's. Warming of the Arctic has occurred during the 1970's, but is not yet of sufficient duration to be considered long term, except, perhaps, in spring. The average temperature of the Arctic during the 1970's was equal to that of the 1960's, indicating a cessation of the long-term cooling trend but not, as yet, a shift to long-term warming. Short-term variations in temperature appear to be most pronounced close to major regions of sea-ice production and decay.

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Philip W. Mote
,
Myles R. Allen
,
Richard G. Jones
,
Sihan Li
,
Roberto Mera
,
David E. Rupp
,
Ahmed Salahuddin
, and
Dean Vickers

Abstract

Computing resources donated by volunteers have generated the first superensemble of regional climate model results, in which the Hadley Centre Regional Model, version 3P (HadRM3P), and Hadley Centre Atmosphere Model, version 3P (HadAM3P), were implemented for the western United States at 25-km resolution. Over 136,000 valid and complete 1-yr runs have been generated to date: about 126,000 for 1960–2009 using observed sea surface temperatures (SSTs) and 10,000 for 2030–49 using projected SSTs from a global model simulation. Ensemble members differ in initial conditions, model physics, and (potentially, for future runs) SSTs. This unprecedented confluence of high spatial resolution and large ensemble size allows high signal-to-noise ratio and more robust estimates of uncertainty. This paper describes the experiment, compares model output with observations, shows select results for climate change simulations, and gives examples of the strength of the large ensemble size.

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M. N. Raphael
,
G. J. Marshall
,
J. Turner
,
R. L. Fogt
,
D. Schneider
,
D. A. Dixon
,
J. S. Hosking
,
J. M. Jones
, and
W. R. Hobbs

Abstract

The Amundsen Sea low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example, in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, the authors summarize the current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, as well as ice-core chemistry and climate model projections, to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the twenty-first century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases.

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G. K. Grice
,
R. J. Trapp
,
S. F. Corfidi
,
R. Davies-Jones
,
C. C. Buonanno
,
J. P. Craven
,
K. K. Droegemeier
,
C. Duchon
,
J. V. Houghton
,
R. A. Prentice
,
G. Romine
,
K. Schlachter
, and
K. K. Wagner
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