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Sultan Hameed


Ph.D., 1968, University of Manchester

Climate change: analysis, impacts, and predictability

Atmospheric Centers of Action Indices

Research Interests

My recent work has focused on the study of the large-scale semi-permanent high and low pressure systems known as the Atmospheric Centers of Action. The major centers are the Icelandic Low, the Azores High, the Aleutian Low, the Hawaiian High and the Siberian High in the Northern Hemisphere and the Subtropical Highs in the southern Atlantic, Pacific and the Indian Oceans. Variations in the global circulation cause variations in the intensities and the morphologies of the atmospheric centers of action (COA). In turn, these systems influence atmospheric and oceanic circulations over their respective domains. The centers of action therefore provide links between variations on the global and regional scales.

In addition, I have interest in investigating mechanisms through which changes in the Sun’s output influence climate at the surface of the earth.

Re-interpretation of the North Atlantic Oscillation

A standard approach to characterizing large-scale fluctuations in atmospheric circulation over the North Atlantic is to use the NAO index.  The NAO index is based on measurements of the pressure difference between the Icelandic Low and Azores High at two fixed locations, Lisbon, Portugal and Stykkisholmur, Iceland. However, it is known that these two pressure systems have extended structures that migrate considerably and whose motions are not entirely coupled. As such, a better estimate of the influence of atmospheric pressure fluctuations on regional climate can be attained through a more quantitative assessment of the fluctuations in the pressures and the locations of these Centers of Action .We have illustrated the usefulness of this reinterpretation of the NAO in several recent investigations by considering the variations of the Azores High and the Icelandic Low systems separately.

Sea Level Rise

Determining the rate of global sea level rise (GSLR) during the past century is important to understanding the impact of climate change. However, this is complicated by local sea-level variability that is one to two orders of magnitude greater than the trend.  Kolker and Hameed (2007) showed that the position and intensity of the Azores High and the Icelandic Low explain major fraction of the variability and trend at key Atlantic Ocean tide gauges over the past century. The COA changes influence winds, pressure and sea-surface temperatures, thereby influencing sea level. Since the role of the COA as meteorological drivers of sea level change was not previously recognized, it is likely that estimates of GSLR by IPCC (2007) are over-estimates.

Transport of Dust over the Atlantic

Previous studies suggested that the variability of the Saharan dust over the North Atlantic can be explained by the North Atlantic Oscillation. In an alternative approach, we correlated TOMS aerosol data with the positions and the pressure of the Azores High and the Icelandic Low (Riemer 2006). This showed that the latitudinal position of the Azores High is the parameter that displays the highest correlation with the dust over the specific region of the North Atlantic, and provides a consistent physical explanation of transport of dust into this region.

Each summer large amounts of mineral dust from the Sahara are transported across the Atlantic and arrive at the Caribbean with far-reaching implications for climate in this region. North Atlantic Oscillation and ENSO are not correlated with dust over the Caribbean. Our results showed that  Saharan dust transport into the Caribbean is related to longitudinal displacement and pressure fluctuation of the Hawaiian High as well as longitudinal displacement of the Azores High, each center of action controlling a different transport route from a distinct source regions are responsible for transporting mineral dust into the Caribbean ( Doherty et. al, 2008). A northern mode in which dust mobilized from the Sahara travels westward is controlled primarily by the Azores High and a southern mode in which intense dust clouds originating in the Sahel region travel over the Gulf of Guinea to reach the Caribbean is controlled primarily by teleconnections with the Hawaiian High.

Sun-Climate Mechanism

Mechanisms by which small changes in the sun’s energy output during the solar cycle can cause changes in weather and climate have been a puzzle and the subject of intense research in recent decades. Hameed and Lee (2005) showed that differences in surface circulation conditions during solar maximum and minimum periods are caused by differences in the frequencies with which circulation perturbations in the stratosphere reach the surface. A much greater fraction of stratospheric perturbations penetrate to the surface during solar maximum conditions than during minimum conditions. This difference is more striking when the zonal wind direction in the tropics is from the west: no stratospheric signals reach the surface when equatorial 50 hPa winds are from the west under solar minimum conditions, and over 50 percent reach the surface under solar maximum conditions. Lee and Hameed (2007) showed that the leading EOF patterns in summer are distinct from the patterns in winter. The summer Northern Hemisphere annular mode (NAM) in the stratosphere has the same sign everywhere but shows higher variability at low latitudes unlike the dipolar structure of the winter NAM. A physical interpretation of the summer NAM in the stratosphere is readily apparent because low (high) values of its principal component correspond to warmer (colder) than climatological mean summer conditions in the stratosphere.  Also, the summer NAM in the stratosphere and upper troposphere is correlated with the solar ultraviolet flux, such that in solar maximum conditions the stratospheric circulation is more ‘‘summer-like’’ than average and it is less summer-like in solar minimum conditions.

Selected Publications

S.M. Gianelli, A. A. Lacis, B.E. Carlson and S. Hameed, Evidence of a weakly absorbing intermediate mode of aerosols in AERONET data from Saharan and Sahelian sites. J. Geophys. Res. Vol 118, 2,661–12,672, doi:10.1002/2013JD020342, 2013

Hameed, S. and N. Riemer, Relationship of Sahel Precipitation and Atmospheric centers of Action, Advances in Meteorology, Volume 2012, doi:10.1155/2012/953853, 2012

Hameed, S. and S. Piontkovski, The dominant influence of the Icelandic Low on the position of the Gulf Stream northwall, Geophysical Research Letters, 31: , 2004. (pdf version)

S. Hameed and J. N. Lee,” A mechanism for sun-climate connection”, GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L23817, doi: 10.1029/2005GL024393, 2005 (pdf version)

N. Riemer, O. Doherty and S. Hameed,” On the variability of African dust transport across the Atlantic”, GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L13814, doi: 10.1029/2006GL026163, 2006 (pdf version)

F. Bakalian, S. Hameed, and R. Pickart, “Influence of the Icelandic Low latitude on the frequency of Greenland tip jet events: Implications for Irminger Sea convection” JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, C04020, doi:10.1029/2006JC003807, 2007 (pdf version)

J. N. Lee and S. Hameed ,”The Northern Annular Mode in Summer, Its Physical Significance and Its Relation to Solar Activity Variations”. Jour. Geophys. Res., VOL. 112, D15111, doi:10.1029/2007JD008394, 2007 (pdf version)

J. N. Lee, S. Hameed and D. T. Shindell,” The Northern Annular Mode in Summer and Its Relation to Solar Activity Variations in the GISS ModelE”,  Journal of Atmospheric and Solar-Terrestrial Physics  vol. 70 , 730–741, 2008 (
pdf version)

Doherty, O. M., N. Riemer, and S. Hameed (2008), Saharan mineral dust transport into the Caribbean: Observed atmospheric controls and trends, J. Geophys. Res., 113, D07211, doi:10.1029/2007JD009171. (pdf version)

A. Kolker and S. Hameed,” Meteorologically Driven Trends in Sea Level Rise”, Geophys. Res. Lettrs, VOL. 34, L23616, doi:10.1029/2007GL031814, 2007 (pdf version)

David G. Kimmel and Sultan Hameed,”An update on the relationship between the North Atlantic Oscillation and Calanus finmarchicus, Marine Ecology Progress Series, 2008 (in press)

Undergraduates Share Summer Research at REU Symposium

Undergraduate students from universities across the country who spent their summer conducting research at SoMAS presented their work at the 2009 Research Experience for Undergraduates (REU) Symposium on Thursday July 30, 2009. The theme of this summers REU program was...

Owen Doherty and Charles Wall Awarded 2008 Schubel Fellowships

Owen Doherty  and Charles Wall have been selected as the two new Schubel Fellows for 2008. The J.R. Schubel Graduate Fellowship, named for former Dean and Director of MSRC Jerry Schubel, provides an annual stipend supplement for SoMAS graduate students committed to...

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