Bio/Research

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. Indices for monthly values of intensity, latitude and longitude of the major centers of action are at the link at the top of this page.

Re-interpretation of the North Atlantic Oscillation

The NAO is usually represented by an index based either on the pressure anomaly difference between two stations chosen to represent the Icelandic Low (Reykjavik, Iceland) and the Azores High (Azores or Lisbon), or an empirical orthogonal function of surface pressure or geopotential over the north Atlantic.  These representations of the NAO overlook the fact that the Icelandic Low and the Azores High change positions continuously. An improved estimate of the influence of  pressure fluctuations on regional climate can be attained through a  quantitative assessment of the fluctuations in the pressures and the locations of the Icelandic Low and the Azores High .We have illustrated the usefulness of this approach in several investigations. (Hameed and Piontovski , 2004; Sanchez-Franks et al., 2016; Riaz et al., 2017; Berdahl et al., 2018).

West Coast Drought and Subtropical Highs

Subtropical high pressure centers of action exist to the west of continents and modulate the influx of moisture into coastal regions. Leonardo and Hameed (2015) showed that rainfall across California is dominated by variations in the pressure of the Hawaiian high;  when the high’s pressure increases, the rainfall in California decreases, and vice versa. Also, east–west shifts in the position of this subtropical high significantly influence winter rainfall. When the Hawaiian high shifts to the west, rainfall increases, and vice versa. The South Pacific High modulates interannual variation of winter precipitation over Chile in a similar manner (Barrett and Hameed, 2017).

Atmosphere’s impact on Gulf Stream Position

The path of the Gulf Stream as it leaves the coast near Cape Hatteras is marked by a sharp gradient in ocean temperature known as the North Wall.  Previous work in the literature has considered processes related to the North Atlantic Oscillation (NAO) in triggering latitudinal displacements of North Wall position.  Hameed et al. (2018)  presented evidence that the Atlantic Meridional Mode (AMM) also impacts interannual variations of   the North Wall position.  The AMM signal from the tropics propagates to the Gulf Stream near 200 m depth and there are two different processes  for this interaction.

It has generally been assumed in oceanography literature that the variations of the North Wall and the Gulf Stream are similar. By utilizing satellite data on sea surface height to identify the Gulf Stream and the 15ºC isotherm at 200 m depth to represent the North Wall, we found that the North Wall and Gulf Stream diverge rapidly east of 71^oW, and their separation is caused by  the presence of mesoscale eddies between them (Chi et al., 2019).

Sea Level Rise

Determining the rate of global sea level rise 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.

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