Development of a Seasonally –Inclusive NAO Index
In a 1987 paper, Lamb and Peppler presented evidence linking the NAO to the interannual variability of Moroccan rainfall. From this study emerged an extended scientific collaboration with several Moroccan scientists. One area of subsequent research with one of these Moroccan scientists, Mostafa El Hamly, was the construction of a seasonally and geographically varying “mobile” index of the North Atlantic Oscillation (NAOm). Monthly sea level pressure (SLP) data from the NCEP reanalysis for 1948-1999 were used to identify the locations of the maximum monthly out-of-phase signal between the subpolar and subtropical North Atlantic SLP (Portis et al. 2001, Fig. 1). There is a general migration of the subtropical node from southeast-northwest from winter to summer and return from summer to winter with exceptions in February-March and November. As a validation of the NAOm, we evaluated its ability to capture the North Atlantic surface westerly flow by correlating it with historical measures of westerly wind intensity across North Atlantic midlatitudes (Barry and Perry, 1973; Lamb, 1972). This seasonally inclusive NAO maintains a higher out-of-phase signal from winter to summer than traditional NAO indices based on fixed stations in the eastern North Atlantic (Azores, Lisbon, Iceland). When the NAOm index is extended back to 1873 using the UEA-Jones (1987) dataset, its annual values during the late 1800s are strongly negative relative to those of the winter-only indices due to negative contributions from all seasons. In contrast, after the mid–1950s, the values for different seasons sufficiently offset each other to make the annually averaged NAOm smaller than those of winter-only indices. Comparison of monthly hemispheric teleconnection maps of the NAOm and teleconnection maps of a more traditional fixed nodal index show the wider influence during the spring-summer-autumn when the NAOm is used -- particularly in the western North Atlantic, eastern North America, and Arctic. In a later study, the NAOm was used to analyze the timescale behavior of the seasonal signal of the NAO from 1873-2001. This was a first step in establishing a frame of reference to explore any seasonal coordination between the NAO and the ocean/cryosphere.
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Lamb, P. J. and R. A. Peppler, 1987: North Atlantic Oscillation: Concept and Application. Bull. Amer. Meteor. Soc., 68, 1218-1225.
Portis, D. H., J. E. Walsh, M. El Hamly and P. J. Lamb, 2001: Seasonality of the North Atlantic Oscillation. J. Climate, 14, 2069–2078.
Fig. 1. Locations of nodes defining the three NAO indices considered – Mobile, Rogers, and Hurrell. The monthly nodal locations of the Mobile index are labeled by month number; the station-based nodes of Rogers's and Hurrell's indexes are located by “WR” and “WH,” respectively. Arrows indicate the annual march of the mobile NAO index nodes.