The influence of the Northern Hemisphere annular mode (NAM) on the El-Nino Southern Oscillation (ENSO) was examined using 41 years of NCEP/NCAR reanalysis data set. The NAM index in spring has significant correlation with the Nino-3 index in the following winter. The NAM's signature on an anomalous westerly in the tropical Pacific corresponds with the modulation of the westerly wind burst (WWB). The signature of the spring NAM on the tropical westerly anomalies follows the modulated WWB and continuously broadens eastward from summer to winter. The NAM-related SST anomalies in the tropical Pacific also broaden from summer to winter, and then, it consists with an anomaly pattern of ENSO in winter. An AGCM experiment, in which SSTs are fixed as a climatological-mean monthly distribution, also shows same result for the relationship between the NAM and tropical westerlies in spring. An agreement between the observational result and the model experiment indicates the possibility of that the NAM actively influences on the tropical circulations. A probable mechanism how the NAM affects tropics was examined using 26 years of NCEP/DOE reanalysis data set. The frequency of the cold and dry surge defined by northerly and temperature dropping increases associated with the positive phase of the NAM. The cold and dry air gains heat and moisture from the warmer ocean in the tropical Pacific. Since the tropical atmospheric heating induces a Rossby response through the Matsuno-Gill theory, surface pressure anomalies related to the spring NAM consists with the twin cyclone. The westerly between the cyclones intensifies the WWB. As a result, the NAM could be an indirect trigger of ENSO through the modulation of the WWB.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1620 Climate dynamics (0429;
- 3305 Climate change and variability (1616;
- 3339 Ocean/atmosphere interactions (0312;