Influence of the Madden-Julian Oscillation on the Gulf Stream

The Gulf Stream is a western boundary current and a major part of the global conveyor belt. The Gulf Stream is driven primarily by atmospheric circulation around the semi-permanent high pressure systems over the North Atlantic. Long-term variability of the Gulf Stream has been studied extensively. For example, the position and flow rate of the Gulf Stream are known to respond to atmospheric forcing on the yearly to decadal time scales. There is also some evidence that the Gulf Stream responds to synoptic-scale atmospheric forcing. However, variability of the Gulf Stream on those shorter time scales has received relatively less attention. The hypothesis tested in this study is that the Gulf Stream position is affected by the leading mode of atmospheric intraseasonal variability, the Madden-Julian Oscillation (MJO). The MJO is an equatorially centered convective cell that propagates eastward at a speed of approximately 5 m s^-1. Latent heating in the MJO’s convective envelope excites Rossby waves that propagate eastward and poleward. These MJO-driven Rossby waves are known to affect circulation in the extratropics, including in the North Atlantic. Thus, because the MJO influences surface wind speed and direction in the North Atlantic, the MJO may also modulate the Gulf Stream.

In this study, daily values of 300-, 500-, and 850-hPa height and wind, and MSLP and 10-m wind, from the ERA-interim reanalysis were composited by phase of the Revised Real-time Multivariate MJO index (RMM-r). These composites were used to examine the MJO’s influence on the atmosphere. The daily change in the position and structure of the southern branch of the Gulf Stream, from the Bahamas northeastward to Cape Hatteras, was also composited using the RMM-r index. Time lags of 10 days were introduced to both sets of composites, following recent work suggesting a delay in MJO influence over the North Atlantic. Results suggest that the strongest Gulf Stream response seems to occur in the MJO phases when surface wind is the strongest. Similarly, phases with the weakest surface wind seem to be associated with the least Gulf Stream response. Implications of these results will be explored in detail.