Madden-Julian Oscillation (MJO) Modulation of Surface Ocean Currents Off the U.S. West Coast

Intraseasonal variability of ocean currents is a relatively new area of study. One major internal mode of atmospheric intraseasonal variability, the Madden-Julian Oscillation (MJO), modulates global tropospheric circulation on intraseasonal (30–45-day) time scales. Via its modulation of upper-troposphere Rossby Waves, the MJO has direct and indirect influences on many smaller-scale atmospheric features, including surface winds. Because the MJO is known to modulate surface wind speed and direction, the hypothesis tested in this study was that MJO would also modulate surface ocean currents off the U.S. west coast. To test this hypothesis, composite anomalies of ocean surface currents were created for each active phase of the MJO for each month.  The surface currents anomalies were derived from the NASA Ocean Surface Current Analysis--Relatime (OSCAR) data product.  Our analysis indicated that anomalous southeasterly currents approximately 1 cm/s slower than normal were found during MJO phase 2 along the coast of Washington and British Columbia.  In comparison, anomalous northwesterly currents approximately 1 cm/s faster than normal were found in the same region during MJO phase 6. To better understand physical mechanisms for these observed anomalies, composite anomalies of upper-troposphere height, lower-troposphere height and wind, mean sea level pressure, and surface winds from the NASA MERRA reanalysis were also calculated for each active phase for each month. To our knowledge, this is the first study attempting to connect surface ocean current variability in this region to the MJO. Thus, our study improves our understanding of atmosphere and upper-ocean processes on the intraseasonal time scale.