Meteorol. Atmos. Phys. 80, 1±18 2002) School of Earth Sciences, University of Melbourne, Australia Surface¯uxesofmomentumandmechanicalenergy overtheNorthPaci®candNorthAtlanticOceans I.Simmonds and K.Keay With 11 Figures Received November 2, 2001 Revised December 24, 2001 Summary Using 6-hourly data from the National Centers for Environmental Prediction=National Center for Atmospheric Research reanalysis set 1958±1997) we have determined the winter and summer mean ¯uxes of momentum and mechanical energy into the Northern Hemisphere NH) oceans. We have also diagnosed the contribution made to these by the mean wind speed and the covariances due to the temporal variability. In both seasons the greatest oceanic ¯ux of momentum is found in the region to the south of Greenland and Iceland. The contributions to the total made by the transient term exhibited maxima in the north central Paci®c and Atlantic and in winter, and ac- counts for about 15% of the mean stress in both extra- tropical ocean basins and both seasons. The rate at which mechanical energy is imparted to the ocean shows a similar spatial structure. The ¯uxes are typically three times larger in winter, and about one third of the input is associated with the transient part of the low level wind. The spatial and temporal structure of the part of ¯uxes contributed by the temporal variability shows a strong relationship with mean cyclone depth, a parameter known to represent an unbiased measure of cyclone activity. The ¯uxes exhibit signi®cant positive winter trends many of which are signi®cant) over the extratropical Paci®c and in the Atlantic north of about 40  N, and these have been found to result from reinforcing trends in the components asso- ciated with the mean wind speed and the temporally varying part. The changes are broadly in line with those in observed signi®cant wave height over the northern oceans in recent decades, and are closely related to secular increases in the mean depth of cyclones. Positive trends in the number of extreme cyclones in key regions of the Paci®c and Atlantic have been found. The trend is signi®cant in the relevant part of the Paci®c, but whether the increase in the Atlantic subregion should be regarded as above the noise is seen to depend on how such extremes are de®ned. We discuss how conclusions drawn in speci®c studies may depend critically on how cyclones and extreme events are characterized. 1.Introduction Documenting and explaining the nature and con- sequences of the interaction between the atmo- sphere and ocean is one of the great challenges of modern weather and climate analysis. These two media are coupled in a host of ways and this leads to a rich array of variabilities covering timescales from a few minutes to millennia and longer. It is the purpose of this paper to explore with a modern atmospheric data set the nature of the ¯uxes of momentum and mechanical energy across the air±sea boundary over the North- ern Hemisphere NH) oceans. These ¯uxes are important determinants of current systems and sea state conditions over these domains. One of our goals is also to explore the extent to which these atmospheric forcings have changed in recent decades, and the degree to which any such changes are re¯ected in the observed changes in the sea state in the North Paci®c and North Atlantic. Fluxes across the air±sea boundary usually depend nonlinearly on the relevant atmospheric or oceanic parameters. An implication of this nonlinearity is that the temporal mean of these