Northern Hemisphere Atmospheric Transient Eddy Fluxes and Their Co-variability with Ocean Frontal Variability near the Western Boundary Current Regions

Time series of winter (January-March) meridional transient eddy heat and moisture fluxes for 1979-2009 in the Northern Hemisphere from the surface to 100 hPa are examined based on the daily mean atmospheric variables from the NASA Modern Era Retrospective-analysis for Research and Applications (MERRA) at 1/2 degrees latitude by 2/3 degrees longitude resolution. Two separate frequency bands, i.e. the synoptic (2-8 days) and intra-seasonal (8-90 days), are investigated, separately.

The climatological mean transient eddy fluxes in the two frequency bands exhibit markedly distinct spatial patterns. The synoptic transient eddy fluxes show conventional storm-track variability, of which maxima are co-located with the Gulf Stream and the Kuroshio-Oyashio Extensions, respectively in each basin. On the other hand, the mean transient eddy fluxes for the intra-seasonal band exhibit maxima co-located with the major orography, e.g. the Rockies and Greenland, which are primarily associated with the Pacific-North America (PNA) teleconnection and Arctic Oscillation (AO). For the vertically and zonally integrated poleward heat transport, the maximum heat transports in the two frequency bands are similar, while the sensible heat fluxes are twice greater than the latent heat fluxes.

Mean spatial patterns of the lateral divergence of the transient eddy fluxes are also distinct between the two frequency bands. However, the spatial pattern for the sum of the two frequency bands seamlessly reveals a remarkable coherence with the paths of ocean fronts across the both ocean basins.

In addition, co-variability between the meridional transient eddy fluxes and the changes in the position of ocean fronts associated with the Kuroshio Extension, Oyashio Extension and Gulf Stream is examined with a focus on the interannual to decadal time scale. In general, the northerly storm track leads the northerly ocean fronts by 1-3 years, which is consistent with the wind-driven ocean response. On the other hand, the northerly ocean fronts lead the southward shift of the storm tracks. Significant transient eddy flux anomalies following the ocean frontal shift are found coherently from the surface to mid-troposphere. The co-variability explains approximately half of the interannual and longer variance in the synoptic band, while only ~20 % for the intra-seasonal band.