The weather pattern and trends associated with heat flux events (HFEs) over the Gulf Stream (GS) are examined as part of the CLIMODE project. A large portion of GS turbulent heat loss occurs during distinct synoptic scale events with a duration of one to two days. Forty percent (40%) of wintertime sensible heat loss over the CLIMODE Region (CR) (30°-42° N, 50°-70° W) occurs on 17.5% of days. The overturning convection caused by ocean surface buoyancy loss leads to a deepening mixed layer and the formation of eighteen-degree water (EDW). EDW is a homogenous slab of subtropical mode water with vertically uniform temperature and salinity characteristics. Mode water is formed in regions of large ocean-atmosphere heat exchange and serves as a potential memory source for the climate system. Analysis of sounding data acquired from a ship in the GS region in February and March of 2007 shows a distinct atmospheric vertical thermal profile during HFEs. A typical event day profile exhibits a well mixed boundary layer (BL) featuring a dry adiabatic lapse rate extending up to around 850 millibars (mb). A sharp inversion layer separates the
BL from the dry free atmosphere. Satellite imagery confirms that open and closed cell cumulus cloud structures are present during events, which indicate strong surface-based instability and shallow convection. An examination of the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) Reanalysis data is performed in order to understand the planetary and synoptic scale weather patterns that induce HFEs. A large 500 mb trough is present over the Western Atlantic Ocean on event days. A surface pressure couplet consisting of high (low) pressure located to the west (northeast) of the CR causes anomalous northwesterly flow that advects cold and dry continental air over the GS. Long term trends in CR Reanalysis data indicate that an increase in sea surface temperature (SST) has significantly increased autumn sensible and latent heat flux since the mid 1960s. Spring is dominated by large decadal variability in thermal and flux fields. Thus, increasing turbulent flux may not lead to greater EDW production since the increased oceanic heat loss serves to remove a seasonal thermocline which is warming over time. Global climate models (GCMs) do not adequately simulate observed CR Reanalysis period climate. The modeled trends in heat flux correlate poorly with Reanalysis. This investigation suggests important areas for potential future research including the effect of turbulent flux on atmosphere and ocean dynamics, and understanding the influence of BL entrainment on heat flux magnitude.
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