The Climode Field Campaign: Observing the Cycle of Convection and Restratification over the Gulf Stream

1 SeptembeR 2009 AmeRICAN meteOROLOGICAL SOCIetY| dissipation cycle is clearly evident in the vertical temperature profiles (from the Argo1 array) shown in Fig. 1 (bottom left). We observe a rapid deepening of the surface mixed layer during February–April and a slower restratification over the remainder of the year. Note that the well-mixed fluid formed in winter has a temperature close to 18 C; hence, North Atlantic STMW is also known as Eighteen Degree Water (EDW). In spite of a relatively well-developed observational history, many fundamental questions remain. For example, Speer and Tziperman (1992), following an approach pioneered by Walin (1982), inferred a formation rate from air–sea fluxes of roughly 15 Sv (1 Sv= 106 m3 s− 1) in the density class of 26–27 kg m− 3. If the annual-mean “head” of EDW is in steady state, it must be dissipated at this rate. However, assuming subsurface diapycnal mixing rates of 10− 5 m2 s− 1, typical of measurements in the thermocline made by, for example, Ledwell et al.(1993), annual mode water dissipation rates are at most roughly 2 Sv (see Marshall 2005). Thus, it is difficult to see how the ocean could accommodate such a large formation rate assuming canonical interior mixing rates. Moreover, subsurface float observations of EDW volume changes suggest an annual ventilation rate only of order 5 Sv (see Kwon and Riser 2005). We see then that there is a considerable mismatch in mode water mass budgets, which belies a series of questions about how much is formed, how it is formed, and where mode waters are lost to other density classes. A prevailing opinion is that the resolution of the …