ENSO is, among other things, an anomalous warming of the eastern Pacific. In order to define the anomalies, it is crucial to define the background, normal conditions against which the anomalies are measured. While simple in concept, this observational and definitional problem is complicated by the existence of both interannual (and longer) variability and intraseasonal (and shorter) variability. In the implicit definition used throughout this report, the climatology of a quantity is the sequence of monthly averages of that quantity for all the months of the year. Because both warm and cold phases of ENSO contribute to the monthly averages, a record long enough to include the effects of the slow interannual variability must be obtained. If the annual average is not stationary, the climatology will be unstable—i.e., different climatologies will arise from different averaging periods. Attempts to define a climatology by averaging only during “normal” periods—i.e., those without significant warm or cold phases of ENSO—will give an incorrect climatology if ENSO produces rectified effects.

Sea surface temperature is one of the key quantities that change during ENSO, and its climatology is therefore one of the most crucial. To date, climatologies of sea surface temperature have been obtained predominantly from historical records of in situ data (e.g., Reynolds 1982, Slutz et al. 1985), or by accumulating statistics from operational analyses (e.g., Reynolds and Smith 1994). Climatological winds have usually been obtained from historical records (e.g., Hellerman and Rosenstein 1983, Harrison 1989). The TOGA Observing System is providing large numbers of tropical data, which future climatologies will reflect.

The TOGA Observing System has produced some remarkable results on the climatologies of the oceanic subsurface thermal structure and subsurface circulation. Current-meter moorings have been in place at 110°W since 1980 and at 140°W since 1983. In conjunction with ATLAS moorings, they have yielded a remarkable picture of the behavior of the near-surface circulation (see, e.g., McPhaden and McCarty 1992). At 110°W (see Figure 7, middle), for example, the undercurrent is strongest in boreal spring when the winds are weakest, the sea surface temperature is warmest, and the surface currents have reversed to eastward. The warm sea surface temperature has no thermocline motion associated with it—indeed, the thermocline stays pretty much flat throughout the year. Clearly then, the main processes available to change sea surface temperature are the surface fluxes and surface advection, both affecting (and affected by) the mixing processes that determine the mixed-layer depth.

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