The Coupled Ocean-Atmosphere System in the East Pacific

  

Figure 1: Reynolds SST (Reynolds, 1988; Reynolds and Marsico, 1993) averaged over $90^{\circ} - 100^{\circ} \mbox{ W}$ for the years 1982-1998.

As illustrated in Figure 1, during March the sea surface temperatures (SSTs) near $95^{\circ} \mbox{ W}$ are nearly symmetric about the equator, with $27^{\circ} - 28^{\circ} \mbox{ C}$ water being found between $10^{\circ} \mbox{ S}$ and $10^{\circ} \mbox{ N}$. Waters are slightly cooler on the equator as a result of equatorial upwelling. This upwelling becomes stronger from April onward, resulting in significant additional cooling on the equator. Subsequently, cooling rapidly spreads into the southern hemisphere, as a result of a poorly understood mix of oceanic processes. Maximum SSTs occur in the northern hemisphere in May in a region we call the east Pacific warm pool, with a gradual cooling trend which persists until the following February. At this point southern hemisphere waters abruptly warm in conjunction with warming on and north of the equator.

  

Figure 2: Zonal surface wind and depth of oceanic $20^\circ \mbox{ C}$contour at $95^{\circ} \mbox{ W}$ during 1996. Results are from TAO moorings, courtesy of NOAA/PMEL. The switch from easterly winds near the equator to westerlies centered near $7^\circ \mbox{ N}$ generates anticyclonic wind stress curl and downwelling in this latitude range during the northern summer. Further north the wind stress curl is cyclonic, resulting in upwelling.

Figure 2 shows the depth of the thermocline, as defined by the $20^\circ \mbox{ C}$ contour, from $8^\circ \mbox{ S}$ to $8^\circ \mbox{ N}$ along $95^{\circ} \mbox{ W}$ for 1996. Also shown are the zonal surface winds on this line. The thermocline depth along the equator is not locally related to equatorial zonal winds at this longitude, which indicates that transport by equatorial waves dominates local wind stress-induced upwelling in this region. However, north of the equator, the thermocline depth appears to respond promptly to changes in the wind stress curl associated with meridional variations in the zonal wind. In particular, the thermocline deepens at $4^\circ \mbox{ N}$ with the onset of westerlies north of this latitude. The ITCZ is just at the northern limit of the TAO buoys, but there is an indication that the thermocline becomes drastically shallower at this time under the ITCZ, presumably as a result of the associated cyclonic wind stress curl. The overall shallowness of the thermocline in this region means that the thermocline depth, the thickness of the ocean mixed layer, and the SST respond quickly to changes in atmospheric forcing. As a consequence, the dynamics of the ocean and the atmosphere are tightly coupled by two-way interactions.

  

Figure 3: Outgoing longwave radiation (OLR) through the annual cycle (January to December) averaged over $90^{\circ} - 100^{\circ} \mbox{ W}$ for the years 1979-1995. These data provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from their Web site at http://www.cdc.noaa.gov/.

The seasonal march of cloudiness and precipitation in the east Pacific is highly asymmetric about the equator as a result of these interactions. As is illustrated in Figure 3, significant northern hemisphere deep convection, indicated by low values of outgoing longwave radiation (OLR), typically begins in April over the east Pacific warm pool. Its northern boundary rapidly migrates northward, while its southern boundary slowly retreats to the north in close correspondence with the northward march of the $27^{\circ} \mbox{ C}$ SST contour. The convection cuts off abruptly in late October or early November. Very little deep convection and precipitation occurs south of the equator -- what little there is typically develops in March and April near $5^{\circ} \mbox{ S}$.

The reasons for the highly asymmetric seasonal march of cloudiness and precipitation in the eastern tropical Pacific are the subject of intense current discussion. The most likely ultimate origin of the above asymmetry is the geographical asymmetry of the Central and South American land masses with respect to the equator and its impact on upwelling (Philander, et al., 1996) or on the American monsoon systems (Mitchell and Wallace, 1992; Yu and Mechoso, 1999). Land exists to the north and east of this domain, but not to the south. In addition, the bounding land mass to the east is narrow in the northern hemisphere, but is very substantial in the southern. An understanding of this issue will have to arise through large scale theoretical work as well as atmospheric and ocean modeling, combined with studies such as this one, which are designed to improve model parameterizations of physical processes. Such modeling and theoretical work is underway under the auspices of NOAA's Pan American Climate Studies Program (PACS, 1999) and in other venues. For example, there is some theoretical evidence that the east Pacific asymmetry results from the westward march of the effects of equatorially asymmetric coastal upwelling via a coupled ocean-atmosphere instability (Xie, 1996).

The ultimate goal of the EPIC program is to provide the observational elements required to either verify one of the current theories or contribute to the development of new ones, and thus to validate the performance of coupled ocean-atmosphere global circulation models in this region.

While bearing the above larger picture in mind, EPIC2001 will concentrate on a more limited domain, namely the atmosphere and ocean during the late northern summer along $95^{\circ} \mbox{ W}$ from $16^{\circ} \mbox{ N}$ to $5^{\circ} \mbox{ S}$ and thence southeastward through the southern hemisphere stratus region. This will allow us to obtain a detailed picture of oceanic and atmospheric processes at several latitudes in the problematic region at a time when the cross-equatorial SST gradient and resulting cross-equatorial atmospheric Hadley circulation are strong. Under these conditions rising motion and deep convection occur over the east Pacific warm pool, while subsidence dominates the region of stratocumulus-topped cold water to the south.