PROJECT GOALS
Our goals are to use these enhanced observations, especially the wind profiles from Galápagos and the balloon soundings from ships travelling in the near-equatorial East Pacific cold tongue, to:

RESULTS AND ACCOMPLISHMENTS
Weather balloon sounding data collected during several TOGA-TAO mooring recovery and deployment cruises at 125E, 110E, and 95EW from 1994 through 1999 have been processed and are being analyzed. We have also assessed the degree of local (e.g., surface flux) versus remote (e.g., large-scale subsidence) control of PBL structure using correlations between the PBL and other properties (e.g., specific humidity above the PBL). One type of perspective on the lower-tropospheric structure in this region, with a focus on the variability, is provided by Figure 1. This figure consists of a series of strip charts showing the values of a host of PBL and other characteristics from each individual sounding collected along 110EW during August 1995, plotted against latitude. The results shown in Figure 1 are tentative, but indicate that the local thermodynamic forcing accounts for only a modest fraction of the PBL variability. For example, linear correlation analysis found that SST was generally the best predictor of PBL characteristics such as height, but it still explained no more than ~30% of the variance in these characteristics.

The composite annual cycle of the observed meridional flow seen at Christmas Island has been compared with the corresponding meridional flow evident in the NCEP/NCAR reanalyses (Figure 2). The gross structure of the meridional winds observed at Christmas Island reveals a distinct annual cycle which can be related to the local mass circulation and the vertical structure associated with the ITCZ; over the central Pacific the northern ITCZ is most pronounced during northern summer, accounting for the strong meridional flow observed at Christmas Island. The observed annual cycle of winds is in reasonable overall agreement with that of the reanalyses for (2.5EN, 157.5EW) above the PBL. However, there are some important systematic differences in the meridional component especially in the lower troposphere. The mid-tropospheric relative maximum in northerlies observed at Christmas Island is only weakly reflected in the reanalyses, and in the PBL the reanalyses usually shows northerly flow while the profiler shows southerly flow. TAO near-surface winds observed at (2EN, 155EW) have been compared with the lower-level profiler winds. The meridional winds are in good agreement; both sets of observations exhibit a pronounced annual cycle with northerlies during northern spring and strongest southerlies during the months of June through October. The balloon soundings taken along 125EW, while limited in number, show that a level of enhanced static stability commonly occurs at about 5 km, near the freezing level. This layer may be due in large part to diabatic effects associated with the melting of falling hydrometeors, and may help cap deep convection in this region. These results are consistent with the profiler winds from Christmas Island, which show a systematic signature in the meridional wind at this level.

The San Cristóbal profiler data show no sign of a sea/land breeze over this small island. Seasonal mean wind profiles reveal the basic vertical structure over the eastern near-equatorial cold tongue and its annual cycle. In general, there is a predominantly southerly jet near 500 m, a speed minimum in easterlies between 1.0 and 1.5 km, and another speed maximum in easterlies above 4.0 km. Comparisons with the balloon soundings from 95EW indicate that the lower jet is generally near the top of the well-mixed layer while the speed minimum is at or just above the top of the trade inversion that caps the PBL. The low-level southerlies peak July through November, immediately following the cold season when upwelling is most pronounced. The easterly minimum exhibits a semi-annual cycle, being strongest during the cold season and briefly weakest (and highest) a few months later during SON, with a secondary strengthening mid-year. The vertical shear from 0.5 to 1.5 km is weakest in DJF, when the easterly minimum is weakest. The profiles from DJF98, at the peak of the 97-98 warm event, are strikingly different, showing low-level northeasterlies between the surface and about 2 km, where the typical easterlies prevail (Figure 3). During January through March 1998 the lowest tropospheric flow became northerly and quite deep; this northerly flow accompanied the relocation of the ITCZ along 90EW to positions south of the equator. The Southern Hemisphere convection and the northerly flow at San Cristóbal lagged the largest SST anomalies at and south of the equator along 95EW but led the largest absolute values of SST in that region (Figure 4).

FUTURE WORK
In the next several months, we will be completing some final interplatform comparisons, particularly the profiler/profiler comparisons at Christmas Island and the balloon/Galápagos profiler comparisons. The latter will permit us to outline an integrated picture of the structure and variability of the lower troposphere over the cold tongue. We will also be continuing our comparisons of balloon and profiler data with reanalyses. Finally, we will be following up on some initial explorations of the variability of Galápagos winds on daily and MJO timescales.

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