Horizontal and vertical structure of synoptic waves in the eastern tropical Pacific ITCZ: a combined observational and reanalysis study

Outgoing longwave radiation (OLR) and low-level wind fields in the tropical east Pacific intertropical convergence zone (ITCZ) are dominated by variability on synoptic time scales primarily associated with convectively coupled tropical depression waves (TD-waves) or easterly waves during boreal summer and fall. A subset of the low pressure centers within such waves also spin up hurricanes, giving easterly waves additional significance for the climate in the Pan American region. This study uses spectral filtering on observed OLR data to capture the convective variability coupled to east Pacific easterly waves. The TD-filtered OLR data are then used to isolate the horizontal and vertical structure in wind, temperature, and humidity associated with these waves. Kinematic and thermodynamic fields for Jun-Nov 1979-2002 are provided by the National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis project. Data from the Tropical Atmosphere-Ocean (TAO)/Triangle Trans-Ocean Buoy Network (TRITON) buoys for Jun-Nov 1990-2003 and from the TAO/East Pacific Investigation of Climate (EPIC) buoys for Jun-Nov 2000-2003 are also analyzed and compared with the NCEP/NCAR 1000 mb fields.

Our results suggest that east of the dateline and north of 2°N, 3-6 day variability is a significant portion of the total sub-seasonal variability in surface zonal and meridional winds, temperature, humidity, wind stress, and latent heat flux fields at the TAO buoys. In addition, 3-6 day variability is a significant portion of the observed total sub-seasonal variability in the surface meridional pressure gradients along 95°W north of 2°N during the EPIC extended observing period.

Results of regressing NCEP/NCAR fields onto TD-filtered OLR suggest that easterly waves in the east Pacific have wavelengths of ~4000-5000 km and phase speeds of ~14 m s^-1. These results are consistent with regressions of buoy winds and TD-filtered OLR, giving confidence in synoptic-scale variability of the reanalysis wind products. The minimum in OLR occurs within the trough in the far eastern portion of the basin near 95°W, shifting about 1/4 wavelength to the east for disturbances west of about 125°W in both the observations and reanalyses. The shift in the position of the convection with respect to the trough is found to result from the difference between the phase speed of the wave and that of the convection, with the latter being closer to 12 m s^-1. The observed tilt of the wave horizontal and vertical structure suggests that these disturbances loose energy to the mean flow as they propagate westward. Our results support the interpretation of these disturbances as free Rossby-like disturbances,which appear to transform into mixed Rossby-gravity (MRG) waves as they propagate westward due to changes in the background state.