Top or Bottom-Heavy? Observational Constraints on the Vertical Structure of the Eastern Pacific ITCZ

Abstract:

The Intertropical Convergence Zone (ITCZ) is a key component of the eastern Pacific ocean-atmosphere system and its variability on seasonal to inter-annual and longer time scales. This feature is generally misrepresented in climate models, which show an excessively strong branch south of the equator. On the other hand, there is debate on what is the structure of the ITCZ in nature, particularly whether the latent heating and vertical velocity profiles are top or bottom-heavy. This knowledge is probably key to validate and improve the models.

Most methods for estimating the vertical structure of the rate of latent heating
rely on profiles from field campaigns in other regions, combined with convective/stratiform fractions from the TRMM satellite.
In this study we use the precipitation profiles from the TRMM Precipitation Radar (PR), with approximations to the moisture conservation equation and the first law of thermodynamic, to directly estimate the vertical profiles of latent heating and vertical air velocity, respectively, in the ITCZ for the period 1998-2010.

Due to limitations in the PR sensitivity and the inability to quantify solid precipitation, our results are restricted to the layer between the altitudes of 2 and 2.75 km. Nevertheless, we show that our results provide a strong constraint on the profiles and help determine which of the other estimates are more realistic.

Our preliminary results for the northern hemisphere ITCZ in austral winter/spring are closer to the top-heavy estimations using TRMM-based algorithms (CSH, SLH and PRH) than to the bottom-heavy atmospheric reanalysis (ERA Interim and NCEP-NCAR), providing indirect evidence for a top-heavy profile. However, using the meridional wind measurements during the EPIC field campaign we find evidence that shallow ascent does exist below 2 km, consistent with the previously reported shallow meridional circulation but not as strong as the Reanalysis products indicate. Thus, our results support the existence of a dual circulation structure in the eastern Pacific ITCZ consisting in both shallow (1000-800 hPa) and deep overturning (450-350 hPa).