Objective Identification of the Intertropical Convergence Zone: Climatology and Trends from the ERA-Interim

Abstract

An objective method for the identification of the intertropical convergence zone (ITCZ) in gridded numerical weather prediction datasets is presented. This technique uses layer- and time-averaged winds in the lower troposphere to automatically detect the location of the ITCZ and is designed for use with datasets including operational forecasts and climate model output. The method is used to create a climatology of ITCZ properties from the Interim ECMWF Re-Analysis (ERA-Interim) dataset for the period 1979–2009 to serve as an indicator of the technique's ability and a benchmark for future comparisons. The automatically generated objective climatology closely matches the results from subjective studies, showing a seasonal cycle in which the oceanic ITCZ migrates meridionally and the land-based ITCZ features are predominantly summertime phenomena. Composites based on the phase of the El Niño–Southern Oscillation index show a major shift in the mean position and changes in intensity of the ITCZ in all ocean basins as the index varies. Under La Niña conditions, the ITCZ intensifies over the Maritime Continent and eastern Pacific, where the ITCZ weakens over the central and equatorial eastern Pacific. An analysis of changes in the ITCZ and its divergence during the period 1979–2009 indicates that the mean position of the ITCZ shifts southward in the western Pacific and a broad global intensification of the convergence into ITCZ regions. The relationship between tropical cyclogenesis and the ITCZ is also examined, finding that more than 50% of all tropical cyclones form within 600 km of the ITCZ.

Energetic Constraints on the Position of the Intertropical Convergence Zone

“…Paleoclimatological evidence and more recent observations and simulations show that the ITCZ has shifted in the past and suggest that the ITCZ can shift with variations in atmospheric greenhouse gas concentrations, aerosol loadings, and El Niño (Dai and Wigley 2000;Sachs et al 2009;Hwang and Frierson 2010;Hwang et al 2013;Berry and Reeder 2014). The anticorrelation between ITCZ position and cross-equatorial energy transport has been noted in many previous studies.…”

“…For example, the increased net energy input to the equatorial atmosphere during El Niño by itself (apart from any change in cross-equatorial energy transport) implies an equatorward shift of the ITCZ according to Eq. (3), as is in fact observed (e.g., Dai and Wigley 2000;Berry and Reeder 2014). Changes in the net energy input to the equatorial atmosphere may also explain why the ITCZ shifts as tropical cloud parameterizations are varied in GCMs (Kang et al 2008(Kang et al , 2009.…”

“…It is located around 68N latitude in the annual and zonal mean but migrates seasonally between a more northern position in boreal summer and a more southern position in austral summer (Waliser and Gautier 1993;Adler et al 2003;Berry and Reeder 2014). Additionally, the annual-mean position of the ITCZ has shifted on geological time scales (e.g., Haug et al 2001;Arbuszewski et al 2013), mimicking its seasonal migration insofar as it tends to shift toward a differentially warming and away from a differentially cooling hemisphere (e.g., Koutavas and LynchStieglitz 2004;Chiang and Friedman 2012).…”

“…However, the ITCZ also shifts southward during El Niño (e.g., Dai and Wigley 2000;Berry and Reeder 2014), an observation not easily related to changes in crossequatorial energy transport. A quantitative understanding of the factors controlling the ITCZ position, including the sensitivity of the ITCZ to changes in cross-equatorial energy transport, has remained elusive.…”

Coherent Potential Vorticity Maxima and Their Relationship to Extreme Summer Rainfall in the Australian and North African Tropics

“…This suggests that the large-scale conditions could be substantially modified in the monsoon region in the wake of an extreme rainfall event. Figure 5 shows the frequency of the objectively detected ITCZ as defined by Berry and Reeder (2013), which is used here as a proxy for the monsoon trough. The first three panels show different snapshots (at day -4, day 0 and day +6) and the bottom right panel is the latitude-time distribution of the frequency of the monsoon trough averaged over the longitudes 120…”