Long‐term characterization of the Pacific ITCZ using TRMM, GPCP, and ERA‐Interim

Abstract

An objective, automated Intertropical Convergence Zone (ITCZ) identification and characterization algorithm is developed and applied to European Centre for Medium-Range Weather Forecasts Reanalysis Interim (ERA-Interim) variables, and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Global Precipitation Climatology Project (GPCP) rain rates (RRs) to create 15 and 36 year climatologies, respectively, in the tropical Pacific ocean. Characteristics derived from the algorithm include ITCZ latitude, northern and southern ITCZ convection boundaries and extent, and precipitation intensity. The climatological location of the ITCZ was found near 8 ∘ N, consistent with previous studies, with a preferred southern boundary location of 4 ∘ N. The northern ITCZ boundary did not exhibit a preferred boundary with locations between 7 ∘ and 15 ∘ N having similar frequencies of occurrence. The northern and southern extents of the ITCZ were symmetric in the central Pacific and asymmetric in the east Pacific. Long-term trends in the ITCZ width and precipitation intensity showed significant narrowing and intensifying. Separation of characteristics by season and location (i.e., central and eastern Pacific) revealed negative trends in ITCZ width in both domains, with trends in the east Pacific weaker than those in the central Pacific. Trends in precipitation intensity near the center of the ITCZ indicate a strong intensification, with slightly stronger trends in the central Pacific than in the east Pacific. These findings show that while the location of the ITCZ has not changed significantly over the past three decades, the ITCZ has narrowed and convection has intensified.

An Observational View of Relationships Between Moisture Aggregation, Cloud, and Radiative Heating Profiles

“…This study supports the view that aggregated convection on the synoptic scale produces an environment with a bimodal moisture distribution including dry regions that produce a positive radiative cooling feedback on the convective circulations. These reinforcing radiative feedbacks on convection have also been noted in interannual variability as manifested in the El Nino Southern Oscillation (Rädel et al 2016) and are implicit in the global-scale narrowing of the inter-tropical convergence zone (Wodzicki and Rapp 2016).…”

Response of the Intertropical Convergence Zone to Climate Change: Location, Width, and Strength

“…We focus on two aspects of the ITCZ which have received relatively little attention: its width and strength. In contrast to the ITCZ location, which has not shifted substantially over the past three decades in the Pacific [27] and which shows no robust response in simulations with elevated CO 2 concentrations [26], the ITCZ width and tropical circulation strength have changed significantly over recent decades [27,28]. And, the ITCZ width and circulation strength are predicted to continue changing into the future [29,30].…”

“…1a). A number of studies have automated identification and developed climatologies of ITCZ location using satellite cloud or radiation observations [43,44] and atmospheric reanalyses [27,45,46]. However, many of these earlier studies focused primarily on ITCZ location rather than on ITCZ width or strength.…”

Tracking the Choco jet since the 19th Century by using historical wind direction measurements

“…1.a and 1.c). Recently, Wodzicki and Rapp (2016) showed that the central latitude of the eastern Pacific ITCZ ranges between 4ºN and 10-12ºN. So, along the year, the ITCZ displaces between these limits, modifying the relation between the areas affected by winds with 5 westward/eastward components and therefore the relative number of wind observations with wind blowing from the southwest inside the area where the CHOCO-D index is defined.…”

“…Forced by the north-south orientation of the South American coastland and the predominant position of the ITCZ north of the Equator between April and November, the southerly trade winds over the eastern Pacific cross the Equator acquiring a predominant westerly direction and entering the continent with maximum zonal velocity around the 5º N to 7º N latitude characteristic of the Choco jet core 30 (Poveda and Mesa, 2000;Sakamoto et al, 2011;Arias et al, 2015). During the boreal winter, the southward migration of the ITCZ allows the north-easterly Northern Hemisphere trade winds to blow as southward as 4º N (Wodzicki and Rapp, 2016 In this way, in this part of the world, the ITCZ separates the domain of winds with westward/eastward component always in the same hemisphere and its migration is the ultimate reason of the seasonal wind reversal observed in the tropical eastward Pacific and consequently its characterization as a "monsoonal" area by wind-based criteria (see Figs. 1.a and 1.c).…”