The Longitudinal Position of the Atlantic ITCZ

An object-based analysis of precipitation over the tropical Atlantic in CMIP5 models is used to investigate factors controlling the longitudinal position of the Atlantic ITCZ. The analysis reveals that even with SST prescribed, models fail to capture the longitudinal position of the Atlantic ITCZ in the mean state. While observations show a central Atlantic ITCZ maximum, models rain excessively either over the West Atlantic coast or over the East Atlantic coast. We find that models with the East Atlantic bias tend to be high resolution models which rain excessively over the east coast during boreal spring and summer. The excessive precipitation maintains an anomalous atmospheric circulation locking the precipitation maximum on the eastern side of the Atlantic basin for the two seasons.

Sensitivity studies are performed with ECHAM to understand the effect of horizontal resolution on the west-east partitioning of precipitation over the Atlantic. A set of three simulations with T255 resolution but 1) T63 orography, 2) T63 surface, and 3) T63 orography and surface, is used to explore the relative contributions of a high resolution atmosphere, orography, and surface. The default T255 model version exhibits an East Atlantic bias, whereas the default T63 model puts the precipitation maximum over the West Atlantic coast. Results show two distinct contributions: a high resolution atmosphere decreases rain on the west coast, whereas a high resolution orography increases rain on the east coast. A high resolution surface alone has a minimal effect. Mechanisms behind these west-east precipitation responses are explored.