The Meridional SST gradient, Low-Level Clouds, and the Latitudinal Location of the ITCZ in the Eastern Pacific: Climatology, Seasonal Cycle, and Interannual Variability.

摘要

The meridional shift of the Intertropical Convergence Zone (ITCZ) in the eastern Pacific does not completely follow that of the surface convergence or the local maximum SST, especially in cold season and equatorial cold years when the SST front in the eastern tropical Pacific is strong. This study focuses on the physical processes and mechanisms that predominantly control the meridional shifts of the ITCZ in the eastern Pacific based on high resolution satellite observational data and regional atmospheric model simulations.;Observational analyses show that the latitudinal location of the ITCZ in the tropical eastern Pacific is highly correlated with the intensity of the SST gradient (SSTG). The SSTG over the sharp SST front is responsible for the formation of a shallow meridional circulation (SMC) from the ITCZ to the cold tongue. The SMC is significant characterized by a returning flow at around 700 hPa from the ITCZ to the cold tongue, descending over the cold tongue with a maximum at around 0-2oN, and a strong nearsurface southerly between the cold tongue and the ITCZ. In the ITCZ, the upward branch of the SMC and the typical Hadley circulation are associated two maximums of vertical velocity at the 850 hPa and 300 hPa. The SMC provides moisture to deep convection and thus largely controls the latitudinal location of the ITCZ and its meridional shift. It is evidenced by the stronger SMC in September, associating surface southerly maxima from the SST front near 3oN to the ITCZ, compared with that in June. Meanwhile, low-level clouds form downstream of the SST front under the returning flow of the SMC and just above the maximum surface northward wind.;Since the SSTG across the eastern Pacific cold tongue displays a strong seasonal cycle, the control of ITCZ convection by the SMC and the related cloud-radiative forcing also experiences a strong seasonal cycle. The SMC is significant year-round except in boreal spring when SSTG weakens. In boreal spring, the SSTG is relatively weak and provides a weak control on the meridional location of the ITCZ, and low-level and high-level clouds are collocated with surface convergence, indicating the dominant control of surface convergence on the ITCZ convection/precipitation. The overlaying of high-level and low-level clouds persists until June when deep convection in the ITCZ is largely affected by the continental monsoon over the Central and North America. The vertical structure of the ITCZ is stronger coupled in boreal summer. In fall when the SSTG is the strongest, the SMC is also the strongest, associated with more northern location of the ITCZ with a large northward vertical tilting from the surface to the upper troposphere. In winter, the ITCZ moves southward when the gap wind from Central America is stronger. The more northward tilted convergence and vertical cloud structure in the ITCZ may be related the tropical waves. In cold seasons when the SSTG is stronger, the meridional gradient of CRF within the atmosphere is amplified due to the increased low-level clouds downstream of the SST front together with the increased middle/high-level clouds in the ITCZ, positively feedback to the SMC and the ITCZ precipitation. The above relations are also applicable in the interannual timescale with more northern location of the ITCZ in the equatorial cold years and vice versa. The relative location changes of the ITCZ to normal years are asymmetrical between El Nino years and La Nina years with larger amplification in El Nino years. The SMC seems more significant in the La Nina events and it is not significant in the El Nino events when the tropical convections are stronger and the SSTG is weaker.;iRAM simulations are used to help to understand the physical processes. The iRAM results are evaluated in climatological mean, seasonal, and interannual time scale. It simulated well in these timescale on multiple variables including the latitudinal location of the ITCZ, clouds, SMC, and the CRF. Comparison in the meridional circulation between the ENSO years and Non-ENSO years apparently show the enhanced SMC in the La Nino events. Sensitivity experiments with SSTA cooling or warming are added over the cold tongue to produce different intensity of the SSTG. The analyses of the heating budget show that the low-level cloud radiative cooling and the vertical mixing due to change of the low-level cloud produce similar magnitude as the diabatic heating in the change of the SMC, which contribute the northward or southward shift of the ITCZ. (Abstract shortened by UMI.).