Wednesday, 17 January 2007: 9:00 AM
Evidence for interannual to decadal variations in Hadley and Walker Circulations and links to water and energy fluxes
214B (Henry B. Gonzalez Convention Center)
Mass and energy transports associated with the Hadley and Walker circulations are important components of the earth's climate system and are strongly linked to hydrologic processes. Interannual to decadal variation in these flows likely signify a combination of natural “climate noise” as well as a response to anthropogenic forcing. There remains considerable uncertainty in quantifying variations in these flows. Evidence in the surface pressure record supports a weakening of the Walker circulation over the Pacific in recent decades. Conversely the NCEP / NCAR and ERA 40 reanalyses indicate that the Hadley circulation has increased in strength over the last two decades, though these analyses depict significantly different mass circulation changes. Interestingly, the NCEP / DOE Reanalysis II contains essentially no Hadley circulation changes. Most climate model integrations anticipate an overall weakening of tropical circulations associated with SST rises and associated stronger static stability. Notably, integrations forced by SSTs observed over recent decades suggest a strengthening of the Hadley circulation. Clearly there is much uncertainty not only with the mass transports, but also how they are linked to water and energy balance of the planet through variations in turbulent heat and radiative fluxes and horizontal exports / imports of energy.
Here we examine heat and water budget variations from a number of reanalysis products and focus on the linear and nonlinear response of ENSO warm and cold events as opportunities to study budget variations over the past 15-20 years. Our analysis addresses such questions as “To what extent do Hadley and Walker Cell variations compensate each other on mass and energy transport? Do static stability adjustments appear to constrain fractional precipitation response vs. fractional water vapor response? We appeal to constraints offered by GPCP precipitation, SSM/I ocean evaporation estimates, and ISCCP-FD radiative fluxes, and other satellite data sets to interpret and confirm reanalysis-based diagnostics. Using our findings we also attempt to place in context the recent findings that tropical ocean evaporation increased by order 5% or more during the 1990s, reconciling this with GPCP precipitation variations.