Recorded presentation14.3
Free and forced variability of the tropical Atlantic Ocean: Role of the wind-evaporation-sea surface temperature (WES) Feedback
Salil Mahajan, Texas A&M University, College Station, TX; and R. Saravanan and P. Chang
The Wind-Evaporation-Sea Surface Temperature (WES) feedback is an important component of tropical air-sea interaction. The WES feedback has been invoked to explain phenomena such as the meridional mode in the tropical Atlantic and the Pacific. The role of the thermodynamic WES feedback in low-frequency natural variability of the tropical Atlantic is studied using an atmospheric global climate model, the NCAR Community Climate Model (CCM3), coupled to a slab ocean model. A modified version of CCM3 is created, where the WES feedback is explicitly suppressed. A comparison of coupled integrations using the modified CCM3 to those carried out using the standard CCM3 conclusively identifies the role of the WES feedback in enhancing the interannual variability in the deep tropics. A singular vector decomposition (SVD) analysis over the tropical Atlantic reveals that the coupled meridional mode of the Atlantic Ocean is amplified in the presence of the WES feedback. In the absence of the WES feedback, tropical Atlantic still exhibits the mode, albeit with weaker amplitude. A new feedback mechanism that involves near-surface humidity is proposed to explain the existence of the meridional mode even in the absence of the influence of winds on the ocean mixed layer.
A similar analysis of coupled model integrations that incorporate an El Nino-Southern Oscillation (ENSO)-like SST cycle in the Pacific reveals that in the presence of the WES feedback, the meridional mode is the preferred mode of response of the Atlantic to ENSO forcings. In the absence of the WES feedback, a non-coherent response in the tropical Atlantic is seen, indicating that other processes, such as large-scale subsidence, dominate over the effect of surface humidity. Regression analysis of the tropical Atlantic against the ENSO index reveals a similar behavior, with the development of a strong cross-equatorial SST gradient (CESG) being dependent on the WES feedback. The CESG peaks in the boreal spring season and can be maintained until the fall in the coupled model in the presence of this feedback. The WES feedback also appears to be responsible for the northward migration of the ITCZ during ENSO events. Surprisingly, a role for the WES feedback is also indicated in the mid-latitude Atlantic off the coast of Canada in response to ENSO events, with the SSTs, surface fluxes and winds being amplified by this feedback.
Session 14, Coupled ocean-atmosphere interactions and their contribution to climate variability on all time scales: Part 3
Thursday, 15 January 2009, 1:30 PM-3:00 PM, Room 128AB
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