A cold tongue without upwelling: how an equatorial continent forces a split ITCZ over a motionless ocean.
Abstract
The Tropical Rain belts with an Annual cycle and Continent Model Intercomparison Project (TRACMIP) ensemble of simulations includes slab-ocean aquaplanet control runs and experiments with an idealized tropical continent, consisting of modified aquaplanet grid cells with reduced evaporation, increased albedo, reduced heat capacity, and no ocean heat transport (as specified by q-fluxes). The annual mean response to the introduction of the land is strongest over the ocean. Along the equator, a broad equatorial cold tongue develops west (downstream) of the continent and rainfall in the oceanic Inter-Tropical Convergence Zone (ITCZ) is reduced, creating an annual mean climate with a split ITCZ in the eastern ocean basin. We explain the pattern of rainfall anomalies as directly due to changes in sea surface temperature (SST). Specifically, at the tail end of the cold tongue the prominent control mechanism is through boundary layer dynamics, as indicated by the correspondence of rainfall changes to changes in the Laplacian of SST, while closer to the coastline the negative rainfall anomalies coincide with a reduction in the boundary-layer moist static energy that derives from SST cooling at fixed relative humidity. Changes in the surface energy budget suggest that the oceanic cooling can be attributed to a feedback between specific humidity, longwave radiation, and temperature initiated by advection of dry continental air over the ocean. Simulations with a gray-radiation model and simulations with full-physics GCMs that diverge from the TRACMIP protocol confirm the importance of moist-radiative feedbacks and also highlight how the annual mean response is mostly the result of the rectification of seasonal effects: winter-time cold anomalies are responsible for most of the dry advection that initiates the cold tongue response. This suggests that the exact shape and location of the continent are not a key parameter for the existence of this mechanism, as long as subtropical dry air is carried from the continent over the ocean by the Trades. Cloud changes are not robust across models, and contribute to the spread in responses across the ensemble.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMGC129..05B
- Keywords:
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- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSES;
- 3339 Ocean/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 1620 Climate dynamics;
- GLOBAL CHANGE;
- 1627 Coupled models of the climate system;
- GLOBAL CHANGE