Air-sea interactions and dynamical processes associated with the midsummer drought

Abstract

Summer precipitation over most of the tropical Americas exhibits maxima in June and September, and a relative minimum in July. The minimum in summer precipitation over the northeastern (NE) tropical Pacific is known as the midsummer drought (MSD). Several theories have been put forward to explain the origin of the MSD, but most of them fail at addressing simultaneously the various elements that characterize it. The temporal evolution of summer precipitation over the NE tropical Pacific warm pool and over the Intra-Americas Seas (IAS) is related, but not the result of the same process. Over the NE Pacific warm pool, the temporal evolution of convective activity is closely related to sea surface temperature (SST) variations, whereas over the IAS, the Caribbean low-level jet (CLLJ) with maximum intensity in July appears to be the key factor. The CLLJ produces ‘gap flow’ over Central America that reaches the NE tropical Pacific and stronger easterly winds. This low-level flow leads to a decrease in SSTs and a westward shift of low-level moisture convergence that combined with subsidence result in the MSD. At the CLLJ exit region, intense tropical convection enhances during July reducing the chances that large-scale circulations, such as the North Atlantic Subtropical High (NASH) inhibit precipitation over the entire tropical Americas. The strong ascending motion off the Caribbean coast of Nicaragua and Costa Rica contributes to enhanced subsidence in the surrounding regions and to the MSD. As the CLLJ and tropical convection in the western Caribbean weaken during August and September, tropical convection increases once more over most of the tropical Americas and produces the second maximum in precipitation that completes the two peaks distribution of summer precipitation. Therefore, the fluctuations in the intensity of the CLLJ appear to be a key dynamical element to explain the MSD.