The Annual Cycle of Air-Sea Fluxes in the Northwest Tropical Atlantic

Sebastien P Bigorre,Albert J Plueddemann

doi:10.3389/fmars.2020.612842

Sebastien P Bigorre, Albert J Plueddemann

Open Access

https://doi.org/10.3389/fmars.2020.612842

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Abstract

In this article we analyze 11 years of near-surface meteorology using observations from an open-ocean surface mooring located in the Northwestern Tropical Atlantic (51°W, 15°N). Air-sea fluxes of heat, freshwater, and momentum are derived from these observations using the Coupled Ocean–Atmosphere Response Experiment (COARE) bulk parameterization. Using this dataset, we compute a climatology of the annual cycle of near-surface meteorological conditions and air-sea fluxes. These in situ data are then compared with three reanalyses: the National Centers for Environmental Prediction-Department of Energy [NCEP-DOE (hereafter referred to as NCEP-2)], the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim and the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) reanalyses. Products from the Clouds and the Earth’s Radiant Energy System (CERES) and the Tropical Rainfall Measuring Mission (TRMM) are also used for comparison. We identify the agreements and characterize the discrepancies in the annual cycles of meteorological variables and the different components of air-sea heat fluxes (latent, sensible, shortwave, and longwave radiation). Recomputing the reanalyses fluxes by applying the COARE algorithm to the reanalyses meteorological variables results in better agreement with the in situ fluxes than using the reanalyses fluxes directly. However, the radiative fluxes (longwave and shortwave) from some of the reanalyses show significant discrepancies when compared with the in situ measurements. Longwave radiation from MERRA-2 is biased high (too much oceanic heat loss), and NCEP-2 longwave does not correlate to in situ observations and other reanalyses. Shortwave radiation from NCEP-2 is biased low in winter and does not track the observed variability in summer. The discrepancies in radiative fluxes versus in situ fluxes are explored, and the potential regional implications are discussed using maps of satellite and reanalyses products, including radiation and cloud cover.

Highlights

The northwest tropical Atlantic hosts a multitude of air-sea interaction phenomena that impact the climate, ecosystem and society on a wide array of temporal and spatial scales
Most surface meteorological variables observed with the ASIMET instrumentation at NTAS are reproduced well by the three reanalyses MERRA-2, ERA-Interim, and NCEP-2
Measurements of net longwave and shortwave radiations are similar between ASIMET and CERES remote sensing, three issues were found when comparing ASIMET to reanalyses: (1) longwave radiation in MERRA-2 is biased low by 10 to 15 W m−2 and this bias is present in most of the tropical Atlantic, (2) downward shortwave radiation in NCEP-2 is biased low (50 W m−2) in the boreal winter, and departs from other datasets in summer, (3) the annual cycle of downward longwave in NCEP-2 does not show an annual cycle consistent with the in situ data

Summary

Introduction

The northwest tropical Atlantic hosts a multitude of air-sea interaction phenomena that impact the climate, ecosystem and society on a wide array of temporal and spatial scales. Numerous hurricanes are created or intensified there, due to high sea surface temperature (SST) and low wind shear (Wang et al, 2006). Oceanic barriers layers in the Northwest Tropical Atlantic are the thickest in the world (Mignot et al, 2012) and contain subsurface heat anomalies that intensify hurricanes (Balaguru et al, 2012). The trade winds converge in the region, giving rise to the Hadley circulation, which feeds the atmospheric meridional heat transport from Equator to poles. Anomalous ascent in the upper troposphere above the AWP leads to an anomalous Hadley-type circulation, with interhemispheric exchange and anomalous subsidence in the Southeast Pacific (Wang et al, 2010). The westward trade winds carry aerosols from SubSaharan Africa (Weinzierl et al, 2017), which affect albedo, cloud nucleation, and fertilization in the ocean and Amazonian forest (Bristow et al, 2010)

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