Examining multidecadal trends in the surface heat balance over the tropical and subtropical oceans in atmospheric reanalyses

Abstract

Trends in the components of the annual‐mean surface heat balance for 1979–2018 over the tropical and subtropical oceans are examined in multiple atmospheric reanalyses to understand how they are changing with current sea surface temperature (SST) trends. Confidence in the reanalysis values is evaluated through statistical significance, agreement among datasets, and physical analysis. While the climatology of the net surface heat flux is similar in the three reanalyses examined (ERAI, JRA‐55, and NCEP2), net heat flux trends agree only in the two second‐generation reanalyses. Trends in the 10‐m winds, which are assimilated in JRA‐55 and ERAI but not in NCEP2, are largely responsible for the disagreement. To first order, trends in the latent heat flux explain trends in the net surface heat flux over the tropical oceans. Trends in sensible heat are smaller, and trends in the net radiative components are relevant only regionally. The latent heat flux is decomposed into thermally, dynamically, and hydrologically driven components. Trends in the thermal component of the latent heat flux simply dampen SST trends through the Clausius–Clapeyron relationship. Dynamically driven trends are associated with an intensification of the tropical easterly trade winds, primarily of the equator and with greater enhancement in the Southern Hemisphere. They are generally supported by hydrologically driven trends, which are similar in magnitude to the wind‐driven trends.

Evaluation of the J-OFURO3 Sea Surface Net Radiation and Inconsistency Correction

“…Comparing with the other two datasets, the results from the corrected J-OFURO3 dataset (Figure 14a) were more consistent with the results from the ERA5 dataset (Figure 14c), which was similar to previous studies [39,40], but it had a large discrepancy with the results from the original J-OFURO3 dataset (Figure 14b), especially in the regions with increasing trends shown in Figure 14a,c. Therefore, this result again emphasized the importance and necessity of correcting the inconsistency in the original J-OFURO3 sea-surface R n time series.…”

“…Therefore, this result again emphasized the importance and necessity of correcting the inconsistency in the original J-OFURO3 sea-surface R n time series. Specifically, based on the results obtained from the corrected J-OFURO3, the seasurface R n increased at a mean rate of 0.257 Wm −2 over most regions of the Western and Central Tropical Pacific, which was consistent with the studies of Liu [39] and Cook [40]. According to Cook [40], the increased sea-surface R n in this region was possibly caused by the increased R ns , which were resulted from the decreased cloud amounts and the enhanced aerosol distributions or reduced water vapor droplet size.…”

“…Specifically, based on the results obtained from the corrected J-OFURO3, the seasurface R n increased at a mean rate of 0.257 Wm −2 over most regions of the Western and Central Tropical Pacific, which was consistent with the studies of Liu [39] and Cook [40]. According to Cook [40], the increased sea-surface R n in this region was possibly caused by the increased R ns , which were resulted from the decreased cloud amounts and the enhanced aerosol distributions or reduced water vapor droplet size. It was also seen that the sea-surface R n over the Eastern Tropical Pacific mostly decreased at a mean rate of 0.316 Wm −2 per year, which was possibly caused by the decreased top-of-atmosphere (TOA) R n within the same region as discussed by Allan [41].…”

Challenges with interpreting the impact of Atlantic Multidecadal Variability using SST-restoring experiments

“…The observational heat-flux estimate shows negative surface heat-flux trends over the subtropical North Atlantic region in recent decades (Supplementary Fig. 18); whilst this is quite noisy and uncertain, it is consistent with other observational analysis 43 . In the transient pacemaker, the restoring heat-flux is seemingly very weak compared to internal variability (and possibly externally forced surface heat-flux variability) and therefore the positive surface heat-flux that is present in the constant SST-restoring experiment -which is crucial for generating the remote response in the Tropical Pacific -is not present in the North Atlantic pacemaker experiment and there is a much weaker remote influence on the Pacific.…”