storage at low latitudes. Tropical warming results from the interplay between increased stratification and equatorward heat transport by the subtropical gyres, which redistributes heat from the subtropics to the tropics (Dias et al. 2020).
Projections (Bindoff et al. 2019) are that by 2100 the ocean is very likely to warm by 2–4 times as much for low emissions (IPCC scenario RCP2.6) and 5–7 times as much for the IPCC's ‘business‐as‐usual’ scenario (RCP8.5) compared with the observed changes since 1970. The top 200 m of the upper ocean will continue to stratify to 2100 in the very likely range of 1–9% and 12–30% for scenarios RCP2.6 and RCP8.5, respectively.
Projections of future precipitation (Table 2.1) in the tropics indicate widely variable changes with regions. Rainfall is forecast to decline in northern South America, the Caribbean and western Central America, South Asia, and northern and eastern Australia. Increased precipitation is expected in all other tropical areas, except in the Arabian Sea where little change is expected. Much of these forecasted changes are linked to forecasted changes in the intensity and frequency of tropical cyclones (Knutson et al. 2020). There is at least medium‐to‐high confidence in an increase globally in tropical cyclone precipitation, with a median projected increase of 14% or close to the rate of tropical water valour increase with warming, at constant relative humidity (Knutson et al. 2020); cyclone intensity will increase with medium‐to‐high confidence. Knutson et al. (2020) indicate that expert opinion was more mixed and confidence levels lower for (i) a further poleward expansion of the latitude of maximum cyclone intensity in the western North Pacific; (ii) a decrease in global tropical cyclone frequency; and (iii) an increase in very intense global tropical cyclone frequency (category 4–5). These changes in tropical precipitation are linked to changes in surface land‐ocean temperature and changes in near‐surface relative humidity (Lambert et al. 2017). High precipitation under climate change is associated with the highest surface relative humidity and temperatures.
TABLE 2.1 IPCC projected changes for tropical regions in salinity, precipitation, and sea‐level rise for 2081–2100 (relative to the 1986–2005 reference period).
Source: Church et al. (2013), Collins et al. (2013), Bindoff et al. (2019) and Oppenheimer et al. (2019). © John Wiley & Sons.
| Region | Salinity a | Precipitation b | Sea‐level rise c |
|---|---|---|---|
| N. South America | 0 to 5 ↑ | −10 to 40% ↓ | 0.22 to 0.24 m |
| E. South America | 0 to 5 ↑ | 0 to 10% ↑ | 0.18 to 0.20 m |
| Caribbean and W. Central America | 0.5 to 1.0 ↑ | −20 to 10% ↓ | 0.18 to 0.20 m |
| Central West Africa | 0 to 1.5 ↓ | 10 to 20% ↑ | 0.20 to 0.24 m |
| Central East Africa | 0 to 2.0 ↓ | 10 to 50% ↑ | 0.20 to 0.24 m |
| Red Sea/Arabian Peninsula | No change | −10 to 10% ↔ | 0.22 to 0.24 m |
| South Asia | 0 to 5 ↑ | −40 to 10% ↓ | 0.18 to 0.24 m |
| SE Asia | 0 to 1.0 ↓ | 0 to 20% ↑ | 0.18 to 0.20 m |
| N. Australia | No change | 0 to 10% ↓ | 0.18 to 0.20 m |
| E. Australia | No change | −10 to 0% ↓ | 0.18 to 0.20 m |
| Oceania | No change | 0 to 10% ↑ | 0.18 to 0.22 m |
a Range of projected sea surface salinity changes for 2081–2100 relative to the 1986–2005 reference period.
b Range of projected changes in December to February precipitation for 2081–2100 relative to 1986–2005. Data from Collins et al. (2013).
c Range of ensemble mean projections of the time‐averaged dynamic and steric sea‐level changes for the period 2081–2100 relative to 1986–2005. Data from Church et al. (2013) and Oppenheimer et al. (2019).
2.7.4 Changes in Ocean Circulation
Increasing ocean heat is resulting in changes in ocean circulation. Models of ocean heat content (OHC) indicate that the poleward OHC substantially reduce (increase) in the Northern (Southern) Hemisphere, with circulation changes varying among subtropical gyres and among western and eastern boundary currents (Dias et al. 2020). In the North Atlantic Current, ocean heat transport (OHT) weakens at all depths, whereas it strengthens at the surface and weakens at mid‐depth in the subtropical gyre. The Gulf Stream has weakened but the Canary and North Equatorial Currents have increased. Changes in the North Atlantic subtropical gyre and associated OHT reduction suggest that heat moving poleward with the Gulf Stream/North Atlantic Current has reduced, and the extra heat, stored passively in the gyres, transported equatorward via eastern boundary currents, and equatorial currents. Similar changes have also been observed in the Pacific and Indian Ocean. The intensification of the equatorial currents should transport extra heat both westward and eastward via the complex system of equatorial currents and counter currents. OHT associated with the Brazil Current should intensify, contributing to the poleward OHT in the Southern Hemisphere. In the South Atlantic, a shift from equatorward OHT to poleward is predicted to result in an intensification of the Brazil Current.
Some studies have suggested that the surface warming trend in the open ocean is opposite to the trends nearshore and that the poleward expansion of the Hadley cells would increase upwelling at the poleward end and decrease it equatorward (Rykaczewski et al. 2015). Although stratification has intensified over most subtropical gyres and the global ocean, the model results of Dias et al. (2020) indicate an opposite trend in tropical regions, especially in the eastern Pacific and Atlantic basins.
Whether there is an emerging trend of global ocean circulation is not yet clear, but there is a significant increasing trend in the globally integrated, ocean kinetic energy since the early 1990s, indicating a substantial acceleration of global mean ocean circulation (Hu et al. 2020). This increase in kinetic energy is especially prominent in the global tropical ocean, reaching depths of several thousand meters, and induced by a planetary intensification of surface winds. However, regional trends are diverse; the Agulhas Current has not intensified, but shallow cells in the Pacific Ocean have accelerated in response to intensified trade winds since the early 2000s, contributing to a recent warming hiatus and leading to an increased leakage
