Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.2.3.3 Tropical Oceans ====

<div id="h3-9-siblings" class="h3-siblings"></div>

The tropics are a tightly coupled ocean-atmosphere system with tightly interconnected basins ( [[#Cai--2019|Cai et al., 2019]] ). The zonal atmospheric Walker Circulation and the Indonesian Throughflow (Figure 9.11) are key connections between the Pacific and Indian oceans, and variations in the Walker and Hadley Circulations are tightly linked to the tropical Pacific SST and currents. The tropics have a profound influence on the climate system through the multiple modes of variability they host, which have widespread global influence at seasonal to annual time scale (Annex IV).

The effect of tropical modes of variability on climate and their long-term changes are reviewed in detail in Annex IV, while changes to the tropical ocean are assessed throughout the report and briefly summarized here. [[IPCC:Wg1:Chapter:Chapter-2#2.4|Section 2.4]] concludes that a sustained shift beyond multi-centennial variability has not been observed for El Niño–Southern Oscillation (ENSO) ( ''medium confidence'' ) and that there is ''limited evidence'' and ''limited agreement'' about the long-term behaviour of other tropical modes. [[IPCC:Wg1:Chapter:Chapter-3#3.7|Section 3.7]] assesses with ''high confidence'' that human influence has not affected the principal tropical modes of interannual climate variability and their associated regional teleconnections beyond the range of internal variability. [[IPCC:Wg1:Chapter:Chapter-4#4.3.3.2|Section 4.3.3.2]] assesses with ''medium confidence'' that there is no consensus from models for a systematic change in the amplitude of ENSO sea surface temperature variability over the 21st century. The related change in tropical SSTs is covered in [[#9.2.1.1|Section 9.2.1.1]] . The projected changes in SST have implications for marine heat wave characteristics, which are assessed in Box 9.2. SST changes in the tropics are related to changes in the atmospheric circulation, including surface equatorial easterly trade winds and Walker Circulation ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.3.2|Section 4.5.3.2]] ), and the weakening Indonesian Throughflow and strengthening Agulhas Extension and leakage ( [[#9.2.3.4|Section 9.2.3.4]] ). Weakening trade winds under climate change ( [[#Vecchi--2007|Vecchi and Soden, 2007]] ) will tend to decrease upwelling, along isopycnals in the eastern Pacific and diapycnal upwelling in the central Pacific, and thus the meridional temperature gradients that drive tropical instability waves ( [[#Terada--2020|Terada et al., 2020]] ), along with a weakening, flattening and shoaling of the tropical thermocline and equatorial undercurrent ( [[#Luo--2011|Luo and Rothstein, 2011]] ). A weak or absent equatorial undercurrent ( [[#Kuntz--2020|Kuntz and Schrag, 2020]] ) and a too-diffuse and incorrectly sloped tropical thermocline ( [[#Zhu--2020|Zhu et al., 2020]] ) remain issues in most CMIP6 models. In summary, while future changes in tropical modes of variability remain unclear, change in atmospheric and ocean circulation will drive continued change in tropical ocean temperature in the 21st century ( ''medium confidence'' ), with part of the region experiencing drastic marine heat wave conditions ( ''high confidence'' ).

<div id="9.2.3.4" class="h3-container"></div>

<span id="gyres-western-boundary-currents-and-inter-basin-exchanges"></span>