Tropical rainfall patterns driven by reduced sea ice in high boreal latitudes

Satellite data enabled the Intergovernmental Panel on Climate Change (IPCC), through Report V, to indicate that the regional distribution of sea ice has been reducing in the Northern hemisphere high latitudes. This study assimilated that reduction into a general circulation model of intermediate complexity to simulate the tropical rainfall response. The Northern hemisphere tropospheric wind field simulations presented a clear similarity to the Northern Annular Mode negative phase. In particular, the meridional wind anomalies of the Northern hemisphere Ferrel cell suggest that the energy upsurge due to the boreal sea ice decrease results in an increase in the amplitude of the Rossby waves, thus connecting the polar zone to the tropics. The 500 hPa vertical motion and the rainfall distribution in the tropical belt simulations show a southward displacement of the Atlantic Intertropical Convergence Zone and also the South Atlantic Convergence Zone. Although several studies indicate the Intertropical Convergence Zone is shifted towards the hemisphere most heated by climatic variations, the apparent disagreement with our results can be understood by considering that some continental sectors in the Northern Hemisphere mid-latitudes have shown cooling in recent years, probably in response to the boreal sea ice decrease. doi: 10.2166/wcc.2018.066 om http://iwaponline.com/jwcc/article-pdf/11/1/74/677646/jwc0110074.pdf 021 Isimar de Azevedo Santos (corresponding author) Maria Gertrudes Alvarez Justi da Silva Laboratorio de Meteorologia, CCT, Universidade Estadual do Norte Fluminense, PO Box 119562, 27910-970 Macaé, RJ, Brazil E-mail: isimar.uenf@gmail.com Alfredo Silveira da Silva Departamento de Meteorologia, CCMN, Universidade Federal do Rio de Janeiro, Av. Horacio Macedo, 2030 bloco B – sala 101, 21941-450 Rio de Janeiro, RJ, Brazil Otto Corrêa Rotunno Filho Programa de Engenharia Civil, COPPE, Universidade Federal do Rio de Janeiro, Av. Horacio Macedo, 2030 bloco B – sala 101, 21941-450 Rio de Janeiro, RJ, Brazil


INTRODUCTION
Sea ice is an important Earth climate system component and its obvious reduction in the Northern hemisphere high latitudes is one of the most anticipated consequences of global warming, and a disturbing decrease is expected by the end of this century if current greenhouse gas emission rates continue without effective containment. Without a doubt, Arctic sea ice melting has important consequences for atmospheric circulation and climate due to the energetic imbalance provoked in the boreal seas, as the highly reflective ice cover is replaced by liquid water with a much smaller albedo (Deser et al. ; Sorokina et al. ).

According to the recent Intergovernmental Panel on Climate
Change (IPCC) Report V (IPCC ), there has been a 3.8% per decade downward trend in the annual average Arctic sea ice coverage over the last four decades.
The connection between polar energy balance variations and tropical atmospheric behaviour occurs through teleconnections, which denote significant temporal correlations between meteorological parameter fluctuations at distant points. These correlations allow establishment of relationships between apparently unrelated climatic anomalies at great distances. Such connections are sustained by the energy transportation and wave propagation both in the atmosphere and the oceans, with the former acting as a bridge and the latter acting as tunnels for these global climate connections (Liu & Alexander ).

METHODOLOGY
Since 1979, satellite sensors have shown that sea ice cover is declining throughout the four annual seasons in practically all the Northern Hemisphere high latitude seas, with the exception of the Bering Sea during the winter. The IPCC Report V (IPCC ) indicates the regional distribution of the high boreal latitude sea ice coverage decrease observed per decade: 9.3% in the Barents Sea; 6.1% East of Greenland; 7.0% West of Greenland; 2.5% in Foxe Bay; 4.6% in the Hudson Bay; and 2.2% in the Arctic Sea (see Figure 1).   Table 1 shows the projected sea ice cover decrease in each region indicated by the IPCC by 2050.

Global model simulations
The expected variation was then applied to the SPEEDY model code in the corresponding areas, using the variation factors as presented in the last column of Table 1. Due to model code requirements, these variation factors actually represent the sea ice that is expected to remain, not the ice that will be lost.
This study presents two experiments to generate global climatologies from the model. In the first, identified as IPCC, the Table 1

Data from National Centers for Environmental Prediction Climate Forecast System Reanalysis
The use of reanalyses in meteorology was a huge step forward in the 1990s, because the gridded datasets available before 1995 were created by constantly changed models and by different methods of analysis, even manual analyses prior to 1965. The most prominent reanalyses in world meteorology were conducted by the US environmental agencies National  In short, blue indicates the Ferrel cell strengthening, while red indicates its weakening.
In Figure 2, regarding the Ferrel cell inferior branch, blue represents the strengthening of the winds from the

Effects of sea ice reduction on tropical vertical motion
The last dynamic link between the Arctic sea ice reduction and the tropical rain patterns is diagnosed herein through the vertical motion field throughout the tropical belt shown in Figure 7.   in northern Argentina, which is a major food producer.
Another highlight is the apparent displacement to the south of the west sector of the Atlantic ITCZ, favouring the rains in Northeast Brazil.   Effects of sea ice reduction on tropical precipitation Figure 9 illustrates the rainfall simulations on tropical South America and its surroundings, following the boreal sea ice reduction indicated in the IPCC Report V and also with respect to the Radical 90% abatement of Northern Hemisphere polar marine ice.
There are some discrepancies between these two simulations, but the Atlantic ITCZ and the SACZ south-   to expect that the Amazon and Northeastern Brazilian rainfall has been and will continue to be affected by global climate change (Kucharski et al. ). Furthermore, the SACZ appears to be shifted by remote forces such as the changes explored herein.

ACKNOWLEDGEMENTS
The authors would like to express appreciation for collaboration and research funding provided by the Special thanks goes to Natalia Tasso Signorelli who kindly prepared Figure 1 for this paper. Finally, we would like to recognize the comments and suggestions provided by the anonymous reviewers which improved the previous versions of the manuscript.