Marine Atlas: Maximizing Benefits for Tonga

Normal conditions

El Niño conditions

Westerly Winds

Strong Trade Winds

Weaker Trade Winds

Water Heated by the Sun

Weak Upwelling

Strong Upwelling

WarmWater

WarmWater

Thermocline

Thermocline

Deep Cold Water

Deep Cold Water

120°E

140°E

160°E

180°

160°W

140°W

120°W

100°W

80°W

120°E

140°E

160°E

180°

160°W

140°W

120°W

100°W

80°W

Darwin, Australia

Tonga

Lima, Peru

Darwin, Australia

Lima, Peru

Tonga

El Niño is an example of the big impact that re- gional climate variability related to ocean currents has on Tonga (see graphs and chapter “Hotter and higher”). Normally, strong trade winds blow from east to west across the Pacific Ocean around the equator. As the winds push warm surface water from South America west towards Asia and Aus- tralia, cold water wells up from below in the east to take its place along the west coast of South Amer- ica. This creates a temperature disparity across the Pacific, which also keeps the trade winds blowing. The accumulation of warm water in the west heats the air, causing it to rise and create un- stable weather, making the western Pacific region warm and rainy. Cool, drier air is usually found on the eastern side of the Pacific. In an El Niño year, the trade winds weaken or break down. The warm water that is normally pushed towards the western Pacific washes back across, piling up on the east side of the Pacific from California to Chile, causing rain and storms and increasing the risk of cyclone formation over the tropical Pacific Ocean (Climate Prediction Center, 2005). On the other side, the western Pacific experiences particularly dry conditions. The periods 1997–1998 and 2014–2016 witnessed some of the most ex- treme events on record in the region. Between 2015 and 2017, Tonga experienced its worst and most sustained drought in decades. Many of the worst affected areas were also those severely hit by Cy- clone Pam, itself one of the worst natural disasters in the history of Tonga. Throughout this period, a food security crisis loomed that saw many commu- nities struggle to survive, with young children the most acutely affected. Moreover, El Niño contrib- utes to an increase in global temperatures. In the particularly hot year of 2015, El Niño was respon- sible for about 10 per cent of the temperature rise. In turn, rising global and ocean temperatures may intensify El Niño (Cai et al., 2014). In summary, sea currents driven by wind, heat and salinity influence not only Tonga’s marine biodiver- sity, but also its rainfall patterns and temperature on land.

14°S

176°W

172°W

SEA SURFACE CURRENTS

Direction and velocity (m/s)

0.05

0.03

75 150 km

Sources : Becker et al, 2009; Claus et al, 2016; ESR, 2009; Smith and Sandwell, 1997. Copyright © MACBIO Map produced by GRID-Arendal

18°S

22°S

26°S

MAXIMIZING BENEFITS FOR TONGA

SUPPORTING VALUES

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