Why is the Caribbean climate changing?
Mark R Jury <jury@uprm.edu>
Physics Dept, UPRM, Mayaguez, PR
The climate of the Caribbean is changing and we are one of the few places in the world
where IPCC models consistently predict both a warmer and drier future. We understand
some aspects of this change: 1) atmospheric composition, and 2) physical processes. Here
the rate of climate change in the Caribbean is studied considering long-term records of
temperature, rainfall and vertical motion.
Caribbean rainfall anomalies in the period 1900-2006 reveal a decline of –1.9
mm/decade, based on smoothed area-averaged data (Figure 1). A feature of individual
records for each Caribbean island is the high level of noise caused by tropical flood
events, for example in 1901 in Antigua and in the Dominican Republic in 1958. These
events induce a range from neutral to very negative that depends on location and length
of record analyzed. Support for the downtrend of rainfall is found in coral proxy records
of Caribbean hurricanes which show a downtrend in the period 1730–1990.
The upward trend in temperature is found in the local records (Figure 2). What is
interesting is the variation in trend depending on elevation. Caribbean Sea temperature
trends are 0.04 C/decade (similar to the global average), whilst air temperature trends are
three times greater (0.14 C/decade) over the period 1888–2006. The fastest rate of
warming occurs at the 1500 m (the level of the trade wind inversion), where temperatures
increase 0.29 C/decade over the period 1949-2006. The linear trend ‘fit’ increases with
elevation, rising from 55% at the sea surface to 87-88% in the air.
Regional variations in air temperature trends have been mapped. Trends are positive and
significant across the Caribbean Islands, with a maximum over the Windward Islands (50
– 60 W). Whilst some researchers attribute this to aerosol loading, an analysis of seasonal
trends indicates it may be due to ozone derived from African smoke plumes. However a
physical mechanism is more plausible: the elevated warming coincides with sinking
motions driven by an accelerating Hadley circulation linked with the Amazon monsoon.
Regional variations in 500 hPa vertical motion trends over the Caribbean (Figure 3),
exhibit a broad NW–SE axis of subsidence lying across the Caribbean. Trends correlation
values are in the range +0.6 to +0.8 from Cuba to Barbados, indicative of accelerated
sinking motions in the period 1976–2005. The subsidence extends over a deep layer.
Elsewhere trends are weak.
Long-term records of air and sea temperature, rainfall and vertical motion exhibit
significant warming and drying trends across the Caribbean. Air temperatures at 1500 m
are rising six times faster than the sea temperatures! Rainfall is gradually declining across
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the Caribbean, whilst upward trends are found over South America. This is related to
acceleration of the Hadley circulation, with enhanced sinking motions over the
Caribbean. The sinking motions induce a faster rate of warming and drying in the trade
wind inversion than at other levels. Although much of the trend in Caribbean climate is
attributable to physical mechanisms, changes in atmospheric composition also play a
role. Smoke and dust plumes from Africa drift westward across the Atlantic and enhance
the greenhouse effect in an elevated (2-4 km) layer. Deposition of larger reflective
particles leaves ozone that induces radiative absorption and a warming of the trade wind
inversion over the Caribbean. A stabilized lower atmosphere tends to suppress tropical
cyclones and contributes to regional drying and warming trends that are greater than the
5 yr smoothed E Caribbean rainfall
global mean.
40
y = -0.0144x + 7.3807
R2 = 0.2832
30
mm / month
20
10
0
-10
-20
-30
-40
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
year
Fig 1 Caribbean rainfall anomalies smoothed with a 5 year running mean, fitted
with a linear trend.
2
Caribbean temperature anomalies
3.5
3
degrees C
2.5
2
1.5
y = 0.0024x
R2 = 0.8778
y = 0.0012x
R2 = 0.8667
y = 0.0004x
R2 = 0.5486
1
0.5
0
-0.5
1890 Jan
1900 Jan
1910 Jan
1920 Jan
1930 Jan
1940 Jan
1950 Jan
1960 Jan
1970 Jan
1980 Jan
1990 Jan
2000
Jan
year
1888 1898 1908 1918 1928 1938 1948 1958 1968 1978 1988 1998
Fig 2 Caribbean temperature anomalies year
and trends: 1500 m (upper), 10 m
(middle) and sea surface (lower).
Fig 3 Linear trend correlation of mid-level vertical motion. Values > 0.4 (yellow)
are significant and indicate accelerating subsidence.
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