Comments on “Warming of Eurasian Landmass Is Making the
Arabian Sea More Productive”
Prabir K. Patra,1 Takakiyo Nakazawa,2,1 and Kentaro Ishijima1
1. Frontier Research Center for Global Change/JAMSTEC, Yokohama 236 0001, Japan
2. CAOS, Graduate School of Science, Tohoku University, Sendai 980 8578, Japan
The increasing trends in chlorophyll-a over the western Arabian Sea based on
satellite data during 1996-2003 are intriguing(1). Their suggested teleconnection
between Eurasian snow-cover and southwest monsoon is unlikely to have caused
this increase. Here an alternative mechanism is given to explain the observed
interannual variability in chlorophyll-a in the period 1997-2004.
This paper(1) relies entirely on correlation statistics to infer the underlying processes;
the causal relationship may not be as straight forward as the correlations. For example
the authors noted that the summer monsoon strength should be more closely related to
the heating gradient between Tibetan plateau and the Indian Ocean, but they showed the
correlation between the Eurasian snow cover. In addition, there is hardly any other
evidence about increase in the southwest (summer) monsoon strength. They also did not
find associated deepening of mixed layer depth (MLD) with time, while upwelling is
suggested to be the main cause for increased nutrient supply to the surface water from
bottom. On the other hand the weakening of the summer monsoon strength has been the
focus of many studies in the recent years (e.g., 2,3). Though no single cause for this
weakening is identified, there are suggestions that dynamical, radiation, and
microphysical processes might be acting in concert to weaken the summer monsoon(2).
The decrease in SST with time (1, Fig. 2) is probably observed due to abstraction of the
incoming solar radiation by absorbing aerosols(2). Finally, it is not fair to make
judgment about ‘year-by-year’ increase in phytoplankton based on 7 years of data and
using only the extreme values (e.g. 1996 and 2003). For instance, more recent
observations show the summer chlorophyll-a values in 2004 are smaller than those in
2003. Thus, we believe, using such a short timeseries study of the interannual variability
is more appropriate than a trend analysis.
In this comment we propose an alternative process, co-occurring with the
regional dynamics and composition change, to explain the interannual variabilities in
chlorophyll-a abundance over the Arabian Sea as a whole (not any particular area).
Figure 1 shows the area-averaged timeseries of chlorophyll-a observed by the SeaWIFS
in the period 1997-2004. It is clearly seen that the increase in chlorophyll-a
concentration is not restricted to any particular area in the Arabian Sea. However, a
decrease in chlorophyll-a is observed in 2004 compared to previous years over some
regions (Fig. 1B), and strong interannual variability is found in the northern Arabian Sea
(Fig. 1A). Fig. 2 shows TOMS aerosol index (AI; positive values only) for the same
period and area as that in Fig. 1. In northern Arabian Sea the chlorophyll-a and AI
interannual variability exhibit good correlations for the summer months during
1998-2001 (Fig. 2A). About a month delay in time for chlorophyll-a maxima are
observed from the AI maxima. Recently, it is shown (2) that the variations in AI over the
Arabian Sea is linked to the transport of aerosols from African continent and the Gulf
region during the monsoon season. Thus interannual changes meteorological conditions
associated with the climate oscillations lead to the interannual variability in production
aerosols over the land and subsequent transport to the Arabian Sea and the tropical
Indian Ocean(2).
In southern Arabian Sea (Fig. 2B) an increase in both chlorophyll-a and AI are
observed for all the months. More interestingly, there is a bi-modal behaviour in the
seasonal cycle of both, one occurring in the pre-monsoon season and the other being
associated with the southwest monsoon. Though such a bimodal structure is also present
in the mixed-layer depth (MLD) timeseries, but the MLD show swallowing trends
between 1997 and 2004 (1, Fig. 2). This is opposite to the observed correlation for
Chlorophyll-a and AI. Finally, secular increases in the baseline of both the chlorophyll-a
concentration and AI values are observed during the period of this analysis (Fig. 1B and
2B). Such changes can be attributed to the enhanced supply of nutrients, through land
based aerosols emission and transport, to the Arabian Sea and the tropical Indian Ocean.
A similar analysis for the Bay of Bengal region also reveal gradual shift in the baseline
chlorophyll-a and AI but with much smaller magnitudes. Our results are in overall
agreement with the findings of increase in primary production over parts of the Indian
and Atlantic Oceans between the periods 1979–1986 and 1997–2002(4).
References:
1. J. I. Goes, P. G. Thoppil, H. do R Gomes, J. T. Fasullo, Warming of the Eurasian
Landmass Is Making the Arabian Sea More Productive, Science 308, 545 (2005).
2. P. K. Patra, S. K. Behera, J. R. Herman, S. Maksyutov, H. Akimoto, T. Yamagata, The
Indian summer monsoon rainfall: interplay of coupled dynamics, radiation and cloud
microphysics, Atmos. Chem. Phys. Discuss. 5, 2879 (2005).
3. V. Ramanathan, C. Chung, D. Kim, T. Bettge, L. Buja, J. T. Kiehl, W. M. Washington,
Q. Fu, D. R. Sikka, and M. Wild, Atmospheric brown clouds: Impacts on South Asian
climate and hydrological cycle, Proc. Natl. Acad. Sci.(USA) 102, 5326 (2005).
4. W. W. Gregg, M. E. Conkright, P. Ginoux, J. E. O'Reilly, and N. W. Casey, Ocean
primary production and climate: Global decadal changes, Geophys. Res. Lett. 30(15),
1809 (2003).
Fig. 1. Area-averaged timeseries of Chlorophyll-a during 1997-2004 as observed from the Sea
viewing Wide Field-of-view Sensor (SeaWiFS) on the Sea Star spacecraft in since September of
1997 (Level-3 9-km global mapped data, Monthly-averages; source: ftp://oceans.gsfc.nasa.gov).
Several area- averaging is done for this diagram in two latitude bands – northern (10-20oN; panel A)
and tropical (5oS-10oN; panel B) Arabian Sea, and four longitude bands – western (47-52oE),
west-central (52-57oE), east-central (57-62oE), and western (62-67oE) within each latitude band. The
region (52-57oE, 5oS-10oN) is identical to that used in Fig 2 of Goes et al. (1).
Fig. 2. Same as Fig. 1, but for the Total Ozone Mapping Spectrometer (TOMS) on the Earth Probe
spacecraft measured aerosol index (panels A and B are for northern and southern Arabian Sea,
respectively). Only positive AIs, representing land/absorbing aerosol particles (airborne microscopic
dust/smoke), are used in this diagram (Level-3, Version-8, Monthly averages; source:
ftp://toms.gsfc.nasa.gov/). Note here that because of continuing changes in the optical properties of
the front scan mirror, the AI data since 2002 should not be used for trend analysis in strict sense
(toms.gsfc.nasa.gov/news). But for the sake of completeness and some discussions AI values are
shown in the plots. Since, it also states that the calibration appears to be stable near the equator, and
by 50o latitude, there is a -2% to -4% errors in TOMS.
Supplementary Materials (only for review process):
Fig. S1. Same as Fig. 1 & 2, but for the Bay of Bengal region.
Fig. S2. Timeseries of mixed-layer depths (MLDs) over several regions of the Arabian
Sea and Bay of Bengal (data source: White et al., 1988; http://jedac.ucsd.edu/).
Fig. S3. Same as Fig. S2, but for NCEP/NCAR wind speed (source: Kalnay et al., 1996;
www.cdc.noaa.gov/cdc).
References:
1. W. B. White, S. E. Pazan, G. W. Withee, and C. Noe, The Joint Environmental Data Analysis
(JEDA) Center for scientific quality control of upper-ocean thermal data in support of TOGA and
WOCE, EOS 69, 122 (1988).
2. Kalnay, E., and Coauthors, The NCEP/NCAR Reanalysis 40-year Project, Bull. Amer.
Meteor. Soc. 77, 437, 1996.