Desert Air Incursions, an Overlooked Aspect, for the Dry Spells of the Indian Summer Monsoon: BY T....

Desert Air Incursions, an Overlooked Aspect, for the Dry Spellsof the Indian Summer Monsoon

T. N. KRISHNAMURTI, A. THOMAS, ANU SIMON, AND VINAY KUMAR

Department of Meteorology, The Florida State University, Tallahassee, Florida

(Manuscript received 7 January 2010, in final form 22 March 2010)

ABSTRACT

The year 2009 was a major drought year for the Indian summer monsoon with a seasonal deficit of rainfall

by 21.6%. Standard oceanic predictors such as ENSO and the Indian Ocean dipole are not consistent for these

dry spells. There are a host of other parameters such as the Himalayan ice cover, the Eurasian snow cover, the

passage of intraseasonal waves, and even accumulated effects of Asian pollution that have been considered

for analysis of the dry spells of the monsoon. This paper presents another factor, the western Asian desert air

incursions toward central India, and emphasizes the formation of a blocking high over western Asia as an

important feature for these dry spells. The blocking high advects descending very dry air toward central India,

portrayed using swaths of three-dimensional trajectories. This is a robust indicator for dry spells of the

monsoon during the last several decades. This dry air above the 3-km level over central India strongly inhibits

the vertical growth of deep convection. Some of the interesting antecedents of the formation of the blocking

high include an eastward and somewhat northward extension of the ITCZ over North Africa, a stronger than

normal local Hadley cell over North Africa, a strong subtropical jet stream over the southern Mediterranean,

and strong conversions of anticyclonic shear vorticity to anticyclonic curvature vorticity. The dynamical

antecedents of the aforementioned scenario in this study are related to many aspects of North African

weather features. They are portrayed using both reanalysis datasets and ensemble modeling using a suite of

coupled atmosphere–ocean models.

1. Introduction

The intervals between rain spells during the Indian

summer rainy season, called breaks, generally last from

one to two weeks. Some years there may be longer-

lasting dry spells and those are often defined as drought

years. Climatological aspects of breaks including their

duration, geographic distributions, preferred periods dur-

ing summer, and other statistics have been addressed

by a number of authors (Krishnan et al. 2000; Rajeevan

et al. 2006; Gadgil and Rupakumar 2006).

The range of external parameters that impact the be-

havior of the summer monsoon rains over India includes

the effects of the El Nino (Rasmusson and Carpenter

1982; Krishnakumar et al. 2006), the Indian Ocean tem-

peratures expressed by the index called the Indian Ocean

dipole (IOD) (Saji et al. 1999), the meridional passage of

intraseasonal oscillation (ISO) waves (Krishnamurti and

Subrahmanyam 1982; Sikka and Gadgil 1980; Wang

and Rui 1990), the Himalayan ice cover (Dickson 1984;

Fasullo 2004), and the Eurasian snow cover (Barnett et al.

1989; Bhanu Kumar 1988). A number of internal indices

have been discussed in this context (Dumenil et al. 1994;

Harzallah and Sadourny 1995), as have possible effects

of monsoonal pollution (Ramanathan et al. 2005; Lau

et al. 2006; Lau and Kim 2006; Krishnamurti et al. 2009).

The extreme dryness of the troposphere above the 2-km

level over central India during periods of dry spells bears

a strong resemblance to desert air rather than to the

drying from a slow subsidence related to El Nino events.

The warm SST anomalies of the El Nino reside in the

eastern and/or the central equatorial Pacific Ocean. The

compensating descent of the El Nino–related rising mo-

tions is described by east–west divergent circulations. The

El Nino–related droughts are known to occur anywhere

throughout eastern Australia, the Philippines, Indonesia,

India, East Africa, Indo-China, and China. The strongest

descending lobe of the east–west circulation anomaly is

known to meander from one El Nino event to the next.

The El Nino monsoon relationship over India is not very

Corresponding author address: T. N. Krishnamurti, Dept. of

Meteorology, The Florida State University, Tallahassee, FL 32310.

E-mail: [email protected]

OCTOBER 2010 K R I S H N A M U R T I E T A L . 3423

DOI: 10.1175/2010JAS3440.1

consistent: whereas some major El Nino years such as

1972 and 1987 have been associated with significantly

below-normal monsoon rainfall years, many El Nino years

have been near-normal rainfall years for the Indian

summer monsoon (Rasmusson and Carpenter 1982). The

dry spells of the Indian summer monsoon last for several

weeks. To portray the teleconnections between the re-

gion of the El Nino and a region of drought, it is not

advisable to look at daily charts of divergent circula-

tions, since short space time transients mask the rela-

tionships being sought. Generally that teleconnection

becomes very apparent after some space–time smoothing

(Krishnamurti et al. 1986). From such an analysis of sev-

eral years of data, it becomes apparent that the El Nino–

Indian monsoon rainfall relationship is not very robust.

The El Nino–related warm SST anomalies also occur in

the central equatorial Pacific ocean (Krishnakumar et al.

2006), and these seem to bear a somewhat stronger re-

lationship to the droughts over India compared to the

warm SST anomalies of El Nino that reside only over the

equatorial eastern Pacific Ocean. The IOD is a normal-

ized SST anomaly difference between the western and

the eastern equatorial Indian Ocean (Saji et al. 1999).

Above-normal rainfall anomalies (over India during the

summer monsoon) have been related to a positive IOD

index (Saji et al. 1999), with the converse being the case

during periods of a negative IOD index. Gadgil et al.

(2003, 2004) noted stronger than normal trades over the

equatorial Indian Ocean during periods of the positive

IOD index and proposed the role of such winds and

related evaporation in the enhancement of monsoon

rains. Gadgil et al. (2004) proposed the name Equatorial

Indian Ocean Oscillation (EQUINO) to define their

wind-related index for assessing the monsoon rainfall

behavior. The variations of the IOD index and summer

monsoon rainfall relationship during recent decades are

illustrated in this paper. Overall the impression one gets

from these indices is that no single parameter is a robust

indicator of the seasonal rainfall behavior.

Another observed feature is the northward arrival of

a high sea level pressure anomaly extending from south

of India northward to around 258N latitude (Krishnan et al.

2000). This northward-moving feature has been identi-

fied as a component of the meridionally propagating

ISO of the monsoon (Waliser 2006). The passage of these

meridionally propagating waves from near the equator

to the foothills of the Himalayas brings in alternate spells

of dry and wet monsoon weather. The ISO is character-

ized by a meridional passage of cyclonic and anticyclonic

eddies on a time scale of roughly 30–50 days in the

lower troposphere over southern and southeastern Asia

(Yasunari 1980, 1981; Krishnamurti and Subrahmanyam

1982; Sikka and Gadgil 1980). The scale of these eddies

is on the order of 3000 km; these propagate meridionally

at a speed of roughly 18 day21. The arrival of an anti-

cyclonic eddy from the near-equatorial latitudes of the

Indian Ocean to near 258N takes nearly 15 days and is

known to bring about a dry spell. Since the space–time

scales and amplitudes of the ISO vary considerably from

case to case, the relationship of the ISO to the occur-

rences of the dry spells of the summer monsoon is not

very robust. Other factors such as the Himalayan ice

cover and the Eurasian snow cover are also not consid-

ered to be consistent robust parameters for the behavior

of the Indian summer monsoon rainfall (ISMR).

There is an important overlooked aspect of dry spells

of the Indian monsoon that is highlighted in this study.

This relates to the formation of a blocking high, around

the Arabian Peninsula, that is located roughly above the

3-km level to almost 9 km above sea level. This intense

blocking high advects descending very dry air over the

Arabian Sea and toward central India and strongly in-

hibits the vertical growth of deep convection. It is pos-

sible to portray the eastward passage of this dry air

through the construction of swaths of three-dimensional

trajectories, which turn out to be a robust indicator for

the dry spells of the last several decades. Our study also

presents an analysis of some of the salient antecedents of

the blocking high that suggests that the phenomenology

to the west of India deserves a closer examination. A

modeling of the proposed scenario, covering dry spells

of the Indian summer monsoon rains, is also addressed

in this paper. This modeling entails the use of a suite of

coupled atmosphere–ocean models for examining the

dry spells of the 2009 season.

2. Results

a. The El Nino factor

The relationship of seasonal monsoon rains to the

El Nino is a major topic of considerable interest

(Rasmusson and Carpenter 1982). The current under-

standing of this problem is that whereas some droughts

or below-normal monsoon rains over India seem to be

related to El Nino years, many El Nino years experience

near normal rainfall over India. The warm SST during

an El Nino reside over the equatorial central and east-

ern Pacific Ocean and divergent east–west circulations

(Krishnamurti 1971) carry an ascending branch of air

over these warm belts of SST anomalies. The compen-

sating descending lobe is known to meander considerably

from one El Nino to the next. That region of strongest

descent can be seen over eastern Australia, Malaysia,

the Philippines, eastern and central China, Indochina,

India, or even as far away as East Africa during dif-

ferent El Nino years. Thus, the prominent response to

3424 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 67

El Nino–related dry weather is not a consistent feature

for any given region. Krishnakumar et al. (2006) noted

the importance of the central Pacific SST during El Nino

years for deficient rainfall over India. The 2009 El Nino

was also characterized by a central Pacific warm SST

anomaly, but that season did not carry a strong descending

lobe of divergent circulation over India. Strong factors

contributing to the varied locations of the descending lobe

are the prevailing atmospheric winds, wind shears, and

thermal structures during these various El Nino years.

The total seasonal rainfall (for June–September) over

all of India [the all-India summer monsoon rainfall

(AISMR)] is used as a measure of the Indian monsoon

behavior. Figure 1 illustrates the total rainfall covering

the years 1960–2009. The El Nino years are shown by an

open bar. While some deficient rainfall years were clearly

El Nino years, many were characterized by near-normal

rains. Thus, the monsoon–El Nino relationship is not very

straightforward. The climatology of the divergent cir-

culations, based on 51 yr of reanalysis datasets, is best

portrayed from charts of the velocity potential (whose

gradient indicates the divergent wind) (Fig. 2). This carries

a monsoonal ascent of air and a primary descent of air

over the eastern Pacific Ocean to the east and over the

Mascarene high toward the south. This is a characteristic

feature of the global summer monsoon.

The divergent circulations in any El Nino year can

be parallel, antiparallel, or even perpendicular to these

climatological divergent circulations. Thus, these can

strengthen or weaken the climatological divergent mo-

tions and, in principle, contribute to an enhancement or

reduction of monsoon rains (Krishnamurti et al. 1989).

Figures 3a–e show the divergent wind anomalies (with

respect to the climatology of the 51-yr mean, 1958–2008)

for some of the recent major dry spells of the Indian

summer monsoon.

The El Nino–monsoon teleconnection requires the

mapping of divergent wind anomalies that would show

an ascent and divergent wind anomalies over the region

of maximum convection/El Nino of the eastern Pacific

Ocean and its teleconnections with a convergent (with

descent) wind anomaly over the monsoon regions. This

can be mapped by first computing the averaged velocity

potential for each of the dry events and subtracting the

climatological picture of the velocity potential shown in

Fig. 2. It is not advisable to carry out such an exercise by

subtracting the climatology from an entire seasonal

mean of a dry monsoon year. That season can still carry

some heavy wet spells as well and the anomalies seem to

average out such details. This statement has some im-

plications for climate modeling where monthly or sea-

sonal averages are often used. In this paper, the velocity

potential of the climatology at the 200-hPa level is sub-

tracted from the averaged velocity potential of dry epi-

sodes, and the related divergent wind vector anomalies

is derived for that period, to examine the teleconnec-

tions of the El Nino and the Asian monsoon. If many

transient disturbances on smaller scales are present be-

tween the regions of the El Nino warming and India,

then the teleconnection will not be apparent. Further-

more, it is noted that the teleconnection is a planetary-

scale phenomenon and requires an examination of the

large-scale component of the divergent wind. Those are

obtained by utilizing zonal harmonics. In Figs. 3a–e the

long wave (wavenumbers 0, 1, 2, and 3) for the differ-

ence of the divergent wind anomalies of the dry epi-

sodes, over India, and the climatological counterparts is

shown. The strongest ascending motions are illustrated

FIG. 1. The total seasonal rainfall (mm) for the months June–

September over all of India (AISMR). The El Nino years are

shown as white bars; the remaining years are marked as black bars.

The wet (W) and dry (D) monsoon years are also marked.

FIG. 2. Climatology of the velocity potential (106 m2 s21) at the

200-hPa level covering JJA for the recent 50 yr. The gradient of

this field denotes the divergent wind. Positive values are shaded.


as dashed lines and the strongest descending motions are

indicated as solid lines in these figures.

The divergent wind anomalies during strong dry spells

for a sequence of El Nino years are examined next.

Divergent wind anomalies at the 200-hPa level are gen-

erally illustrated to portray the Walker and east–west

circulations (Krishnamurti et al. 1985). During El Nino

years the Hadley-type overturning is generally stronger

FIG. 3. The divergent wind anomalies (with respect to the climatology of the 51-yr mean, 1958–2008) for some of

the major dry spells. The lengths of the arrows are defined in units of meters per second. The largest ascending

motions are shown as dashed lines, and the largest descending motions are shown as solid lines.


compared to La Nina years (Wallace and Gutzler 1981).

The divergent wind anomalies for a major dry spell (2–

12 July 1966) are shown in Fig. 3a. The divergent winds

from the Southern Hemisphere ITCZ were strong dur-

ing this period and the divergent winds imply an ascent

of air near 108S latitude and a descent over India. No

direct teleconnections of the monsoonal region with the

belt of warm SST anomalies of the eastern equatorial

Pacific Ocean were noted during this period. The di-

vergent winds of the period 18 July–3 August 1972 (Fig. 3b)

show divergent wind emanations from the ITCZ over

Africa near 108N that carried a descent over India, and

the divergent wind teleconnections with the central or

eastern equatorial Pacific Ocean were not present. In

1982 the period 1–8 July was characterized by a strong

sea surface anomaly over the central Pacific Ocean and

the divergent wind emanation from this region of ascent

could be clearly seen (Fig. 3c) and a teleconnection with

central India where the descending lobe existed was

noted. The vertical motions are not shown; they were

examined from reanalysis datasets (Kalnay et al. 1996).

The year 1997 carried a major dry spell over India during

the period 9–14 August. The SSTs of this El Nino year

showed the warm anomalies over the central Pacific

Ocean. This was a textbook case, where the divergent

winds (Fig. 3d) emanated from this region of warm SST

anomalies and the descending lobe occurred over cen-

tral India. During the period 4–17 July 2002, another

El Nino year, a significant dry spell was noted over India

(Gadgil et al. 2004; Mitra et al. 2005). This was a year

with warm SST anomalies that extended between the

eastern and the central equatorial Pacific Ocean. This

was another year with a clear monsoonal teleconnection

with India (Fig. 3e), with the ascending lobe of the di-

vergent wind located around 1208W and the descending

lobe of the divergent wind over central India. Thus, the

relationships of the dry spells of the Indian monsoon

with reference to the El Nino are not always very direct.

FIG. 3. (Continued)


b. Indian Ocean dipole

The Indian Ocean dipole index is a normalized dif-

ference in sea surface temperature between the near-

equatorial western Arabian Sea and the eastern Bay of

Bengal. Figure 4a shows the time history of this index

during the year 2009. A positive index signifies warmer

SSTs over the southwestern Arabian Sea compared to

those over the southeastern Bay of Bengal, and it relates

to above-normal monsoon activity over India. During

the 2009 season this index was not a good indicator of

monsoon behavior: the dry spells prior to 13 June, the

dry spell between 30 July and 10 August, and the wet

spells after 10 August 2009 were not reflected by this in-

dex. However, when the longer record is studied, Fig. 4b,

this index displays some usefulness for the monsoon be-

havior. Using a symbol ‘‘A’’ for above normal, ‘‘B’’ for

below normal, and ‘‘N’’ for normal seasonal rainfall [June–

August (JJA) for the years 1990–2009] produces the year-

by-year sequence AABBAABNANNBBBNBAAAA.

The corresponding IOD index (‘‘A’’ for positive, ‘‘B’’

for negative, and ‘‘N’’ for neutral) produces the sequence

BAANANBABNNBBANBAAAA. Clearly the rela-

tionship of the seasonal rainfall to the seasonal IOD in-

dex is not very strong. Often the IOD index carries more

skill in predicting the dry spells when it is also a strong

El Nino year, with the warm SST anomalies residing over

the equatorial central Pacific Ocean. The correlation of

the IOD index and ISMR, for the last 30 years, was noted

to be around 0.1.

This paper addresses the land influences from western

Asia for their contributions of very dry middle tropo-

spheric air toward India. Some of the antecedents of that

dry air incursion are also illustrated here. These are

additional features of the dry spells of the Indian sum-

mer monsoon that deserve to be considered in any overall

evaluation of the problem.

c. Trajectories during the dry and wet spells

The daily totals of rainfall (mm day21) over India for

2009 are shown by a histogram in Fig. 5a, covering the

entire rainy season. This illustration for the summer of

2009 identifies the dry and wet spells and the long term

(40-yr) average daily rainfall climatology. In year 2009,

except for short wet spells, most of the season carried

below-normal rains. The distribution of humidity aver-

aged for the dry and wet spells of the Indian monsoon,

during 2009, shows a marked drop of humidity over

central India at the 700-hPa level when we compare the

respective dry and wet spells (Figs. 5b,c). The dry spell

event for 10–19 June clearly shows an extension of a

dry tongue extending toward India from the northwest.

Figures 5d–f show Moderate Resolution Imaging Spec-

troradiometer (MODIS) imagery composites at the fre-

quency 550 nm. This is in the visible part of the spectrum.

This shows the incursion of fine dust from the Arabian

Sea into all of northern India during periods of com-

posite dry spells. This includes the dry spell composite

for the years 2000–09 (Rajeevan et al. 2006) of the sum-

mer monsoon for the period between days 23, 0, and 13

of the dry spell composite. Day 0 is the day of onset of

the dry spell defined following Rajeevan et al. (2006).

The desert regions do not carry a large dust content in

the MODIS imagery because the reflected solar radia-

tion over these high-albedo regions masks the presence

of large dust levels dust even though the dust originates

from there. It is quite clear from these composites that

a large incursion of dust from the west makes its way into

northern India during the dry spells.

This suggested the need for the construction of a

swath of trajectories (Krishnamurti and Bounoua 1996,

259–266) that terminate over central India during the

dry and wet spells. Three-dimensional wind datasets were

obtained from the reanalysis fields (Kalnay et al. 1996).

Several 10-day long back trajectories were constructed.

Parcels of air that terminate over central India at the

850-hPa level (roughly 1.5 km above the earth’s surface)

were distinctly different from those terminating at the

700-hPa level (roughly 3 km above the earth’s surface).

During the dry spells, back trajectories, terminating at

the 850-hPa level (and above) over central India were

of oceanic origin, whereas those arriving at the 700-hPa

level had their origin over western Asia. During the

wet spells most of the trajectories terminating over

central India at 850- and 700-hPa levels were of oceanic

origin. Figures 6a–h show several examples of these

back trajectories for a number of wet and dry spells of

FIG. 4. Indian Ocean dipole mode index (DMI) from Reynolds

Optimum Interpolation, version 2 (OIv2) for (a) 2009 and (b) 1990–

2008.


(a)

FIG. 5. (a) The daily rainfall (mm day21) for the 2009 season over India. The dry and wet spells for the season are identified. (b),(c) The

vertically integrated lower tropospheric (950–700 hPa) specific humidity (kg kg21) for dry and wet spells of monsoon rains over central

India. This covers an average for (b) 10–19 Jun 2009 for the dry spell and (c) 14–20 Jul 2009 for a wet spell. (d)–(f) Aerosol optical depth

(AOD) at 550 nm, composited for break days (2002–09) from MODIS (Terra/Aqua) at (d) 3 days before (e), the zeroth day, and (f) 3 days

after the break. The 550-nm wavelength being invisible, MODIS is not able to capture AOD at deserts.


the Indian summer monsoon covering different years

based on the data provided in Rajeevan et al. (2006).

The swaths of trajectories from western Asia during the

dry spells show a marked descent, as they arrive over

central India at the 700-hPa level from levels as high

as the 400-hPa level (roughly 6 km above the earth’s

surface) over western Asia. The air in the lowest kilo-

meter is consistently of oceanic origin. Clouds do not

show a vertical buildup because of the entrainment of

very dry air of desert origin above the 3-km level. This

is a unique aspect of the dry spells. During the wet

spells almost all the trajectories examined carried more

moisture at the 3-km level and a vertical growth of deep

convection was not inhibited. Nearly all 10-day-long

trajectories terminating over central India during dry

spells, between the 700- and 300-hPa levels, arrive from

FIG. 6. Ten-day back trajectories from central India. (left) Dry spell for trajectories terminating at 850 (green) and

700 (red) hPa; (right) as at left, but for the wet spells of the Indian summer monsoon, for (a) 18 Jun 2009, (b) 14 Aug

2005, (c) 16 Jul 2002, (d) 30 Aug 2001, (e) 14 Jul 2009, (f) 1 Aug 2005, (g) 1 Aug 2004, and (h) 12 Jul 2001.


western Asia, whereas most of the trajectories of the

wet spells terminating over central India are of oceanic

origin.

d. Blocking high over western Asia

The dry air descending from western Asia was part of

the circulation of a blocking high that was nearly always

present during periods of the dry spells of the rainy season.

Figures 7a–d show the vertically integrated horizontal

motion field between the 3- and 9-km levels in the verti-

cal, which is also integrated in time covering several epi-

sodes when a dry spell prevailed over central India. This

blocking high was a persistent feature during these entire

dry spells. It is the air to the north of this high over western

Asia that descends during the traverse into central India.

The passage of very dry air over the central Arabian Sea is

another important aspect of the circulation around the

blocking high that limits the passage of the moist oceanic

air to the lowest 2 km, above which the dry air advection

seems to predominate. This descending air (Kalnay et al.

1996) north and east of the blocking high brings in very dry

desert air from the upper troposphere of western Asia to

central India. The typical specific humidity of air at the

400-hPa level over the northern parts of the Arabian

Peninsula is around 0.3 g kg21. This dry air descends and

arrives over central India near the 700-hPa level with low

specific humidity on the order of 5 g kg21. That increase in

specific humidity, in the absence of significant evaporation

or precipitation, is largely from subgrid-scale vertical eddy

convergence of flux of moisture. The specific humidity

over central India during active monsoon periods is typi-

cally on the order of 11 g kg21 at 700 hPa.

FIG. 7. The blocking high of western Asia for different dry spells of the Indian summer monsoon. These are based

on vertically integrated horizontal winds from the 700- to 300-hPa levels. The lengths of the arrows are defined in

the bottom inset in units of meters per second. Dates are (a) 10–19 Jun 2009, (b) 4–17 Jul 2002, (c) 7–14 Aug 2005, and

(d) 26–30 Aug 2001. The rainfall (mm day21) for these dates is also included in the figure.


e. Antecedents of the western Asian blocking high

There are both tropical and extratropical antecedents

of the blocking high. Looking at several dry spells of the

last three decades, the following tropical scenario can

be noted. The ITCZ of the eastern Atlantic Ocean,

discerned from the remotely sensed rainfall estimates

(mm day21) from the National Aeronautics and Space

Administration’s (NASA’s) Tropical Rainfall Measur-

ing Mission (TRMM), shows a somewhat more north-

ward location (98N) as compared to a normal position

(78N; Figs. 8a,b), extending much farther inland to al-

most 408E over Africa. It should be noted that this con-

figuration of the ITCZ supports a stronger local Hadley

cell over Africa during these dry spells. Composites of

the local Hadley cell for North Africa were prepared

for the periods of the dry and wet spells of the Indian

rainy season covering the longitudes 108–408E. The dif-

ference of Hadley cell streamfunction over Africa be-

tween the longitudes 208W and 408E, computed following

Oort and Yienger (1996) for the dry minus wet spells

of rains over central India, is shown in Fig. 9a. This

difference shows an active and stronger Hadley cell

over the upper troposphere near the latitudes 158–208N.

The individual panels are presented in Figs. 9b,c. These

respectively show the vertical display of the local Hadley

cell between the longitudes 208W and 408E for a dry

spell and the wet spell composites. These illustrations

show a greater vertical extent for the local Hadley cell

FIG. 8. Antecedents of the blocking high. (a) The climatological

mean (JJA) of the ITCZ (mm day21); (b) ITCZ for composite of

several dry spells of the Indian monsoon (mm day21).

FIG. 9. (a) The difference in the local Hadley cell streamfunction

(109 m2 s22) over Africa for the dry and the wet spells over central

India. (b),(c) The local Hadley cell streamfunction (109 m2 s22) over

Africa (b) for the wet and (c) for the dry spells over central India.


(between 208 and 308N) over Africa that contributes to

an intensification of the subtropical westerly jet stream

of the southern Mediterranean.

A stronger Hadley cell, from the angular momentum

perspective, implies strong south–north-directed diver-

gent wind leading to a stronger subtropical jet stream

and the possibility for its breakdown into eddies from

local shear flow instabilities. The location of the sub-

tropical jet stream is closely linked to the northern limit

of the local Hadley cell (Palmen 1951). Those winds

often contribute to the presence of strong supergradient

westerly winds along the subtropical westerly jet streams

(Mogil and Holle 1972; Krishnamurti et al. 2005). This is

illustrated in Table 1. These are called anomalous winds

and approach a speed close to the anomalous solution of

the local gradient wind equation (Holton 2004, 34–53).

These strong anomalous winds often lead to the for-

mation of blocking highs and wave trains downstream

from such regions (Krishnamurti et al. 2005). When the

wind datasets over North Africa are examined, during

periods of the formation of the blocking high the pres-

ence of such anomalous winds along the subtropical jet

stream over North Africa can be observed on numerous

occasions. Table 1 includes the location, dates, and ra-

tios of the observed to the geostrophic wind speeds at

the 200-hPa level (roughly 12 km above the earth’s sur-

face), which is close to the level of the strongest winds.

This suggests that the formation of the blocking high over

western Asia could be related to this strong jet stream

that favors a transformation of the anticyclonic shear vor-

ticity to anticyclonic curved flows (Bell and Keyser 1993;

Viudez and Haney 1996). Appendixes A and B provide

the theoretical background for the shear to curvature and

generation of eddy kinetic energy discussed below.

The supergradient winds contribute to the shear flow

instability from the increased anticyclonic shear vorticity

and its transformation to anticyclonic curvature vorticity

(Fig. 10a). This illustration confirms such an exchange

of vorticity from shear to curvature (s21) for the period

1–10 June 2009, when the blocking high was persistent

over western Asia. Figures 10b and 10c illustrate these

features for this transformation of vorticity during several

other dry spells of the monsoon over India. The various

examples all support this exchange. Large exchange from

shear to curvature is the kinematics of the prevailing

flows during the formation of the blocking high over

western Asia. This conversion from horizontal shear leads

to an increase of curvature, which is the inverse of the

radius, thus contributing to smaller-sized closed circula-

tions, which appears to be the blocking high of this region.

In several of these instances an increasing middle

latitudinal influence from the passage of waves in the

westerlies can also be noted. Figure 11 illustrates a se-

quence of maps during the dry spell of monsoon rains

over India. This midlatitude wave activity over the re-

gion of the southern Mediterranean is best seen from

TABLE 1. The dates, locations, observed winds V, geostrophic

winds Vg, and their ratio. All cases are during 2009.

Date Latitude (8 N) V (m s21) Vg (m s21) V/Vg (m s21)

10 Jun 27.5 4 1.01 3.97

12 Jun 27.5 7 1.46 4.80

17 Jun 27.5 31 12.55 2.47

18 Jun 27.5 24 10.95 2.19

20 Jun 27.5 14 5.71 2.45

15 Jun 27.2 15.9 5.39 2.95

16 Jun 27.2 18 5.45 3.30

17 Jun 27.2 15.4 6.26 2.46

18 Jun 27.2 16 3.92 4.09

19 Jun 27.2 12 2.35 5.11

20 Jun 27.2 12.3 1.46 8.41

10 Jun 27.6 4 1.01 3.97

11 Jun 27.6 11.2 5.19 2.16

17 Jun 27.6 30.9 12.55 2.46

18 Jun 27.6 23.7 10.95 2.17

20 Jun 27.6 13.9 5.71 2.43

2 Jun 22.8 30 14.98 2.00

17 Jun 22.8 14 6.55 2.14

18 Jun 22.8 20 8.84 2.26

19 Jun 22.8 19 5.43 3.50

FIG. 10. Conversion of shear vorticity into curvature vorticity

(along ordinate; 10210 s22) for dry spells during (a) 2009, (b) 2002,

and (c) 2005.


FIG. 11. A sequence of 500-hPa charts of the geopotential heights (m) when a blocking high was present over western

Asia. The dashed line denotes a southwest–northeast-tilted trough.


the geopotential height fields at the 500-hPa (6 km) level.

A sequence of days of such maps covering the period

7–12 June 2009 is shown here. The waves in the mid-

latitude westerlies show a tilt from southwest to north-

east that signify a northward flux of westerly momentum.

Midlatitude wave activity over eastern Europe leading

to the dry spells of the Indian summer monsoon was also

noted by Krishnan et al. (2009). From the latitudes

308–458N there was an increase of this momentum flux,

signifying a divergence of westerly momentum flux from

this belt. When the subtropical jet encounters this re-

moval of westerly momentum, the result is a significant

barotropic energy exchange from the zonal kinetic en-

ergy (i.e., the subtropical jet) to the eddy kinetic (i.e.,

the blocking high). This results in an energy exchange

(Manabe et al. 1965) from the eddies to the zonal flows

(the results for period 1–20 June 2009 are illustrated in

Fig. 12a). The preponderance of negative values implies

the growth of eddies, which is related to the blocking

high in this region (108–508N, 108W–508E). These cal-

culations cover the vertical layer between the 500- and

200-hPa levels where the blocking high was noted to

form. This is a consistent feature during the formation

of the blocking high.

Figures 12b and 12c show similar results for dry spells

over India during 21–31 July 2002 and during 24–30

August 2005 for such energy exchanges. These were both

strong dry spells over India with rainfall amounts around

30% below normal. In both instances we noted a strong

negative energy exchange, implying a transfer of energy

from the zonal kinetic energy toward the eddies. This

calls for a weakening of the subtropical jet over North

Africa and a strengthening of the eddies. The prominent

eddy of this region was the blocking high. Thus, it is

perceived that the growth of the blocking high of west-

ern Asia carries contributions from the divergent winds

of the local tropical Hadley cell over Africa and from the

drainage of westerly momentum of the subtropical jet

stream from midlatitude waves, which then results in that

growth of the eddy.

f. Modeling aspects

A suite of four Florida State University (FSU) cou-

pled atmosphere–ocean models are used in this study;

they are described in detail in Krishnamurti et al. (2002,

2006a,b) and Chakraborty and Krishnamurti (2006).

These models differ only in the prescription of physical

parameterizations (Table 2). All other features of these

models were kept identical. Table 2 also describes the

horizontal and vertical resolutions of these models and

the rest of the model specifications. Seasonal climate

forecasts were started on the first day of each month

covering the years 1987–2001. The construction of the

multimodel superensemble for seasonal climate forecasts

follows our earlier studies (Krishnamurti et al. 2006c;

Chakraborty and Krishnamurti 2006). Superensemble

forecasts include training and a forecast phase, which are

carried out using the cross-validation method that al-

lowed for the handling of smaller data lengths covering

these forecast years.

Figure 13a illustrates the daily march of the predicted

daily rainfall (mm day21) from this ensemble suite cov-

ering the Indian monsoon domain (188–228N, 718–808E)

for the summer months of 2009. This cannot be literally

compared with an observed counterpart, since daily

forecasts lose their predictability by about a week. How-

ever, this forecast shows that the ensemble suite does

carry some interesting dry and wet spells of rains, the

dates and amplitude of which not expected to be any-

where close to the corresponding dates for the observed

events. The climatological rainfall over the belt 188–228N,

718–808E during the summer monsoon months (June–

September) was around 837 mm day21. During the 2009

season that figure was around 660 mm day21, suggesting

a 21.6% deficit for rain. The climatological ensemble

mean rains from the suite of four coupled models was

around 990 mm day21, whereas the corresponding model

FIG. 12. The energy exchange from eddy kinetic energy to

the zonal kinetic energy (m2 s21) for dry spells during (a) 2009,

(b) 2002, and (c) 2005.


rains for the 2009 season was around 880 mm day21,

suggesting a rain deficit for the models of around 11.1%.

For the sake of displaying the dynamics of the mod-

eled features of the dry and wet spells, the dates 8–

27 June for the dry spell and 19–29 August for the wet

spell were selected using the model results of Fig. 13a.

We shall next illustrate the model-based dynamical

features that were noted in the observations. The multi-

model superensemble (Krishnamurti et al. 1999) gener-

ally carries somewhat higher prediction skills compared

to member models. In Fig. 13a we show the daily totals

for the central Indian rainfall for the 2009 summer rainy

season, as obtained from the multimodel superensemble.

This illustration shows that on a season-long forecast the

predicted rainfall totals carry some interesting dry and

wet spells. It is of interest to ask whether model-based

trajectories for these dry and wet spells correspond to

the features illustrated earlier in Fig. 2. The model-

based trajectories for the dry and wet spells of the 2009

season are shown in Figs. 13b,c. These results seem to

convey the same message: trajectories arriving at the

850-hPa level are all of oceanic origin, in contrast to the

700-hPa level where a preponderance of trajectories

during the dry spells over India arrives from central Asia,

even though during the wet spells they were of oceanic

origin. This modeling result is encouraging in that it sug-

gests the possibility for seasonal prediction of dry spells

from a suite of global atmosphere–ocean coupled models.

The model rains exhibited a dry spell during 8–27 June

2009. The vertically integrated winds between the 700-

and 300-hPa levels averaged over this period showed

a prominent blocking high over western Asia (Fig. 14).

The circulations around this high bring in very dry desert

air over the Arabian Sea and toward India in this fore-

cast. The model results shown here pertain to the en-

semble mean of the four-member suite of FSU models.

The results from the single models did simulate this

blocking high, but the ensemble mean carried infor-

mation quite close to what we had illustrated earlier

(Fig. 7a) from the observed fields.

The model-based ITCZ (as seen from the model-based

heavy rains) over North Africa (from the ensemble mean

of the four-member suite) showed a clear extension of the

rain area eastward to almost the Arabian Sea (Fig. 15).

The location of the ITCZ east of 308E was near 88N. This

contributed to a strong local Hadley cell near 408E (not

shown), which appears to be in agreement with the ob-

served strengthening of the local Hadley cell during the

period of the blocking high over western Asia.

Figures 16a and 16b show the daily time histories for

the conversions of shear to the curvature vorticity and

for the rates of generation of eddy kinetic energy (i.e., its

conversion from the zonal mean kinetic energy). These

illustrations are derived from model-based forecast data-

sets during the dry spells of the summer monsoon over

India; these antecedent histories are calculated over North

Africa. The units, domain, and vertical levels for these

model-based computations are identical to those pre-

sented in section 2e that pertained to estimates for such

conversions based on analyzed datasets. These model-

based results for the dry spells of the year 2009 are very

closely in line with the results derived from observed

fields. During an entire period immediately prior to and

during the break spell of the model monsoon of the year

TABLE 2. Salient features of the four FSU coupled atmosphere–ocean general circulation models used in this study.

Atmospheric component Oceanic component

Name (Source) Model Resolution Initial condition Model Resolution

Initial

condition

ANR (FSU) FSU Global Spectral

Model (FSUGSM) with

Arakawa–Schubert

convection and new

radiation (band model)

T63L14 ECMWF

with physical

initialization

Hamburg Ocean

Primitive Equation

(HOPE) global

58 longitude,

0.58–58 latitude,

17 levels

Coupled

assimilation

relaxed to

observed SST

AOR (FSU) FSUGSM with

Arakawa–Schubert

convection and old

radiation (emissivity–

absorptivity based)

T63L14 ECMWF

with physical

initialization

HOPE global 58 longitude,

0.58–58 latitude,

17 levels

Coupled

assimilation

relaxed to

observed SST

KNR (FSU) FSUGSM with Kuo

convection and new

radiation (band model)

T63L14 ECMWF

with physical

initialization


0.58–58 latitude,

17 levels

Coupled

assimilation

relaxed to

observed SST

KOR (FSU) FSUGSM with Kuo

convection and old

radiation (emissivity–

absorptivity based)

T63L14 ECMWF

with physical

initialization


0.58–58 latitude,

17 levels

Coupled

assimilation

relaxed to

observed SST


2009 there was a significant and persistent conversion of

anticyclonic shear vorticity into anticyclonic curvature

vorticity. This same period saw significant conversions of

zonal kinetic energy into eddy kinetic energy. The only

significant eddy that grew over this latitude belt was the

model-predicted blocking high. This was a region that

encountered consistently large values for the convergence

of flux of eddy kinetic energy, again in essential agree-

ment with the observed results presented in section 2e.

3. Discussion

The extreme dryness of air in the middle troposphere

over central India during the dry spells of the summer

FIG. 13. (a) Daily rainfall (mm day21) from the model output. (b),(c) Ten-day back trajectories from central India

from model output for trajectories terminating at 850 (green) and 700 (red) hPa for (b) a dry spell, 27 Jun 2009, and

(c) a wet spell, 22 Aug 2009.


rainy season cannot be explained by the advection of

air from oceanic regions. The specific humidity of an

air parcel is a quasi-conservative property, except for the

effects of condensation, evaporation, and subgrid-scale

vertical eddy convergence of flux of moisture. Lacking

much precipitation during these dry spells, values of

specific humidity as low as 5 g kg21 at the 700-hPa level

over central India are clearly not of oceanic origin. This

is best illustrated from the construction of 10-day-long

back trajectories that terminate over central India at

different vertical levels. We show that during the dry

spells the parcels of air that arrive over central India,

below the 850-hPa level, are of oceanic origin, whereas

nearly all trajectories between 700- to 400-hPa levels

arrive over central India from western Asia. During the

wet spells nearly all trajectories from the surface to the

400-hPa levels were noted to be of oceanic origin. A

persistent feature that leads to such dry air intrusions

from western Asia is a blocking high that extends ver-

tically from the 700-hPa level to the 300-hPa level. De-

scending air circulations around this deep blocking high

contribute to the advection of very dry upper tropo-

spheric air from western Asia to the Arabian Sea and

central India. The air in the upper troposphere over

Arabia is extremely dry and descends toward central

India during these dry spells. The often accepted notion

for the dry monsoon years is to look to the Pacific and

Indian Oceans for finding an explanation for these anom-

alies of the Indian summer monsoon rains. Rasmusson

and Carpenter (1983) showed that many dry monsoon

years were also El Nino years. Saji et al. (1999) noted

that the Indian Ocean dipole (in sea surface temperature

anomalies) was also a contributor to the strength of the

monsoon rains. It is possible that such sea surface

anomalies of the Pacific and Indian Oceans influence the

formation of the western Asian blocking high, but that is

not supported by datasets of the seasonal breaks that

occur during all phases of the El Nino and of the Indian

Ocean dipole. The role of these oceanic features cannot

be entirely discounted, since they do seem to describe

some of the dry spells. The El Nino is characterized by

an east–west circulation anomaly that carries an ascending

FIG. 14. The blocking high of western Asia from the model

output for the dry spell during 8–27 Jun 2009, based on vertically

integrated horizontal winds from the 700- to 300-hPa levels. The

lengths of the arrows are defined by the bottom inset in meters per

second.

FIG. 15. ITCZ from the model output for the dry spell during

8–27 Jun (mm day21).

FIG. 16. (a) Conversion of shear vorticity into curvature vor-

ticity (along ordinate; 10210 s22) from model output for dry spell

during 2009. (b) The energy exchange from eddy kinetic energy to

the zonal kinetic energy from model output for the dry spell during

2009 (m2 s21).


lobe in the equatorial central and/or eastern Pacific

Ocean. The compensating descending lobe meanders

with its strongest descending anomaly over different

parts of Asia, Australia, and East Africa during different

El Nino years. As we look west of India, we see the

possibility of midlatitude waves in the westerlies and the

behavior of the eastern Atlantic Ocean ITCZ providing

mechanisms for the maintenance of a blocking high over

western Asia. During and immediately preceding weeks

of the dry spells of the Indian monsoon, an increased

midlatitude wave activity was noted. These waves are

located north of the subtropical jet stream of North

Africa and consistently show a southwest–northeast tilt

that provides a divergence of flux of westerly momen-

tum away from the subtropical jet stream. This is a sig-

nature for dynamical instability in which energy from

the subtropical jet would go toward the eddies and is

manifested in the formation of the blocking high, which

is the most prominent eddy over that region. The ITCZ

over the eastern Atlantic Ocean, during the periods of

the dry spells of the Indian monsoon, was located some-

what north of its normal position (by about 28 latitude)

and extended much farther inland to almost 408E over

Africa. We noted that this configuration of the ITCZ

supports a stronger local Hadley cell over Africa. This

leads to the generation of zonal kinetic energy from

the vertical overturning of the local Hadley cell. Those

winds are often supergradient and carry large anticy-

clonic shear vorticity. This shows that over this region

of North Africa a significant transfer of anticyclonic

shear vorticity into anticyclonic curvature vorticity is sup-

ported by the flow kinematics. This is another support-

ing argument for the formation of the blocking high over

western Asia. Each of the above interrelated arguments

is supported by a number of diagnostic computations

that are presented in this study. Ultimately, the useful-

ness of any such study requires a modeling of this entire

scenario sequentially. We show several results using

ensemble-averaged datasets to portray the entire sce-

nario, from a suite of coupled global atmosphere–ocean

coupled models for the simulation of dry spells of the

Indian summer monsoon. Furthermore, we show that

trajectories leading to central India, at the 700-hPa level,

during the dry spells bring in very dry air from western

Asia, whereas those arriving at the 700-hPa level during

wet spells are of oceanic origin.

Figure 17 is a schematic for a possible watch for the

antecedents of the dry spells of the Indian summer mon-

soon. Roughly 3 days prior to the onset of the dry spell,

the blocking high always appears, with the center of that

high generally located over the western Arabian Penin-

sula. A strong subtropical westerly jet near the 200-hPa

level precedes (day 25) the formation of the blocking

high. That in turn seems to be preceded by the north-

ward and eastward location of the North African ITCZ

(day 27). Monitoring these elements can provide some

guidance for the eventual occurrence of a dry spell.

The year-by-year changes in the summer monsoon

rains over India, the El Nino index, and the Indian Ocean

dipole index show a mixed history. Many El Nino years

and negative IOD index years were not drought years.

This was also true for several El Nino years that coincided

with periods of negative IOD indices, such as 1991 and

2007. Almost all of these dry years carried a deep tro-

pospheric blocking high over western Asia. None of the

past indices, including as the ENSO, IOD, passage of

ISO, Himalayan ice cover, or Eurasian snow cover, seem

to consistently predict the dry spells of the Indian summer

monsoon.

In any final analysis the relative contributions for the

Indian monsoon rainfall deficits of a given rainy sea-

son need to be predicted in advance. Suites of global

coupled models will be needed to place in proper per-

spective the relative importance of features such as the

El Nino, the Indian Ocean dipole, Eurasian snow cover

(Barnett et al. 1989), Tibetan ice cover (Bhanu Kumar

1988), the intraseasonal oscillation (Waliser 2006), and

the western influences from the blocking high. Carefully

designed numerical experimentation and postprocessing

is required for addressing these questions. Is it possible

that oceanic conditions such as El Nino and IOD can

affect the global weather patterns to promote blocking

conditions over western Asia and invite the advection of

very dry air toward India? Complex questions of that

nature remain to be addressed. Such an indirect re-

sponse is somewhat unlikely since the correlation of the

ENSO indices to the all-India summer monsoon rainfall

(AISMR) is only on the order of 0.1. The dry spells of

the Indian summer monsoon seem to prefer the import

FIG. 17. Schematic for the antecedents for the dry spells for the

Indian summer monsoon.


of deep layers of dry air from dry land areas, whereas the

wet spells seem to prefer imports of moist air of oceanic

origin. This study provides a different perspective for

the intraseasonal oscillations of the Indian summer mon-

soon (i.e., the disruption of the rains on intraseasonal time

scales arising from the incursion of desert dry air of the

northwest of India). The meridional passage of ISO

waves is very well defined, Waliser (2006), but further

work is needed to relate that passage to the chronology

of events over Africa presented here.

Acknowledgments. The research reported here is sup-

ported by a creativity award of NSF Grant ATM 0419618.

APPENDIX A

Transformation of Shear to Curvature Vorticity

The transformation of shear vorticity to curvature

vorticity is one of the parameters that is important for

the rapid intensification of a hurricane. The curvature

vorticity and the shear vorticity in natural coordinates

can be written, following Bell and Keyser (1993) and

Viudez and Haney (1996), as

d

dtf 1 V

›a

›s

� �5�›V

›s

da

dt� ›

›n

›f

›s

� �� f 1 V

›a

›s

� �

3 $p�V�V

›v

›s

›a

›p, (A1)

d

dt�›V

›n

� �5

›V

›s

da

dt1

›

›n

›f

›s

� �� ›V

›n

� �$

p�V

1›v

›n

›V

›p, (A2)

and the tendency equation for absolute vorticity can be

written as

d

dtf 1 V

›a

›s� ›V

›n

� �5 � f 1 V

›a

›s� ›V

›n

� �$

p� V

� V›v

›s

›a

›p1

›v

›n

›V

›p. (A3)

Here, V and f denote the scalar wind and geopotential,

respectively, and a is the angle subtended by the velocity

vector with respect to the x axis (positive in the anti-

clockwise orientation). The first and second terms on the

right-hand sides of (A1) and (A2) describe the conver-

sion between shear and curvature vorticity. A compu-

tational form for the shear to curvature conversion term

in the Cartesian coordinates is also given by Bell and

Keyser (1993).

APPENDIX B

Energy Exchange from the Zonal to Eddy Flow

The following equation uses standard variable nota-

tion to express the energy exchange from the zonal flow

to eddies. These are tropospheric averages over the

African monsoon domain.

CK

5�1

g

ðPs

100

[u9y9]›[u]

›ydp� 1

g

ðPs

100

[y92]›[y]

›ydp

� 1

g

ðPs

100

[u9v9]›[u]

›ydp� 1

g

ðPs

100

[y9v9]›[y]

›pdp.

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Desert Air Incursions, an Overlooked Aspect, for the Dry Spells of the Indian Summer Monsoon: BY T....

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