Global Summer Monsoon Rainy Season

Suping Zhang Ocean University of China

Bin WangDepartment of Meteorology and International Pacific Research Institute

School of Ocean and Earth Science and Technology, University of Hawaiiic

Introduction

• Monsoon originally means seasonal reversals in prevailing winds. (e.g.Ramage 1971) .

• It is now more generally applied to tropical and subtropical seasonal reversals in atmospheric circulation and the contrasting wet-dry seasons (Trenberth 2002,2006).

Numerous literatures have devoted to the regional monsoons, such as

• Indian monsoon (Rao 1976, Lau and Yang 1997),• East Asian monsoon (e.g. Chen et. al. 1991,Wang

and Liho,2002), • Indonesian and Australian monsoon (e.g.

Wheeler and McBride 2005, Wang et.al. 2003), • South African monsoon (e.g. Cook 2000), • South American monsoon (e.g. Rao et al. 1996,

Charles et.al. 2002), • West African monsoon (e.g. Grodsky and Carton

2001, Sulton and Janicot 2003) • North American monsoon (e.g. Adams and

Comrie 1997, Anderson et.al. 2000).

North African monsoon from Reddiman 2001From Simona 2004

From Tao and Chen 1987 From Ramage 1971

Regional summer monsoon domains

From Berbery et al. 2005,

From Wang and LinHo 2002

From Wang 1997 (using OLR and HRC, the criteria are complex )

Problems• Our knowledge falls short when talking about the

global monsoon system due to a lack of long-term reliable data, especially the observations over the monsoon ocean regions.

• Some proxies like OLR (outgoing longwave radiation) and HRC (highly reflective clouds) have some limitations.

• Definitions of monsoon domain, onset, peak and withdrawal lead to different results .

• This is a challenging issue to objectively define the global monsoon domains, which has just recently become possible in a climatological time-scale by utilizing global homogeneous rainfall and wind data sets.

• To discuss the categories of global monsoons. • The results provide an observational basis for

validation of the existing GCMs and climate system models (NCEP, JRA-25, and ERA-40).

Purpose of this study•To build up a unified picture of the spatial and temporal structures of the global monsoon rainy seasons. •To investigate the feasibility of using a set of universal criteria to quantify rainy season characteristics for the entire global monsoon domain, which are simple and easy for application.

Outlines

• Section 1 introduction• Section 2, the dataset and the processing

procedure.• Section 3, to define the global rainy season

domain • Section 4, to presents the onset, peak, and

withdrawal patterns of the global monsoon rainy season.

• Section 5, to discuss on the global summer monsoon regimes. And the results derived from different datasets are compared.

• Section 6, to summarizes our conclusions

2 Dataset • CPC Merged Analysis of Precipitation

(CMAP) from 1979-2004. • NCEP, JRA-25, ERA-40.• Reconstructed climatological pentad mean

(RCPM)) from the Fourier truncation.

3 Definition of summer monsoon rainy season3.1 The length of the summer monsoon rainy

season• The annual cycle is usually divided into four

seasons.• In China the 24 Solar Terms have long been used

to depict the annual cycle since the ancient time.• Summer monsoon may start as early as in May

and as late as in July. • How to measure the length of the monsoon season?

From the first modes of EOF analysis, MJJAS is summer monsoon rainy season for the NH, NDJFM for the SH

1 2 3 4 5 6 7 8 9 10 11 12

-2

-1

0

1

2

PC1-UPC1-VPC1-Precip

account for 66%, 77%, and 78% of the total variances respectively

The first EOF modes reflect the effects of the earth’s orbit around the sun and of the thermal characters of the earth’s surface (ocean and continent). In this sense, the first modes represent the global monsoon. The same phase persists in MJJAS in annual cycle being the boreal summer monsoon time. Similarly, NDJFM is taken as the austral summer monsoon time.

Rain

u

v

3.2 Definition of the summer monsoon rainy season

Three factors are considered here:• the total amount of summer rainfall to

measure the intensity of the rainy season ,• the annual range to measure the amplitude

of annual rainfall variation,• the ratio of summer to yearly rainfall to

measure the concentration of summer precipitation .

Fig.2 Precipitation rate (mm/day) (a) Summer (b) Winter

The precipitation is below 3 mm/day in semiarid and arid regions or over cold tongues

(a) Ratio of summer to annual precipitation.

(b) Annual range (the pentad max minus min)

The level of 55% provides a reasonable demarcation for the equatorial perennial and monsoon rainy seasons.

• The domain of the summer monsoon rainy season is defined as :

• the summer (MJJAS for the NH, NDJFM for the SH) rainfall must be equal to or above 3 mm/day and the ratio must be greater than 55%.

1) the domains are off the equatorial zone, thus excluding the equatorial perennial rainfall regions or bimodal variations, e.g. ITCZ in the Pacific and the Atlantic,2) extratropical perennial or semi-perennial rainfall regions are excluded, e.g. SACZ,SPCZ, and storm tracks, 3) the ASM domain stretches northward into the middle latitudes in northeast China and Korea peninsula.

3.3 Distributions of global summer monsoon rainy season

Fig.5 CPM precipitation rate (solid line with cycle mark) and RCPM precipitation rate (dashed line) The two straight lines mark the rainfall rate of 3 mm/day and 5 mm/day, represent the rainy season domain and onset criteria respectively. The CPM climatology is derived from CMAP data for the period 1979-2004. Unit: mm/day

Samples to convince the present resultant summer monsoon rainy season domains. The annual rainfall variation in the Northeast China (Fig.5a) is almost the duplication of the annual rainfall cycle in the West Africa (Fig.5b) that is a pronounced monsoon region.

• These resultant domains are compatible with previous literatures (Drosdaowsky 1996, Ramage 1971, Wang 1994, Sultan and Janicot 2000, 2003, 2003, Grodsky and Carton 2001 , Berbery et al. 2005,Wang and Linho 2002 , etc.) , but the criteria are simple and easy for application.

4. The onset, peak, and withdrawal

• 4.1 Definitions • Rainfall rate exceeding 6 mm/day (Lau and Yang

1997) for onset.• OLR (CPM) below 230W/m2 (Murakami and

Matsumoto 1994) for onset. • Such a uniform criterion is perhaps suitable for the

tropical monsoon regions but is not applicable in a domain with a large latitudinal extent.

• The relative RCPM rainfall rate is defined by

RRi =Ri -RJAN , i=1,2,…,73 pentad, (for the NH)RRi =Ri -RJUL , i=1,2,…,73 pentad, (for the SH)

• The criterion, RRi≥5mm/day, is adapted from Wang and LinHo for Asian monsoon (2002).

4.2 The onset patterns

The Asian summer monsoon domian

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39 0 37 43 34 35 33 33 320 0 0 38 35 34 33

39 35 3438 0

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JUL

AUGJULJUNMAY

APR

JUN

The earliest onset is in Apr. over the southeast Bay of Bengal. The onset in the SCS in P27-P28 agreeing with the mean onset date of P28.

The onset phase progresses northward gradually in the ISM and eastward in a stepwise way in the WNPSM.

Over East Asia, the rainband moves to the Yangtze River and southern Japan by P32- P33, staring the Chinese mei- yu and Japanese baiu season. By late June (P34-P35), to South Korea, starting the Korean Changma.

The synchronized onset of the Indian rainy season and the mei-yu/baiu forms a grand onset pattern in early June.

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0 01 73 8 9 10 10

4 5 4 73 1 73 72 73 3 2 109 2 70 1 70 71 72 73 73 2

4 1 68 68 68 67 73 70 1 21 61 66 66 69 68 68 72 7167 62 66 63 67 68 69

66 67 71

DECNOV

JANFEB

Monsoon rain starts in the tropical Java in early November (P61), and moves southeastward.

This eastward advance of monsoon onset phase is primarily related to the eastward propagation of the envelopes of the Madden Julian Oscillation (MJO; McBride 1987, Hendon and Liebmann 1990, Wheeler and McBride 2005).

The Indonesian and Australian summer monsoon rain

The African monsoon

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34 29 29 37 33 30 0 2835 33 30 41 4837 39 38 39 0 41 3941 39 38 39

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JUNAPRJUL

MAY MAYJUN

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00

0 0 0 0 00 0 0 5 6 90 7 1 70 2 0 8

5 2 3 1 68 1 0 5 7 11 11 1670 70 70 2 1 70 72 7 7 5 6 8 1162 66 67 70 71 71 71 73 72 5 5 4 1 1

60 60 67 67 73 2 71 72 72 71 2 73 6415 60 60 62 67 7 16 16 71 71 71 71

13 61 60 60 67 73 72 7112 63 60 62 0

10 11

NOVDEC

JAN

FEBJAN

The onset phases in the NAfSM domain are obviously discontinuous reflecting its variable rainfall, which is the most significant climatic feature in the region.

There are two steps for the onset of the NAfSM.

The westward progression agrees with the westward propagating wave-like fluctuations (Grodsky and Carton 2001) The local rainy season starts

southeastward in the SWISM

The American summer monsoon regions

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30

20 20 2725 24 23 21 23 0 28 0

23 23 24 25 26 26 25 29 33 028 30 26 27 28 28 53 28 58 0 0

38 35 29 29 29 0 30 54 0 0 044 35 36 0 0 53 0 0

48 0 37 0 5437 0 0 0 0

0 0 0 0

APRMAY

JUN

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0 0 3 71 01 0 69 68 63 0

0 67 65 62 670 1 62 61 62 67

3 67 62 60 62 63 066 64 63 61 60 62 662 66 60 60 60 60 9

68 68 66 65 172 3

OCT

JAN

NOV

DEC

The onset phase propagates northward in the NAmSM region

In the SAmSM, the grand onset phases spread rapidly from P60 to P63 over the vast landmass of Brazil and stop to the east of the Andes.

The peak pattern

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31 3131 32 33 30 54 36 36

32 33 31 32 30 54 55 35 36 35 46 45 52 53 51 5233 34 41 34 32 55 52 58 37 36 44 47 46 46 52 53 52 52

34 55 41 34 43 50 51 47 44 44 47 46 47 46 52 52 53 4633 34 44 41 34 30 49 51 53 45 50 44 46 45 45 45 43 46 46 5242 41 42 42 35 37 42 36 37 32 30 49 46 44 44 45 45 42 46 47

42 43 43 37 41 41 36 26 33 31 45 44 42 41 4243 39 41 38 41 36 3446 40 38 40 43 35 35

41 36 37 45 38 37 36 36 3536 38 44 40 38 37 36

41 39 3642 48

42 41 4741 41 47

44 4443

AUG

JUN SEPAUG

SEPJUN

JULJUN

JUN

AUG

The rainy season peaks first in June in four regions in the ASM domain. The zone extending from the mid-lower reaches of the Yangtze River to the southern Kyushu, Japan is obviously associated with the subtropical front activities (mei-yu/baiu front) in East Asia.

peak

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8 112 9 10 11 12 12

4 11 11 10 8 11 9 12 12 12 159 4 11 4 6 8 11 13 10 10

6 6 4 4 5 5 6 7 8 97 66 6 5 10 3 8 7 8

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JANFEB

FEB

In the I-ASM, the peak of the rainy season, similar to the onset phase, progresses southeastward crossing the region from January to February.

peak

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15

41 60 56 43 27 273233 42 49 48 47 28 2662 30 31 32 33

49 49 51 48 49 45 38 4347 41 42 49 5048 45 40 47 51 44 4450 42 48 48

4449 48 44 45

AUGMAY

JULJUN SEP

SEP

0 20 40 60 80-40

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98

5 8 11 13 69 9 10 11 6 97 8 10 5 12 5 10

6 6 8 10 5 10 9 12 8 19 12 186 6 8 7 4 5 9 9 13 7 7 11 184 6 8 6 4 10 9 8 7 7 7 7 10 3

62 69 70 9 8 17 2 9 8 6 5 6 1116 63 68 69 70 8 17 17 1 1 5 5

20 63 63 63 17 73 73 415 15 63 19 18

18 17MARNOV

DEC

FEB MARJAN

JAN

FEB

MARMAR

JAN

peak

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41 35 41 35 35 37 30 52 34 3449 50 39 51 36 31 54 35 58 39 59

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MAY

SEPJUL

SEP

JUNJUN

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2 14 8 73 12 3 70 7 72 1

2 2 73 73 692 2 8 2 72 68

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17 18MAR

DEC

FEB

JAN

The NAmSM is distinctive for the discontinuity in its peak patterns around the latitude of 10°N between 60- 120°W .

The withdrawal pattern

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36 58 48 60 65 65 59 73 64 54 57 61 61 63 62 6136 51 69 68 59 59 59 67 66 50 54 56 60 61 61 61 61 60

51 57 59 66 53 57 61 66 62 54 62 56 58 61 61 60 60 6037 50 49 58 58 31 54 61 61 61 53 54 56 58 56 55 59 56 50 5345 50 51 54 57 40 52 52 52 55 38 53 55 49 48 49 52 50 49 50

51 53 54 57 44 50 49 38 36 36 47 46 44 43 052 53 50 56 44 42 3850 42 41 0 49 40 38

46 0 38 48 42 40 40 39 390 0 0 42 42 42 41

43 51 4051 0

46 43 045 0 0

0 00

JUL

NOV

AUGSEP

SEP

NOVOCTOCT

NOV

SEP

JUN

AUGSEP

DEC

There are three major equatorward withdrawal patterns in the ASM.

A northward retreat of the rainy season is found in East Asia .

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6 13 13 13 16 15 19 16 18 16 2111 16 15 14 15 21 16 17 21 19

9 13 17 16 15 17 17 20 18 2013 21 16 17 15 15 23 17 1715 21 20 23 16 12 26

3 10 10

MARFEB

APR

APR

A large-scale withdrawal of rainy season occurs in March in the main body of the I-ASM domain , which is close to the mean retreat date of 13 March given by Drosdowsky (1996).

withdrawal

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62 63 61 52 30 293939 63 59 60 61 49 072 70 58 38 38

56 59 58 52 58 52 0 5468 68 59 53 5454 50 44 50 0 51 5053 52 53 52

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SEPOCTDEC

SEP

NOVJUL

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19 19 19 20 17 20 13 12 12 13 10 14 1420 21 20 21 21 11 20 20 8 12 12 13

22 70 20 24 20 2 26 924 24 71 23 0

25 25

MAR

APR APR FEB

MAR

FEB

The withdrawal dates vary widely from July (P38-P39) to December (P68-P72) in the tropical North Africa

An equatorward withdrawal in the SAfSM

withdrawal

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39 42 3743 52 66 65 61 0 50 0

52 55 67 66 64 63 61 56 35 059 59 62 61 61 59 55 67 59 0 0

54 58 59 59 58 0 63 55 0 0 055 56 39 0 0 56 0 0

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0 0 0 0

OCTNOV

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0 0 10 9 03 0 72 11 16 0

0 6 17 16 30 4 18 18 16 71

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MAR

APRMAY

JAN

The withdrawals of rainy season in North and South America are apparently characterized by the regularly equatorward progression.

The length of the rainy seasonExamples:• The longest rainy season is found in the Bay

of Bengal, in the SCS , which lasts about 40 pentads.

• The rainy season in the EASM is about 6 -7 pentads

• The rainy season in the central-eastern Arabian Sea is about 7 pentads though the area is located about 10°N.

The length of the summer monsoon rainy season

shorter

longer

5 discussions• 5.1 Global monsoon regimes• Both rainfall and winds are considered.• Ramage (1971): 120 degrees for seasonal

wind reversals.

Seasonal overturning of winds

90 degrees between summer and winter in East Asian. 90 degree is adopted here.

The definitions of the seasonal wind reversals are proposed in this paper as follows:

• a) the winds alter signs at least in one component (zonal or meridional) from summer to winter,

• b) the differences between the wind directions are equal to or greater than 90º and the differences between the wind speed exceed 2 m/s.

Monsoonal or non-monsoonal

The dark gray shaded areas in Fig.4 denote the monsoon regions demarcated by the above criteria using ERA-40 925hPa wind data. The monsoon regime from Ramage 1971 (purple dotted rectangles)

Mediterranean&not steady

5.2 Contrast between results deriving from different datasets

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orange: NCEP red: CMAP blue: JRA-25 green: ERA-40

The datasets show good capabilities of the reanalyses in the demarcation of the major monsoon rainy season domains in the tropics and subtropics

• But they are less realistic in the mid- latitudes of Eurasia and North America.

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orange: NCEP red: CMAP blue: JRA-25 green: ERA-40

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NCEP

JRA-25

ERA-40

6 conclusions• In this paper we have proposed a set of universal

criteria for defining the domain, onset, peak and withdrawal of the global mean monsoon rainy season.

• The criteria of summer rainfall intensity (≥3mm/day) and the rainfall ratio (>55%) are combined to produce the six major domains of the summer monsoon rainy season.

• The onset, peak, and withdrawal can reflect the spatial-temporal structure of the summer monsoon precipitation on a global perspective, and are essential for our understanding of monsoon dynamics.

• Rainy season starts in the tropics and then progresses poleward in the continental monsoon domains, while the propagation is rather zonal from west to east in the oceanic monsoon areas in both Hemispheres.

• The rainy season retreats equatorward causing longer wet summer in the tropics.

• The rainy season lasts shorter close to the margins of the warm water.

• The longer rainy season may last about 40 pentads. While the shorter one may be only a month.

• There are exceptions, e.g. in the EASM, in the WASM, and in Korea.

• The results are definite, quantitative and primarily compatible with the other individuals who focus on local monsoons.

• The global summer monsoon rainy season domains can be divided into two regimes: a) strong: contrasting wet-dry seasons and reversals in winds, e.g. the ISM, the SCSSM; b) weak: contrasting wet-dry seasons but no wind reversals as a whole, e.g. the WNPSM, the SASM.

• Some areas in the tropics, though without pronounced wet- dry season, are highly monsoonal for the seasonal reversals in winds and cross-equatorial flows are very much linked with the typical strong monsoons, e.g. the vicinity of the Somali jet.

• Some areas with overturning in winds are not monsoonal because the overturning is opposite to the monsoon reversal and the rainfall patterns are more or less Mediterranean.

• The ERA-40, JAR-25 and NCEP show good capabilities in the identification of the major summer monsoon rainy season domains in the tropics and subtropics, but are less realistic in the mid- latitudes of Eurasia and North America. The resultS are useful for validation of GCMs.

• The global monsoon systems are very much enriched in their local structures. New observations, particularly, estimated from satellites could be able to supply more reliable data to modify this study results.

• Acknowledgments• The authors deeply appreciate Miss Xuxu and Mr.

Wushu for their assistance in figure plotting. This research is supported by the Scientific and Technological Programs of Shandong Province, China, No. 2004GG2208111.