3. Large Scale Tropical Circulations
3.1 Introduction
3.2 Scale Analysis of the Thermodynamic Equation
3.3 Large-Scale Circulations in Convecting Atmospheres 3.4 Held and Hou theory for the Hadley Cell
3.5 Monsoons
(i) West African Monsoon (ii) Asian Monsoon
(iii) North American Monsoon
3.6 Comparing ITCZ over land and ocean
3.1 Introduction
Very wet:
Intertropical Convergence Zone (ITCZ)
East Pacific ITCZ from the Geostationary Operational Environmental Satellite 11 (GOES-11) July 2000
SSTs and Rainfall
Seasonal Variations in Rainfall
Zonal and time mean circulations
Zonal and time mean circulations
Zonal and time mean circulations
Zonal and time mean circulations
Figure 2.19
3.2 Scale Analysis of Thermodynamic Equation
See Notes
3.3 Large Scale Circulations in Convecting Atmospheres
See Notes
3.4 Held and Hou Model for the Hadley Cell
8 Solution for q
MEq. Pole
q
M, q
Eq
Eprescribed for radiative equilibrium u = U
Mu = 0
y 2 a gH
2 0y
2 3
y
M M
a gH y
0
2 0 2
4
2
the equatorial temperature
_M` used to remind us that q has been derived using conservation of angular momentum
Equilibrium temperature, actual temperature
From J ames (1994)
M M
a gH y
0
2 0 2
4
2
E y E y
0 a
2 2
Y
cooling heating cooling
8 Solution for qM
Eq. Pole
qM , qE
qE prescribed for radiative equilibrium u = UM
u = 0
y 2a gH2 0 y
2
3
y
M M
a gH y
0
2 0
2 4
2
the equatorial temperature
_M` used to remind us that qhas been derived using conservation of angular momentum
Equilibrium temperature, actual temperature
From J ames (1994)
M M
a gH y
0
2 0
2 4
2
E y E y
0 a
2 2
Y
cooling heating cooling
3.5 MONSOONS
SEASONAL PRECIPITATION DISTRIBUTION (DELHI v.s. NEW YORK)
Delhi, India
Monsoon climate
New York
(mid-latitude temperate climate)
Large-scale sea-breeze!!!
The approximate global precipitation domain – defined here as where the local summer minus winter precipitation rate exceeds 2.5mm/day and the local
summer precipitation exceeds 55% of the annual total (in red). Courtesy Wang.
MONSOON SYSTEMS
• Indian monsoon
– India, Sri Lanka, Bangladesh, Indochina…
• East Asian monsoon
– China, Korea, Japan
• Australian monsoon
– Northern Australia, Indonesia
• West African monsoon
– West Africa (Sahel: between the Sahara and the tropical rain forests)
• North American monsoon
– Southwest US, Northern Mexico
• South American monsoon
BASIC DRIVING MECHANISMS OF A MONSOON
• Differential heating between land and ocean
– Different heat capacity (specific heat of water >> dry land)
• Moist processes
– Brings moist air from ocean to land
• Rotational effects
– Coriolis Force
• Topography effects
– Brings elevated heating (Himalayas in summer)
Monsoon is a climatological feature covering roughly half the tropics (1/4 of the global surface)
Strictly, a system where the winds and precipitation reverses (summer rain, winter dry)
Host 65% of the world’s population
Small changes in year-to-year climate can be catastrophic
Section 3.5(i) Introduction to the West African Monsoon
-10%
-20%
20%
10%
0
Wet period
Dry Period The largest regional deficit of rainfall observed during the last century
The largest regional deficit of rainfall observed during the last century
Uncertainties about the future Uncertainties about the future
Climate Variability Impacts:
Water
Agriculture Health
Demographics Security
Introduction to the West African Monsoon
Annual Cycle of Rainfall and associated Water Vapour Transport Thorncroft et al (2011) QJRMS, 137, 129-147
The Coupled Monsoon System
Cold Tongue SAL
ITCZ
Heat Low Key features of the WAM Climate
System during Boreal summer
AEJ
North-South Section along the Greenwich Meridian
θ 50oC
20oC θe θ
90oC
60oC θe
AEJ
Meridional Circulations
Shallow Meridional Circulation (SMC) over ocean, especially in Spring
θ 50oC
20oC θe θ
90oC
60oC θe
AEJ
Meridional Circulations
The monsoon flow is active at night and in the morning.
Parker et al 2005 “The diurnal cycle of the West African
Monsoon circulation”, QJRMS
Fig.1 Schematic
of Ekman Spiral
in the Ocean
Spring 2013
Equatorial Upwelling
Spring 2013
Coastal Upwelling
• Motion of surface waters away from coast requires
upwelling of water from below to
satisfy continuity of mass.
Andes Mts.
S. Pacific Ocean
Tropical Atlantic Climate System
Atmosphere:
o Atlantic marine ITCZ (AMI), Amazonian convection center, and western African monsoon (WAM)
o Trades and surface wind convergence o Stratus deck over colder tropical oceans
o Meridional (Hadley) and zonal (Walker) circulations o mid/low-level jets
o Transients (easterly waves, tropical storms, African dust)
Ocean:
oAir-sea fluxes
oCold tongue, SST gradient, upper- ocean-heat-content
oOcean circulation (surface currents, eq.
and subtropical upwelling, STCs, THC)
oTransients (TIW) Differences between the
Atlantic and Pacific ITCZ:
• position, intensity, and seasonal cycle in the ITCZ;
• SST gradient;
• land effects;
• remote influences
Impacts: NE Brazil rainfall, African monsoon onset; GG rainfall;
ITCZ: warm SST centered on eq with weak gradients; strong surface wind convergence with a relatively weak and broad marine convective region close to the equator;
External influences: ENSO; previous winter NAO; previous summer S. Atlantic; African dust and dry-air outbreaks;
Impacts: W. African monsoon; tropical storm activity; rainfall in northern S. America;
ITCZ: colder SST with strong gradients;
strong and concentrated marine convection positioned furthest from the equator;
External influences: ENSO state; ongoing S.
Atlantic circulation; Saharan Air Layer (SAL), African easterly waves (AEW);
Boreal spring
Boreal summer
Seasonal dependence of AMI
Data: GPCP (Global Precipitation Climatological Project).
Resolution: pentad on a 2.5o grid.
Averaged from 10oW to 10oE over 23 years (1979-2001).
c.f. Gu and Adler (2004)
Annual Cycle of Mean Rainband
Sultan and Janicot (2000) “Abrupt shift of the ITCZ over West Africa and
intraseasonal variability” GRL
Annual Cycle of Mean Rainband
Daily Rainfall March to November 1978 – 5N- green curve, 10N – black curve, 15N- red curve
• Observations, reanalysis and operational analysis data including:
– pentad 2.5o GPCP
– Reynolds SST 1o, weekly and daily
– Reanalysis from the ECMWF: daily 2.5o ERA40
• The period of study is 1979-2001
Data
Relationship between SKT and surface meridional wind
SKT and rainfall
MSLP and VWND
Warming over the continent due to the surface solar heating.
Rapid cooling of the ocean surface south of the
equator between April and June rapid rise in MSLP:
Acceleration of southerly winds across the equator.
c.f. Okumara and Xie (2004)
Relationship between rainfall and surface conditions
Relationship between rainfall and surface conditions
Equivalent potential temperature
• Peak rainfall always lies south of thetae peak
• Gradient in thetae still important
• Location of heat low important for poleward extent
Total Column Moisture Flux Convergence
Total Column Moisture Flux Convergence
Peak in moisture flux convergence linked to heat low shallow meridional circulation – acts to moisten the column and extend the rainfall polewards (c.f. Sultan and Janicot (2000,2003), Hagos and Cook (2008))
Total Column Moisture Flux Convergence
Peak in moisture flux convergence over ocean
Total Column Moisture Flux Convergence
Rapid shift and increase in moisture flux convergence towards coast between April and May
Total Column Moisture Flux Convergence
Rapid reduction in moisture flux convergence during June – linked to end of coastal rains
Total Column Moisture Flux Convergence
Rapid increase in moisture flux convergence beginning of July linked to Sahelian rainfall onset
Meridional Moisture Fluxes
Mid-levels (850-500mb)
Low-levels (sfc-850mb)
Impact of Heat Low SMC
Meridional Moisture Fluxes
Mid-levels (850-500mb)
Low-levels (sfc-850mb)
Equatorward moisture flux at mid-levels enhances moisture flux convergence in rainy zone : enhances rainfall there?
Polewards of this there is dry advection: inhibits rainfall there?
Meridional Moisture Fluxes
Mid-levels (850-500mb)
Low-levels (sfc-850mb)
Marked increase in cross- equatorial moisture fluxes during April-May
Linked to cold tongue development and coastal onset
Schematic evolution
SST
SMC
ITCZ 1. Ocean phase (Feb-April):
-Main rainband is broad with peak values just poleward of the Equator (~1oN ). The rainfall is located mostly over the warmest water (>28oC) with little over the land.
-At the end of this period the cold tongue starts to develop, resulting in a broad region of SSTs close to the equator falling below 28oC.
- Does the heat low SMC impact the surface winds?
HL
2. Coastal phase (May-mid-June):
-Cold tongue development associated with a rise in equatorial surface pressure, and an acceleration of southerlies and associated moisture flux
towards the coast.
-Marked moisture flux convergence, just equatorward of the land (~4oN) is associated with the highest rainfall of the annual cycle, and the first rainy season for coastal regions of West Africa.
c.f. Zheng, Eltahir and Emanuel (1999) Okumara and Xie (2004)
Gu and Adler (2004) Caniaux et al (2009)
- Peak rainfall is located over warmest water
Schematic evolution
SST
SMC ITCZ
HL
3. Transitional Phase (End of June)
- June represents a period where the environment becomes less favorable for convection in the coastal region. This is consistent with coastal upwelling and a reduction of SSTs there.
- Intense coastal rainfall can only be transient?
- Why doesn’t it rain more in June?
- Does this weakening promote the perception of a
“jump” often discussed in the literature?
Schematic evolution
SST
SMC ITCZ
HL
4. Sahelian Phase (July-August):
- Between June and July the peak in moisture flux convergence reaches 10oN and increases rapidly consistent with the observed Sahelian rainfall onset.
- In July and August moisture flux divergence is present over the coastal region consistent with continued suppression of rainfall there.
c.f. Sultan and Janicot (2000.2003) Sijikumar et al (2006)
Ramel et al (2006) Hagos and Cook (2007)
Schematic evolution
SST
SMC ITCZ
HL
Wet bias in Spring?
Dry bias in Sahel in Summer
Dry bias in Spring?
ERA40 vs NCEP1
rainfall
2005
2006
2007
11 20
21 3
23 29
Large variation in the coastal onset.
Earliest cold tongue
development in Spring 2005 – earliest coastal onset.
Strongest HL in Spring 2007 during the oceanic regime possible role in delaying the coastal onset via subsidence
Concluding remarks on Annual Cycle
• At some level the coastal onset seems easier to understand than the Sahelian onset – with peak rainfall following the peak in SSTs
• What processes determine the nature and variability of the cold tongue (role of heat low, sub-surface ocean structure, Atlantic ocean variability, radiation)?
• Why is cold tongue development more rapid in the Atlantic than in the Pacific?
• Can climate models represent these coupled processes?
• Need more in situ observations in the tropical East Atlantic!
• Need more work on nature and causes of variability of coastal rains
Concluding remarks on Annual Cycle
3590 J O U R N A L O F C L I M A T E VO L U M E 17
FI G. 1. (a) Scatter diagram of the observed zonal wind stress to the west (48N–48S, 1308–1108W), the meridional wind stress to the north (88N–08, 1208–1008W) of the Pacific cold tongue, and the cold tongue SST (8C; 48N–48S, 1048–868W). (b) As in (a), but for the zonal wind stress in the western equatorial Atlantic (48N–48S, 348–268W), the meridional wind stress in the Gulf of Guinea (68N–08, 168W–48E), and the cold tongue SST (48N–48S, 168W–48E). Adopted from Mitchell and Wallace (1992).
(weakening) of these southerlies lowers (raises) equa- torial SST in summer (spring) by enhancing (reducing) the vertical mixing, surface evaporation (Xie 1994), and upwelling1 (Philander and Pacanowski 1981). Such southerly wind-induced changes in SST are greatest in the eastern ocean where the thermocline is shallow. The resultant anomalous SST gradients intensify (weaken) the equatorial easterly winds in summer (spring). Dis- placed west of the maximum SST changes, the easterly wind acceleration helps to propagate the coupled SST–
wind anomalies westward through upwelling and evap- oration (Mitchell and Wallace 1992; Xie 1994; Nigam and Chao 1996). With regard to the origin of the north- ward displacement of the ITCZ, continental asymmetry, such as the bulge of western Africa for the Atlantic and the northwest orientation of the Pacific coast of the Americas for the Pacific, is the trigger (Philander et al.
1 Near the equator, surface ocean currents flow downwind, while they become perpendicular to the wind direction 28–38 away from the equator due to the Coriolis effect. Because of this change in flow regimes, cross-equatorial southerly winds generate ocean upwelling south and downwelling north of the equator.
1996), with positive feedback between the ocean and atmosphere helping sustain the climatic asymmetry far away from the coast into the west (see Xie 2004 for a recent review).
While the seasonal warming and cooling are roughly symmetric in their durations in the eastern equatorial Pacific, they are highly asymmetric in the Atlantic, where the cooling takes only three months to reach the peak but takes 3 times longer to warm up (Fig. 1). The tropical Atlantic, with a narrow basin, is strongly influ- enced by the major continents on both sides. Li and Philander (1997) suggest that the annual cycle in the eastern equatorial Atlantic is largely a passive response to continental monsoons through their effect on oceanic winds, with local air–sea interaction playing a minor role. Comparing to a control simulation forced by sea- sonally varying SST, their atmospheric general circu- lation model (AGCM), with annual mean SST pre- scribed globally, reproduces the seasonal cycle in the meridional (zonal) wind component over the eastern (western) equatorial Atlantic. In the equatorial Pacific, by contrast, they report a substantial reduction in equa- torial wind variations, confirming that they result from
Annual Cycle of Synoptic Weather Systems
TD-filtered OLR (AEW-activity)
Peaks in summer
We know little about the nature and causes of AEW-variability
Kelvin-filtered OLR Peaks in Spring
Key synoptic system for pre-coastal phase and possibly the coastal
phase
