Why the ITCZ Is Mostly North of the Equator

Philander, S. George; Gu, Daifang; Lambert, G.; Li, T.; Halpern, David; Lau, Ngar‐Cheung; Pacanowski, R. C.

doi:10.1175/1520-0442(1996)009<2958:wtiimn>2.0.co;2

Cited by 468 publications

(306 citation statements)

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“…contrast is in SST); the tropical North Atlantic; and the South Atlantic (where the contrast is in SLP). The SST anomaly pattern is reminiscent of the Atlantic Niño mode, one of the two most common patterns of variability in the tropical Atlantic (Covey and Hastenrath, 1978;Philander et al, 1996;Chang et al, 2000;Latif and Grötzner, 2000;Wang and Carton, 2003). This mode is marked by the disappearance of the equatorial cold tongue in the eastern Atlantic, and it is most developed in the boreal summer, coincident with the Sahel rainy season.…”

Section: Discussionmentioning

confidence: 99%

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A physical basis for the interannual variability of rainfall in the Sahel

Nicholson¹,

Webster²

2007

Quart J Royal Meteoro Soc

108

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A major factor in rainfall variability over Sahelian West Africa is the latitudinal location of the tropical rainbelt. When it is displaced abnormally far northward, the Sahel experiences a wet year. An anomalous southward displacement results in drought. In this paper we examine the question of what controls the location during the boreal summer, hypothesizing that inertial instability plays a role. An analysis of surface pressure and temperature fields, wind fields, divergence and vertical motion show that the criteria for inertial instability are satisfied in wet Augusts but not in dry ones. The key determinant appears to be the surface pressure gradient between the continent and the equatorial Atlantic. When this is large, inertial instability results in the development of a low-level westerly jet. The presence of this jet enhances the horizontal and vertical shear, and displaces the African Easterly Jet northwestward. Associated with this situation is strong vertical motion over the Sahel and subsidence over the Guinea Coast, producing dry conditions over the latter. The result is a rainfall dipole, one of two major modes of variability over West Africa. Important factors include sea surface temperatures (SSTs) in the equatorial Atlantic and pressure in the South Atlantic. The first of these factors suggests a link with the Atlantic Niño mode of tropical Atlantic variability; while the second suggests a possible link with the Pacific and the extratropical South Atlantic. Overall, our study relates the well-known SST influence on Sahel rainfall to atmospheric dynamics over the continent.

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Section: Discussionmentioning

confidence: 99%

“…Charney, 1971;Wang and Li, 1994). One aspect of this is the reason why the ITCZ tends to form away from the Equator (Philander et al, 1996). Kraus (1977aKraus ( , 1977b argues that the ITCZ is a 'thermal equator', separating integral areas of equal heat receipt.…”

Section: Off-equatorial Convectionmentioning

confidence: 99%

A physical basis for the interannual variability of rainfall in the Sahel

Nicholson¹,

Webster²

2007

Quart J Royal Meteoro Soc

108

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“…The cold tongue is less well developed during DJF compared with JJA, and the minimum in h does not extend as far down to the surface. There is still northward flow at the surface over the eastern Pacific and Atlantic oceans, because the intertropical convergence zone (ITCZ) is north of the Equator in these areas throughout the year (Philander et al, 1996), although it is weaker than in JJA. In fact in some parts of these colder, drier regions, the direction of the flow reverses up to three times.…”

Section: December-februarymentioning

confidence: 99%

Deconstructing the Hadley cell heat transport

Heaviside

Czaja

2012

Quart J Royal Meteoro Soc

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The mechanisms responsible for poleward atmospheric heat transport at low latitudes are examined in the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset. Deep meridional overturning circulations are found in regions experiencing frequent convection (i.e. the Indo-Pacific 'warm pool') and it is suggested that these are the primary reason why atmospheric heat transport seems dominated by axisymmetric motions or 'Hadley cell heat transport'. In contrast, the more complex distribution of meridional mass transport by the circulations over the 'cold tongue' regions (such as the eastern Atlantic area) and the presence of a pronounced minimum in moist static energy at mid-levels constrain these regions to contribute little to poleward heat transport year round. Regions experiencing frequent convection do not, however, account for all the annual mean poleward atmospheric heat transport. At the Equator, an annual net southward transport of heat of about 0.4 PW (1 PW = 10 15 W) is found, but the deep overturning cells found in the most convectively active regions contribute only 25% of this. This small contribution of ∼0.1 PW is understood to reflect a seasonal cancellation between large (∼1 PW) northward and southward heat transports in boreal winter and summer respectively. During December-February, most of the cross-equatorial heat transport is attributable to the convective regions, whereas in June-August, only half is attributable to these regions. In this season, there is a significant amount of heat transport by the strong Somali jet associated with the Asian summer monsoon. Rather than attributing the net southward crossequatorial heat transport to asymmetries in the climatology associated with the northerly position of the intertropical convection zones throughout the year over the eastern Pacific and Atlantic oceans, our diagnostics suggest that the Somali jet is a significant contributor to the annual net southward atmospheric heat transport at the Equator.

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“…1). A fundamental reason for the north-south asymmetric distribution of SST is considered to be the northwest tilt of the American continent's coastline (Philander et al 1996). Ekman transport of ocean water driven by northeasterly surface winds in the NH is directed northwestward and that driven by southeasterly winds in the SH is directed southwestward.…”

Section: Introductionmentioning

confidence: 99%

“…Klein and Hartmann (1993) reported that low-level stratus cloud cover is negatively correlated with SST over the southeastern Pacific. A SST cooling increases the atmospheric stability and hence the stratus cloud cover that reflects the solar heating back, thereby decreasing SST further (Philander et al 1996). On the other hand, in climate models, the cross-equatorial southerly winds over the southeastern Pacific are underestimated and the positive SST bias appears (Mechoso et al 1995;Xie et al 2007;de Szoeke and Xie 2008).…”

Section: Introductionmentioning

confidence: 99%

Reproducibility of precipitation distribution over the tropical oceans in CMIP5 multi-climate models compared to CMIP3

Hirota

Takayabu

2013

Clim Dyn

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Reproducibility of precipitation distribution over the tropical oceans for the recent dataset of the Coupled Model Intercomparison Project phase 5 (CMIP5) is investigated and compared to CMIP3. The Taylor skill score for the reproducibility of the CMIP5 multi-model ensemble mean (0.64) is slightly higher than that of CMIP3 (0.60), but the difference is not statistically significant. Still, there is some evidences that the double intertropical convergence zone (ITCZ) bias is mitigated from CMIP3 to CMIP5, whereas the cold tongue bias remains similar. An inter-model empirical orthogonal function analysis shows that these two biases are closely related to the dominant inter-model discrepancies of precipitation patterns. The two biases are attributed to two factors, respectively. In the CMIP5 models with the prominent double ITCZ, the deep convection is not sensitive enough to environmental air humidity at the lower-mid troposphere, as is in CMIP3. Thus, the deep convection is not suppressed even over the dry subsidence region of the southeastern Pacific, forming the double ITCZ bias. Conversely, models with the severe cold tongue bias have lower ocean model resolution with too strong equatorial trades. Therefore, proper representation of the sensitivity of deep convection to humidity and higher resolution of the ocean models with better equatorial trades are important for reducing the double ITCZ and the cold tongue biases.

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Why the ITCZ Is Mostly North of the Equator

Cited by 468 publications

References 0 publications

A physical basis for the interannual variability of rainfall in the Sahel

A physical basis for the interannual variability of rainfall in the Sahel

Deconstructing the Hadley cell heat transport

Reproducibility of precipitation distribution over the tropical oceans in CMIP5 multi-climate models compared to CMIP3

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