Tropical Pacific impacts of convective momentum transport in the SNU coupled GCM

Abstract

Impacts of convective momentum transport (CMT) on tropical Pacific climate are examined, using an atmospheric (AGCM) and coupled GCM (CGCM) from Seoul National University. The CMT scheme affects the surface mainly via a convection-compensating atmospheric subsidence which conveys momentum downward through most of the troposphere. AGCM simulations—with SSTs prescribed from climatological and El Nino Southern Oscillation (ENSO) conditions—show substantial changes in circulation when CMT is added, such as an eastward shift of the climatological trade winds and west Pacific convection. The CMT also alters the ENSO wind anomalies by shifting them eastward and widening them meridionally, despite only subtle changes in the precipitation anomaly patterns. During ENSO, CMT affects the low-level winds mainly via the anomalous convection acting on the climatological westerly wind shear over the central Pacific—so that an eastward shift of convection transfers more westerly momentum toward the surface than would occur without CMT. By altering the low-level circulation, the CMT further alters the precipitation, which in turn feeds back on the CMT. In the CGCM, CMT affects the simulated climatology by shifting the mean convection and trade winds eastward and warming the equatorial SST; the ENSO period and amplitude also increase. In contrast to the AGCM simulations, CMT substantially alters the El Nino precipitation anomaly patterns in the CGCM. Also discussed are possible impacts of the CMT-induced changes in climatology on the simulated ENSO.

Appendix

Budget calculation for anomalous vertical advection

Here, we derive Eq. (2) using daily averaged convective mass flux (M _c ) and zonal wind (u). First the daily-mean variables from the climatological run (( )^C) and El Nino SST run (( )^E) are divided as follows,

$$ \begin{aligned} M^{C}_{c} &= \overline{{(M^{C}_{c})}} + (M^{C}_{c})^{\prime\prime}\\u^{C}_{c} &= \overline{{(u^{C}_{c})}} + (u^{C}_{c})^{\prime\prime} \end{aligned} $$

(1a)

$$ \begin{aligned} M^{E}_{c} &= \overline{{(M^{C}_{c})}} + (M^{E}_{c})^{\prime}\,\, + \,\,(M^{E}_{c})^{\prime\prime}\\u^{E}_{c} &= \overline{{(u^{C}_{c})}} + (u^{E}_{c})^{\prime}\,\, + \,\,(u^{E}_{c})^{\prime\prime} \end{aligned} $$

(2a)

where an overbar $\overline{{(\;)}}$ denotes a seasonal mean value. A prime ( )′ denotes a seasonal-mean anomaly, and a double prime ( )′′ a daily fluctuation. Note that the seasonal mean from the El Nino SST run is decomposed into a climatology and seasonal mean anomaly.

The vertical advection term in Eq. (1) is

$$ - M_{c} \frac{{\partial u}}{{\partial p}} $$

(3a)

Substituting Eqs. (1a) and (2a) into (3a) and taking a seasonal mean, we get the seasonal mean momentum forcing by the CMT vertical advection term in Eq. (1).

For the climatology run,

$$ - \overline{{M^{C}_{c} \frac{{\partial u^{C}}}{{\partial p}}}} = - \overline{{(M^{C}_{c})}} \frac{\partial}{{\partial p}}\overline{{(u^{C})}} - \overline{{(M^{C}_{c})^{\prime\prime}\frac{\partial}{{\partial p}}(u^{C})^{\prime\prime}}} $$

(4a)

and for the El Nino SST run,

$$ \begin{aligned} - \overline{{M^{E}_{c} \frac{{\partial u^{E}}}{{\partial p}}}} &= - \,\overline{{(M^{C}_{c})}} \frac{\partial}{{\partial p}}\overline{{(u^{C})}} - \overline{{(M^{C}_{c})}} \frac{\partial}{{\partial p}}(u^{E})^{\prime}\, - (M^{E}_{c})^{\prime}\frac{\partial}{{\partial p}}\overline{{u^{C}}} \\ & \quad - (M^{E}_{c})^{\prime}\frac{\partial}{{\partial p}}(u^{E})^{\prime}\, - \overline{{(M^{E}_{c})^{\prime\prime}\frac{\partial}{{\partial p}}(u^{E})^{\prime\prime}}} \\\end{aligned} $$

(5a)

We then obtain the seasonally anomalous momentum forcing due to the CMT vertical advection term, by subtracting (5a) from (4a):

$$ \begin{aligned} - \overline{{M^{E}_{c} \frac{{\partial u^{E}}}{{\partial p}}}} + \overline{{M^{C}_{c} \frac{{\partial u^{C}}}{{\partial p}}}} &= - \overline{{(M^{C}_{c})}} \frac{\partial}{{\partial p}}(u^{E})^{\prime}\, - (M^{E}_{c})^{\prime}\frac{\partial}{{\partial p}}\overline{{u^{C}}} \\ & \quad - (M^{E}_{c})^{\prime}\frac{\partial}{{\partial p}}(u^{E})^{\prime}\, - \overline{{(M^{E}_{c})^{\prime\prime}\frac{\partial}{{\partial p}}(u^{E})^{\prime\prime}}} + \overline{{(M^{C}_{c})^{\prime\prime}\frac{\partial}{{\partial p}}(u^{C})^{\prime\prime}}} \\\end{aligned} $$

(6a)

Equation (6a) is repeated in the main text as Eq. (2). We label the four terms in the right hand side of Eq. (6a) as follows:

Term A—climatological mass flux and seasonally anomalous wind shear

$$ - \overline{{(M^{C}_{c})}} \frac{\partial}{{\partial p}}(u^{E})^{\prime} $$

(7a)

Term B—seasonally anomalous mass flux and climatological wind shear

$$ - (M^{E}_{c})^{\prime}\frac{\partial}{{\partial p}}\overline{{u^{C}}} $$

(8a)

Term C—seasonally anomalous mass flux and seasonally anomalous wind shear

$$ - (M^{E}_{c})^{\prime}\frac{\partial}{{\partial p}}(u^{E})^{\prime} $$

(9a)

Term D—change in transient eddy momentum

$$ - \overline{{(M^{E}_{c})^{\prime\prime}\frac{\partial}{{\partial p}}(u^{E})^{\prime\prime}}} + \overline{{(M^{C}_{c})^{\prime\prime}\frac{\partial}{{\partial p}}(u^{C})^{\prime\prime}}} $$

(10a)