Wengel, Christian (2015) Tropical Pacific mean state and ENSO sensitivity to stratiform cloud representation. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 107 pp.

Text Msc_Wengel_christian_W_2015.pdf - Reprinted Version Restricted to Registered users only Available under License German copyright act UrhG. Download (13MB) | Contact

Abstract

The simulation of tropical Pacific mean state and El Nino / Southern Oscillation (ENSO) is investigated in the Kiel Climate Model (KCM). Compared to observations, the KCM produces an excessive equatorial cold tongue, too strong equatorial wind stress and too warm surface and subsurface conditions in the western equatorial Pacific as well as on both sides of the equator. These biases in the mean state impact the simulation of ENSO. The KCM tends to overestimate ENSO amplitude as a result of too warm surface conditions, in particular in the western tropical Pacific. ENSO asymmetry is too weak, because the Bjerknes feedback is underestimated which is in tum suspected to be a result of the equatorial cold bias. Moreover, the period of ENSO is too short because ENSO dynamics are too narrowly confined about the equator and due to the coupled ocean-atmosphere system being overdamped. The latter is a result of too strong equatorial winds. The recharge/discharge mechanism associated with ENSO is favorably well represented. ENSO phase-locking to the annual cycle is not captured. This is assumed to be related to the underestimation of thermodynamical damping. It is argued that the equatorial cold bias acts to suppress the dynamical response of the atmosphere to pertubations of eastern equatorial sea surface temperature (SST) and, thus, accounts for an incorrect representation of the shortwave feedback. The sensitivity of the tropical Pacific mean state and ENSO to stratiform cloud cover representation is furthermore assessed by comparing two KCM control experiments with different cloud cover parameterizations. In a warmer climate, as a result of stronger shortwave radiation induced by weaker climatological mean cloud cover, ENSO is more intense. This is characterized by a higher ENSO amplitude and skewness which is the consequence of a less stable coupled system under warmer climatological mean conditions.

Actions (login required)

View Item