ATOC 5051: Introduction to Physical Oceanography HW #6

Home , Current (stream), Ekman layer, Oceanography, Thermocline

The oceanic surface boundary layer, sometimes called the Ekman layer, is in direct contact with the atmosphere, therefore it is directly forced by winds, heat and salinity fluxes. In this layer, the observed temperature and salinity vary little and thus they are well mixed. Consequently, this layer is often referred to as the surface mixed layer.

(a) For Coriolis parameter and eddy viscosity calculate the Ekman layer thickness. (5pts)

The Ekman layer thickness

(b) In the Northern Hemisphere, the continuously stratified ocean is forced by spatially varying winds, with southerly wind in the west (into the paper) and northerly wind in the east (out of the paper). Sketch the thermocline variability and Ekman pumping velocity (5 points). Discuss how the deep ocean (the region below the surface mixed layer) is set in motion (5 points).

See below for the sketch.

isopycnals

Surface Ekman convergence causes a downward Ekman pumping velocity, pushing the thermocline downward and thus generating horizontal pressure gradient force. This pressure gradient force drives geostrophic current in the thermocline, setting the deep ocean in motion.

2. Thermodynamics (40pts)

The mixed layer temperature (often referred to as sea surface temperature) equation is:

1 (1) where are surface radiative, sensible, and latent heat fluxes, respectively; is horizontal current, is upwelling velocity, is entrainment rate, is temperature in the thermocline, is mixed layer thickness, is density of mixed layer, and is specific heat of sea water. Note that effect of precipitation on is ignored in the above equation.

(a) In the eastern equatorial Pacific where southeasterly trade winds prevail. Using equation (1) to discuss the processes for the cold tongue formation and its westward extension. (10 pts)

The easterly wind components of the southeasterly trades drive poleward (off-equatorial) Ekman divergence, shoal the thermocline, and thus produce equatorial upwelling. The colder, subsurface water is upwelled to the surface mixed layer, cools the SST. In equation (1), this process is represented by term. Meanwhile, strong winds associated with the easterly trades act to enhance the cooling by increasing evaporation (latent heat loss) and entrainment. The latent heat flux is and entrainment cooling is term in equation (1). Additionally, the easterly winds also forces a westward surface current, which is called south equatorial current (SEC). The SEC advects the cold SST in the eastern basin westward, extending the cold water to the west and thus forms the cold tongue ( term). (Note that air/sea coupling tends to enhance the easterly winds on the equator, and thus helps to produce the cold tongue and its westward extension. This part is not required. There won't be point deduction if the students do not say the air/sea coupling effect.)

(b) Assume the eastern boundary of the Pacific is oriented north-south, and strong northeasterly trades cause an anticyclonic wind (clockwise or negative wind curl) in the off-equatorial region near 15oN (Fig. 1). Draw a schematic diagram showing the Ekman transport and thermocline variability (up or down) in the anomalous wind region. (5 pts) Wind anomaly North

15N Fig. 1. Wind anomaly over the north tropical Pacific Ocean.

West EQ east

Sout h

Below is the schematic diagram showing surface Ekman convergence and thermocline variability.

Ekman convergence North z 15N 15N

EQ east

Downwelling: thermocline depressed

(c) Through what mechanisms can this thermocline variability affect the mixed layer temperature of the eastern equatorial Pacific cold tongue? Will it increase or decrease the cold tongue temperature, and why? (10 pts)

The anticyclonic winds cause convergence and thus deepen the thermocline. The deepened thermocline signal can propagate westward as Rossby waves. As the Rossby waves arrive at the western boundary, the thermocline signal propagates to the equator as coastal Kelvin waves. At the equator, the deepened thermocline can propagate eastward as equatorial Kelvin waves to the eastern Pacific cold tongue region, where they suppress the thermocline and thus reduce upwelling, resulting in an increased SST (or mixed layer temperature).

(d) During winter of the far North Atlantic, net surface cooling due to radiation and turbulent heat fluxes dominates oceanic processes. Use equation (1) to discuss how this surface cooling affects the deep water formation so as to influence the global thermohaline circulation. (15 points)

Since surface heat fluxes dominate the oceanic processes, the last three terms in equation (1) can be dropped. The equation is simplified to . During winter, and thus Mixed layer temperature decreases and therefore density increases according to the equation of state. When the mixed layer is cooled so much that density inversion occurs in some regions, deep convection happens and deep water forms, which is believed to drive the global thermohaline circulation.

3. The El Nino and Southern Oscillation (ENSO) (35pts) (a) [6pts total] Describe the ocean-atmospheric circulation in the equatorial Pacific Ocean for a normal year: including atmospheric convection, surface wind, Walker circulation (3pts); oceanic upwelling, thermocline, SST, sea level, surface currents and biological activities (3pts).

3 During normal years, the air rises in the region of the warm water in the western equatorial Pacific (warm pool) and the rising air is characterized by low pressure at the surface. The winds across the surface of the tropical Pacific blow westward into the region of low pressure, consistent with the westward trade winds, which is the surface branch of the Walker cell. The Walker Cell has a rising branch (convection) in the western Pacific warm pool, an eastward high-level wind, a sinking branch in the eastern equatorial Pacific, and a westward surface wind. Corresponding to the equatorial easterly and southeasterly trades, upwelling occurs along the west coast of South America and in the eastern equatorial basin, where thermocline shoals, sea level drops and SST reduces (Figs 2 and 6 of chapter 6). Upwelling brings nutrient up to the euphotic zone, favoring biological activities and fishery. The surface current (South Equatorial Current) flows westward.

(b) [26pts total] Provide detailed discussions on ENSO recharge mechanism (20pts), and provide schematic diagrams to show key features of anomalous convection, zonal surface wind and thermocline depth for each phase of the ENSO cycle (6pts). When you discuss each phase and phase transition of ENSO, please include the discussion on: • east-west displacement of the warm pool (28.5C isotherm), and its associated convection and surface wind anomaly; • east-west thermocline depth anomaly and its associated oceanic processes; • change of mean thermocline depth and upper-ocean heat (warm water volume) in the equatorial basin; • SST anomalies and associated processes.

Let’s begin with phase 1, the mature warm phase of ENSO, which is El Nino. During the peak phase of El Nino, the warm pool (28.5C isotherm) and its associated convection move eastward. The anomalous convection in the central-eastern Pacific drives equatorial westerly wind anomalies in the western and central Pacific basin. The anomalous equatorial westerlies cause equatorial Ekman convergence and off-equatorial divergence, increasing sea level and deepening the thermocline on the equator, and reducing sea level and shoaling the thermocline on both sides of the equator. The deepened thermocline (rising sea level) on the equator propagates eastward as Kelvin waves, and the off-equatorial shoaling thermocline (falling sea level) signals propagate westward as Rossby waves, setting up the east-west sea level and thermocline tilts. The deepened thermocline reduces upwelling and thus increases SST (positive SST anomaly) in the eastern equatorial Pacific. The Kelvin and Rossby wave processes are fast, which take only ~8months. Comparing with the 2~7 year ENSO period, the east-west thermocline tilt set-up is fast and is in quasi-equilibrium with the westerly wind anomalies. The geostrophic currents associated with the sea level and thermocline tilt are poleward, and thus the currents driven by the anomalous wind stress curl. These poleward currents discharge heat out of the equatorial region, causing a shallower equatorial mean thermocline depth. This shallower mean thermocline reduces the positive SST anomaly (SSTA) in the eastern equatorial Pacific associated with El Nino. Eventually, the small positive SSTA can be balanced by the negative feedbacks, such as reduced solar radiation associated with the increased convection and increased evaporation associated with the positive SSTA, resulting in a zero SSTA in the eastern Pacific, zero convection westerly wind anomalies. ENSO enters transition phase - phase 2 in the Figure below.

Because of the shallower mean thermocline, the mean climatological winds will cause colder SSTA in the eastern equatorial Pacific by entraining colder subsurface water into the surface. The colder SSTA increases sea level pressure and induces an easterly wind anomaly along the equator. Similar to the El

4 Nino phase but with an opposite sign, the anomalous equatorial easterlies set up the east-west thermocline and sea level tilt, with a shoaled thermocline in the east and deepened thermocline in the west via Kelvin and Rossby wave processes. The warm pool moves westward, and thus its associated convection. The colder SSTA in the central Pacific reduces convection there, and ENSO reaches its peak negative phase, which is La Nina (phase 3).

Meanwhile, the geostrophic currents and currents induced by the anomalous wind stress curl are equatorward, recharging heat into the equatorial region and producing a deepened mean thermocline. The deepened thermocline reduces the negative SSTA in the east. At some point, the weakened negative SSTA is balanced by the negative feedback processes, such as increased solar heating associated with reduced convection and reduced evaporation due to decreased SST, producing zero SSTA, convection and surface wind. ENSO enters phase 4.

Due to the deepened mean thermocline, climatological winds will entrain relative less cold water into the surface, and thus cause a positive SST. Then the system will transit to phase 1.

(c) Which phase transition has not been observed in Kessler 2002 paper (1pt), and what does this physically indicate (2pts)?

Kessler (2002) did not observe the transition from phase 4 to phase I, indicating that in the coupled climate system requires a “kick” to initiate El Nino. This is true at least for some El Nino events, such as the 1997-98 El Nino, which is likely initiated by the MJO (McPhaden, 1999).

A schematic diagram showing ENSO recharge mechanism.

4. Salinity budget (15pts)

Similar to the temperature equation, the mixed layer salinity equation can be written as:

6 where is salinity flux due to precipitation, is salinity flux due to evaporation, is salinity flux due to river runoff, is salinity flux due to sea ice freezing and melting. The rest terms are salinity fluxes due to entrainment, horizontal advection, and upwelling.

In the Mediterranean Sea, salinity fluxes are dominated by evaporation and precipitation, and evaporation is greater than precipitation. Using the above equation to discuss how the excessive evaporation affects the Mediterranean Deep Water formation. (15 points)

In the Mediterranean Sea, since salinity changes are dominated by evaporation and precipitation, the mixed layer salinity equation can be simplified as: When evaporation exceeds precipitation, and thus causing sea surface salinity to increase. The increased surface salinity increases density. As the surface density is higher than that of the deeper ocean, convection occurs and thus deep water forms.